It seems that Documentation/DMA-mapping.txt was supposed to be renamed
to Documentation/PCI/PCI-DMA-mapping.txt.
Signed-off-by: Kusanagi Kouichi <[email protected]>
---
Documentation/DMA-mapping.txt | 766 ---------------------------------
Documentation/PCI/PCI-DMA-mapping.txt | 766 +++++++++++++++++++++++++++++++++
Documentation/block/biodoc.txt | 2 +-
3 files changed, 767 insertions(+), 767 deletions(-)
delete mode 100644 Documentation/DMA-mapping.txt
create mode 100644 Documentation/PCI/PCI-DMA-mapping.txt
diff --git a/Documentation/DMA-mapping.txt b/Documentation/DMA-mapping.txt
deleted file mode 100644
index 01f24e9..0000000
--- a/Documentation/DMA-mapping.txt
+++ /dev/null
@@ -1,766 +0,0 @@
- Dynamic DMA mapping
- ===================
-
- David S. Miller <[email protected]>
- Richard Henderson <[email protected]>
- Jakub Jelinek <[email protected]>
-
-This document describes the DMA mapping system in terms of the pci_
-API. For a similar API that works for generic devices, see
-DMA-API.txt.
-
-Most of the 64bit platforms have special hardware that translates bus
-addresses (DMA addresses) into physical addresses. This is similar to
-how page tables and/or a TLB translates virtual addresses to physical
-addresses on a CPU. This is needed so that e.g. PCI devices can
-access with a Single Address Cycle (32bit DMA address) any page in the
-64bit physical address space. Previously in Linux those 64bit
-platforms had to set artificial limits on the maximum RAM size in the
-system, so that the virt_to_bus() static scheme works (the DMA address
-translation tables were simply filled on bootup to map each bus
-address to the physical page __pa(bus_to_virt())).
-
-So that Linux can use the dynamic DMA mapping, it needs some help from the
-drivers, namely it has to take into account that DMA addresses should be
-mapped only for the time they are actually used and unmapped after the DMA
-transfer.
-
-The following API will work of course even on platforms where no such
-hardware exists, see e.g. arch/x86/include/asm/pci.h for how it is implemented on
-top of the virt_to_bus interface.
-
-First of all, you should make sure
-
-#include <linux/pci.h>
-
-is in your driver. This file will obtain for you the definition of the
-dma_addr_t (which can hold any valid DMA address for the platform)
-type which should be used everywhere you hold a DMA (bus) address
-returned from the DMA mapping functions.
-
- What memory is DMA'able?
-
-The first piece of information you must know is what kernel memory can
-be used with the DMA mapping facilities. There has been an unwritten
-set of rules regarding this, and this text is an attempt to finally
-write them down.
-
-If you acquired your memory via the page allocator
-(i.e. __get_free_page*()) or the generic memory allocators
-(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
-that memory using the addresses returned from those routines.
-
-This means specifically that you may _not_ use the memory/addresses
-returned from vmalloc() for DMA. It is possible to DMA to the
-_underlying_ memory mapped into a vmalloc() area, but this requires
-walking page tables to get the physical addresses, and then
-translating each of those pages back to a kernel address using
-something like __va(). [ EDIT: Update this when we integrate
-Gerd Knorr's generic code which does this. ]
-
-This rule also means that you may use neither kernel image addresses
-(items in data/text/bss segments), nor module image addresses, nor
-stack addresses for DMA. These could all be mapped somewhere entirely
-different than the rest of physical memory. Even if those classes of
-memory could physically work with DMA, you'd need to ensure the I/O
-buffers were cacheline-aligned. Without that, you'd see cacheline
-sharing problems (data corruption) on CPUs with DMA-incoherent caches.
-(The CPU could write to one word, DMA would write to a different one
-in the same cache line, and one of them could be overwritten.)
-
-Also, this means that you cannot take the return of a kmap()
-call and DMA to/from that. This is similar to vmalloc().
-
-What about block I/O and networking buffers? The block I/O and
-networking subsystems make sure that the buffers they use are valid
-for you to DMA from/to.
-
- DMA addressing limitations
-
-Does your device have any DMA addressing limitations? For example, is
-your device only capable of driving the low order 24-bits of address
-on the PCI bus for SAC DMA transfers? If so, you need to inform the
-PCI layer of this fact.
-
-By default, the kernel assumes that your device can address the full
-32-bits in a SAC cycle. For a 64-bit DAC capable device, this needs
-to be increased. And for a device with limitations, as discussed in
-the previous paragraph, it needs to be decreased.
-
-pci_alloc_consistent() by default will return 32-bit DMA addresses.
-PCI-X specification requires PCI-X devices to support 64-bit
-addressing (DAC) for all transactions. And at least one platform (SGI
-SN2) requires 64-bit consistent allocations to operate correctly when
-the IO bus is in PCI-X mode. Therefore, like with pci_set_dma_mask(),
-it's good practice to call pci_set_consistent_dma_mask() to set the
-appropriate mask even if your device only supports 32-bit DMA
-(default) and especially if it's a PCI-X device.
-
-For correct operation, you must interrogate the PCI layer in your
-device probe routine to see if the PCI controller on the machine can
-properly support the DMA addressing limitation your device has. It is
-good style to do this even if your device holds the default setting,
-because this shows that you did think about these issues wrt. your
-device.
-
-The query is performed via a call to pci_set_dma_mask():
-
- int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask);
-
-The query for consistent allocations is performed via a call to
-pci_set_consistent_dma_mask():
-
- int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask);
-
-Here, pdev is a pointer to the PCI device struct of your device, and
-device_mask is a bit mask describing which bits of a PCI address your
-device supports. It returns zero if your card can perform DMA
-properly on the machine given the address mask you provided.
-
-If it returns non-zero, your device cannot perform DMA properly on
-this platform, and attempting to do so will result in undefined
-behavior. You must either use a different mask, or not use DMA.
-
-This means that in the failure case, you have three options:
-
-1) Use another DMA mask, if possible (see below).
-2) Use some non-DMA mode for data transfer, if possible.
-3) Ignore this device and do not initialize it.
-
-It is recommended that your driver print a kernel KERN_WARNING message
-when you end up performing either #2 or #3. In this manner, if a user
-of your driver reports that performance is bad or that the device is not
-even detected, you can ask them for the kernel messages to find out
-exactly why.
-
-The standard 32-bit addressing PCI device would do something like
-this:
-
- if (pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
- printk(KERN_WARNING
- "mydev: No suitable DMA available.\n");
- goto ignore_this_device;
- }
-
-Another common scenario is a 64-bit capable device. The approach
-here is to try for 64-bit DAC addressing, but back down to a
-32-bit mask should that fail. The PCI platform code may fail the
-64-bit mask not because the platform is not capable of 64-bit
-addressing. Rather, it may fail in this case simply because
-32-bit SAC addressing is done more efficiently than DAC addressing.
-Sparc64 is one platform which behaves in this way.
-
-Here is how you would handle a 64-bit capable device which can drive
-all 64-bits when accessing streaming DMA:
-
- int using_dac;
-
- if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
- using_dac = 1;
- } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
- using_dac = 0;
- } else {
- printk(KERN_WARNING
- "mydev: No suitable DMA available.\n");
- goto ignore_this_device;
- }
-
-If a card is capable of using 64-bit consistent allocations as well,
-the case would look like this:
-
- int using_dac, consistent_using_dac;
-
- if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
- using_dac = 1;
- consistent_using_dac = 1;
- pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(64));
- } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
- using_dac = 0;
- consistent_using_dac = 0;
- pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(32));
- } else {
- printk(KERN_WARNING
- "mydev: No suitable DMA available.\n");
- goto ignore_this_device;
- }
-
-pci_set_consistent_dma_mask() will always be able to set the same or a
-smaller mask as pci_set_dma_mask(). However for the rare case that a
-device driver only uses consistent allocations, one would have to
-check the return value from pci_set_consistent_dma_mask().
-
-Finally, if your device can only drive the low 24-bits of
-address during PCI bus mastering you might do something like:
-
- if (pci_set_dma_mask(pdev, DMA_BIT_MASK(24))) {
- printk(KERN_WARNING
- "mydev: 24-bit DMA addressing not available.\n");
- goto ignore_this_device;
- }
-
-When pci_set_dma_mask() is successful, and returns zero, the PCI layer
-saves away this mask you have provided. The PCI layer will use this
-information later when you make DMA mappings.
-
-There is a case which we are aware of at this time, which is worth
-mentioning in this documentation. If your device supports multiple
-functions (for example a sound card provides playback and record
-functions) and the various different functions have _different_
-DMA addressing limitations, you may wish to probe each mask and
-only provide the functionality which the machine can handle. It
-is important that the last call to pci_set_dma_mask() be for the
-most specific mask.
-
-Here is pseudo-code showing how this might be done:
-
- #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32)
- #define RECORD_ADDRESS_BITS 0x00ffffff
-
- struct my_sound_card *card;
- struct pci_dev *pdev;
-
- ...
- if (!pci_set_dma_mask(pdev, PLAYBACK_ADDRESS_BITS)) {
- card->playback_enabled = 1;
- } else {
- card->playback_enabled = 0;
- printk(KERN_WARN "%s: Playback disabled due to DMA limitations.\n",
- card->name);
- }
- if (!pci_set_dma_mask(pdev, RECORD_ADDRESS_BITS)) {
- card->record_enabled = 1;
- } else {
- card->record_enabled = 0;
- printk(KERN_WARN "%s: Record disabled due to DMA limitations.\n",
- card->name);
- }
-
-A sound card was used as an example here because this genre of PCI
-devices seems to be littered with ISA chips given a PCI front end,
-and thus retaining the 16MB DMA addressing limitations of ISA.
-
- Types of DMA mappings
-
-There are two types of DMA mappings:
-
-- Consistent DMA mappings which are usually mapped at driver
- initialization, unmapped at the end and for which the hardware should
- guarantee that the device and the CPU can access the data
- in parallel and will see updates made by each other without any
- explicit software flushing.
-
- Think of "consistent" as "synchronous" or "coherent".
-
- The current default is to return consistent memory in the low 32
- bits of the PCI bus space. However, for future compatibility you
- should set the consistent mask even if this default is fine for your
- driver.
-
- Good examples of what to use consistent mappings for are:
-
- - Network card DMA ring descriptors.
- - SCSI adapter mailbox command data structures.
- - Device firmware microcode executed out of
- main memory.
-
- The invariant these examples all require is that any CPU store
- to memory is immediately visible to the device, and vice
- versa. Consistent mappings guarantee this.
-
- IMPORTANT: Consistent DMA memory does not preclude the usage of
- proper memory barriers. The CPU may reorder stores to
- consistent memory just as it may normal memory. Example:
- if it is important for the device to see the first word
- of a descriptor updated before the second, you must do
- something like:
-
- desc->word0 = address;
- wmb();
- desc->word1 = DESC_VALID;
-
- in order to get correct behavior on all platforms.
-
- Also, on some platforms your driver may need to flush CPU write
- buffers in much the same way as it needs to flush write buffers
- found in PCI bridges (such as by reading a register's value
- after writing it).
-
-- Streaming DMA mappings which are usually mapped for one DMA transfer,
- unmapped right after it (unless you use pci_dma_sync_* below) and for which
- hardware can optimize for sequential accesses.
-
- This of "streaming" as "asynchronous" or "outside the coherency
- domain".
-
- Good examples of what to use streaming mappings for are:
-
- - Networking buffers transmitted/received by a device.
- - Filesystem buffers written/read by a SCSI device.
-
- The interfaces for using this type of mapping were designed in
- such a way that an implementation can make whatever performance
- optimizations the hardware allows. To this end, when using
- such mappings you must be explicit about what you want to happen.
-
-Neither type of DMA mapping has alignment restrictions that come
-from PCI, although some devices may have such restrictions.
-Also, systems with caches that aren't DMA-coherent will work better
-when the underlying buffers don't share cache lines with other data.
-
-
- Using Consistent DMA mappings.
-
-To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
-you should do:
-
- dma_addr_t dma_handle;
-
- cpu_addr = pci_alloc_consistent(pdev, size, &dma_handle);
-
-where pdev is a struct pci_dev *. This may be called in interrupt context.
-You should use dma_alloc_coherent (see DMA-API.txt) for buses
-where devices don't have struct pci_dev (like ISA, EISA).
-
-This argument is needed because the DMA translations may be bus
-specific (and often is private to the bus which the device is attached
-to).
-
-Size is the length of the region you want to allocate, in bytes.
-
-This routine will allocate RAM for that region, so it acts similarly to
-__get_free_pages (but takes size instead of a page order). If your
-driver needs regions sized smaller than a page, you may prefer using
-the pci_pool interface, described below.
-
-The consistent DMA mapping interfaces, for non-NULL pdev, will by
-default return a DMA address which is SAC (Single Address Cycle)
-addressable. Even if the device indicates (via PCI dma mask) that it
-may address the upper 32-bits and thus perform DAC cycles, consistent
-allocation will only return > 32-bit PCI addresses for DMA if the
-consistent dma mask has been explicitly changed via
-pci_set_consistent_dma_mask(). This is true of the pci_pool interface
-as well.
-
-pci_alloc_consistent returns two values: the virtual address which you
-can use to access it from the CPU and dma_handle which you pass to the
-card.
-
-The cpu return address and the DMA bus master address are both
-guaranteed to be aligned to the smallest PAGE_SIZE order which
-is greater than or equal to the requested size. This invariant
-exists (for example) to guarantee that if you allocate a chunk
-which is smaller than or equal to 64 kilobytes, the extent of the
-buffer you receive will not cross a 64K boundary.
-
-To unmap and free such a DMA region, you call:
-
- pci_free_consistent(pdev, size, cpu_addr, dma_handle);
-
-where pdev, size are the same as in the above call and cpu_addr and
-dma_handle are the values pci_alloc_consistent returned to you.
-This function may not be called in interrupt context.
-
-If your driver needs lots of smaller memory regions, you can write
-custom code to subdivide pages returned by pci_alloc_consistent,
-or you can use the pci_pool API to do that. A pci_pool is like
-a kmem_cache, but it uses pci_alloc_consistent not __get_free_pages.
-Also, it understands common hardware constraints for alignment,
-like queue heads needing to be aligned on N byte boundaries.
-
-Create a pci_pool like this:
-
- struct pci_pool *pool;
-
- pool = pci_pool_create(name, pdev, size, align, alloc);
-
-The "name" is for diagnostics (like a kmem_cache name); pdev and size
-are as above. The device's hardware alignment requirement for this
-type of data is "align" (which is expressed in bytes, and must be a
-power of two). If your device has no boundary crossing restrictions,
-pass 0 for alloc; passing 4096 says memory allocated from this pool
-must not cross 4KByte boundaries (but at that time it may be better to
-go for pci_alloc_consistent directly instead).
-
-Allocate memory from a pci pool like this:
-
- cpu_addr = pci_pool_alloc(pool, flags, &dma_handle);
-
-flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
-holding SMP locks), SLAB_ATOMIC otherwise. Like pci_alloc_consistent,
-this returns two values, cpu_addr and dma_handle.
-
-Free memory that was allocated from a pci_pool like this:
-
- pci_pool_free(pool, cpu_addr, dma_handle);
-
-where pool is what you passed to pci_pool_alloc, and cpu_addr and
-dma_handle are the values pci_pool_alloc returned. This function
-may be called in interrupt context.
-
-Destroy a pci_pool by calling:
-
- pci_pool_destroy(pool);
-
-Make sure you've called pci_pool_free for all memory allocated
-from a pool before you destroy the pool. This function may not
-be called in interrupt context.
-
- DMA Direction
-
-The interfaces described in subsequent portions of this document
-take a DMA direction argument, which is an integer and takes on
-one of the following values:
-
- PCI_DMA_BIDIRECTIONAL
- PCI_DMA_TODEVICE
- PCI_DMA_FROMDEVICE
- PCI_DMA_NONE
-
-One should provide the exact DMA direction if you know it.
-
-PCI_DMA_TODEVICE means "from main memory to the PCI device"
-PCI_DMA_FROMDEVICE means "from the PCI device to main memory"
-It is the direction in which the data moves during the DMA
-transfer.
-
-You are _strongly_ encouraged to specify this as precisely
-as you possibly can.
-
-If you absolutely cannot know the direction of the DMA transfer,
-specify PCI_DMA_BIDIRECTIONAL. It means that the DMA can go in
-either direction. The platform guarantees that you may legally
-specify this, and that it will work, but this may be at the
-cost of performance for example.
-
-The value PCI_DMA_NONE is to be used for debugging. One can
-hold this in a data structure before you come to know the
-precise direction, and this will help catch cases where your
-direction tracking logic has failed to set things up properly.
-
-Another advantage of specifying this value precisely (outside of
-potential platform-specific optimizations of such) is for debugging.
-Some platforms actually have a write permission boolean which DMA
-mappings can be marked with, much like page protections in the user
-program address space. Such platforms can and do report errors in the
-kernel logs when the PCI controller hardware detects violation of the
-permission setting.
-
-Only streaming mappings specify a direction, consistent mappings
-implicitly have a direction attribute setting of
-PCI_DMA_BIDIRECTIONAL.
-
-The SCSI subsystem tells you the direction to use in the
-'sc_data_direction' member of the SCSI command your driver is
-working on.
-
-For Networking drivers, it's a rather simple affair. For transmit
-packets, map/unmap them with the PCI_DMA_TODEVICE direction
-specifier. For receive packets, just the opposite, map/unmap them
-with the PCI_DMA_FROMDEVICE direction specifier.
-
- Using Streaming DMA mappings
-
-The streaming DMA mapping routines can be called from interrupt
-context. There are two versions of each map/unmap, one which will
-map/unmap a single memory region, and one which will map/unmap a
-scatterlist.
-
-To map a single region, you do:
-
- struct pci_dev *pdev = mydev->pdev;
- dma_addr_t dma_handle;
- void *addr = buffer->ptr;
- size_t size = buffer->len;
-
- dma_handle = pci_map_single(pdev, addr, size, direction);
-
-and to unmap it:
-
- pci_unmap_single(pdev, dma_handle, size, direction);
-
-You should call pci_unmap_single when the DMA activity is finished, e.g.
-from the interrupt which told you that the DMA transfer is done.
-
-Using cpu pointers like this for single mappings has a disadvantage,
-you cannot reference HIGHMEM memory in this way. Thus, there is a
-map/unmap interface pair akin to pci_{map,unmap}_single. These
-interfaces deal with page/offset pairs instead of cpu pointers.
-Specifically:
-
- struct pci_dev *pdev = mydev->pdev;
- dma_addr_t dma_handle;
- struct page *page = buffer->page;
- unsigned long offset = buffer->offset;
- size_t size = buffer->len;
-
- dma_handle = pci_map_page(pdev, page, offset, size, direction);
-
- ...
-
- pci_unmap_page(pdev, dma_handle, size, direction);
-
-Here, "offset" means byte offset within the given page.
-
-With scatterlists, you map a region gathered from several regions by:
-
- int i, count = pci_map_sg(pdev, sglist, nents, direction);
- struct scatterlist *sg;
-
- for_each_sg(sglist, sg, count, i) {
- hw_address[i] = sg_dma_address(sg);
- hw_len[i] = sg_dma_len(sg);
- }
-
-where nents is the number of entries in the sglist.
-
-The implementation is free to merge several consecutive sglist entries
-into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
-consecutive sglist entries can be merged into one provided the first one
-ends and the second one starts on a page boundary - in fact this is a huge
-advantage for cards which either cannot do scatter-gather or have very
-limited number of scatter-gather entries) and returns the actual number
-of sg entries it mapped them to. On failure 0 is returned.
-
-Then you should loop count times (note: this can be less than nents times)
-and use sg_dma_address() and sg_dma_len() macros where you previously
-accessed sg->address and sg->length as shown above.
-
-To unmap a scatterlist, just call:
-
- pci_unmap_sg(pdev, sglist, nents, direction);
-
-Again, make sure DMA activity has already finished.
-
-PLEASE NOTE: The 'nents' argument to the pci_unmap_sg call must be
- the _same_ one you passed into the pci_map_sg call,
- it should _NOT_ be the 'count' value _returned_ from the
- pci_map_sg call.
-
-Every pci_map_{single,sg} call should have its pci_unmap_{single,sg}
-counterpart, because the bus address space is a shared resource (although
-in some ports the mapping is per each BUS so less devices contend for the
-same bus address space) and you could render the machine unusable by eating
-all bus addresses.
-
-If you need to use the same streaming DMA region multiple times and touch
-the data in between the DMA transfers, the buffer needs to be synced
-properly in order for the cpu and device to see the most uptodate and
-correct copy of the DMA buffer.
-
-So, firstly, just map it with pci_map_{single,sg}, and after each DMA
-transfer call either:
-
- pci_dma_sync_single_for_cpu(pdev, dma_handle, size, direction);
-
-or:
-
- pci_dma_sync_sg_for_cpu(pdev, sglist, nents, direction);
-
-as appropriate.
-
-Then, if you wish to let the device get at the DMA area again,
-finish accessing the data with the cpu, and then before actually
-giving the buffer to the hardware call either:
-
- pci_dma_sync_single_for_device(pdev, dma_handle, size, direction);
-
-or:
-
- pci_dma_sync_sg_for_device(dev, sglist, nents, direction);
-
-as appropriate.
-
-After the last DMA transfer call one of the DMA unmap routines
-pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_*
-call till pci_unmap_*, then you don't have to call the pci_dma_sync_*
-routines at all.
-
-Here is pseudo code which shows a situation in which you would need
-to use the pci_dma_sync_*() interfaces.
-
- my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
- {
- dma_addr_t mapping;
-
- mapping = pci_map_single(cp->pdev, buffer, len, PCI_DMA_FROMDEVICE);
-
- cp->rx_buf = buffer;
- cp->rx_len = len;
- cp->rx_dma = mapping;
-
- give_rx_buf_to_card(cp);
- }
-
- ...
-
- my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
- {
- struct my_card *cp = devid;
-
- ...
- if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
- struct my_card_header *hp;
-
- /* Examine the header to see if we wish
- * to accept the data. But synchronize
- * the DMA transfer with the CPU first
- * so that we see updated contents.
- */
- pci_dma_sync_single_for_cpu(cp->pdev, cp->rx_dma,
- cp->rx_len,
- PCI_DMA_FROMDEVICE);
-
- /* Now it is safe to examine the buffer. */
- hp = (struct my_card_header *) cp->rx_buf;
- if (header_is_ok(hp)) {
- pci_unmap_single(cp->pdev, cp->rx_dma, cp->rx_len,
- PCI_DMA_FROMDEVICE);
- pass_to_upper_layers(cp->rx_buf);
- make_and_setup_new_rx_buf(cp);
- } else {
- /* Just sync the buffer and give it back
- * to the card.
- */
- pci_dma_sync_single_for_device(cp->pdev,
- cp->rx_dma,
- cp->rx_len,
- PCI_DMA_FROMDEVICE);
- give_rx_buf_to_card(cp);
- }
- }
- }
-
-Drivers converted fully to this interface should not use virt_to_bus any
-longer, nor should they use bus_to_virt. Some drivers have to be changed a
-little bit, because there is no longer an equivalent to bus_to_virt in the
-dynamic DMA mapping scheme - you have to always store the DMA addresses
-returned by the pci_alloc_consistent, pci_pool_alloc, and pci_map_single
-calls (pci_map_sg stores them in the scatterlist itself if the platform
-supports dynamic DMA mapping in hardware) in your driver structures and/or
-in the card registers.
-
-All PCI drivers should be using these interfaces with no exceptions.
-It is planned to completely remove virt_to_bus() and bus_to_virt() as
-they are entirely deprecated. Some ports already do not provide these
-as it is impossible to correctly support them.
-
- Optimizing Unmap State Space Consumption
-
-On many platforms, pci_unmap_{single,page}() is simply a nop.
-Therefore, keeping track of the mapping address and length is a waste
-of space. Instead of filling your drivers up with ifdefs and the like
-to "work around" this (which would defeat the whole purpose of a
-portable API) the following facilities are provided.
-
-Actually, instead of describing the macros one by one, we'll
-transform some example code.
-
-1) Use DECLARE_PCI_UNMAP_{ADDR,LEN} in state saving structures.
- Example, before:
-
- struct ring_state {
- struct sk_buff *skb;
- dma_addr_t mapping;
- __u32 len;
- };
-
- after:
-
- struct ring_state {
- struct sk_buff *skb;
- DECLARE_PCI_UNMAP_ADDR(mapping)
- DECLARE_PCI_UNMAP_LEN(len)
- };
-
- NOTE: DO NOT put a semicolon at the end of the DECLARE_*()
- macro.
-
-2) Use pci_unmap_{addr,len}_set to set these values.
- Example, before:
-
- ringp->mapping = FOO;
- ringp->len = BAR;
-
- after:
-
- pci_unmap_addr_set(ringp, mapping, FOO);
- pci_unmap_len_set(ringp, len, BAR);
-
-3) Use pci_unmap_{addr,len} to access these values.
- Example, before:
-
- pci_unmap_single(pdev, ringp->mapping, ringp->len,
- PCI_DMA_FROMDEVICE);
-
- after:
-
- pci_unmap_single(pdev,
- pci_unmap_addr(ringp, mapping),
- pci_unmap_len(ringp, len),
- PCI_DMA_FROMDEVICE);
-
-It really should be self-explanatory. We treat the ADDR and LEN
-separately, because it is possible for an implementation to only
-need the address in order to perform the unmap operation.
-
- Platform Issues
-
-If you are just writing drivers for Linux and do not maintain
-an architecture port for the kernel, you can safely skip down
-to "Closing".
-
-1) Struct scatterlist requirements.
-
- Struct scatterlist must contain, at a minimum, the following
- members:
-
- struct page *page;
- unsigned int offset;
- unsigned int length;
-
- The base address is specified by a "page+offset" pair.
-
- Previous versions of struct scatterlist contained a "void *address"
- field that was sometimes used instead of page+offset. As of Linux
- 2.5., page+offset is always used, and the "address" field has been
- deleted.
-
-2) More to come...
-
- Handling Errors
-
-DMA address space is limited on some architectures and an allocation
-failure can be determined by:
-
-- checking if pci_alloc_consistent returns NULL or pci_map_sg returns 0
-
-- checking the returned dma_addr_t of pci_map_single and pci_map_page
- by using pci_dma_mapping_error():
-
- dma_addr_t dma_handle;
-
- dma_handle = pci_map_single(pdev, addr, size, direction);
- if (pci_dma_mapping_error(pdev, dma_handle)) {
- /*
- * reduce current DMA mapping usage,
- * delay and try again later or
- * reset driver.
- */
- }
-
- Closing
-
-This document, and the API itself, would not be in it's current
-form without the feedback and suggestions from numerous individuals.
-We would like to specifically mention, in no particular order, the
-following people:
-
- Russell King <[email protected]>
- Leo Dagum <[email protected]>
- Ralf Baechle <[email protected]>
- Grant Grundler <[email protected]>
- Jay Estabrook <[email protected]>
- Thomas Sailer <[email protected]>
- Andrea Arcangeli <[email protected]>
- Jens Axboe <[email protected]>
- David Mosberger-Tang <[email protected]>
diff --git a/Documentation/PCI/PCI-DMA-mapping.txt b/Documentation/PCI/PCI-DMA-mapping.txt
new file mode 100644
index 0000000..01f24e9
--- /dev/null
+++ b/Documentation/PCI/PCI-DMA-mapping.txt
@@ -0,0 +1,766 @@
+ Dynamic DMA mapping
+ ===================
+
+ David S. Miller <[email protected]>
+ Richard Henderson <[email protected]>
+ Jakub Jelinek <[email protected]>
+
+This document describes the DMA mapping system in terms of the pci_
+API. For a similar API that works for generic devices, see
+DMA-API.txt.
+
+Most of the 64bit platforms have special hardware that translates bus
+addresses (DMA addresses) into physical addresses. This is similar to
+how page tables and/or a TLB translates virtual addresses to physical
+addresses on a CPU. This is needed so that e.g. PCI devices can
+access with a Single Address Cycle (32bit DMA address) any page in the
+64bit physical address space. Previously in Linux those 64bit
+platforms had to set artificial limits on the maximum RAM size in the
+system, so that the virt_to_bus() static scheme works (the DMA address
+translation tables were simply filled on bootup to map each bus
+address to the physical page __pa(bus_to_virt())).
+
+So that Linux can use the dynamic DMA mapping, it needs some help from the
+drivers, namely it has to take into account that DMA addresses should be
+mapped only for the time they are actually used and unmapped after the DMA
+transfer.
+
+The following API will work of course even on platforms where no such
+hardware exists, see e.g. arch/x86/include/asm/pci.h for how it is implemented on
+top of the virt_to_bus interface.
+
+First of all, you should make sure
+
+#include <linux/pci.h>
+
+is in your driver. This file will obtain for you the definition of the
+dma_addr_t (which can hold any valid DMA address for the platform)
+type which should be used everywhere you hold a DMA (bus) address
+returned from the DMA mapping functions.
+
+ What memory is DMA'able?
+
+The first piece of information you must know is what kernel memory can
+be used with the DMA mapping facilities. There has been an unwritten
+set of rules regarding this, and this text is an attempt to finally
+write them down.
+
+If you acquired your memory via the page allocator
+(i.e. __get_free_page*()) or the generic memory allocators
+(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
+that memory using the addresses returned from those routines.
+
+This means specifically that you may _not_ use the memory/addresses
+returned from vmalloc() for DMA. It is possible to DMA to the
+_underlying_ memory mapped into a vmalloc() area, but this requires
+walking page tables to get the physical addresses, and then
+translating each of those pages back to a kernel address using
+something like __va(). [ EDIT: Update this when we integrate
+Gerd Knorr's generic code which does this. ]
+
+This rule also means that you may use neither kernel image addresses
+(items in data/text/bss segments), nor module image addresses, nor
+stack addresses for DMA. These could all be mapped somewhere entirely
+different than the rest of physical memory. Even if those classes of
+memory could physically work with DMA, you'd need to ensure the I/O
+buffers were cacheline-aligned. Without that, you'd see cacheline
+sharing problems (data corruption) on CPUs with DMA-incoherent caches.
+(The CPU could write to one word, DMA would write to a different one
+in the same cache line, and one of them could be overwritten.)
+
+Also, this means that you cannot take the return of a kmap()
+call and DMA to/from that. This is similar to vmalloc().
+
+What about block I/O and networking buffers? The block I/O and
+networking subsystems make sure that the buffers they use are valid
+for you to DMA from/to.
+
+ DMA addressing limitations
+
+Does your device have any DMA addressing limitations? For example, is
+your device only capable of driving the low order 24-bits of address
+on the PCI bus for SAC DMA transfers? If so, you need to inform the
+PCI layer of this fact.
+
+By default, the kernel assumes that your device can address the full
+32-bits in a SAC cycle. For a 64-bit DAC capable device, this needs
+to be increased. And for a device with limitations, as discussed in
+the previous paragraph, it needs to be decreased.
+
+pci_alloc_consistent() by default will return 32-bit DMA addresses.
+PCI-X specification requires PCI-X devices to support 64-bit
+addressing (DAC) for all transactions. And at least one platform (SGI
+SN2) requires 64-bit consistent allocations to operate correctly when
+the IO bus is in PCI-X mode. Therefore, like with pci_set_dma_mask(),
+it's good practice to call pci_set_consistent_dma_mask() to set the
+appropriate mask even if your device only supports 32-bit DMA
+(default) and especially if it's a PCI-X device.
+
+For correct operation, you must interrogate the PCI layer in your
+device probe routine to see if the PCI controller on the machine can
+properly support the DMA addressing limitation your device has. It is
+good style to do this even if your device holds the default setting,
+because this shows that you did think about these issues wrt. your
+device.
+
+The query is performed via a call to pci_set_dma_mask():
+
+ int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask);
+
+The query for consistent allocations is performed via a call to
+pci_set_consistent_dma_mask():
+
+ int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask);
+
+Here, pdev is a pointer to the PCI device struct of your device, and
+device_mask is a bit mask describing which bits of a PCI address your
+device supports. It returns zero if your card can perform DMA
+properly on the machine given the address mask you provided.
+
+If it returns non-zero, your device cannot perform DMA properly on
+this platform, and attempting to do so will result in undefined
+behavior. You must either use a different mask, or not use DMA.
+
+This means that in the failure case, you have three options:
+
+1) Use another DMA mask, if possible (see below).
+2) Use some non-DMA mode for data transfer, if possible.
+3) Ignore this device and do not initialize it.
+
+It is recommended that your driver print a kernel KERN_WARNING message
+when you end up performing either #2 or #3. In this manner, if a user
+of your driver reports that performance is bad or that the device is not
+even detected, you can ask them for the kernel messages to find out
+exactly why.
+
+The standard 32-bit addressing PCI device would do something like
+this:
+
+ if (pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
+ printk(KERN_WARNING
+ "mydev: No suitable DMA available.\n");
+ goto ignore_this_device;
+ }
+
+Another common scenario is a 64-bit capable device. The approach
+here is to try for 64-bit DAC addressing, but back down to a
+32-bit mask should that fail. The PCI platform code may fail the
+64-bit mask not because the platform is not capable of 64-bit
+addressing. Rather, it may fail in this case simply because
+32-bit SAC addressing is done more efficiently than DAC addressing.
+Sparc64 is one platform which behaves in this way.
+
+Here is how you would handle a 64-bit capable device which can drive
+all 64-bits when accessing streaming DMA:
+
+ int using_dac;
+
+ if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
+ using_dac = 1;
+ } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
+ using_dac = 0;
+ } else {
+ printk(KERN_WARNING
+ "mydev: No suitable DMA available.\n");
+ goto ignore_this_device;
+ }
+
+If a card is capable of using 64-bit consistent allocations as well,
+the case would look like this:
+
+ int using_dac, consistent_using_dac;
+
+ if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
+ using_dac = 1;
+ consistent_using_dac = 1;
+ pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(64));
+ } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
+ using_dac = 0;
+ consistent_using_dac = 0;
+ pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(32));
+ } else {
+ printk(KERN_WARNING
+ "mydev: No suitable DMA available.\n");
+ goto ignore_this_device;
+ }
+
+pci_set_consistent_dma_mask() will always be able to set the same or a
+smaller mask as pci_set_dma_mask(). However for the rare case that a
+device driver only uses consistent allocations, one would have to
+check the return value from pci_set_consistent_dma_mask().
+
+Finally, if your device can only drive the low 24-bits of
+address during PCI bus mastering you might do something like:
+
+ if (pci_set_dma_mask(pdev, DMA_BIT_MASK(24))) {
+ printk(KERN_WARNING
+ "mydev: 24-bit DMA addressing not available.\n");
+ goto ignore_this_device;
+ }
+
+When pci_set_dma_mask() is successful, and returns zero, the PCI layer
+saves away this mask you have provided. The PCI layer will use this
+information later when you make DMA mappings.
+
+There is a case which we are aware of at this time, which is worth
+mentioning in this documentation. If your device supports multiple
+functions (for example a sound card provides playback and record
+functions) and the various different functions have _different_
+DMA addressing limitations, you may wish to probe each mask and
+only provide the functionality which the machine can handle. It
+is important that the last call to pci_set_dma_mask() be for the
+most specific mask.
+
+Here is pseudo-code showing how this might be done:
+
+ #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32)
+ #define RECORD_ADDRESS_BITS 0x00ffffff
+
+ struct my_sound_card *card;
+ struct pci_dev *pdev;
+
+ ...
+ if (!pci_set_dma_mask(pdev, PLAYBACK_ADDRESS_BITS)) {
+ card->playback_enabled = 1;
+ } else {
+ card->playback_enabled = 0;
+ printk(KERN_WARN "%s: Playback disabled due to DMA limitations.\n",
+ card->name);
+ }
+ if (!pci_set_dma_mask(pdev, RECORD_ADDRESS_BITS)) {
+ card->record_enabled = 1;
+ } else {
+ card->record_enabled = 0;
+ printk(KERN_WARN "%s: Record disabled due to DMA limitations.\n",
+ card->name);
+ }
+
+A sound card was used as an example here because this genre of PCI
+devices seems to be littered with ISA chips given a PCI front end,
+and thus retaining the 16MB DMA addressing limitations of ISA.
+
+ Types of DMA mappings
+
+There are two types of DMA mappings:
+
+- Consistent DMA mappings which are usually mapped at driver
+ initialization, unmapped at the end and for which the hardware should
+ guarantee that the device and the CPU can access the data
+ in parallel and will see updates made by each other without any
+ explicit software flushing.
+
+ Think of "consistent" as "synchronous" or "coherent".
+
+ The current default is to return consistent memory in the low 32
+ bits of the PCI bus space. However, for future compatibility you
+ should set the consistent mask even if this default is fine for your
+ driver.
+
+ Good examples of what to use consistent mappings for are:
+
+ - Network card DMA ring descriptors.
+ - SCSI adapter mailbox command data structures.
+ - Device firmware microcode executed out of
+ main memory.
+
+ The invariant these examples all require is that any CPU store
+ to memory is immediately visible to the device, and vice
+ versa. Consistent mappings guarantee this.
+
+ IMPORTANT: Consistent DMA memory does not preclude the usage of
+ proper memory barriers. The CPU may reorder stores to
+ consistent memory just as it may normal memory. Example:
+ if it is important for the device to see the first word
+ of a descriptor updated before the second, you must do
+ something like:
+
+ desc->word0 = address;
+ wmb();
+ desc->word1 = DESC_VALID;
+
+ in order to get correct behavior on all platforms.
+
+ Also, on some platforms your driver may need to flush CPU write
+ buffers in much the same way as it needs to flush write buffers
+ found in PCI bridges (such as by reading a register's value
+ after writing it).
+
+- Streaming DMA mappings which are usually mapped for one DMA transfer,
+ unmapped right after it (unless you use pci_dma_sync_* below) and for which
+ hardware can optimize for sequential accesses.
+
+ This of "streaming" as "asynchronous" or "outside the coherency
+ domain".
+
+ Good examples of what to use streaming mappings for are:
+
+ - Networking buffers transmitted/received by a device.
+ - Filesystem buffers written/read by a SCSI device.
+
+ The interfaces for using this type of mapping were designed in
+ such a way that an implementation can make whatever performance
+ optimizations the hardware allows. To this end, when using
+ such mappings you must be explicit about what you want to happen.
+
+Neither type of DMA mapping has alignment restrictions that come
+from PCI, although some devices may have such restrictions.
+Also, systems with caches that aren't DMA-coherent will work better
+when the underlying buffers don't share cache lines with other data.
+
+
+ Using Consistent DMA mappings.
+
+To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
+you should do:
+
+ dma_addr_t dma_handle;
+
+ cpu_addr = pci_alloc_consistent(pdev, size, &dma_handle);
+
+where pdev is a struct pci_dev *. This may be called in interrupt context.
+You should use dma_alloc_coherent (see DMA-API.txt) for buses
+where devices don't have struct pci_dev (like ISA, EISA).
+
+This argument is needed because the DMA translations may be bus
+specific (and often is private to the bus which the device is attached
+to).
+
+Size is the length of the region you want to allocate, in bytes.
+
+This routine will allocate RAM for that region, so it acts similarly to
+__get_free_pages (but takes size instead of a page order). If your
+driver needs regions sized smaller than a page, you may prefer using
+the pci_pool interface, described below.
+
+The consistent DMA mapping interfaces, for non-NULL pdev, will by
+default return a DMA address which is SAC (Single Address Cycle)
+addressable. Even if the device indicates (via PCI dma mask) that it
+may address the upper 32-bits and thus perform DAC cycles, consistent
+allocation will only return > 32-bit PCI addresses for DMA if the
+consistent dma mask has been explicitly changed via
+pci_set_consistent_dma_mask(). This is true of the pci_pool interface
+as well.
+
+pci_alloc_consistent returns two values: the virtual address which you
+can use to access it from the CPU and dma_handle which you pass to the
+card.
+
+The cpu return address and the DMA bus master address are both
+guaranteed to be aligned to the smallest PAGE_SIZE order which
+is greater than or equal to the requested size. This invariant
+exists (for example) to guarantee that if you allocate a chunk
+which is smaller than or equal to 64 kilobytes, the extent of the
+buffer you receive will not cross a 64K boundary.
+
+To unmap and free such a DMA region, you call:
+
+ pci_free_consistent(pdev, size, cpu_addr, dma_handle);
+
+where pdev, size are the same as in the above call and cpu_addr and
+dma_handle are the values pci_alloc_consistent returned to you.
+This function may not be called in interrupt context.
+
+If your driver needs lots of smaller memory regions, you can write
+custom code to subdivide pages returned by pci_alloc_consistent,
+or you can use the pci_pool API to do that. A pci_pool is like
+a kmem_cache, but it uses pci_alloc_consistent not __get_free_pages.
+Also, it understands common hardware constraints for alignment,
+like queue heads needing to be aligned on N byte boundaries.
+
+Create a pci_pool like this:
+
+ struct pci_pool *pool;
+
+ pool = pci_pool_create(name, pdev, size, align, alloc);
+
+The "name" is for diagnostics (like a kmem_cache name); pdev and size
+are as above. The device's hardware alignment requirement for this
+type of data is "align" (which is expressed in bytes, and must be a
+power of two). If your device has no boundary crossing restrictions,
+pass 0 for alloc; passing 4096 says memory allocated from this pool
+must not cross 4KByte boundaries (but at that time it may be better to
+go for pci_alloc_consistent directly instead).
+
+Allocate memory from a pci pool like this:
+
+ cpu_addr = pci_pool_alloc(pool, flags, &dma_handle);
+
+flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
+holding SMP locks), SLAB_ATOMIC otherwise. Like pci_alloc_consistent,
+this returns two values, cpu_addr and dma_handle.
+
+Free memory that was allocated from a pci_pool like this:
+
+ pci_pool_free(pool, cpu_addr, dma_handle);
+
+where pool is what you passed to pci_pool_alloc, and cpu_addr and
+dma_handle are the values pci_pool_alloc returned. This function
+may be called in interrupt context.
+
+Destroy a pci_pool by calling:
+
+ pci_pool_destroy(pool);
+
+Make sure you've called pci_pool_free for all memory allocated
+from a pool before you destroy the pool. This function may not
+be called in interrupt context.
+
+ DMA Direction
+
+The interfaces described in subsequent portions of this document
+take a DMA direction argument, which is an integer and takes on
+one of the following values:
+
+ PCI_DMA_BIDIRECTIONAL
+ PCI_DMA_TODEVICE
+ PCI_DMA_FROMDEVICE
+ PCI_DMA_NONE
+
+One should provide the exact DMA direction if you know it.
+
+PCI_DMA_TODEVICE means "from main memory to the PCI device"
+PCI_DMA_FROMDEVICE means "from the PCI device to main memory"
+It is the direction in which the data moves during the DMA
+transfer.
+
+You are _strongly_ encouraged to specify this as precisely
+as you possibly can.
+
+If you absolutely cannot know the direction of the DMA transfer,
+specify PCI_DMA_BIDIRECTIONAL. It means that the DMA can go in
+either direction. The platform guarantees that you may legally
+specify this, and that it will work, but this may be at the
+cost of performance for example.
+
+The value PCI_DMA_NONE is to be used for debugging. One can
+hold this in a data structure before you come to know the
+precise direction, and this will help catch cases where your
+direction tracking logic has failed to set things up properly.
+
+Another advantage of specifying this value precisely (outside of
+potential platform-specific optimizations of such) is for debugging.
+Some platforms actually have a write permission boolean which DMA
+mappings can be marked with, much like page protections in the user
+program address space. Such platforms can and do report errors in the
+kernel logs when the PCI controller hardware detects violation of the
+permission setting.
+
+Only streaming mappings specify a direction, consistent mappings
+implicitly have a direction attribute setting of
+PCI_DMA_BIDIRECTIONAL.
+
+The SCSI subsystem tells you the direction to use in the
+'sc_data_direction' member of the SCSI command your driver is
+working on.
+
+For Networking drivers, it's a rather simple affair. For transmit
+packets, map/unmap them with the PCI_DMA_TODEVICE direction
+specifier. For receive packets, just the opposite, map/unmap them
+with the PCI_DMA_FROMDEVICE direction specifier.
+
+ Using Streaming DMA mappings
+
+The streaming DMA mapping routines can be called from interrupt
+context. There are two versions of each map/unmap, one which will
+map/unmap a single memory region, and one which will map/unmap a
+scatterlist.
+
+To map a single region, you do:
+
+ struct pci_dev *pdev = mydev->pdev;
+ dma_addr_t dma_handle;
+ void *addr = buffer->ptr;
+ size_t size = buffer->len;
+
+ dma_handle = pci_map_single(pdev, addr, size, direction);
+
+and to unmap it:
+
+ pci_unmap_single(pdev, dma_handle, size, direction);
+
+You should call pci_unmap_single when the DMA activity is finished, e.g.
+from the interrupt which told you that the DMA transfer is done.
+
+Using cpu pointers like this for single mappings has a disadvantage,
+you cannot reference HIGHMEM memory in this way. Thus, there is a
+map/unmap interface pair akin to pci_{map,unmap}_single. These
+interfaces deal with page/offset pairs instead of cpu pointers.
+Specifically:
+
+ struct pci_dev *pdev = mydev->pdev;
+ dma_addr_t dma_handle;
+ struct page *page = buffer->page;
+ unsigned long offset = buffer->offset;
+ size_t size = buffer->len;
+
+ dma_handle = pci_map_page(pdev, page, offset, size, direction);
+
+ ...
+
+ pci_unmap_page(pdev, dma_handle, size, direction);
+
+Here, "offset" means byte offset within the given page.
+
+With scatterlists, you map a region gathered from several regions by:
+
+ int i, count = pci_map_sg(pdev, sglist, nents, direction);
+ struct scatterlist *sg;
+
+ for_each_sg(sglist, sg, count, i) {
+ hw_address[i] = sg_dma_address(sg);
+ hw_len[i] = sg_dma_len(sg);
+ }
+
+where nents is the number of entries in the sglist.
+
+The implementation is free to merge several consecutive sglist entries
+into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
+consecutive sglist entries can be merged into one provided the first one
+ends and the second one starts on a page boundary - in fact this is a huge
+advantage for cards which either cannot do scatter-gather or have very
+limited number of scatter-gather entries) and returns the actual number
+of sg entries it mapped them to. On failure 0 is returned.
+
+Then you should loop count times (note: this can be less than nents times)
+and use sg_dma_address() and sg_dma_len() macros where you previously
+accessed sg->address and sg->length as shown above.
+
+To unmap a scatterlist, just call:
+
+ pci_unmap_sg(pdev, sglist, nents, direction);
+
+Again, make sure DMA activity has already finished.
+
+PLEASE NOTE: The 'nents' argument to the pci_unmap_sg call must be
+ the _same_ one you passed into the pci_map_sg call,
+ it should _NOT_ be the 'count' value _returned_ from the
+ pci_map_sg call.
+
+Every pci_map_{single,sg} call should have its pci_unmap_{single,sg}
+counterpart, because the bus address space is a shared resource (although
+in some ports the mapping is per each BUS so less devices contend for the
+same bus address space) and you could render the machine unusable by eating
+all bus addresses.
+
+If you need to use the same streaming DMA region multiple times and touch
+the data in between the DMA transfers, the buffer needs to be synced
+properly in order for the cpu and device to see the most uptodate and
+correct copy of the DMA buffer.
+
+So, firstly, just map it with pci_map_{single,sg}, and after each DMA
+transfer call either:
+
+ pci_dma_sync_single_for_cpu(pdev, dma_handle, size, direction);
+
+or:
+
+ pci_dma_sync_sg_for_cpu(pdev, sglist, nents, direction);
+
+as appropriate.
+
+Then, if you wish to let the device get at the DMA area again,
+finish accessing the data with the cpu, and then before actually
+giving the buffer to the hardware call either:
+
+ pci_dma_sync_single_for_device(pdev, dma_handle, size, direction);
+
+or:
+
+ pci_dma_sync_sg_for_device(dev, sglist, nents, direction);
+
+as appropriate.
+
+After the last DMA transfer call one of the DMA unmap routines
+pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_*
+call till pci_unmap_*, then you don't have to call the pci_dma_sync_*
+routines at all.
+
+Here is pseudo code which shows a situation in which you would need
+to use the pci_dma_sync_*() interfaces.
+
+ my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
+ {
+ dma_addr_t mapping;
+
+ mapping = pci_map_single(cp->pdev, buffer, len, PCI_DMA_FROMDEVICE);
+
+ cp->rx_buf = buffer;
+ cp->rx_len = len;
+ cp->rx_dma = mapping;
+
+ give_rx_buf_to_card(cp);
+ }
+
+ ...
+
+ my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
+ {
+ struct my_card *cp = devid;
+
+ ...
+ if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
+ struct my_card_header *hp;
+
+ /* Examine the header to see if we wish
+ * to accept the data. But synchronize
+ * the DMA transfer with the CPU first
+ * so that we see updated contents.
+ */
+ pci_dma_sync_single_for_cpu(cp->pdev, cp->rx_dma,
+ cp->rx_len,
+ PCI_DMA_FROMDEVICE);
+
+ /* Now it is safe to examine the buffer. */
+ hp = (struct my_card_header *) cp->rx_buf;
+ if (header_is_ok(hp)) {
+ pci_unmap_single(cp->pdev, cp->rx_dma, cp->rx_len,
+ PCI_DMA_FROMDEVICE);
+ pass_to_upper_layers(cp->rx_buf);
+ make_and_setup_new_rx_buf(cp);
+ } else {
+ /* Just sync the buffer and give it back
+ * to the card.
+ */
+ pci_dma_sync_single_for_device(cp->pdev,
+ cp->rx_dma,
+ cp->rx_len,
+ PCI_DMA_FROMDEVICE);
+ give_rx_buf_to_card(cp);
+ }
+ }
+ }
+
+Drivers converted fully to this interface should not use virt_to_bus any
+longer, nor should they use bus_to_virt. Some drivers have to be changed a
+little bit, because there is no longer an equivalent to bus_to_virt in the
+dynamic DMA mapping scheme - you have to always store the DMA addresses
+returned by the pci_alloc_consistent, pci_pool_alloc, and pci_map_single
+calls (pci_map_sg stores them in the scatterlist itself if the platform
+supports dynamic DMA mapping in hardware) in your driver structures and/or
+in the card registers.
+
+All PCI drivers should be using these interfaces with no exceptions.
+It is planned to completely remove virt_to_bus() and bus_to_virt() as
+they are entirely deprecated. Some ports already do not provide these
+as it is impossible to correctly support them.
+
+ Optimizing Unmap State Space Consumption
+
+On many platforms, pci_unmap_{single,page}() is simply a nop.
+Therefore, keeping track of the mapping address and length is a waste
+of space. Instead of filling your drivers up with ifdefs and the like
+to "work around" this (which would defeat the whole purpose of a
+portable API) the following facilities are provided.
+
+Actually, instead of describing the macros one by one, we'll
+transform some example code.
+
+1) Use DECLARE_PCI_UNMAP_{ADDR,LEN} in state saving structures.
+ Example, before:
+
+ struct ring_state {
+ struct sk_buff *skb;
+ dma_addr_t mapping;
+ __u32 len;
+ };
+
+ after:
+
+ struct ring_state {
+ struct sk_buff *skb;
+ DECLARE_PCI_UNMAP_ADDR(mapping)
+ DECLARE_PCI_UNMAP_LEN(len)
+ };
+
+ NOTE: DO NOT put a semicolon at the end of the DECLARE_*()
+ macro.
+
+2) Use pci_unmap_{addr,len}_set to set these values.
+ Example, before:
+
+ ringp->mapping = FOO;
+ ringp->len = BAR;
+
+ after:
+
+ pci_unmap_addr_set(ringp, mapping, FOO);
+ pci_unmap_len_set(ringp, len, BAR);
+
+3) Use pci_unmap_{addr,len} to access these values.
+ Example, before:
+
+ pci_unmap_single(pdev, ringp->mapping, ringp->len,
+ PCI_DMA_FROMDEVICE);
+
+ after:
+
+ pci_unmap_single(pdev,
+ pci_unmap_addr(ringp, mapping),
+ pci_unmap_len(ringp, len),
+ PCI_DMA_FROMDEVICE);
+
+It really should be self-explanatory. We treat the ADDR and LEN
+separately, because it is possible for an implementation to only
+need the address in order to perform the unmap operation.
+
+ Platform Issues
+
+If you are just writing drivers for Linux and do not maintain
+an architecture port for the kernel, you can safely skip down
+to "Closing".
+
+1) Struct scatterlist requirements.
+
+ Struct scatterlist must contain, at a minimum, the following
+ members:
+
+ struct page *page;
+ unsigned int offset;
+ unsigned int length;
+
+ The base address is specified by a "page+offset" pair.
+
+ Previous versions of struct scatterlist contained a "void *address"
+ field that was sometimes used instead of page+offset. As of Linux
+ 2.5., page+offset is always used, and the "address" field has been
+ deleted.
+
+2) More to come...
+
+ Handling Errors
+
+DMA address space is limited on some architectures and an allocation
+failure can be determined by:
+
+- checking if pci_alloc_consistent returns NULL or pci_map_sg returns 0
+
+- checking the returned dma_addr_t of pci_map_single and pci_map_page
+ by using pci_dma_mapping_error():
+
+ dma_addr_t dma_handle;
+
+ dma_handle = pci_map_single(pdev, addr, size, direction);
+ if (pci_dma_mapping_error(pdev, dma_handle)) {
+ /*
+ * reduce current DMA mapping usage,
+ * delay and try again later or
+ * reset driver.
+ */
+ }
+
+ Closing
+
+This document, and the API itself, would not be in it's current
+form without the feedback and suggestions from numerous individuals.
+We would like to specifically mention, in no particular order, the
+following people:
+
+ Russell King <[email protected]>
+ Leo Dagum <[email protected]>
+ Ralf Baechle <[email protected]>
+ Grant Grundler <[email protected]>
+ Jay Estabrook <[email protected]>
+ Thomas Sailer <[email protected]>
+ Andrea Arcangeli <[email protected]>
+ Jens Axboe <[email protected]>
+ David Mosberger-Tang <[email protected]>
diff --git a/Documentation/block/biodoc.txt b/Documentation/block/biodoc.txt
index 8d2158a..6fab97e 100644
--- a/Documentation/block/biodoc.txt
+++ b/Documentation/block/biodoc.txt
@@ -186,7 +186,7 @@ a virtual address mapping (unlike the earlier scheme of virtual address
do not have a corresponding kernel virtual address space mapping) and
low-memory pages.
-Note: Please refer to Documentation/DMA-mapping.txt for a discussion
+Note: Please refer to Documentation/PCI/PCI-DMA-mapping.txt for a discussion
on PCI high mem DMA aspects and mapping of scatter gather lists, and support
for 64 bit PCI.
--
1.6.5.3
On Sat, 28 Nov 2009 13:35:39 +0900 Kusanagi Kouichi wrote:
> It seems that Documentation/DMA-mapping.txt was supposed to be renamed
> to Documentation/PCI/PCI-DMA-mapping.txt.
>
> Signed-off-by: Kusanagi Kouichi <[email protected]>
applied, thanks.
> ---
> Documentation/DMA-mapping.txt | 766 ---------------------------------
> Documentation/PCI/PCI-DMA-mapping.txt | 766 +++++++++++++++++++++++++++++++++
> Documentation/block/biodoc.txt | 2 +-
> 3 files changed, 767 insertions(+), 767 deletions(-)
> delete mode 100644 Documentation/DMA-mapping.txt
> create mode 100644 Documentation/PCI/PCI-DMA-mapping.txt
>
> diff --git a/Documentation/DMA-mapping.txt b/Documentation/DMA-mapping.txt
> deleted file mode 100644
> index 01f24e9..0000000
> --- a/Documentation/DMA-mapping.txt
> +++ /dev/null
> @@ -1,766 +0,0 @@
> - Dynamic DMA mapping
> - ===================
> -
> - David S. Miller <[email protected]>
> - Richard Henderson <[email protected]>
> - Jakub Jelinek <[email protected]>
> -
> -This document describes the DMA mapping system in terms of the pci_
> -API. For a similar API that works for generic devices, see
> -DMA-API.txt.
> -
> -Most of the 64bit platforms have special hardware that translates bus
> -addresses (DMA addresses) into physical addresses. This is similar to
> -how page tables and/or a TLB translates virtual addresses to physical
> -addresses on a CPU. This is needed so that e.g. PCI devices can
> -access with a Single Address Cycle (32bit DMA address) any page in the
> -64bit physical address space. Previously in Linux those 64bit
> -platforms had to set artificial limits on the maximum RAM size in the
> -system, so that the virt_to_bus() static scheme works (the DMA address
> -translation tables were simply filled on bootup to map each bus
> -address to the physical page __pa(bus_to_virt())).
> -
> -So that Linux can use the dynamic DMA mapping, it needs some help from the
> -drivers, namely it has to take into account that DMA addresses should be
> -mapped only for the time they are actually used and unmapped after the DMA
> -transfer.
> -
> -The following API will work of course even on platforms where no such
> -hardware exists, see e.g. arch/x86/include/asm/pci.h for how it is implemented on
> -top of the virt_to_bus interface.
> -
> -First of all, you should make sure
> -
> -#include <linux/pci.h>
> -
> -is in your driver. This file will obtain for you the definition of the
> -dma_addr_t (which can hold any valid DMA address for the platform)
> -type which should be used everywhere you hold a DMA (bus) address
> -returned from the DMA mapping functions.
> -
> - What memory is DMA'able?
> -
> -The first piece of information you must know is what kernel memory can
> -be used with the DMA mapping facilities. There has been an unwritten
> -set of rules regarding this, and this text is an attempt to finally
> -write them down.
> -
> -If you acquired your memory via the page allocator
> -(i.e. __get_free_page*()) or the generic memory allocators
> -(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
> -that memory using the addresses returned from those routines.
> -
> -This means specifically that you may _not_ use the memory/addresses
> -returned from vmalloc() for DMA. It is possible to DMA to the
> -_underlying_ memory mapped into a vmalloc() area, but this requires
> -walking page tables to get the physical addresses, and then
> -translating each of those pages back to a kernel address using
> -something like __va(). [ EDIT: Update this when we integrate
> -Gerd Knorr's generic code which does this. ]
> -
> -This rule also means that you may use neither kernel image addresses
> -(items in data/text/bss segments), nor module image addresses, nor
> -stack addresses for DMA. These could all be mapped somewhere entirely
> -different than the rest of physical memory. Even if those classes of
> -memory could physically work with DMA, you'd need to ensure the I/O
> -buffers were cacheline-aligned. Without that, you'd see cacheline
> -sharing problems (data corruption) on CPUs with DMA-incoherent caches.
> -(The CPU could write to one word, DMA would write to a different one
> -in the same cache line, and one of them could be overwritten.)
> -
> -Also, this means that you cannot take the return of a kmap()
> -call and DMA to/from that. This is similar to vmalloc().
> -
> -What about block I/O and networking buffers? The block I/O and
> -networking subsystems make sure that the buffers they use are valid
> -for you to DMA from/to.
> -
> - DMA addressing limitations
> -
> -Does your device have any DMA addressing limitations? For example, is
> -your device only capable of driving the low order 24-bits of address
> -on the PCI bus for SAC DMA transfers? If so, you need to inform the
> -PCI layer of this fact.
> -
> -By default, the kernel assumes that your device can address the full
> -32-bits in a SAC cycle. For a 64-bit DAC capable device, this needs
> -to be increased. And for a device with limitations, as discussed in
> -the previous paragraph, it needs to be decreased.
> -
> -pci_alloc_consistent() by default will return 32-bit DMA addresses.
> -PCI-X specification requires PCI-X devices to support 64-bit
> -addressing (DAC) for all transactions. And at least one platform (SGI
> -SN2) requires 64-bit consistent allocations to operate correctly when
> -the IO bus is in PCI-X mode. Therefore, like with pci_set_dma_mask(),
> -it's good practice to call pci_set_consistent_dma_mask() to set the
> -appropriate mask even if your device only supports 32-bit DMA
> -(default) and especially if it's a PCI-X device.
> -
> -For correct operation, you must interrogate the PCI layer in your
> -device probe routine to see if the PCI controller on the machine can
> -properly support the DMA addressing limitation your device has. It is
> -good style to do this even if your device holds the default setting,
> -because this shows that you did think about these issues wrt. your
> -device.
> -
> -The query is performed via a call to pci_set_dma_mask():
> -
> - int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask);
> -
> -The query for consistent allocations is performed via a call to
> -pci_set_consistent_dma_mask():
> -
> - int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask);
> -
> -Here, pdev is a pointer to the PCI device struct of your device, and
> -device_mask is a bit mask describing which bits of a PCI address your
> -device supports. It returns zero if your card can perform DMA
> -properly on the machine given the address mask you provided.
> -
> -If it returns non-zero, your device cannot perform DMA properly on
> -this platform, and attempting to do so will result in undefined
> -behavior. You must either use a different mask, or not use DMA.
> -
> -This means that in the failure case, you have three options:
> -
> -1) Use another DMA mask, if possible (see below).
> -2) Use some non-DMA mode for data transfer, if possible.
> -3) Ignore this device and do not initialize it.
> -
> -It is recommended that your driver print a kernel KERN_WARNING message
> -when you end up performing either #2 or #3. In this manner, if a user
> -of your driver reports that performance is bad or that the device is not
> -even detected, you can ask them for the kernel messages to find out
> -exactly why.
> -
> -The standard 32-bit addressing PCI device would do something like
> -this:
> -
> - if (pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
> - printk(KERN_WARNING
> - "mydev: No suitable DMA available.\n");
> - goto ignore_this_device;
> - }
> -
> -Another common scenario is a 64-bit capable device. The approach
> -here is to try for 64-bit DAC addressing, but back down to a
> -32-bit mask should that fail. The PCI platform code may fail the
> -64-bit mask not because the platform is not capable of 64-bit
> -addressing. Rather, it may fail in this case simply because
> -32-bit SAC addressing is done more efficiently than DAC addressing.
> -Sparc64 is one platform which behaves in this way.
> -
> -Here is how you would handle a 64-bit capable device which can drive
> -all 64-bits when accessing streaming DMA:
> -
> - int using_dac;
> -
> - if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
> - using_dac = 1;
> - } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
> - using_dac = 0;
> - } else {
> - printk(KERN_WARNING
> - "mydev: No suitable DMA available.\n");
> - goto ignore_this_device;
> - }
> -
> -If a card is capable of using 64-bit consistent allocations as well,
> -the case would look like this:
> -
> - int using_dac, consistent_using_dac;
> -
> - if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
> - using_dac = 1;
> - consistent_using_dac = 1;
> - pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(64));
> - } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
> - using_dac = 0;
> - consistent_using_dac = 0;
> - pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(32));
> - } else {
> - printk(KERN_WARNING
> - "mydev: No suitable DMA available.\n");
> - goto ignore_this_device;
> - }
> -
> -pci_set_consistent_dma_mask() will always be able to set the same or a
> -smaller mask as pci_set_dma_mask(). However for the rare case that a
> -device driver only uses consistent allocations, one would have to
> -check the return value from pci_set_consistent_dma_mask().
> -
> -Finally, if your device can only drive the low 24-bits of
> -address during PCI bus mastering you might do something like:
> -
> - if (pci_set_dma_mask(pdev, DMA_BIT_MASK(24))) {
> - printk(KERN_WARNING
> - "mydev: 24-bit DMA addressing not available.\n");
> - goto ignore_this_device;
> - }
> -
> -When pci_set_dma_mask() is successful, and returns zero, the PCI layer
> -saves away this mask you have provided. The PCI layer will use this
> -information later when you make DMA mappings.
> -
> -There is a case which we are aware of at this time, which is worth
> -mentioning in this documentation. If your device supports multiple
> -functions (for example a sound card provides playback and record
> -functions) and the various different functions have _different_
> -DMA addressing limitations, you may wish to probe each mask and
> -only provide the functionality which the machine can handle. It
> -is important that the last call to pci_set_dma_mask() be for the
> -most specific mask.
> -
> -Here is pseudo-code showing how this might be done:
> -
> - #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32)
> - #define RECORD_ADDRESS_BITS 0x00ffffff
> -
> - struct my_sound_card *card;
> - struct pci_dev *pdev;
> -
> - ...
> - if (!pci_set_dma_mask(pdev, PLAYBACK_ADDRESS_BITS)) {
> - card->playback_enabled = 1;
> - } else {
> - card->playback_enabled = 0;
> - printk(KERN_WARN "%s: Playback disabled due to DMA limitations.\n",
> - card->name);
> - }
> - if (!pci_set_dma_mask(pdev, RECORD_ADDRESS_BITS)) {
> - card->record_enabled = 1;
> - } else {
> - card->record_enabled = 0;
> - printk(KERN_WARN "%s: Record disabled due to DMA limitations.\n",
> - card->name);
> - }
> -
> -A sound card was used as an example here because this genre of PCI
> -devices seems to be littered with ISA chips given a PCI front end,
> -and thus retaining the 16MB DMA addressing limitations of ISA.
> -
> - Types of DMA mappings
> -
> -There are two types of DMA mappings:
> -
> -- Consistent DMA mappings which are usually mapped at driver
> - initialization, unmapped at the end and for which the hardware should
> - guarantee that the device and the CPU can access the data
> - in parallel and will see updates made by each other without any
> - explicit software flushing.
> -
> - Think of "consistent" as "synchronous" or "coherent".
> -
> - The current default is to return consistent memory in the low 32
> - bits of the PCI bus space. However, for future compatibility you
> - should set the consistent mask even if this default is fine for your
> - driver.
> -
> - Good examples of what to use consistent mappings for are:
> -
> - - Network card DMA ring descriptors.
> - - SCSI adapter mailbox command data structures.
> - - Device firmware microcode executed out of
> - main memory.
> -
> - The invariant these examples all require is that any CPU store
> - to memory is immediately visible to the device, and vice
> - versa. Consistent mappings guarantee this.
> -
> - IMPORTANT: Consistent DMA memory does not preclude the usage of
> - proper memory barriers. The CPU may reorder stores to
> - consistent memory just as it may normal memory. Example:
> - if it is important for the device to see the first word
> - of a descriptor updated before the second, you must do
> - something like:
> -
> - desc->word0 = address;
> - wmb();
> - desc->word1 = DESC_VALID;
> -
> - in order to get correct behavior on all platforms.
> -
> - Also, on some platforms your driver may need to flush CPU write
> - buffers in much the same way as it needs to flush write buffers
> - found in PCI bridges (such as by reading a register's value
> - after writing it).
> -
> -- Streaming DMA mappings which are usually mapped for one DMA transfer,
> - unmapped right after it (unless you use pci_dma_sync_* below) and for which
> - hardware can optimize for sequential accesses.
> -
> - This of "streaming" as "asynchronous" or "outside the coherency
> - domain".
> -
> - Good examples of what to use streaming mappings for are:
> -
> - - Networking buffers transmitted/received by a device.
> - - Filesystem buffers written/read by a SCSI device.
> -
> - The interfaces for using this type of mapping were designed in
> - such a way that an implementation can make whatever performance
> - optimizations the hardware allows. To this end, when using
> - such mappings you must be explicit about what you want to happen.
> -
> -Neither type of DMA mapping has alignment restrictions that come
> -from PCI, although some devices may have such restrictions.
> -Also, systems with caches that aren't DMA-coherent will work better
> -when the underlying buffers don't share cache lines with other data.
> -
> -
> - Using Consistent DMA mappings.
> -
> -To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
> -you should do:
> -
> - dma_addr_t dma_handle;
> -
> - cpu_addr = pci_alloc_consistent(pdev, size, &dma_handle);
> -
> -where pdev is a struct pci_dev *. This may be called in interrupt context.
> -You should use dma_alloc_coherent (see DMA-API.txt) for buses
> -where devices don't have struct pci_dev (like ISA, EISA).
> -
> -This argument is needed because the DMA translations may be bus
> -specific (and often is private to the bus which the device is attached
> -to).
> -
> -Size is the length of the region you want to allocate, in bytes.
> -
> -This routine will allocate RAM for that region, so it acts similarly to
> -__get_free_pages (but takes size instead of a page order). If your
> -driver needs regions sized smaller than a page, you may prefer using
> -the pci_pool interface, described below.
> -
> -The consistent DMA mapping interfaces, for non-NULL pdev, will by
> -default return a DMA address which is SAC (Single Address Cycle)
> -addressable. Even if the device indicates (via PCI dma mask) that it
> -may address the upper 32-bits and thus perform DAC cycles, consistent
> -allocation will only return > 32-bit PCI addresses for DMA if the
> -consistent dma mask has been explicitly changed via
> -pci_set_consistent_dma_mask(). This is true of the pci_pool interface
> -as well.
> -
> -pci_alloc_consistent returns two values: the virtual address which you
> -can use to access it from the CPU and dma_handle which you pass to the
> -card.
> -
> -The cpu return address and the DMA bus master address are both
> -guaranteed to be aligned to the smallest PAGE_SIZE order which
> -is greater than or equal to the requested size. This invariant
> -exists (for example) to guarantee that if you allocate a chunk
> -which is smaller than or equal to 64 kilobytes, the extent of the
> -buffer you receive will not cross a 64K boundary.
> -
> -To unmap and free such a DMA region, you call:
> -
> - pci_free_consistent(pdev, size, cpu_addr, dma_handle);
> -
> -where pdev, size are the same as in the above call and cpu_addr and
> -dma_handle are the values pci_alloc_consistent returned to you.
> -This function may not be called in interrupt context.
> -
> -If your driver needs lots of smaller memory regions, you can write
> -custom code to subdivide pages returned by pci_alloc_consistent,
> -or you can use the pci_pool API to do that. A pci_pool is like
> -a kmem_cache, but it uses pci_alloc_consistent not __get_free_pages.
> -Also, it understands common hardware constraints for alignment,
> -like queue heads needing to be aligned on N byte boundaries.
> -
> -Create a pci_pool like this:
> -
> - struct pci_pool *pool;
> -
> - pool = pci_pool_create(name, pdev, size, align, alloc);
> -
> -The "name" is for diagnostics (like a kmem_cache name); pdev and size
> -are as above. The device's hardware alignment requirement for this
> -type of data is "align" (which is expressed in bytes, and must be a
> -power of two). If your device has no boundary crossing restrictions,
> -pass 0 for alloc; passing 4096 says memory allocated from this pool
> -must not cross 4KByte boundaries (but at that time it may be better to
> -go for pci_alloc_consistent directly instead).
> -
> -Allocate memory from a pci pool like this:
> -
> - cpu_addr = pci_pool_alloc(pool, flags, &dma_handle);
> -
> -flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
> -holding SMP locks), SLAB_ATOMIC otherwise. Like pci_alloc_consistent,
> -this returns two values, cpu_addr and dma_handle.
> -
> -Free memory that was allocated from a pci_pool like this:
> -
> - pci_pool_free(pool, cpu_addr, dma_handle);
> -
> -where pool is what you passed to pci_pool_alloc, and cpu_addr and
> -dma_handle are the values pci_pool_alloc returned. This function
> -may be called in interrupt context.
> -
> -Destroy a pci_pool by calling:
> -
> - pci_pool_destroy(pool);
> -
> -Make sure you've called pci_pool_free for all memory allocated
> -from a pool before you destroy the pool. This function may not
> -be called in interrupt context.
> -
> - DMA Direction
> -
> -The interfaces described in subsequent portions of this document
> -take a DMA direction argument, which is an integer and takes on
> -one of the following values:
> -
> - PCI_DMA_BIDIRECTIONAL
> - PCI_DMA_TODEVICE
> - PCI_DMA_FROMDEVICE
> - PCI_DMA_NONE
> -
> -One should provide the exact DMA direction if you know it.
> -
> -PCI_DMA_TODEVICE means "from main memory to the PCI device"
> -PCI_DMA_FROMDEVICE means "from the PCI device to main memory"
> -It is the direction in which the data moves during the DMA
> -transfer.
> -
> -You are _strongly_ encouraged to specify this as precisely
> -as you possibly can.
> -
> -If you absolutely cannot know the direction of the DMA transfer,
> -specify PCI_DMA_BIDIRECTIONAL. It means that the DMA can go in
> -either direction. The platform guarantees that you may legally
> -specify this, and that it will work, but this may be at the
> -cost of performance for example.
> -
> -The value PCI_DMA_NONE is to be used for debugging. One can
> -hold this in a data structure before you come to know the
> -precise direction, and this will help catch cases where your
> -direction tracking logic has failed to set things up properly.
> -
> -Another advantage of specifying this value precisely (outside of
> -potential platform-specific optimizations of such) is for debugging.
> -Some platforms actually have a write permission boolean which DMA
> -mappings can be marked with, much like page protections in the user
> -program address space. Such platforms can and do report errors in the
> -kernel logs when the PCI controller hardware detects violation of the
> -permission setting.
> -
> -Only streaming mappings specify a direction, consistent mappings
> -implicitly have a direction attribute setting of
> -PCI_DMA_BIDIRECTIONAL.
> -
> -The SCSI subsystem tells you the direction to use in the
> -'sc_data_direction' member of the SCSI command your driver is
> -working on.
> -
> -For Networking drivers, it's a rather simple affair. For transmit
> -packets, map/unmap them with the PCI_DMA_TODEVICE direction
> -specifier. For receive packets, just the opposite, map/unmap them
> -with the PCI_DMA_FROMDEVICE direction specifier.
> -
> - Using Streaming DMA mappings
> -
> -The streaming DMA mapping routines can be called from interrupt
> -context. There are two versions of each map/unmap, one which will
> -map/unmap a single memory region, and one which will map/unmap a
> -scatterlist.
> -
> -To map a single region, you do:
> -
> - struct pci_dev *pdev = mydev->pdev;
> - dma_addr_t dma_handle;
> - void *addr = buffer->ptr;
> - size_t size = buffer->len;
> -
> - dma_handle = pci_map_single(pdev, addr, size, direction);
> -
> -and to unmap it:
> -
> - pci_unmap_single(pdev, dma_handle, size, direction);
> -
> -You should call pci_unmap_single when the DMA activity is finished, e.g.
> -from the interrupt which told you that the DMA transfer is done.
> -
> -Using cpu pointers like this for single mappings has a disadvantage,
> -you cannot reference HIGHMEM memory in this way. Thus, there is a
> -map/unmap interface pair akin to pci_{map,unmap}_single. These
> -interfaces deal with page/offset pairs instead of cpu pointers.
> -Specifically:
> -
> - struct pci_dev *pdev = mydev->pdev;
> - dma_addr_t dma_handle;
> - struct page *page = buffer->page;
> - unsigned long offset = buffer->offset;
> - size_t size = buffer->len;
> -
> - dma_handle = pci_map_page(pdev, page, offset, size, direction);
> -
> - ...
> -
> - pci_unmap_page(pdev, dma_handle, size, direction);
> -
> -Here, "offset" means byte offset within the given page.
> -
> -With scatterlists, you map a region gathered from several regions by:
> -
> - int i, count = pci_map_sg(pdev, sglist, nents, direction);
> - struct scatterlist *sg;
> -
> - for_each_sg(sglist, sg, count, i) {
> - hw_address[i] = sg_dma_address(sg);
> - hw_len[i] = sg_dma_len(sg);
> - }
> -
> -where nents is the number of entries in the sglist.
> -
> -The implementation is free to merge several consecutive sglist entries
> -into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
> -consecutive sglist entries can be merged into one provided the first one
> -ends and the second one starts on a page boundary - in fact this is a huge
> -advantage for cards which either cannot do scatter-gather or have very
> -limited number of scatter-gather entries) and returns the actual number
> -of sg entries it mapped them to. On failure 0 is returned.
> -
> -Then you should loop count times (note: this can be less than nents times)
> -and use sg_dma_address() and sg_dma_len() macros where you previously
> -accessed sg->address and sg->length as shown above.
> -
> -To unmap a scatterlist, just call:
> -
> - pci_unmap_sg(pdev, sglist, nents, direction);
> -
> -Again, make sure DMA activity has already finished.
> -
> -PLEASE NOTE: The 'nents' argument to the pci_unmap_sg call must be
> - the _same_ one you passed into the pci_map_sg call,
> - it should _NOT_ be the 'count' value _returned_ from the
> - pci_map_sg call.
> -
> -Every pci_map_{single,sg} call should have its pci_unmap_{single,sg}
> -counterpart, because the bus address space is a shared resource (although
> -in some ports the mapping is per each BUS so less devices contend for the
> -same bus address space) and you could render the machine unusable by eating
> -all bus addresses.
> -
> -If you need to use the same streaming DMA region multiple times and touch
> -the data in between the DMA transfers, the buffer needs to be synced
> -properly in order for the cpu and device to see the most uptodate and
> -correct copy of the DMA buffer.
> -
> -So, firstly, just map it with pci_map_{single,sg}, and after each DMA
> -transfer call either:
> -
> - pci_dma_sync_single_for_cpu(pdev, dma_handle, size, direction);
> -
> -or:
> -
> - pci_dma_sync_sg_for_cpu(pdev, sglist, nents, direction);
> -
> -as appropriate.
> -
> -Then, if you wish to let the device get at the DMA area again,
> -finish accessing the data with the cpu, and then before actually
> -giving the buffer to the hardware call either:
> -
> - pci_dma_sync_single_for_device(pdev, dma_handle, size, direction);
> -
> -or:
> -
> - pci_dma_sync_sg_for_device(dev, sglist, nents, direction);
> -
> -as appropriate.
> -
> -After the last DMA transfer call one of the DMA unmap routines
> -pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_*
> -call till pci_unmap_*, then you don't have to call the pci_dma_sync_*
> -routines at all.
> -
> -Here is pseudo code which shows a situation in which you would need
> -to use the pci_dma_sync_*() interfaces.
> -
> - my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
> - {
> - dma_addr_t mapping;
> -
> - mapping = pci_map_single(cp->pdev, buffer, len, PCI_DMA_FROMDEVICE);
> -
> - cp->rx_buf = buffer;
> - cp->rx_len = len;
> - cp->rx_dma = mapping;
> -
> - give_rx_buf_to_card(cp);
> - }
> -
> - ...
> -
> - my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
> - {
> - struct my_card *cp = devid;
> -
> - ...
> - if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
> - struct my_card_header *hp;
> -
> - /* Examine the header to see if we wish
> - * to accept the data. But synchronize
> - * the DMA transfer with the CPU first
> - * so that we see updated contents.
> - */
> - pci_dma_sync_single_for_cpu(cp->pdev, cp->rx_dma,
> - cp->rx_len,
> - PCI_DMA_FROMDEVICE);
> -
> - /* Now it is safe to examine the buffer. */
> - hp = (struct my_card_header *) cp->rx_buf;
> - if (header_is_ok(hp)) {
> - pci_unmap_single(cp->pdev, cp->rx_dma, cp->rx_len,
> - PCI_DMA_FROMDEVICE);
> - pass_to_upper_layers(cp->rx_buf);
> - make_and_setup_new_rx_buf(cp);
> - } else {
> - /* Just sync the buffer and give it back
> - * to the card.
> - */
> - pci_dma_sync_single_for_device(cp->pdev,
> - cp->rx_dma,
> - cp->rx_len,
> - PCI_DMA_FROMDEVICE);
> - give_rx_buf_to_card(cp);
> - }
> - }
> - }
> -
> -Drivers converted fully to this interface should not use virt_to_bus any
> -longer, nor should they use bus_to_virt. Some drivers have to be changed a
> -little bit, because there is no longer an equivalent to bus_to_virt in the
> -dynamic DMA mapping scheme - you have to always store the DMA addresses
> -returned by the pci_alloc_consistent, pci_pool_alloc, and pci_map_single
> -calls (pci_map_sg stores them in the scatterlist itself if the platform
> -supports dynamic DMA mapping in hardware) in your driver structures and/or
> -in the card registers.
> -
> -All PCI drivers should be using these interfaces with no exceptions.
> -It is planned to completely remove virt_to_bus() and bus_to_virt() as
> -they are entirely deprecated. Some ports already do not provide these
> -as it is impossible to correctly support them.
> -
> - Optimizing Unmap State Space Consumption
> -
> -On many platforms, pci_unmap_{single,page}() is simply a nop.
> -Therefore, keeping track of the mapping address and length is a waste
> -of space. Instead of filling your drivers up with ifdefs and the like
> -to "work around" this (which would defeat the whole purpose of a
> -portable API) the following facilities are provided.
> -
> -Actually, instead of describing the macros one by one, we'll
> -transform some example code.
> -
> -1) Use DECLARE_PCI_UNMAP_{ADDR,LEN} in state saving structures.
> - Example, before:
> -
> - struct ring_state {
> - struct sk_buff *skb;
> - dma_addr_t mapping;
> - __u32 len;
> - };
> -
> - after:
> -
> - struct ring_state {
> - struct sk_buff *skb;
> - DECLARE_PCI_UNMAP_ADDR(mapping)
> - DECLARE_PCI_UNMAP_LEN(len)
> - };
> -
> - NOTE: DO NOT put a semicolon at the end of the DECLARE_*()
> - macro.
> -
> -2) Use pci_unmap_{addr,len}_set to set these values.
> - Example, before:
> -
> - ringp->mapping = FOO;
> - ringp->len = BAR;
> -
> - after:
> -
> - pci_unmap_addr_set(ringp, mapping, FOO);
> - pci_unmap_len_set(ringp, len, BAR);
> -
> -3) Use pci_unmap_{addr,len} to access these values.
> - Example, before:
> -
> - pci_unmap_single(pdev, ringp->mapping, ringp->len,
> - PCI_DMA_FROMDEVICE);
> -
> - after:
> -
> - pci_unmap_single(pdev,
> - pci_unmap_addr(ringp, mapping),
> - pci_unmap_len(ringp, len),
> - PCI_DMA_FROMDEVICE);
> -
> -It really should be self-explanatory. We treat the ADDR and LEN
> -separately, because it is possible for an implementation to only
> -need the address in order to perform the unmap operation.
> -
> - Platform Issues
> -
> -If you are just writing drivers for Linux and do not maintain
> -an architecture port for the kernel, you can safely skip down
> -to "Closing".
> -
> -1) Struct scatterlist requirements.
> -
> - Struct scatterlist must contain, at a minimum, the following
> - members:
> -
> - struct page *page;
> - unsigned int offset;
> - unsigned int length;
> -
> - The base address is specified by a "page+offset" pair.
> -
> - Previous versions of struct scatterlist contained a "void *address"
> - field that was sometimes used instead of page+offset. As of Linux
> - 2.5., page+offset is always used, and the "address" field has been
> - deleted.
> -
> -2) More to come...
> -
> - Handling Errors
> -
> -DMA address space is limited on some architectures and an allocation
> -failure can be determined by:
> -
> -- checking if pci_alloc_consistent returns NULL or pci_map_sg returns 0
> -
> -- checking the returned dma_addr_t of pci_map_single and pci_map_page
> - by using pci_dma_mapping_error():
> -
> - dma_addr_t dma_handle;
> -
> - dma_handle = pci_map_single(pdev, addr, size, direction);
> - if (pci_dma_mapping_error(pdev, dma_handle)) {
> - /*
> - * reduce current DMA mapping usage,
> - * delay and try again later or
> - * reset driver.
> - */
> - }
> -
> - Closing
> -
> -This document, and the API itself, would not be in it's current
> -form without the feedback and suggestions from numerous individuals.
> -We would like to specifically mention, in no particular order, the
> -following people:
> -
> - Russell King <[email protected]>
> - Leo Dagum <[email protected]>
> - Ralf Baechle <[email protected]>
> - Grant Grundler <[email protected]>
> - Jay Estabrook <[email protected]>
> - Thomas Sailer <[email protected]>
> - Andrea Arcangeli <[email protected]>
> - Jens Axboe <[email protected]>
> - David Mosberger-Tang <[email protected]>
> diff --git a/Documentation/PCI/PCI-DMA-mapping.txt b/Documentation/PCI/PCI-DMA-mapping.txt
> new file mode 100644
> index 0000000..01f24e9
> --- /dev/null
> +++ b/Documentation/PCI/PCI-DMA-mapping.txt
> @@ -0,0 +1,766 @@
> + Dynamic DMA mapping
> + ===================
> +
> + David S. Miller <[email protected]>
> + Richard Henderson <[email protected]>
> + Jakub Jelinek <[email protected]>
> +
> +This document describes the DMA mapping system in terms of the pci_
> +API. For a similar API that works for generic devices, see
> +DMA-API.txt.
> +
> +Most of the 64bit platforms have special hardware that translates bus
> +addresses (DMA addresses) into physical addresses. This is similar to
> +how page tables and/or a TLB translates virtual addresses to physical
> +addresses on a CPU. This is needed so that e.g. PCI devices can
> +access with a Single Address Cycle (32bit DMA address) any page in the
> +64bit physical address space. Previously in Linux those 64bit
> +platforms had to set artificial limits on the maximum RAM size in the
> +system, so that the virt_to_bus() static scheme works (the DMA address
> +translation tables were simply filled on bootup to map each bus
> +address to the physical page __pa(bus_to_virt())).
> +
> +So that Linux can use the dynamic DMA mapping, it needs some help from the
> +drivers, namely it has to take into account that DMA addresses should be
> +mapped only for the time they are actually used and unmapped after the DMA
> +transfer.
> +
> +The following API will work of course even on platforms where no such
> +hardware exists, see e.g. arch/x86/include/asm/pci.h for how it is implemented on
> +top of the virt_to_bus interface.
> +
> +First of all, you should make sure
> +
> +#include <linux/pci.h>
> +
> +is in your driver. This file will obtain for you the definition of the
> +dma_addr_t (which can hold any valid DMA address for the platform)
> +type which should be used everywhere you hold a DMA (bus) address
> +returned from the DMA mapping functions.
> +
> + What memory is DMA'able?
> +
> +The first piece of information you must know is what kernel memory can
> +be used with the DMA mapping facilities. There has been an unwritten
> +set of rules regarding this, and this text is an attempt to finally
> +write them down.
> +
> +If you acquired your memory via the page allocator
> +(i.e. __get_free_page*()) or the generic memory allocators
> +(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
> +that memory using the addresses returned from those routines.
> +
> +This means specifically that you may _not_ use the memory/addresses
> +returned from vmalloc() for DMA. It is possible to DMA to the
> +_underlying_ memory mapped into a vmalloc() area, but this requires
> +walking page tables to get the physical addresses, and then
> +translating each of those pages back to a kernel address using
> +something like __va(). [ EDIT: Update this when we integrate
> +Gerd Knorr's generic code which does this. ]
> +
> +This rule also means that you may use neither kernel image addresses
> +(items in data/text/bss segments), nor module image addresses, nor
> +stack addresses for DMA. These could all be mapped somewhere entirely
> +different than the rest of physical memory. Even if those classes of
> +memory could physically work with DMA, you'd need to ensure the I/O
> +buffers were cacheline-aligned. Without that, you'd see cacheline
> +sharing problems (data corruption) on CPUs with DMA-incoherent caches.
> +(The CPU could write to one word, DMA would write to a different one
> +in the same cache line, and one of them could be overwritten.)
> +
> +Also, this means that you cannot take the return of a kmap()
> +call and DMA to/from that. This is similar to vmalloc().
> +
> +What about block I/O and networking buffers? The block I/O and
> +networking subsystems make sure that the buffers they use are valid
> +for you to DMA from/to.
> +
> + DMA addressing limitations
> +
> +Does your device have any DMA addressing limitations? For example, is
> +your device only capable of driving the low order 24-bits of address
> +on the PCI bus for SAC DMA transfers? If so, you need to inform the
> +PCI layer of this fact.
> +
> +By default, the kernel assumes that your device can address the full
> +32-bits in a SAC cycle. For a 64-bit DAC capable device, this needs
> +to be increased. And for a device with limitations, as discussed in
> +the previous paragraph, it needs to be decreased.
> +
> +pci_alloc_consistent() by default will return 32-bit DMA addresses.
> +PCI-X specification requires PCI-X devices to support 64-bit
> +addressing (DAC) for all transactions. And at least one platform (SGI
> +SN2) requires 64-bit consistent allocations to operate correctly when
> +the IO bus is in PCI-X mode. Therefore, like with pci_set_dma_mask(),
> +it's good practice to call pci_set_consistent_dma_mask() to set the
> +appropriate mask even if your device only supports 32-bit DMA
> +(default) and especially if it's a PCI-X device.
> +
> +For correct operation, you must interrogate the PCI layer in your
> +device probe routine to see if the PCI controller on the machine can
> +properly support the DMA addressing limitation your device has. It is
> +good style to do this even if your device holds the default setting,
> +because this shows that you did think about these issues wrt. your
> +device.
> +
> +The query is performed via a call to pci_set_dma_mask():
> +
> + int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask);
> +
> +The query for consistent allocations is performed via a call to
> +pci_set_consistent_dma_mask():
> +
> + int pci_set_consistent_dma_mask(struct pci_dev *pdev, u64 device_mask);
> +
> +Here, pdev is a pointer to the PCI device struct of your device, and
> +device_mask is a bit mask describing which bits of a PCI address your
> +device supports. It returns zero if your card can perform DMA
> +properly on the machine given the address mask you provided.
> +
> +If it returns non-zero, your device cannot perform DMA properly on
> +this platform, and attempting to do so will result in undefined
> +behavior. You must either use a different mask, or not use DMA.
> +
> +This means that in the failure case, you have three options:
> +
> +1) Use another DMA mask, if possible (see below).
> +2) Use some non-DMA mode for data transfer, if possible.
> +3) Ignore this device and do not initialize it.
> +
> +It is recommended that your driver print a kernel KERN_WARNING message
> +when you end up performing either #2 or #3. In this manner, if a user
> +of your driver reports that performance is bad or that the device is not
> +even detected, you can ask them for the kernel messages to find out
> +exactly why.
> +
> +The standard 32-bit addressing PCI device would do something like
> +this:
> +
> + if (pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
> + printk(KERN_WARNING
> + "mydev: No suitable DMA available.\n");
> + goto ignore_this_device;
> + }
> +
> +Another common scenario is a 64-bit capable device. The approach
> +here is to try for 64-bit DAC addressing, but back down to a
> +32-bit mask should that fail. The PCI platform code may fail the
> +64-bit mask not because the platform is not capable of 64-bit
> +addressing. Rather, it may fail in this case simply because
> +32-bit SAC addressing is done more efficiently than DAC addressing.
> +Sparc64 is one platform which behaves in this way.
> +
> +Here is how you would handle a 64-bit capable device which can drive
> +all 64-bits when accessing streaming DMA:
> +
> + int using_dac;
> +
> + if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
> + using_dac = 1;
> + } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
> + using_dac = 0;
> + } else {
> + printk(KERN_WARNING
> + "mydev: No suitable DMA available.\n");
> + goto ignore_this_device;
> + }
> +
> +If a card is capable of using 64-bit consistent allocations as well,
> +the case would look like this:
> +
> + int using_dac, consistent_using_dac;
> +
> + if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
> + using_dac = 1;
> + consistent_using_dac = 1;
> + pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(64));
> + } else if (!pci_set_dma_mask(pdev, DMA_BIT_MASK(32))) {
> + using_dac = 0;
> + consistent_using_dac = 0;
> + pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(32));
> + } else {
> + printk(KERN_WARNING
> + "mydev: No suitable DMA available.\n");
> + goto ignore_this_device;
> + }
> +
> +pci_set_consistent_dma_mask() will always be able to set the same or a
> +smaller mask as pci_set_dma_mask(). However for the rare case that a
> +device driver only uses consistent allocations, one would have to
> +check the return value from pci_set_consistent_dma_mask().
> +
> +Finally, if your device can only drive the low 24-bits of
> +address during PCI bus mastering you might do something like:
> +
> + if (pci_set_dma_mask(pdev, DMA_BIT_MASK(24))) {
> + printk(KERN_WARNING
> + "mydev: 24-bit DMA addressing not available.\n");
> + goto ignore_this_device;
> + }
> +
> +When pci_set_dma_mask() is successful, and returns zero, the PCI layer
> +saves away this mask you have provided. The PCI layer will use this
> +information later when you make DMA mappings.
> +
> +There is a case which we are aware of at this time, which is worth
> +mentioning in this documentation. If your device supports multiple
> +functions (for example a sound card provides playback and record
> +functions) and the various different functions have _different_
> +DMA addressing limitations, you may wish to probe each mask and
> +only provide the functionality which the machine can handle. It
> +is important that the last call to pci_set_dma_mask() be for the
> +most specific mask.
> +
> +Here is pseudo-code showing how this might be done:
> +
> + #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32)
> + #define RECORD_ADDRESS_BITS 0x00ffffff
> +
> + struct my_sound_card *card;
> + struct pci_dev *pdev;
> +
> + ...
> + if (!pci_set_dma_mask(pdev, PLAYBACK_ADDRESS_BITS)) {
> + card->playback_enabled = 1;
> + } else {
> + card->playback_enabled = 0;
> + printk(KERN_WARN "%s: Playback disabled due to DMA limitations.\n",
> + card->name);
> + }
> + if (!pci_set_dma_mask(pdev, RECORD_ADDRESS_BITS)) {
> + card->record_enabled = 1;
> + } else {
> + card->record_enabled = 0;
> + printk(KERN_WARN "%s: Record disabled due to DMA limitations.\n",
> + card->name);
> + }
> +
> +A sound card was used as an example here because this genre of PCI
> +devices seems to be littered with ISA chips given a PCI front end,
> +and thus retaining the 16MB DMA addressing limitations of ISA.
> +
> + Types of DMA mappings
> +
> +There are two types of DMA mappings:
> +
> +- Consistent DMA mappings which are usually mapped at driver
> + initialization, unmapped at the end and for which the hardware should
> + guarantee that the device and the CPU can access the data
> + in parallel and will see updates made by each other without any
> + explicit software flushing.
> +
> + Think of "consistent" as "synchronous" or "coherent".
> +
> + The current default is to return consistent memory in the low 32
> + bits of the PCI bus space. However, for future compatibility you
> + should set the consistent mask even if this default is fine for your
> + driver.
> +
> + Good examples of what to use consistent mappings for are:
> +
> + - Network card DMA ring descriptors.
> + - SCSI adapter mailbox command data structures.
> + - Device firmware microcode executed out of
> + main memory.
> +
> + The invariant these examples all require is that any CPU store
> + to memory is immediately visible to the device, and vice
> + versa. Consistent mappings guarantee this.
> +
> + IMPORTANT: Consistent DMA memory does not preclude the usage of
> + proper memory barriers. The CPU may reorder stores to
> + consistent memory just as it may normal memory. Example:
> + if it is important for the device to see the first word
> + of a descriptor updated before the second, you must do
> + something like:
> +
> + desc->word0 = address;
> + wmb();
> + desc->word1 = DESC_VALID;
> +
> + in order to get correct behavior on all platforms.
> +
> + Also, on some platforms your driver may need to flush CPU write
> + buffers in much the same way as it needs to flush write buffers
> + found in PCI bridges (such as by reading a register's value
> + after writing it).
> +
> +- Streaming DMA mappings which are usually mapped for one DMA transfer,
> + unmapped right after it (unless you use pci_dma_sync_* below) and for which
> + hardware can optimize for sequential accesses.
> +
> + This of "streaming" as "asynchronous" or "outside the coherency
> + domain".
> +
> + Good examples of what to use streaming mappings for are:
> +
> + - Networking buffers transmitted/received by a device.
> + - Filesystem buffers written/read by a SCSI device.
> +
> + The interfaces for using this type of mapping were designed in
> + such a way that an implementation can make whatever performance
> + optimizations the hardware allows. To this end, when using
> + such mappings you must be explicit about what you want to happen.
> +
> +Neither type of DMA mapping has alignment restrictions that come
> +from PCI, although some devices may have such restrictions.
> +Also, systems with caches that aren't DMA-coherent will work better
> +when the underlying buffers don't share cache lines with other data.
> +
> +
> + Using Consistent DMA mappings.
> +
> +To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
> +you should do:
> +
> + dma_addr_t dma_handle;
> +
> + cpu_addr = pci_alloc_consistent(pdev, size, &dma_handle);
> +
> +where pdev is a struct pci_dev *. This may be called in interrupt context.
> +You should use dma_alloc_coherent (see DMA-API.txt) for buses
> +where devices don't have struct pci_dev (like ISA, EISA).
> +
> +This argument is needed because the DMA translations may be bus
> +specific (and often is private to the bus which the device is attached
> +to).
> +
> +Size is the length of the region you want to allocate, in bytes.
> +
> +This routine will allocate RAM for that region, so it acts similarly to
> +__get_free_pages (but takes size instead of a page order). If your
> +driver needs regions sized smaller than a page, you may prefer using
> +the pci_pool interface, described below.
> +
> +The consistent DMA mapping interfaces, for non-NULL pdev, will by
> +default return a DMA address which is SAC (Single Address Cycle)
> +addressable. Even if the device indicates (via PCI dma mask) that it
> +may address the upper 32-bits and thus perform DAC cycles, consistent
> +allocation will only return > 32-bit PCI addresses for DMA if the
> +consistent dma mask has been explicitly changed via
> +pci_set_consistent_dma_mask(). This is true of the pci_pool interface
> +as well.
> +
> +pci_alloc_consistent returns two values: the virtual address which you
> +can use to access it from the CPU and dma_handle which you pass to the
> +card.
> +
> +The cpu return address and the DMA bus master address are both
> +guaranteed to be aligned to the smallest PAGE_SIZE order which
> +is greater than or equal to the requested size. This invariant
> +exists (for example) to guarantee that if you allocate a chunk
> +which is smaller than or equal to 64 kilobytes, the extent of the
> +buffer you receive will not cross a 64K boundary.
> +
> +To unmap and free such a DMA region, you call:
> +
> + pci_free_consistent(pdev, size, cpu_addr, dma_handle);
> +
> +where pdev, size are the same as in the above call and cpu_addr and
> +dma_handle are the values pci_alloc_consistent returned to you.
> +This function may not be called in interrupt context.
> +
> +If your driver needs lots of smaller memory regions, you can write
> +custom code to subdivide pages returned by pci_alloc_consistent,
> +or you can use the pci_pool API to do that. A pci_pool is like
> +a kmem_cache, but it uses pci_alloc_consistent not __get_free_pages.
> +Also, it understands common hardware constraints for alignment,
> +like queue heads needing to be aligned on N byte boundaries.
> +
> +Create a pci_pool like this:
> +
> + struct pci_pool *pool;
> +
> + pool = pci_pool_create(name, pdev, size, align, alloc);
> +
> +The "name" is for diagnostics (like a kmem_cache name); pdev and size
> +are as above. The device's hardware alignment requirement for this
> +type of data is "align" (which is expressed in bytes, and must be a
> +power of two). If your device has no boundary crossing restrictions,
> +pass 0 for alloc; passing 4096 says memory allocated from this pool
> +must not cross 4KByte boundaries (but at that time it may be better to
> +go for pci_alloc_consistent directly instead).
> +
> +Allocate memory from a pci pool like this:
> +
> + cpu_addr = pci_pool_alloc(pool, flags, &dma_handle);
> +
> +flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
> +holding SMP locks), SLAB_ATOMIC otherwise. Like pci_alloc_consistent,
> +this returns two values, cpu_addr and dma_handle.
> +
> +Free memory that was allocated from a pci_pool like this:
> +
> + pci_pool_free(pool, cpu_addr, dma_handle);
> +
> +where pool is what you passed to pci_pool_alloc, and cpu_addr and
> +dma_handle are the values pci_pool_alloc returned. This function
> +may be called in interrupt context.
> +
> +Destroy a pci_pool by calling:
> +
> + pci_pool_destroy(pool);
> +
> +Make sure you've called pci_pool_free for all memory allocated
> +from a pool before you destroy the pool. This function may not
> +be called in interrupt context.
> +
> + DMA Direction
> +
> +The interfaces described in subsequent portions of this document
> +take a DMA direction argument, which is an integer and takes on
> +one of the following values:
> +
> + PCI_DMA_BIDIRECTIONAL
> + PCI_DMA_TODEVICE
> + PCI_DMA_FROMDEVICE
> + PCI_DMA_NONE
> +
> +One should provide the exact DMA direction if you know it.
> +
> +PCI_DMA_TODEVICE means "from main memory to the PCI device"
> +PCI_DMA_FROMDEVICE means "from the PCI device to main memory"
> +It is the direction in which the data moves during the DMA
> +transfer.
> +
> +You are _strongly_ encouraged to specify this as precisely
> +as you possibly can.
> +
> +If you absolutely cannot know the direction of the DMA transfer,
> +specify PCI_DMA_BIDIRECTIONAL. It means that the DMA can go in
> +either direction. The platform guarantees that you may legally
> +specify this, and that it will work, but this may be at the
> +cost of performance for example.
> +
> +The value PCI_DMA_NONE is to be used for debugging. One can
> +hold this in a data structure before you come to know the
> +precise direction, and this will help catch cases where your
> +direction tracking logic has failed to set things up properly.
> +
> +Another advantage of specifying this value precisely (outside of
> +potential platform-specific optimizations of such) is for debugging.
> +Some platforms actually have a write permission boolean which DMA
> +mappings can be marked with, much like page protections in the user
> +program address space. Such platforms can and do report errors in the
> +kernel logs when the PCI controller hardware detects violation of the
> +permission setting.
> +
> +Only streaming mappings specify a direction, consistent mappings
> +implicitly have a direction attribute setting of
> +PCI_DMA_BIDIRECTIONAL.
> +
> +The SCSI subsystem tells you the direction to use in the
> +'sc_data_direction' member of the SCSI command your driver is
> +working on.
> +
> +For Networking drivers, it's a rather simple affair. For transmit
> +packets, map/unmap them with the PCI_DMA_TODEVICE direction
> +specifier. For receive packets, just the opposite, map/unmap them
> +with the PCI_DMA_FROMDEVICE direction specifier.
> +
> + Using Streaming DMA mappings
> +
> +The streaming DMA mapping routines can be called from interrupt
> +context. There are two versions of each map/unmap, one which will
> +map/unmap a single memory region, and one which will map/unmap a
> +scatterlist.
> +
> +To map a single region, you do:
> +
> + struct pci_dev *pdev = mydev->pdev;
> + dma_addr_t dma_handle;
> + void *addr = buffer->ptr;
> + size_t size = buffer->len;
> +
> + dma_handle = pci_map_single(pdev, addr, size, direction);
> +
> +and to unmap it:
> +
> + pci_unmap_single(pdev, dma_handle, size, direction);
> +
> +You should call pci_unmap_single when the DMA activity is finished, e.g.
> +from the interrupt which told you that the DMA transfer is done.
> +
> +Using cpu pointers like this for single mappings has a disadvantage,
> +you cannot reference HIGHMEM memory in this way. Thus, there is a
> +map/unmap interface pair akin to pci_{map,unmap}_single. These
> +interfaces deal with page/offset pairs instead of cpu pointers.
> +Specifically:
> +
> + struct pci_dev *pdev = mydev->pdev;
> + dma_addr_t dma_handle;
> + struct page *page = buffer->page;
> + unsigned long offset = buffer->offset;
> + size_t size = buffer->len;
> +
> + dma_handle = pci_map_page(pdev, page, offset, size, direction);
> +
> + ...
> +
> + pci_unmap_page(pdev, dma_handle, size, direction);
> +
> +Here, "offset" means byte offset within the given page.
> +
> +With scatterlists, you map a region gathered from several regions by:
> +
> + int i, count = pci_map_sg(pdev, sglist, nents, direction);
> + struct scatterlist *sg;
> +
> + for_each_sg(sglist, sg, count, i) {
> + hw_address[i] = sg_dma_address(sg);
> + hw_len[i] = sg_dma_len(sg);
> + }
> +
> +where nents is the number of entries in the sglist.
> +
> +The implementation is free to merge several consecutive sglist entries
> +into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
> +consecutive sglist entries can be merged into one provided the first one
> +ends and the second one starts on a page boundary - in fact this is a huge
> +advantage for cards which either cannot do scatter-gather or have very
> +limited number of scatter-gather entries) and returns the actual number
> +of sg entries it mapped them to. On failure 0 is returned.
> +
> +Then you should loop count times (note: this can be less than nents times)
> +and use sg_dma_address() and sg_dma_len() macros where you previously
> +accessed sg->address and sg->length as shown above.
> +
> +To unmap a scatterlist, just call:
> +
> + pci_unmap_sg(pdev, sglist, nents, direction);
> +
> +Again, make sure DMA activity has already finished.
> +
> +PLEASE NOTE: The 'nents' argument to the pci_unmap_sg call must be
> + the _same_ one you passed into the pci_map_sg call,
> + it should _NOT_ be the 'count' value _returned_ from the
> + pci_map_sg call.
> +
> +Every pci_map_{single,sg} call should have its pci_unmap_{single,sg}
> +counterpart, because the bus address space is a shared resource (although
> +in some ports the mapping is per each BUS so less devices contend for the
> +same bus address space) and you could render the machine unusable by eating
> +all bus addresses.
> +
> +If you need to use the same streaming DMA region multiple times and touch
> +the data in between the DMA transfers, the buffer needs to be synced
> +properly in order for the cpu and device to see the most uptodate and
> +correct copy of the DMA buffer.
> +
> +So, firstly, just map it with pci_map_{single,sg}, and after each DMA
> +transfer call either:
> +
> + pci_dma_sync_single_for_cpu(pdev, dma_handle, size, direction);
> +
> +or:
> +
> + pci_dma_sync_sg_for_cpu(pdev, sglist, nents, direction);
> +
> +as appropriate.
> +
> +Then, if you wish to let the device get at the DMA area again,
> +finish accessing the data with the cpu, and then before actually
> +giving the buffer to the hardware call either:
> +
> + pci_dma_sync_single_for_device(pdev, dma_handle, size, direction);
> +
> +or:
> +
> + pci_dma_sync_sg_for_device(dev, sglist, nents, direction);
> +
> +as appropriate.
> +
> +After the last DMA transfer call one of the DMA unmap routines
> +pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_*
> +call till pci_unmap_*, then you don't have to call the pci_dma_sync_*
> +routines at all.
> +
> +Here is pseudo code which shows a situation in which you would need
> +to use the pci_dma_sync_*() interfaces.
> +
> + my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
> + {
> + dma_addr_t mapping;
> +
> + mapping = pci_map_single(cp->pdev, buffer, len, PCI_DMA_FROMDEVICE);
> +
> + cp->rx_buf = buffer;
> + cp->rx_len = len;
> + cp->rx_dma = mapping;
> +
> + give_rx_buf_to_card(cp);
> + }
> +
> + ...
> +
> + my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
> + {
> + struct my_card *cp = devid;
> +
> + ...
> + if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
> + struct my_card_header *hp;
> +
> + /* Examine the header to see if we wish
> + * to accept the data. But synchronize
> + * the DMA transfer with the CPU first
> + * so that we see updated contents.
> + */
> + pci_dma_sync_single_for_cpu(cp->pdev, cp->rx_dma,
> + cp->rx_len,
> + PCI_DMA_FROMDEVICE);
> +
> + /* Now it is safe to examine the buffer. */
> + hp = (struct my_card_header *) cp->rx_buf;
> + if (header_is_ok(hp)) {
> + pci_unmap_single(cp->pdev, cp->rx_dma, cp->rx_len,
> + PCI_DMA_FROMDEVICE);
> + pass_to_upper_layers(cp->rx_buf);
> + make_and_setup_new_rx_buf(cp);
> + } else {
> + /* Just sync the buffer and give it back
> + * to the card.
> + */
> + pci_dma_sync_single_for_device(cp->pdev,
> + cp->rx_dma,
> + cp->rx_len,
> + PCI_DMA_FROMDEVICE);
> + give_rx_buf_to_card(cp);
> + }
> + }
> + }
> +
> +Drivers converted fully to this interface should not use virt_to_bus any
> +longer, nor should they use bus_to_virt. Some drivers have to be changed a
> +little bit, because there is no longer an equivalent to bus_to_virt in the
> +dynamic DMA mapping scheme - you have to always store the DMA addresses
> +returned by the pci_alloc_consistent, pci_pool_alloc, and pci_map_single
> +calls (pci_map_sg stores them in the scatterlist itself if the platform
> +supports dynamic DMA mapping in hardware) in your driver structures and/or
> +in the card registers.
> +
> +All PCI drivers should be using these interfaces with no exceptions.
> +It is planned to completely remove virt_to_bus() and bus_to_virt() as
> +they are entirely deprecated. Some ports already do not provide these
> +as it is impossible to correctly support them.
> +
> + Optimizing Unmap State Space Consumption
> +
> +On many platforms, pci_unmap_{single,page}() is simply a nop.
> +Therefore, keeping track of the mapping address and length is a waste
> +of space. Instead of filling your drivers up with ifdefs and the like
> +to "work around" this (which would defeat the whole purpose of a
> +portable API) the following facilities are provided.
> +
> +Actually, instead of describing the macros one by one, we'll
> +transform some example code.
> +
> +1) Use DECLARE_PCI_UNMAP_{ADDR,LEN} in state saving structures.
> + Example, before:
> +
> + struct ring_state {
> + struct sk_buff *skb;
> + dma_addr_t mapping;
> + __u32 len;
> + };
> +
> + after:
> +
> + struct ring_state {
> + struct sk_buff *skb;
> + DECLARE_PCI_UNMAP_ADDR(mapping)
> + DECLARE_PCI_UNMAP_LEN(len)
> + };
> +
> + NOTE: DO NOT put a semicolon at the end of the DECLARE_*()
> + macro.
> +
> +2) Use pci_unmap_{addr,len}_set to set these values.
> + Example, before:
> +
> + ringp->mapping = FOO;
> + ringp->len = BAR;
> +
> + after:
> +
> + pci_unmap_addr_set(ringp, mapping, FOO);
> + pci_unmap_len_set(ringp, len, BAR);
> +
> +3) Use pci_unmap_{addr,len} to access these values.
> + Example, before:
> +
> + pci_unmap_single(pdev, ringp->mapping, ringp->len,
> + PCI_DMA_FROMDEVICE);
> +
> + after:
> +
> + pci_unmap_single(pdev,
> + pci_unmap_addr(ringp, mapping),
> + pci_unmap_len(ringp, len),
> + PCI_DMA_FROMDEVICE);
> +
> +It really should be self-explanatory. We treat the ADDR and LEN
> +separately, because it is possible for an implementation to only
> +need the address in order to perform the unmap operation.
> +
> + Platform Issues
> +
> +If you are just writing drivers for Linux and do not maintain
> +an architecture port for the kernel, you can safely skip down
> +to "Closing".
> +
> +1) Struct scatterlist requirements.
> +
> + Struct scatterlist must contain, at a minimum, the following
> + members:
> +
> + struct page *page;
> + unsigned int offset;
> + unsigned int length;
> +
> + The base address is specified by a "page+offset" pair.
> +
> + Previous versions of struct scatterlist contained a "void *address"
> + field that was sometimes used instead of page+offset. As of Linux
> + 2.5., page+offset is always used, and the "address" field has been
> + deleted.
> +
> +2) More to come...
> +
> + Handling Errors
> +
> +DMA address space is limited on some architectures and an allocation
> +failure can be determined by:
> +
> +- checking if pci_alloc_consistent returns NULL or pci_map_sg returns 0
> +
> +- checking the returned dma_addr_t of pci_map_single and pci_map_page
> + by using pci_dma_mapping_error():
> +
> + dma_addr_t dma_handle;
> +
> + dma_handle = pci_map_single(pdev, addr, size, direction);
> + if (pci_dma_mapping_error(pdev, dma_handle)) {
> + /*
> + * reduce current DMA mapping usage,
> + * delay and try again later or
> + * reset driver.
> + */
> + }
> +
> + Closing
> +
> +This document, and the API itself, would not be in it's current
> +form without the feedback and suggestions from numerous individuals.
> +We would like to specifically mention, in no particular order, the
> +following people:
> +
> + Russell King <[email protected]>
> + Leo Dagum <[email protected]>
> + Ralf Baechle <[email protected]>
> + Grant Grundler <[email protected]>
> + Jay Estabrook <[email protected]>
> + Thomas Sailer <[email protected]>
> + Andrea Arcangeli <[email protected]>
> + Jens Axboe <[email protected]>
> + David Mosberger-Tang <[email protected]>
> diff --git a/Documentation/block/biodoc.txt b/Documentation/block/biodoc.txt
> index 8d2158a..6fab97e 100644
> --- a/Documentation/block/biodoc.txt
> +++ b/Documentation/block/biodoc.txt
> @@ -186,7 +186,7 @@ a virtual address mapping (unlike the earlier scheme of virtual address
> do not have a corresponding kernel virtual address space mapping) and
> low-memory pages.
>
> -Note: Please refer to Documentation/DMA-mapping.txt for a discussion
> +Note: Please refer to Documentation/PCI/PCI-DMA-mapping.txt for a discussion
> on PCI high mem DMA aspects and mapping of scatter gather lists, and support
> for 64 bit PCI.
>
> --
> 1.6.5.3
>
---
~Randy