Date: Tue, 14 Jun 2011 13:10:33 +1000
From: David Gibson <david@gibson.dropbear.id.au>
To: Grant Likely <grant.likely@secretlab.ca>
Cc: devicetree-discuss@lists.ozlabs.org, Randy Dunlap <rdunlap@xenotime.net>,
        linaro-dev@lists.linaro.org, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org
Subject: Re: [RFC] (early draft) dt: Linux dt usage model documentation
Message-ID: <20110614031033.GA4996@yookeroo.fritz.box>
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	Grant Likely <grant.likely@secretlab.ca>,
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Comments below.  Only really considering gaps in content or emphasis
here, not poor wording or grammatical errors.

On Mon, Jun 13, 2011 at 07:32:15AM -0600, Grant Likely wrote:
> Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
> ---
> 
> Hey all,
> 
> This is an early draft of the usage model document for the device
> tree, but I wanted to get it out there for feedback, and so that some
> of the Linaro engineers could get started on migrating board ports.
> 
> g.
> 
>  Documentation/devicetree/usage-model |  403 ++++++++++++++++++++++++++++++++++
>  1 files changed, 403 insertions(+), 0 deletions(-)
>  create mode 100644 Documentation/devicetree/usage-model
> 
> diff --git a/Documentation/devicetree/usage-model b/Documentation/devicetree/usage-model
> new file mode 100644
> index 0000000..4203119
> --- /dev/null
> +++ b/Documentation/devicetree/usage-model
> @@ -0,0 +1,403 @@
> +Linux and the Device Tree
> +The Linux usage model for device tree data
> +
> +Author: Grant Likely <grant.likely@secretlab.ca>
> +
> +This article describes how Linux uses the device tree.  An overview of
> +the device tree data format can be found at the <a
> +href="http://devicetree.org/Device_Tree_Usage">Device Tree Usage</a>
> +page on <a href="http://devicetree.org">devicetree.org</a>.
> +
> +
> +	All the cool architectures are using device tree.  I want to
> +	use device tree too!
> +
> +The "Open Firmware Device Tree", or simply Device Tree (DT), is a data
> +structure and language for describing hardware.  More specifically, it
> +is an description of hardware that is readable by an operating system
> +so that the operating system doesn't need to hard code details of the
> +machine.
> +
> +Structurally, the DT is a tree, or acyclic graph with named nodes, and
> +nodes may have an arbitrary number of named properties encapsulating
> +arbitrary data.  A mechanism also exists to create arbitrary
> +links from one node to another outside of the natural tree structure..
> +
> +Conceptually, a common set of usage conventions, called 'bindings',
> +is defined for how data should appear in the tree to describe typical
> +hardware characteristics including data busses, interrupt lines, gpio
> +connections, and peripheral devices.
> +
> +As much as possible, hardware is described using existing bindings to
> +maximize use of existing support code, but since property and node
> +names are simply text strings, it is easy to extend existing bindings
> +or create new ones by defining new nodes and properties.

I think this could do with some expansion to mention that a lot of the
core bindings come from the OF working group way back when.  There are
plenty of newer "flat tree era" bindings, but particularly for busses
rather than individual devices, we should look to the old, old
bindings for precedent.

In fact, I think it might be worth explicitly talking about how we
ended up with two different i2c bindings (the old OF one, and the
flat-tree era one) as a cautionary tale of insufficient research into
existing bindings.

> +<h2>History</h2>
> +The DT was originally created by Open Firmware as part of the
> +communication method for passing data from Open Firmware to a client
> +program (like to an operating system).  An operating system used the
> +Device Tree to discover the topology of the hardware at runtime, and
> +thereby support a majority of available hardware without hard coded
> +information (assuming drivers were available for all devices).
> +
> +Since Open Firmware is commonly used on PowerPC and SPARC platforms,
> +the Linux support for those architectures has for a long time used the
> +Device Tree.
> +
> +In 2005, when PowerPC Linux began a major cleanup and to merge 32 bit
> +and 64 support, the decision was made to require DT support on all
> +powerpc platforms, regardless of whether or not they used Open
> +Firmware.  To do this, a DT representation called the Flattened Device
> +Tree (FDT) was created which could be passed to the kernel as a binary
> +blob without requiring a real Open Firmware implementation.  U-Boot,
> +kexec, and other bootloaders were modified to support both passing a
> +Device Tree Binary (dtb) and to modify a dtb at boot time.

And there's the option of having a wrapper for non-dt bootloaders
which patches a canned dtb with whatever info it gets from the
firmware.

> +Some time later, FDT infrastructure was generalized to be usable by
> +all architectures.  At the time of this writing, 5 mainlined
> +architectures (arm, mips, powerpc, sparc, and x86) and 1 out of
> +mainline architecture (nios) have some level of DT support.
> +
> +<h2>Data Model</h2>
> +If you haven't already read the
> +href="http://devicetree.org/Device_Tree_Usage">Device Tree Usage</a>
> +page, then go read it now.  It's okay, I'll wait....
> +
> +<h3>High Level View</h3>
> +The most important thing to understand is that the DT is simply a data
> +structure that describes the hardware.  There is nothing magical about
> +it, and it doesn't magically make all hardware configuration problems
> +go away.  What it does do is provide a language for decoupling the
> +hardware configuration from the board and device driver support in the
> +Linux kernel (or any other operating system for that matter).  Using
> +it allows board and device support to become data driven; to make
> +setup decisions based on data passed into the kernel instead of on
> +per-machine hard coded selections.
> +
> +Ideally, data driven platform setup should result in less code
> +duplication and make it easier to support a wide range of hardware
> +with a single kernel image.
> +
> +Linux uses DT data for three major purposes:
> +1) platform identification,
> +2) runtime configuration, and
> +3) device population.
> +
> +<h4>Platform Identification</h4>
> +First and foremost, the kernel will use data in the DT to identify the
> +specific machine.  In a perfect world, the specific platform shouldn't
> +matter to the kernel because all platform details would be described
> +perfectly by the device tree in a consistent and reliable manner.
> +Hardware is not perfect though, and so the kernel must identify the
> +machine during early boot so that it has the opportunity to run
> +machine specific fixups.
> +
> +In the majority of cases, the machine identity is irrelevant, and the
> +kernel will instead select setup code based on the machines core
> +cpu or SoC.  On ARM for example, setup_arch() in
> +arch/arm/kernel/setup.c will call setup_machine_fdt() in
> +arch/arm/kernel/devicetree.c which searches through the machine_desc
> +table and selects the machine_desc which best matches the device tree
> +data.  It determines the best match by looking at the 'compatible'
> +property in the root device tree node, and comparing it with the
> +dt_compat list in struct machine_desc.
> +
> +The 'compatible' property contains a sorted list of strings starting
> +with the exact name of the machine, followed by an optional list of
> +boards it is compatible with sorted from most compatible to list.  For
> +example, the root compatible properties for the TI BeagleBoard and its
> +successor, the BeagleBoard xM board might look like:
> +
> +	compatible = "ti,omap3-beagleboard", "ti,omap3450", "ti,omap3";
> +	compatible = "ti,omap3-beagleboard-xm", "ti,omap3450", "ti,omap3";
> +
> +Where "ti,omap3-beagleboard-xm" specifies the exact model, it also
> +claims that it compatible with the OMAP 3450 SoC, and the omap3 family
> +of SoCs in general.  You'll notice that the list is sorted from most
> +specific (exact board) to least specific (SoC family).
> +
> +Astute readers might point out that the Beagle xM could also claim
> +compatibility with the original Beagle board.  However, one should be
> +cautioned about doing so at the board level since there is typically a
> +high level of change from one board to another, even within the same
> +product line, that it is hard to nail down exactly is meant when one
> +board claims to be compatible with another.  For the top level, it is
> +better to err on the side of caution and not claim one board is
> +compatible with another.  The notable exception would be when one
> +board is a carrier for another, such as a cpu module attached to a
> +carrier board.
> +
> +One more note on compatible values.  Any string used in a compatible
> +property must be documented as to what it indicates.  Add
> +documentation for compatible strings in Documentation/devicetree/bindings.

Uh.. do we really want to point people to the kernel tree for the
binding documentation?  I thought the idea is that devicetree.org
would take over as the binding repository.  Having them in the kernel
tree encourages Linux-specific behaviour, whereas the device tree
should aim to be an OS-neutral structure.  This is not just
theoretical, there are FreeBSD people working on using DTs for several
of their platforms.

> +Again on ARM, the for each machine_desc, the kernel looks to see if
> +any of the dt_compat list entries appear in the compatible property.
> +If one does, then that machine_desc is a candidate for driving the
> +machine.  After searching the entire table of machine_descs,
> +setup_machine_fdt() returns the 'most compatible' machine_desc based
> +on which entry in the compatible property each machine_desc matches
> +against.  If no matching machine_desc is found, then it returns NULL.
> +
> +The reasoning behind this scheme is the observation that in the majority
> +of cases, a single machine_desc can support a large number of boards
> +if that all use the same SoC, or same family of SoCs.  However,
> +invariably there will be some exceptions where a specific board will
> +require special setup code that is not useful in the generic case.
> +Special cases could be handled by explicitly checking for the
> +troublesome board(s) in generic setup code, but doing so very quickly
> +becomes ugly and/or unmaintainable if it is more than just a couple of
> +cases.
> +
> +Instead, the compatible list allows a generic machine_desc to provide
> +support for a wide common set of boards by specifying "less
> +compatible" value in the dt_compat list.  In the example above,
> +generic board support can claim compatibility with "ti,omap3" or
> +"ti,omap3450".  If a bug was discovered on the original beagleboard
> +that required special workaround code during early boot, then a new
> +machine_desc could be added which implements the workarounds and only
> +matches on "ti,beagleboard".
> +
> +PowerPC uses a slightly different scheme where it calls the .probe()
> +hook from each machine_desc, and the first one returning TRUE is used.
> +However, this approach does not take into account the priority of the
> +compatible list, and probably should be avoided for new architecture
> +support.
> +
> +<h4>Runtime configuration</h4>
> +In most cases, a DT will be the sole method of communicating data from
> +firmware to the kernel, so also gets used to pass in runtime and
> +configuration data like the kernel parameters string and the location
> +of an initrd image.
> +
> +Most of this data is contained in the /chosen node, and when booting
> +Linux it will look something like this:
> +
> +	chosen {
> +		bootargs = "console=ttyS0,115200 loglevel=8";
> +		initrd-start = &lt;0xc8000000&gt;;
> +		initrd-end = &lt;0xc8200000&gt;;
> +	};
> +
> +The bootargs property contains the kernel arguments, and the initrd-*
> +properties define the address and size of an initrd blob.  The
> +chosen node may also optionally contain an arbitrary number of
> +additional properties for platform specific configuration data.

Hrm, yeah.  It is important to talk about this usage, but we should
also discourage adding random linux-configuration specific bits to
random bindings all over the place.

> +During early boot, the architecture setup code calls of_scan_flat_dt()
> +several times with different helper callbacks to parse device tree
> +data before paging is setup.  The of_scan_flat_dt() code scans through
> +the device tree and uses the helpers to extract information required
> +during early boot.  Typically the early_init_dt_scan_chosen() helper
> +is used to parse the chosen node including kernel parameters,
> +early_init_dt_scan_root() to initialize the DT address space model,
> +and early_init_dt_scan_memory() to determine the size and
> +location of usable RAM.
> +
> +On ARM, the function setup_machine_fdt() is responsible for early
> +scanning of the device tree after selecting the correct machine_desc
> +that supports the board.
> +
> +<h4>Device population</h4>
> +After the board has been identified, and after the early configuration data
> +has been parsed, then kernel initialization can proceed in the normal
> +way.  At some point in this process, unflatten_device_tree() is called
> +to convert the data into a more efficient runtime representation.
> +This is also when machine specific setup hooks will get called, like
> +the machine_desc .init_early(), .init_irq() and .init_machine() hooks
> +on ARM.  The remainder of this section uses examples from the ARM
> +implementation, but all architectures will do pretty much the same
> +thing when using a DT.
> +
> +As can be guessed by the names, .init_early() is used for any machine
> +specific setup that needs to be executed early in the boot process,
> +and .init_irq() is used to set up interrupt handling.  Using a DT
> +doesn't materially change the behaviour of either of these functions.
> +If a DT is provided, then both .init_early() and .init_irq() are able
> +to call any of the DT query functions (of_* in include/linux/of*.h) to
> +get additional data about the platform.
> +
> +The most interesting hook in the DT context is .init_machine() which
> +is primarily responsible for populating the Linux device model with
> +data about the platform.  Historically this has been implemented on
> +embedded platforms by defining a set of static clock structures,
> +platform_devices, and other data in the board support .c file, and
> +registering it en-masse in .init_machine().  When DT is used, then
> +instead of hard coding static devices for each platform, the list of
> +devices can be obtained by parsing the DT, and allocating device
> +structures dynamically.
> +
> +The simplest case is when .init_machine() is only responsible for
> +registering a block of platform_devices.  Platform devices are concept
> +used by Linux for memory or io mapped devices which cannot be detected
> +by hardware, and for 'composite' or 'virtual' devices (more on those
> +later).  While there is no 'platform device' terminology for the DT,
> +platform devices roughly correspond to device nodes at the root of the
> +tree and children of simple memory mapped bus nodes.
> +
> +About now is a good time to lay out an example.  Here is part of the
> +device tree for the NVIDIA Tegra board.
> +
> +/{
> +	compatible = "nvidia,harmony", "nvidia,tegra250";
> +	#address-cells = <1>;
> +	#size-cells = <1>;
> +	interrupt-parent = <&intc>;
> +
> +	chosen { };
> +	aliases { };
> +
> +	memory {
> +		device_type = "memory";
> +		reg = <0x00000000 0x40000000>;
> +	};
> +
> +	soc {
> +		compatible = "nvidia,tegra250-soc", "simple-bus";
> +		#address-cells = <1>;
> +		#size-cells = <1>;
> +		ranges;
> +
> +		intc: interrupt-controller@50041000 {
> +			compatible = "nvidia,tegra250-gic";
> +			interrupt-controller;
> +			#interrupt-cells = <1>;
> +			reg = <0x50041000 0x1000>, < 0x50040100 0x0100 >;
> +		};
> +
> +		serial@70006300 {
> +			compatible = "nvidia,tegra250-uart";
> +			reg = <0x70006300 0x100>;
> +			interrupts = <122>;
> +		};
> +
> +		i2s-1: i2s@70002800 {
> +			compatible = "nvidia,tegra250-i2s";
> +			reg = <0x70002800 0x100>;
> +			interrupts = <77>;
> +			codec = <&wm8903>;
> +		};
> +
> +		i2c@7000c000 {
> +			compatible = "nvidia,tegra250-i2c";
> +			#address-cells = <1>;
> +			#size-cells = <1>;
> +			reg = <0x7000c000 0x100>;
> +			interrupts = <70>;
> +
> +			wm8903: codec@1a {
> +				compatible = "wlf,wm8903";
> +				reg = <0x1a>;
> +				interrupts = <347>;
> +			};
> +		};
> +	};
> +
> +	sound {
> +		compatible = "nvidia,harmony-sound";
> +		i2s-controller = <&i2s-1>;
> +		i2s-codec = <&wm8903>;
> +	};
> +};
> +
> +At .machine_init() time, Tegra board support code will need to look at
> +this DT and decide which nodes to create platform_devices for.
> +However, looking at the tree, it is not immediately obvious what kind
> +of device each node represents, or even if a node represents a device
> +at all.  The /chosen, /aliases, and /memory nodes are informational
> +nodes that don't describe devices (although arguably memory could be
> +considered a device).  The children of the /soc node are memory mapped
> +devices, but the codec@1a is an i2c device, and the sound node
> +represents not a device, but rather how other devices are connected
> +together to create the audio subsystem.  I know what each device is
> +because I'm familiar with the board design, but how does the kernel
> +know what to do with each node?

I wonder if we should explain that the /sound node is in some senses a
hack - and thereby an example of compromise in binding design for
practicality.  In theory the "correct" way of doing things would be to
have all the codecs / sound busses / connections and whatnot
individually described in the device tree, with all all the links
between them specified.  But this would be a pain both from the dt
construction and kernel driver point of view, hence the /sound node,
corresponding to the kernel "sound fabric" driver.  It seems to be
this document might be the right place for discussion of this sort of
design trade-off.

> +The trick is that the kernel starts at the root of the tree and looks
> +for nodes that have a 'compatible' property.  First, it is generally
> +assumed that any node with a 'compatible' property represents a device
> +of some kind, and second, it can be assumed that any node at the root
> +of the tree is either directly attached to the processor bus, or is a
> +miscellaneous system device that cannot be described any other way.
> +For each of these nodes, Linux allocates and registers a
> +platform_device, which in turn may get bound to a platform_driver.
> +
> +Why is using a platform_device for these nodes a safe assumption?
> +Well, for the way that Linux models devices, just about all bus_types
> +assume that its devices are children of a bus controller.  For
> +example, each i2c_client is a child of an i2c_master.  Each spi_device
> +is a child of an spi bus.  Similarly for USB, PCI, MDIO, etc.  The
> +same hierarchy is also found in the DT, where i2c device nodes only
> +ever appear as children of an i2c bus node.  Ditto for spi, mdio, usb,
> +etc.  The only devices which do not require a specific type of parent
> +device are platform_devices (and amba_devices, but more on that
> +later), which will happily live at the base of the Linux /sys/devices
> +tree.  Therefore, if a DT node is at the root of the tree, then it
> +really probably is best registered as a platform_device.
> +
> +Linux board support code calls of_platform_populate(NULL, NULL, NULL)
> +to kick of discovery of devices at the root of the tree.  The
> +parameters are all NULL because when starting from the root of the
> +tree, there is no need to provide a starting node (the first NULL), a
> +parent struct device (the last NULL), and we're not using a match
> +table (yet).  For a board that only needs to register devices,
> +.init_machine() can be completely empty except for the
> +of_platform_populate() call.
> +
> +In the Tegra example, this accounts for the /soc and /sound nodes, but
> +what about the children of the soc node?  Shouldn't they be registered
> +as platform devices too?  For Linux DT support, the generic behaviour
> +is for child devices to be registered by the parent's device driver at
> +driver .probe() time.  So, an i2c bus device driver will register a
> +i2c_client for each child node, an spi bus driver will register
> +it's spi_device children, and similarly for other bus_types.
> +According to that model, a driver could be written that binds to the
> +soc node and simply registers platform_devices for each of it's
> +children.  The board support code would allocate and register an soc
> +device, an soc device driver would bind to the soc device, and
> +register platform_devices for /soc/interrupt-controller, /soc/serial,
> +/soc/i2s, and /soc/i2c in it's .probe() hook.  Easy, right?  Although
> +it is a lot of mucking about for just registering platform devices.
> +
> +It turns out that registering children of certain platform_devices as
> +more platform_devices is a common pattern, and the device tree support
> +code reflects that.  The second argument to of_platform_populate() is
> +an of_device_id table, and any node that matches an entry in that
> +table will also get it's child nodes registered.  In the tegra case,
> +the code can look something like this:
> +
> +static struct of_device_id harmony_bus_ids[] __initdata = {
> +	{ .compatible = "simple-bus", },
> +	{}
> +};
> +
> +static void __init harmony_init_machine(void)
> +{
> +	/* ... */
> +	of_platform_populate(NULL, harmony_bus_ids, NULL);
> +}
> +
> +"simple-bus" is defined in the ePAPR 1.0 specification as a property
> +meaning a simple memory mapped bus, so the of_platform_populate() code
> +could be written to just assume simple-bus compatible nodes will
> +always be traversed.  However, we pass it in as an argument so that
> +board support code can always override the default behaviour.
> +
> +<h2>Appendix A: AMBA devices</h2>
> +
> +ARM Primecells are a certain kind of device attached to the ARM AMBA
> +bus which include some support for hardware detection and power
> +management.  In Linux, struct amba_device and the amba_bus_type is
> +used to represent Primecell devices.  However, the fiddly bit is that
> +not all devices on an AMBA bus are Primecells, and for Linux it is
> +typical for both amba_device and platform_device instances to be
> +siblings of the same bus segment.
> +
> +When using the DT, this creates problems for of_platform_populate()
> +because it must decide whether to register each node as either a
> +platform_device or an amba_device.  This unfortunately complicates the
> +device creation model a little bit, but the solution turns out not to
> +be too invasive.  If a node is compatible with "arm,amba-primecell", then
> +of_platform_populate() will register it as an amba_device instead of a
> +platform_device.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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