For newly added streaming mapping APIs, the internal core function
__dma_alloc_pages() should check dev->dma_mask, but not
ev->coherent_dma_mask which is for coherent mapping.
Meanwhile, just filter out gfp flags if they are any of
__GFP_DMA, __GFP_DMA32 and __GFP_HIGHMEM, but not fail it. This change
makes it consistent with coherent mapping allocs.
Signed-off-by: Baoquan He <[email protected]>
---
kernel/dma/mapping.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 9478eccd1c8e..e66847aeac67 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -543,10 +543,11 @@ static struct page *__dma_alloc_pages(struct device *dev, size_t size,
{
const struct dma_map_ops *ops = get_dma_ops(dev);
- if (WARN_ON_ONCE(!dev->coherent_dma_mask))
- return NULL;
- if (WARN_ON_ONCE(gfp & (__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM)))
- return NULL;
+ if (WARN_ON_ONCE(!dev->dma_mask))
+ return NULL;
+
+ /* let the implementation decide on the zone to allocate from: */
+ gfp &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);
size = PAGE_ALIGN(size);
if (dma_alloc_direct(dev, ops))
--
2.31.1
From: Christoph Hellwig
> Sent: 23 February 2022 14:26
>
> On Wed, Feb 23, 2022 at 08:28:13AM +0800, Baoquan He wrote:
> > Could you tell more why this is wrong? According to
> > Documentation/core-api/dma-api.rst and DMA code, __dma_alloc_pages() is
> > the core function of dma_alloc_pages()/dma_alloc_noncoherent() which are
> > obviously streaming mapping,
>
> Why are they "obviously" streaming mappings?
>
> > why do we need to check
> > dev->coherent_dma_mask here? Because dev->coherent_dma_mask is the subset
> > of dev->dma_mask, it's safer to use dev->coherent_dma_mask in these
> > places? This is confusing, I talked to Hyeonggon in private mail, he has
> > the same feeling.
>
> Think of th coherent_dma_mask as dma_alloc_mask. It is the mask for the
> DMA memory allocator. dma_mask is the mask for the dma_map_* routines.
I suspect it is all to allow for things like:
- A 64bit system with memory above 4G.
- A device that can only generate 32bit addresses.
- Some feature of the memory system (or bus bridges) that restricts
cache snooping to the low 1G of address space.
So dma_alloc_coherent() has to allocate memory below 1G.
The dma_map functions have to use bounce-buffers for addresses
above 4G.
dma_alloc_noncoherent() can allocate anything below 4G and so
avoid bounce buffers later on.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
On 02/23/22 at 03:25pm, Christoph Hellwig wrote:
> On Wed, Feb 23, 2022 at 08:28:13AM +0800, Baoquan He wrote:
> > Could you tell more why this is wrong? According to
> > Documentation/core-api/dma-api.rst and DMA code, __dma_alloc_pages() is
> > the core function of dma_alloc_pages()/dma_alloc_noncoherent() which are
> > obviously streaming mapping,
>
> Why are they "obviously" streaming mappings?
Because they are obviously not coherent mapping?
With my understanding, there are two kinds of DMA mapping, coherent
mapping (which is also persistent mapping), and streaming mapping. The
coherent mapping will be handled during driver init, and released during
driver de-init. While streaming mapping will be done when needed at any
time, and released after usage.
Are we going to add another kind of mapping? It's not streaming mapping,
but use dev->coherent_dma_mask, just because it uses dma_alloc_xxx()
api.
>
> > why do we need to check
> > dev->coherent_dma_mask here? Because dev->coherent_dma_mask is the subset
> > of dev->dma_mask, it's safer to use dev->coherent_dma_mask in these
> > places? This is confusing, I talked to Hyeonggon in private mail, he has
> > the same feeling.
>
> Think of th coherent_dma_mask as dma_alloc_mask. It is the mask for the
> DMA memory allocator. dma_mask is the mask for the dma_map_* routines.
I will check code further. While this may need be noted in doc, e.g
dma_api.rst or dma-api-howto.rst.
If you have guide, I can try to add some words to make clear this. Or
leave this to people who knows this clearly. I believe it will be very
helpful to understand DMA api.