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SubjectRe: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page
On 04/05/2017 05:14 PM, Russell King - ARM Linux wrote:
> On Wed, Apr 05, 2017 at 10:02:42AM -0600, Shuah Khan wrote:
>> When coherent DMA memory without struct page is shared, importer
>> fails to find the page and runs into kernel page fault when it
>> tries to dmabuf_ops_attach/map_sg/map_page the invalid page found
>> in the sg_table. Please see
>> for more information on this problem.
>> This solution allows coherent DMA memory without struct page to be
>> shared by providing a way for the exporter to tag the DMA buffer as
>> a special buffer without struct page association and passing the
>> information in sg_table to the importer. This information is used
>> in attach/map_sg to avoid cleaning D-cache and mapping.
>> The details of the change are:
>> Framework:
>> - Add a new dma_attrs field to struct scatterlist.
>> - Add a new DMA_ATTR_DEV_COHERENT_NOPAGE attribute to clearly identify
>> Coherent memory without struct page.
>> - Add a new dma_check_dev_coherent() interface to check if memory is
>> the device coherent area. There is no way to tell where the memory
>> returned by dma_alloc_attrs() came from.
>> Exporter logic:
>> - Add logic to vb2_dc_alloc() to call dma_check_dev_coherent() and set
>> DMA_ATTR_DEV_COHERENT_NOPAGE based the results of the check. This is
>> done in the exporter context.
>> - Add logic to arm_dma_get_sgtable() to identify memory without struct
>> page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute. If this attr is
>> set, arm_dma_get_sgtable() will set page as the cpu_addr and update
>> dma_address and dma_attrs fields in struct scatterlist for this sgl.
>> This is done in exporter context when buffer is exported. With this
> This sentence appears to just end...
> I'm not convinced that coherent allocations should be setting the "page"
> of a scatterlist to anything that isn't a real struct page or NULL. It
> is, after all, an error to look up the virtual address etc of the
> scatterlist entry or kmap it when it isn't backed by a struct page.
> I'm actually already passing non-page backed memory through the DMA API
> in armada-drm, although not entirely correctly, and etnaviv handles it
> fine:
> } else if (dobj->linear) {
> /* Single contiguous physical region - no struct page */
> if (sg_alloc_table(sgt, 1, GFP_KERNEL))
> goto free_sgt;
> sg_dma_address(sgt->sgl) = dobj->dev_addr;
> sg_dma_len(sgt->sgl) = dobj->obj.size;
> This is not quite correct, as it assumes (which is safe for it currently)
> that the DMA address is the same on all devices. On Dove, which is where
> this is used, that is the case, but it's not true elsewhere. Also note
> that I'm avoid calling dma_map_sg() and dma_unmap_sg() - there's no iommus
> to be considered.

I see. That is not the case for the drivers involved in my use-case. exynos
has iommu and this s5p-mfc exporting buffers to exynos-gsc use-case does
work when iommu is enabled.

> I'd suggest that this follows the same pattern - setting the DMA address
> (more appropriately for generic code) and the DMA length, while leaving
> the virtual address members NULL/0. However, there's also the
> complication of setting up any IOMMUs that would be necessary. I haven't
> looked at that, or how it could work.
> I also think this should be documented in the dmabuf API that it can
> pass such scatterlists that are DMA-parameter only.
> Lastly, I'd recommend that anything using this does _not_ provide
> functional kmap/kmap_atomic support for these - kmap and kmap_atomic
> are both out because there's no struct page anyway (and their use would
> probably oops the kernel in this scenario.) I avoided mmap support in
> armada drm, but if there's a pressing reason and real use case for the
> importer to mmap() the buffers in userspace, it's something I could be
> convinced of.
> What I'm quite certain of is that we do _not_ want to be passing
> coherent memory allocations into the streaming DMA API, not even with
> a special attribute. The DMA API is about gaining coherent memory
> (shared ownership of the buffer), or mapping system memory to a
> specified device (which can imply unique ownership.) Trying to mix
> the two together muddies the separation that we have there, and makes
> it harder to explain. As can be seen from this patch, we'd end up
> needing to add this special DMA_ATTR_DEV_COHERENT_NOPAGE everywhere,
> which is added complexity on top of stuff that is not required for
> this circumstance.

The ownership can be tricky as you mentioned. In this particular use-case,
there is a clear ownership definition because of the way v4l2 export/import
works and also the qbuf/dqbuf rules. However, there might be other use-cases
ownership isn't clearly established.

> I can see why you're doing it, to avoid having to duplicate more of
> the generic code in drm_prime, but I don't think plasting over this
> problem in arch code by adding this special flag is a particularly
> good way forward.

Right. I went with this approach to avoid duplicating the code. It does
come with the complexity of needing to check the attribute in a few

With the current code, we still have the issue of pagefault. Your patch
that adds a check for invalid doesn't cover all cases.

My goal behind this patch is two fold. 1. Fix the pagefault with a
definitive test and 2. see if per-device coherent memory can be passed

The first goal is still worth while. Would it be reasonable to use
dma_check_dev_coherent() to test for this case in arm_dma_get_sgtable()
or even from dma_get_sgtable_attrs() and fail early? This will avoid
false negatives with the invalid page test. If this sounds reasonable,
I can spin this work to do that instead.

-- Shuah

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