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SubjectRe: [PATCH 0/8] Convert the intel iommu driver to the dma-iommu api
Hi Logan,

On 29.05.2020 21:05, Logan Gunthorpe wrote:
> On 2020-05-29 6:45 a.m., Christoph Hellwig wrote:
>> On Thu, May 28, 2020 at 06:00:44PM -0600, Logan Gunthorpe wrote:
>>>> This issue is most likely in the i915 driver and is most likely caused by the driver not respecting the return value of the dma_map_ops::map_sg function. You can see the driver ignoring the return value here:
>>>> Previously this didn’t cause issues because the intel map_sg always returned the same number of elements as the input scatter gather list but with the change to this dma-iommu api this is no longer the case. I wasn’t able to track the bug down to a specific line of code unfortunately.
>> Mark did a big audit into the map_sg API abuse and initially had
>> some i915 patches, but then gave up on them with this comment:
>> "The biggest TODO is DRM/i915 driver and I don't feel brave enough to fix
>> it fully. The driver creatively uses sg_table->orig_nents to store the
>> size of the allocate scatterlist and ignores the number of the entries
>> returned by dma_map_sg function. In this patchset I only fixed the
>> sg_table objects exported by dmabuf related functions. I hope that I
>> didn't break anything there."
>> it would be really nice if the i915 maintainers could help with sorting
>> that API abuse out.
> I agree completely that the API abuse should be sorted out, but I think
> that's much larger than just the i915 driver. Pretty much every dma-buf
> map_dma_buf implementation I looked at ignores the returned nents of
> sg_attrs. This sucks, but I don't think it's the bug Tom ran into. See:
> amdgpu_dma_buf_map
> armada_gem_prime_map_dma_buf
> drm_gem_map_dma_buf
> i915_gem_map_dma_buf
> tegra_gem_prime_map_dma_buf
> So this should probably be addressed by the whole GPU community.

Patches are pending:

> However, as Robin pointed out, there are other ugly tricks like stopping
> iterating through the SGL when sg_dma_len() is zero. For example, the
> AMD driver appears to use drm_prime_sg_to_page_addr_arrays() which does
> this trick and thus likely isn't buggy (otherwise, I'd expect someone to
> have complained by now seeing AMD has already switched to IOMMU-DMA.

I'm not sure that this is a trick. Stopping at zero sg_dma_len() was
somewhere documented.

> As I tried to point out in my previous email, i915 does not do this
> trick. In fact, it completely ignores sg_dma_len() and is thus
> completely broken. See i915_scatterlist.h and the __sgt_iter() function.
> So it doesn't sound to me like Mark's fix would address the issue at
> all. Per my previous email, I'd guess that it can be fixed simply by
> adjusting the __sgt_iter() function to do something more sensible.
> (Better yet, if possible, ditch __sgt_iter() and use the common DRM
> function that AMD uses).
> This would at least allow us to make progress with Tom's IOMMU-DMA patch
> set and once that gets in, it will be harder for other drivers to make
> the same mistake.

Best regards
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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