[PATCH 0/8] Convert the intel iommu driver to the dma-iommu api

[Logan Gunthorpe]

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:
>>> https://github.com/torvalds/linux/blob/7e0165b2f1a912a06e381e91f0f4e495f4ac3736/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c#L51
>>>
>>> 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.

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.

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.

Logan