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

[Robin Murphy]

On 2019-12-23 10:37 am, Jani Nikula wrote:
> On Sat, 21 Dec 2019, Tom Murphy <murphyt7@xxxxxx> wrote:
> > This patchset converts the intel iommu driver to the dma-iommu api.
> >
> > While converting the driver I exposed a bug in the intel i915 driver
> > which causes a huge amount of artifacts on the screen of my
> > laptop. You can see a picture of it here:
> > https://github.com/pippy360/kernelPatches/blob/master/IMG_20191219_225922.jpg
> >
> > 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 didnât 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 wasnât able to track the bug down to a specific line of code
> > unfortunately.
> >
> > Could someone from the intel team look at this?
>
> Let me get this straight. There is current API that on success always
> returns the same number of elements as the input scatter gather
> list. You propose to change the API so that this is no longer the case?

No, the API for dma_map_sg() has always been that it may return fewer 
DMA segments than nents - see Documentation/DMA-API.txt (and otherwise, 
the return value would surely be a simple success/fail condition). 
Relying on a particular implementation behaviour has never been strictly 
correct, even if it does happen to be a very common behaviour.

> A quick check of various dma_map_sg() calls in the kernel seems to
> indicate checking for 0 for errors and then ignoring the non-zero return
> is a common pattern. Are you sure it's okay to make the change you're
> proposing?

Various code uses tricks like just iterating the mapped list until the 
first segment with zero sg_dma_len(). Others may well simply have bugs.

Robin.

> Anyway, due to the time of year and all, I'd like to ask you to file a
> bug against i915 at [1] so this is not forgotten, and please let's not
> merge the changes before this is resolved.
>
>
> Thanks,
> Jani.
>
>
> [1] https://gitlab.freedesktop.org/drm/intel/issues/new