Messages in this thread | | | Subject | Re: [PATCH 0/8] Convert the intel iommu driver to the dma-iommu api | From | Robin Murphy <> | Date | Mon, 23 Dec 2019 11:29:17 +0000 |
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On 2019-12-23 10:37 am, Jani Nikula wrote: > On Sat, 21 Dec 2019, Tom Murphy <murphyt7@tcd.ie> 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 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. >> >> 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 > >
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