On Mon, 23 Dec 2019, Robin Murphy <robin.murphy@xxxxxxx> wrote: > 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 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. Thanks for the clarification. BR, Jani. > > 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 >> >> -- Jani Nikula, Intel Open Source Graphics Center
