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Date:   Fri, 29 May 2020 05:45:23 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Logan Gunthorpe <logang@deltatee.com>
Cc:     Tom Murphy <murphyt7@tcd.ie>, iommu@lists.linux-foundation.org,
        kvm@vger.kernel.org, David Airlie <airlied@linux.ie>,
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        Will Deacon <will@kernel.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        linux-samsung-soc@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
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        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Subject: Re: [PATCH 0/8] Convert the intel iommu driver to the dma-iommu api
Message-ID: <20200529124523.GA11817@infradead.org>
References: <20191221150402.13868-1-murphyt7@tcd.ie>
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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.