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Subject: Re: [PATCH 0/8] Convert the intel iommu driver to the dma-iommu api
To:     Jani Nikula <jani.nikula@linux.intel.com>,
        Tom Murphy <murphyt7@tcd.ie>, iommu@lists.linux-foundation.org
Cc:     Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Kukjin Kim <kgene@kernel.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Andy Gross <agross@kernel.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Rob Clark <robdclark@gmail.com>,
        Heiko Stuebner <heiko@sntech.de>,
        Gerald Schaefer <gerald.schaefer@de.ibm.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
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References: <20191221150402.13868-1-murphyt7@tcd.ie>
 <87blrzwcn8.fsf@intel.com>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <432d306c-fe9f-75b2-f0f7-27698f1467ad@arm.com>
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
> 
>