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

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Date:   Mon, 23 Dec 2019 11:29:17 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Tom Murphy <murphyt7@....ie>, iommu@...ts.linux-foundation.org
Cc:     Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Clark <robdclark@...il.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Julien Grall <julien.grall@....com>,
        Marc Zyngier <maz@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-tegra@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org
Subject: Re: [PATCH 0/8] Convert the intel iommu driver to the dma-iommu api

On 2019-12-23 10:37 am, Jani Nikula wrote:
> On Sat, 21 Dec 2019, Tom Murphy <murphyt7@....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
> 
>