linux-kernel - Re: [PATCH v7 04/36] drm: amdgpu: fix common struct sg

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Date:   Tue, 7 Jul 2020 15:06:59 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Christian König <christian.koenig@....com>,
        dri-devel@...ts.freedesktop.org, iommu@...ts.linux-foundation.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-arm-kernel@...ts.infradead.org,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Alex Deucher <alexander.deucher@....com>,
        amd-gfx@...ts.freedesktop.org
Subject: Re: [PATCH v7 04/36] drm: amdgpu: fix common struct sg_table
 related issues

Hi Christian,

On 22.06.2020 15:27, Christian König wrote:
> Am 19.06.20 um 12:36 schrieb Marek Szyprowski:
>> The Documentation/DMA-API-HOWTO.txt states that the dma_map_sg() 
>> function
>> returns the number of the created entries in the DMA address space.
>> However the subsequent calls to the dma_sync_sg_for_{device,cpu}() and
>> dma_unmap_sg must be called with the original number of the entries
>> passed to the dma_map_sg().
>>
>> struct sg_table is a common structure used for describing a 
>> non-contiguous
>> memory buffer, used commonly in the DRM and graphics subsystems. It
>> consists of a scatterlist with memory pages and DMA addresses (sgl 
>> entry),
>> as well as the number of scatterlist entries: CPU pages (orig_nents 
>> entry)
>> and DMA mapped pages (nents entry).
>>
>> It turned out that it was a common mistake to misuse nents and 
>> orig_nents
>> entries, calling DMA-mapping functions with a wrong number of entries or
>> ignoring the number of mapped entries returned by the dma_map_sg()
>> function.
>>
>> To avoid such issues, lets use a common dma-mapping wrappers operating
>> directly on the struct sg_table objects and use scatterlist page
>> iterators where possible. This, almost always, hides references to the
>> nents and orig_nents entries, making the code robust, easier to follow
>> and copy/paste safe.
>>
>> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
>> Reviewed-by: Christian König <christian.koenig@....com>
>
> Any objection that we pick this one and the radeon up into our 
> branches for upstreaming?
>
> That should about clashes with other driver changes.

I'm fine. This one and radeon doesn't depend on the prime changes, so it 
should merge fine via your tree. I will try to ask for more review of 
the remaining patches and then try merging via drm-misc.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland