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In-Reply-To: <20200513133245.6408-24-m.szyprowski@samsung.com>
From:   Eric Anholt <eric@anholt.net>
Date:   Wed, 13 May 2020 14:51:00 -0700
Message-ID: <CADaigPV7sO1pKrspDya+3OLeX0b0mzkZdjb4ox+gQddsmfw0-w@mail.gmail.com>
Subject: Re: [PATCH v5 24/38] drm: v3d: fix common struct sg_table related issues
To:     Marek Szyprowski <m.szyprowski@samsung.com>
Cc:     DRI Development <dri-devel@lists.freedesktop.org>,
        iommu@lists.linux-foundation.org, linaro-mm-sig@lists.linaro.org,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        Robin Murphy <robin.murphy@arm.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        linux-arm-kernel@lists.infradead.org,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>
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On Wed, May 13, 2020 at 6:33 AM Marek Szyprowski
<m.szyprowski@samsung.com> wrote:
>
> 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@samsung.com>

Reviewed-by: Eric Anholt <eric@anholt.net>