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From:   Rob Herring <robh@kernel.org>
Date:   Thu, 14 May 2020 08:51:27 -0500
Message-ID: <CAL_JsqLwEB_YMzDDQ=nt0_Tq-BGGFEW64BagPN-FbDeOnLpYCA@mail.gmail.com>
Subject: Re: [PATCH v5 19/38] drm: panfrost: fix common struct sg_table
 related issues
To:     Marek Szyprowski <m.szyprowski@samsung.com>
Cc:     dri-devel <dri-devel@lists.freedesktop.org>,
        Linux IOMMU <iommu@lists.linux-foundation.org>,
        linaro-mm-sig@lists.linaro.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Robin Murphy <robin.murphy@arm.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Tomeu Vizoso <tomeu.vizoso@collabora.com>
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On Wed, May 13, 2020 at 8:33 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: Steven Price <steven.price@arm.com>
> ---
> For more information, see '[PATCH v5 00/38] DRM: fix struct sg_table nents
> vs. orig_nents misuse' thread:
> https://lore.kernel.org/linux-iommu/20200513132114.6046-1-m.szyprowski@samsung.com/T/
> ---
>  drivers/gpu/drm/panfrost/panfrost_gem.c | 4 ++--
>  drivers/gpu/drm/panfrost/panfrost_mmu.c | 7 +++----
>  2 files changed, 5 insertions(+), 6 deletions(-)

Reviewed-by: Rob Herring <robh@kernel.org>