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Subject: Re: [PATCH v7 08/36] drm: exynos: fix common struct sg_table
 related issues
To:     Marek Szyprowski <m.szyprowski@samsung.com>,
        dri-devel@lists.freedesktop.org, iommu@lists.linux-foundation.org,
        linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org
Cc:     linux-samsung-soc@vger.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        David Airlie <airlied@linux.ie>,
        Seung-Woo Kim <sw0312.kim@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Christoph Hellwig <hch@lst.de>,
        linux-arm-kernel@lists.infradead.org
From:   Andrzej Hajda <a.hajda@samsung.com>
Message-ID: <53d2ae18-3d60-773f-4544-3c8f55749038@samsung.com>
Date:   Tue, 7 Jul 2020 11:40:26 +0200
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On 19.06.2020 12:36, Marek Szyprowski 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 <a.hajda@samsung.com>

Regards
Andrzej


> ---
>   drivers/gpu/drm/exynos/exynos_drm_g2d.c | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/exynos/exynos_drm_g2d.c b/drivers/gpu/drm/exynos/exynos_drm_g2d.c
> index fcee33a43aca..7014a8cd971a 100644
> --- a/drivers/gpu/drm/exynos/exynos_drm_g2d.c
> +++ b/drivers/gpu/drm/exynos/exynos_drm_g2d.c
> @@ -395,8 +395,8 @@ static void g2d_userptr_put_dma_addr(struct g2d_data *g2d,
>   		return;
>   
>   out:
> -	dma_unmap_sg(to_dma_dev(g2d->drm_dev), g2d_userptr->sgt->sgl,
> -			g2d_userptr->sgt->nents, DMA_BIDIRECTIONAL);
> +	dma_unmap_sgtable(to_dma_dev(g2d->drm_dev), g2d_userptr->sgt,
> +			  DMA_BIDIRECTIONAL, 0);
>   
>   	pages = frame_vector_pages(g2d_userptr->vec);
>   	if (!IS_ERR(pages)) {
> @@ -511,10 +511,10 @@ static dma_addr_t *g2d_userptr_get_dma_addr(struct g2d_data *g2d,
>   
>   	g2d_userptr->sgt = sgt;
>   
> -	if (!dma_map_sg(to_dma_dev(g2d->drm_dev), sgt->sgl, sgt->nents,
> -				DMA_BIDIRECTIONAL)) {
> +	ret = dma_map_sgtable(to_dma_dev(g2d->drm_dev), sgt,
> +			      DMA_BIDIRECTIONAL, 0);
> +	if (ret) {
>   		DRM_DEV_ERROR(g2d->dev, "failed to map sgt with dma region.\n");
> -		ret = -ENOMEM;
>   		goto err_sg_free_table;
>   	}
>