Re: [PATCH v9 08/32] drm: i915: fix common struct sg_table related issues

[Robin Murphy]

On 2020-08-26 07:32, 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.
>
> This driver creatively uses sg_table->orig_nents to store the size of the
> allocated scatterlist and ignores the number of the entries returned by
> dma_map_sg function. The sg_table->orig_nents is (mis)used to properly
> free the (over)allocated scatterlist.
>
> This patch only introduces the common DMA-mapping wrappers operating
> directly on the struct sg_table objects to the dmabuf related functions,
> so the other drivers, which might share buffers with i915 could rely on
> the properly set nents and orig_nents values.

This one looks mechanical enough :)

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

> Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> ---
>   drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c       | 11 +++--------
>   drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c |  7 +++----
>   2 files changed, 6 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> index 2679380159fc..8a988592715b 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> @@ -48,12 +48,9 @@ static struct sg_table *i915_gem_map_dma_buf(struct dma_buf_attachment *attachme
>   		src = sg_next(src);
>   	}
>   
> -	if (!dma_map_sg_attrs(attachment->dev,
> -			      st->sgl, st->nents, dir,
> -			      DMA_ATTR_SKIP_CPU_SYNC)) {
> -		ret = -ENOMEM;
> +	ret = dma_map_sgtable(attachment->dev, st, dir, DMA_ATTR_SKIP_CPU_SYNC);
> +	if (ret)
>   		goto err_free_sg;
> -	}
>   
>   	return st;
>   
> @@ -73,9 +70,7 @@ static void i915_gem_unmap_dma_buf(struct dma_buf_attachment *attachment,
>   {
>   	struct drm_i915_gem_object *obj = dma_buf_to_obj(attachment->dmabuf);
>   
> -	dma_unmap_sg_attrs(attachment->dev,
> -			   sg->sgl, sg->nents, dir,
> -			   DMA_ATTR_SKIP_CPU_SYNC);
> +	dma_unmap_sgtable(attachment->dev, sg, dir, DMA_ATTR_SKIP_CPU_SYNC);
>   	sg_free_table(sg);
>   	kfree(sg);
>   
> diff --git a/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c b/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
> index debaf7b18ab5..be30b27e2926 100644
> --- a/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
> +++ b/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
> @@ -28,10 +28,9 @@ static struct sg_table *mock_map_dma_buf(struct dma_buf_attachment *attachment,
>   		sg = sg_next(sg);
>   	}
>   
> -	if (!dma_map_sg(attachment->dev, st->sgl, st->nents, dir)) {
> -		err = -ENOMEM;
> +	err = dma_map_sgtable(attachment->dev, st, dir, 0);
> +	if (err)
>   		goto err_st;
> -	}
>   
>   	return st;
>   
> @@ -46,7 +45,7 @@ static void mock_unmap_dma_buf(struct dma_buf_attachment *attachment,
>   			       struct sg_table *st,
>   			       enum dma_data_direction dir)
>   {
> -	dma_unmap_sg(attachment->dev, st->sgl, st->nents, dir);
> +	dma_unmap_sgtable(attachment->dev, st, dir, 0);
>   	sg_free_table(st);
>   	kfree(st);
>   }