Re: [PATCH v9 04/32] drm: armada: 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.
>
> 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@xxxxxxxxxxx>
> ---
>   drivers/gpu/drm/armada/armada_gem.c | 12 ++++++------
>   1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/armada/armada_gem.c b/drivers/gpu/drm/armada/armada_gem.c
> index 8005614d2e6b..bedd8937d8a1 100644
> --- a/drivers/gpu/drm/armada/armada_gem.c
> +++ b/drivers/gpu/drm/armada/armada_gem.c
> @@ -395,7 +395,7 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment *attach,
>   
>   		mapping = dobj->obj.filp->f_mapping;
>   
> -		for_each_sg(sgt->sgl, sg, count, i) {
> +		for_each_sgtable_sg(sgt, sg, i) {
>   			struct page *page;
>   
>   			page = shmem_read_mapping_page(mapping, i);
> @@ -407,8 +407,8 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment *attach,
>   			sg_set_page(sg, page, PAGE_SIZE, 0);
>   		}
>   
> -		if (dma_map_sg(attach->dev, sgt->sgl, sgt->nents, dir) == 0) {
> -			num = sgt->nents;
> +		if (dma_map_sgtable(attach->dev, sgt, dir, 0)) {
> +			num = count;

I think it might be even nicer to get rid of "num" entirely and convert 
the cleanup path to for_each_sgtable_sg() for completeness - AFAICS it 
should only need an extra "if (sg_page(sg))..." check in that loop. Then 
"count" could possibly be squashed into its one remaining use as well, 
but maybe it's worth keeping for readability.

Robin.

>   			goto release;
>   		}
>   	} else if (dobj->page) {
> @@ -418,7 +418,7 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment *attach,
>   
>   		sg_set_page(sgt->sgl, dobj->page, dobj->obj.size, 0);
>   
> -		if (dma_map_sg(attach->dev, sgt->sgl, sgt->nents, dir) == 0)
> +		if (dma_map_sgtable(attach->dev, sgt, dir, 0))
>   			goto free_table;
>   	} else if (dobj->linear) {
>   		/* Single contiguous physical region - no struct page */
> @@ -449,11 +449,11 @@ static void armada_gem_prime_unmap_dma_buf(struct dma_buf_attachment *attach,
>   	int i;
>   
>   	if (!dobj->linear)
> -		dma_unmap_sg(attach->dev, sgt->sgl, sgt->nents, dir);
> +		dma_unmap_sgtable(attach->dev, sgt, dir, 0);
>   
>   	if (dobj->obj.filp) {
>   		struct scatterlist *sg;
> -		for_each_sg(sgt->sgl, sg, sgt->nents, i)
> +		for_each_sgtable_sg(sgt, sg, i)
>   			put_page(sg_page(sg));
>   	}
>