Re: [PATCH v9 17/32] drm: rockchip: fix common struct sg_table related issues

[Robin Murphy]

On 2020-08-26 07:33, 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.

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

(Until now I hadn't noticed the crimes against the API that 
rockchip_gem_get_pages() is committing, but it's not this patch's 
fault... I'll have to take a closer look at that)

> Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> ---
>   drivers/gpu/drm/rockchip/rockchip_drm_gem.c | 23 +++++++++------------
>   1 file changed, 10 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_gem.c b/drivers/gpu/drm/rockchip/rockchip_drm_gem.c
> index 2970e534e2bb..cb50f2ba2e46 100644
> --- a/drivers/gpu/drm/rockchip/rockchip_drm_gem.c
> +++ b/drivers/gpu/drm/rockchip/rockchip_drm_gem.c
> @@ -36,8 +36,8 @@ static int rockchip_gem_iommu_map(struct rockchip_gem_object *rk_obj)
>   
>   	rk_obj->dma_addr = rk_obj->mm.start;
>   
> -	ret = iommu_map_sg(private->domain, rk_obj->dma_addr, rk_obj->sgt->sgl,
> -			   rk_obj->sgt->nents, prot);
> +	ret = iommu_map_sgtable(private->domain, rk_obj->dma_addr, rk_obj->sgt,
> +				prot);
>   	if (ret < rk_obj->base.size) {
>   		DRM_ERROR("failed to map buffer: size=%zd request_size=%zd\n",
>   			  ret, rk_obj->base.size);
> @@ -98,11 +98,10 @@ static int rockchip_gem_get_pages(struct rockchip_gem_object *rk_obj)
>   	 * TODO: Replace this by drm_clflush_sg() once it can be implemented
>   	 * without relying on symbols that are not exported.
>   	 */
> -	for_each_sg(rk_obj->sgt->sgl, s, rk_obj->sgt->nents, i)
> +	for_each_sgtable_sg(rk_obj->sgt, s, i)
>   		sg_dma_address(s) = sg_phys(s);
>   
> -	dma_sync_sg_for_device(drm->dev, rk_obj->sgt->sgl, rk_obj->sgt->nents,
> -			       DMA_TO_DEVICE);
> +	dma_sync_sgtable_for_device(drm->dev, rk_obj->sgt, DMA_TO_DEVICE);
>   
>   	return 0;
>   
> @@ -350,8 +349,8 @@ void rockchip_gem_free_object(struct drm_gem_object *obj)
>   		if (private->domain) {
>   			rockchip_gem_iommu_unmap(rk_obj);
>   		} else {
> -			dma_unmap_sg(drm->dev, rk_obj->sgt->sgl,
> -				     rk_obj->sgt->nents, DMA_BIDIRECTIONAL);
> +			dma_unmap_sgtable(drm->dev, rk_obj->sgt,
> +					  DMA_BIDIRECTIONAL, 0);
>   		}
>   		drm_prime_gem_destroy(obj, rk_obj->sgt);
>   	} else {
> @@ -476,15 +475,13 @@ rockchip_gem_dma_map_sg(struct drm_device *drm,
>   			struct sg_table *sg,
>   			struct rockchip_gem_object *rk_obj)
>   {
> -	int count = dma_map_sg(drm->dev, sg->sgl, sg->nents,
> -			       DMA_BIDIRECTIONAL);
> -	if (!count)
> -		return -EINVAL;
> +	int err = dma_map_sgtable(drm->dev, sg, DMA_BIDIRECTIONAL, 0);
> +	if (err)
> +		return err;
>   
>   	if (drm_prime_get_contiguous_size(sg) < attach->dmabuf->size) {
>   		DRM_ERROR("failed to map sg_table to contiguous linear address.\n");
> -		dma_unmap_sg(drm->dev, sg->sgl, sg->nents,
> -			     DMA_BIDIRECTIONAL);
> +		dma_unmap_sgtable(drm->dev, sg, DMA_BIDIRECTIONAL, 0);
>   		return -EINVAL;
>   	}
>