Re: [PATCH v10 05/30] drm: etnaviv: fix common struct sg_table related issues

[Lucas Stach]

On Fr, 2020-09-04 at 15:16 +0200, 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>
> Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

Acked-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>

> ---
>  drivers/gpu/drm/etnaviv/etnaviv_gem.c | 12 +++++-------
>  drivers/gpu/drm/etnaviv/etnaviv_mmu.c | 15 ++++-----------
>  2 files changed, 9 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> index f06e19e7be04..eaf1949bc2e4 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> @@ -27,7 +27,7 @@ static void etnaviv_gem_scatter_map(struct etnaviv_gem_object *etnaviv_obj)
>  	 * because display controller, GPU, etc. are not coherent.
>  	 */
>  	if (etnaviv_obj->flags & ETNA_BO_CACHE_MASK)
> -		dma_map_sg(dev->dev, sgt->sgl, sgt->nents, DMA_BIDIRECTIONAL);
> +		dma_map_sgtable(dev->dev, sgt, DMA_BIDIRECTIONAL, 0);
>  }
>  
>  static void etnaviv_gem_scatterlist_unmap(struct etnaviv_gem_object *etnaviv_obj)
> @@ -51,7 +51,7 @@ static void etnaviv_gem_scatterlist_unmap(struct etnaviv_gem_object *etnaviv_obj
>  	 * discard those writes.
>  	 */
>  	if (etnaviv_obj->flags & ETNA_BO_CACHE_MASK)
> -		dma_unmap_sg(dev->dev, sgt->sgl, sgt->nents, DMA_BIDIRECTIONAL);
> +		dma_unmap_sgtable(dev->dev, sgt, DMA_BIDIRECTIONAL, 0);
>  }
>  
>  /* called with etnaviv_obj->lock held */
> @@ -404,9 +404,8 @@ int etnaviv_gem_cpu_prep(struct drm_gem_object *obj, u32 op,
>  	}
>  
>  	if (etnaviv_obj->flags & ETNA_BO_CACHED) {
> -		dma_sync_sg_for_cpu(dev->dev, etnaviv_obj->sgt->sgl,
> -				    etnaviv_obj->sgt->nents,
> -				    etnaviv_op_to_dma_dir(op));
> +		dma_sync_sgtable_for_cpu(dev->dev, etnaviv_obj->sgt,
> +					 etnaviv_op_to_dma_dir(op));
>  		etnaviv_obj->last_cpu_prep_op = op;
>  	}
>  
> @@ -421,8 +420,7 @@ int etnaviv_gem_cpu_fini(struct drm_gem_object *obj)
>  	if (etnaviv_obj->flags & ETNA_BO_CACHED) {
>  		/* fini without a prep is almost certainly a userspace error */
>  		WARN_ON(etnaviv_obj->last_cpu_prep_op == 0);
> -		dma_sync_sg_for_device(dev->dev, etnaviv_obj->sgt->sgl,
> -			etnaviv_obj->sgt->nents,
> +		dma_sync_sgtable_for_device(dev->dev, etnaviv_obj->sgt,
>  			etnaviv_op_to_dma_dir(etnaviv_obj->last_cpu_prep_op));
>  		etnaviv_obj->last_cpu_prep_op = 0;
>  	}
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_mmu.c b/drivers/gpu/drm/etnaviv/etnaviv_mmu.c
> index 3607d348c298..15d9fa3879e5 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_mmu.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_mmu.c
> @@ -73,13 +73,13 @@ static int etnaviv_iommu_map(struct etnaviv_iommu_context *context, u32 iova,
>  			     struct sg_table *sgt, unsigned len, int prot)
>  {	struct scatterlist *sg;
>  	unsigned int da = iova;
> -	unsigned int i, j;
> +	unsigned int i;
>  	int ret;
>  
>  	if (!context || !sgt)
>  		return -EINVAL;
>  
> -	for_each_sg(sgt->sgl, sg, sgt->nents, i) {
> +	for_each_sgtable_dma_sg(sgt, sg, i) {
>  		u32 pa = sg_dma_address(sg) - sg->offset;
>  		size_t bytes = sg_dma_len(sg) + sg->offset;
>  
> @@ -95,14 +95,7 @@ static int etnaviv_iommu_map(struct etnaviv_iommu_context *context, u32 iova,
>  	return 0;
>  
>  fail:
> -	da = iova;
> -
> -	for_each_sg(sgt->sgl, sg, i, j) {
> -		size_t bytes = sg_dma_len(sg) + sg->offset;
> -
> -		etnaviv_context_unmap(context, da, bytes);
> -		da += bytes;
> -	}
> +	etnaviv_context_unmap(context, iova, da - iova);
>  	return ret;
>  }
>  
> @@ -113,7 +106,7 @@ static void etnaviv_iommu_unmap(struct etnaviv_iommu_context *context, u32 iova,
>  	unsigned int da = iova;
>  	int i;
>  
> -	for_each_sg(sgt->sgl, sg, sgt->nents, i) {
> +	for_each_sgtable_dma_sg(sgt, sg, i) {
>  		size_t bytes = sg_dma_len(sg) + sg->offset;
>  
>  		etnaviv_context_unmap(context, da, bytes);