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

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@arm.com

(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@samsung.com

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; }