drm: gem_cma_helper.c: Allow importing of contiguous scatterlists with nents > 1

drm_gem_cma_prime_import_sg_table() will fail if the number of entries
in the sg_table > 1. However, you can have a device that uses an IOMMU
engine and can map a discontiguous buffer with multiple entries that
have consecutive sg_dma_addresses, effectively making it contiguous.
Allow for that scenario by testing the entries in the sg_table for
contiguous coverage.

Reviewed-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Liviu Dudau <[e-mail]>
Signed-off-by: Noralf Trønnes <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_gem_cma_helper.c

···
 {
 	struct drm_gem_cma_object *cma_obj;
 
-	if (sgt->nents != 1)
-		return ERR_PTR(-EINVAL);
+	if (sgt->nents != 1) {
+		/* check if the entries in the sg_table are contiguous */
+		dma_addr_t next_addr = sg_dma_address(sgt->sgl);
+		struct scatterlist *s;
+		unsigned int i;
+
+		for_each_sg(sgt->sgl, s, sgt->nents, i) {
+			/*
+			 * sg_dma_address(s) is only valid for entries
+			 * that have sg_dma_len(s) != 0
+			 */
+			if (!sg_dma_len(s))
+				continue;
+
+			if (sg_dma_address(s) != next_addr)
+				return ERR_PTR(-EINVAL);
+
+			next_addr = sg_dma_address(s) + sg_dma_len(s);
+		}
+	}
 
 	/* Create a CMA GEM buffer. */
 	cma_obj = __drm_gem_cma_create(dev, attach->dmabuf->size);

include/drm/drm_gem_cma_helper.h

···
  * struct drm_gem_cma_object - GEM object backed by CMA memory allocations
  * @base: base GEM object
  * @paddr: physical address of the backing memory
- * @sgt: scatter/gather table for imported PRIME buffers
+ * @sgt: scatter/gather table for imported PRIME buffers. The table can have
+ *       more than one entry but they are guaranteed to have contiguous
+ *       DMA addresses.
  * @vaddr: kernel virtual address of the backing memory
  */
 struct drm_gem_cma_object {