drm/tegra: gem: Properly pin imported buffers

Buffers that are imported from a DMA-BUF don't have pages allocated with
them. At the same time an SG table for them can't be derived using the
DMA API helpers because the necessary information doesn't exist. However
there's already an SG table that was created during import, so this can
simply be duplicated.
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/gem.c

@@ -27,6 +27,29 @@ static void tegra_bo_put(struct host1x_bo *bo)
 	drm_gem_object_put_unlocked(&obj->gem);
 }
 
+/* XXX move this into lib/scatterlist.c? */
+static int sg_alloc_table_from_sg(struct sg_table *sgt, struct scatterlist *sg,
+				  unsigned int nents, gfp_t gfp_mask)
+{
+	struct scatterlist *dst;
+	unsigned int i;
+	int err;
+
+	err = sg_alloc_table(sgt, nents, gfp_mask);
+	if (err < 0)
+		return err;
+
+	dst = sgt->sgl;
+
+	for (i = 0; i < nents; i++) {
+		sg_set_page(dst, sg_page(sg), sg->length, 0);
+		dst = sg_next(dst);
+		sg = sg_next(sg);
+	}
+
+	return 0;
+}
+
 static struct sg_table *tegra_bo_pin(struct device *dev, struct host1x_bo *bo,
 				     dma_addr_t *phys)
 {
@@ -52,11 +75,31 @@ static struct sg_table *tegra_bo_pin(struct device *dev, struct host1x_bo *bo,
 		return ERR_PTR(-ENOMEM);
 
 	if (obj->pages) {
+		/*
+		 * If the buffer object was allocated from the explicit IOMMU
+		 * API code paths, construct an SG table from the pages.
+		 */
 		err = sg_alloc_table_from_pages(sgt, obj->pages, obj->num_pages,
 						0, obj->gem.size, GFP_KERNEL);
 		if (err < 0)
 			goto free;
+	} else if (obj->sgt) {
+		/*
+		 * If the buffer object already has an SG table but no pages
+		 * were allocated for it, it means the buffer was imported and
+		 * the SG table needs to be copied to avoid overwriting any
+		 * other potential users of the original SG table.
+		 */
+		err = sg_alloc_table_from_sg(sgt, obj->sgt->sgl, obj->sgt->nents,
+					     GFP_KERNEL);
+		if (err < 0)
+			goto free;
 	} else {
+		/*
+		 * If the buffer object had no pages allocated and if it was
+		 * not imported, it had to be allocated with the DMA API, so
+		 * the DMA API helper can be used.
+		 */
 		err = dma_get_sgtable(dev, sgt, obj->vaddr, obj->iova,
 				      obj->gem.size);
 		if (err < 0)