drm/tegra: gem: Flush buffer objects upon allocation

Buffers obtained via shmem may still have associated cachelines. If they
aren't properly flushed they may cause framebuffer corruption if the
cache gets flushed after the application has drawn to it.

Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/gem.c

@@ -219,19 +219,47 @@ static void tegra_bo_free(struct drm_device *drm, struct tegra_bo *bo)
 static int tegra_bo_get_pages(struct drm_device *drm, struct tegra_bo *bo,
 			      size_t size)
 {
+	struct scatterlist *s;
+	struct sg_table *sgt;
+	unsigned int i;
+
 	bo->pages = drm_gem_get_pages(&bo->gem);
 	if (IS_ERR(bo->pages))
 		return PTR_ERR(bo->pages);
 
 	bo->num_pages = size >> PAGE_SHIFT;
 
-	bo->sgt = drm_prime_pages_to_sg(bo->pages, bo->num_pages);
-	if (IS_ERR(bo->sgt)) {
-		drm_gem_put_pages(&bo->gem, bo->pages, false, false);
-		return PTR_ERR(bo->sgt);
+	sgt = drm_prime_pages_to_sg(bo->pages, bo->num_pages);
+	if (IS_ERR(sgt))
+		goto put_pages;
+
+	/*
+	 * Fake up the SG table so that dma_map_sg() can be used to flush the
+	 * pages associated with it. Note that this relies on the fact that
+	 * the DMA API doesn't hook into IOMMU on Tegra, therefore mapping is
+	 * only cache maintenance.
+	 *
+	 * TODO: Replace this by drm_clflash_sg() once it can be implemented
+	 * without relying on symbols that are not exported.
+	 */
+	for_each_sg(sgt->sgl, s, sgt->nents, i)
+		sg_dma_address(s) = sg_phys(s);
+
+	if (dma_map_sg(drm->dev, sgt->sgl, sgt->nents, DMA_TO_DEVICE) == 0) {
+		sgt = ERR_PTR(-ENOMEM);
+		goto release_sgt;
 	}
 
+	bo->sgt = sgt;
+
 	return 0;
+
+release_sgt:
+	sg_free_table(sgt);
+	kfree(sgt);
+put_pages:
+	drm_gem_put_pages(&bo->gem, bo->pages, false, false);
+	return PTR_ERR(sgt);
 }
 
 static int tegra_bo_alloc(struct drm_device *drm, struct tegra_bo *bo,