drm/i915: Skip CPU synchronisation on dmabuf attachments

dma-bufs are device coherent, with explicit CPU synchronisation provided
via the begin/end cpu access ioctls. As the coherency of the dma-buf is
explicitly defined to be under user control, flushing any caches on
attach/detach of the dma-buf is additional work that doesn't aide the
user in the slightest.
Suggested-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Dongwon Kim <dongwon.kim@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171130180702.29357-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c

@@ -48,7 +48,9 @@ static struct sg_table *i915_gem_map_dma_buf(struct dma_buf_attachment *attachme
 		src = sg_next(src);
 	}
 
-	if (!dma_map_sg(attachment->dev, st->sgl, st->nents, dir)) {
+	if (!dma_map_sg_attrs(attachment->dev,
+			      st->sgl, st->nents, dir,
+			      DMA_ATTR_SKIP_CPU_SYNC)) {
 		ret = -ENOMEM;
 		goto err_free_sg;
 	}
@@ -71,7 +73,9 @@ static void i915_gem_unmap_dma_buf(struct dma_buf_attachment *attachment,
 {
 	struct drm_i915_gem_object *obj = dma_buf_to_obj(attachment->dmabuf);
 
-	dma_unmap_sg(attachment->dev, sg->sgl, sg->nents, dir);
+	dma_unmap_sg_attrs(attachment->dev,
+			   sg->sgl, sg->nents, dir,
+			   DMA_ATTR_SKIP_CPU_SYNC);
 	sg_free_table(sg);
 	kfree(sg);