drm/prime: skip CPU sync in map/unmap dma_buf

Dma-bufs should already be device coherent, as they are only pulled in the
CPU domain via the begin/end cpu_access calls. As we cache the mapping set
up by dma_map_sg a CPU sync at this point will not actually guarantee proper
coherency on non-coherent architectures, so we can as well stop pretending.

This is an important performance fix for architectures which need explicit
cache synchronization and userspace doing lots of dma-buf imports.
Improves Weston on Etnaviv performance 5x, where before this patch > 90%
of Weston CPU time was spent synchronizing caches for buffers which are
already device coherent.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171130173428.8666-1-l.stach@pengutronix.de

drivers/gpu/drm/drm_prime.c

@@ -218,8 +218,9 @@ static void drm_gem_map_detach(struct dma_buf *dma_buf,
 	sgt = prime_attach->sgt;
 	if (sgt) {
 		if (prime_attach->dir != DMA_NONE)
-			dma_unmap_sg(attach->dev, sgt->sgl, sgt->nents,
-					prime_attach->dir);
+			dma_unmap_sg_attrs(attach->dev, sgt->sgl, sgt->nents,
+					   prime_attach->dir,
+					   DMA_ATTR_SKIP_CPU_SYNC);
 		sg_free_table(sgt);
 	}
 
@@ -277,7 +278,8 @@ static struct sg_table *drm_gem_map_dma_buf(struct dma_buf_attachment *attach,
 	sgt = obj->dev->driver->gem_prime_get_sg_table(obj);
 
 	if (!IS_ERR(sgt)) {
-		if (!dma_map_sg(attach->dev, sgt->sgl, sgt->nents, dir)) {
+		if (!dma_map_sg_attrs(attach->dev, sgt->sgl, sgt->nents, dir,
+				      DMA_ATTR_SKIP_CPU_SYNC)) {
 			sg_free_table(sgt);
 			kfree(sgt);
 			sgt = ERR_PTR(-ENOMEM);