[PATCH RFC 044/111] staging: etnaviv: fix DMA API usage

[Lucas Stach]

From: Russell King <rmk+kernel at arm.linux.org.uk>

We test for write-combine and non-cacheable mappings before calling the
DMA API.  This is werid, because non-cacheable mappings are DMA coherent
by definition, whereas cacheable mappings need cache maintanence
provided by the DMA API.

This seems to be a typo: ETNA_BO_CACHED should be used rather than
ETNA_BO_UNCACHED.

Moreover, add a comment to the dma_unmap_sg() site so to remind people
about the data-corrupting implications of this call if it is abused (as
can happen with the etnaviv DRM code structure as it currently stands
with long-term mapping of the buffer.)

Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
---
 drivers/staging/etnaviv/etnaviv_gem.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/staging/etnaviv/etnaviv_gem.c b/drivers/staging/etnaviv/etnaviv_gem.c
index e508d64aa2d3..034ff732bdf4 100644
--- a/drivers/staging/etnaviv/etnaviv_gem.c
+++ b/drivers/staging/etnaviv/etnaviv_gem.c
@@ -48,7 +48,7 @@ static int etnaviv_gem_shmem_get_pages(struct etnaviv_gem_object *etnaviv_obj)
 	/* For non-cached buffers, ensure the new pages are clean
 	 * because display controller, GPU, etc. are not coherent:
 	 */
-	if (etnaviv_obj->flags & (ETNA_BO_WC|ETNA_BO_UNCACHED))
+	if (etnaviv_obj->flags & (ETNA_BO_WC|ETNA_BO_CACHED))
 		dma_map_sg(dev->dev, etnaviv_obj->sgt->sgl,
 			   etnaviv_obj->sgt->nents, DMA_BIDIRECTIONAL);
 
@@ -60,10 +60,22 @@ static void put_pages(struct etnaviv_gem_object *etnaviv_obj)
 	if (etnaviv_obj->pages) {
 		struct drm_device *dev = etnaviv_obj->base.dev;
 
-		/* For non-cached buffers, ensure the new pages are clean
+		/*
+		 * For non-cached buffers, ensure the new pages are clean
 		 * because display controller, GPU, etc. are not coherent:
+		 *
+		 * WARNING: The DMA API does not support concurrent CPU
+		 * and device access to the memory area.  With BIDIRECTIONAL,
+		 * we will clean the cache lines which overlap the region,
+		 * and invalidate all cache lines (partially) contained in
+		 * the region.
+		 *
+		 * If you have dirty data in the overlapping cache lines,
+		 * that will corrupt the GPU-written data.  If you have
+		 * written into the remainder of the region, this can
+		 * discard those writes.
 		 */
-		if (etnaviv_obj->flags & (ETNA_BO_WC|ETNA_BO_UNCACHED))
+		if (etnaviv_obj->flags & (ETNA_BO_WC|ETNA_BO_CACHED))
 			dma_unmap_sg(dev->dev, etnaviv_obj->sgt->sgl,
 				     etnaviv_obj->sgt->nents,
 				     DMA_BIDIRECTIONAL);
-- 
2.1.4