Skip cache sync for GEM CMA objects?

Skip cache sync for GEM CMA objects?

[Steve Longerbeam]

Hi all,

GEM CMA uses dma_alloc_writecombine() when allocating the CMA objects.
So IINM the GEM buffers are cache coherent. So is there any reason why
DMA_ATTR_SKIP_CPU_SYNC is not passed to dma_map_sg() when GEM CMA
prepares the buffer for dma on behalf of the dma-buf user? Is this just an
oversight or is there something I have missed?

Thanks,
Steve

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Skip cache sync for GEM CMA objects?

[Steve Longerbeam]

Hi all,

GEM CMA uses dma_alloc_writecombine() when allocating the CMA objects.
So IINM the GEM buffers are cache coherent. So is there any reason why
DMA_ATTR_SKIP_CPU_SYNC is not passed to dma_map_sg() when GEM CMA
prepares the buffer for dma on behalf of the dma-buf user? Is this just an
oversight or is there something I have missed?

Thanks,
Steve

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[RFC PATCH] drm: use dma_map_sg_attrs for GEM CMA objects

[Steve Longerbeam]

The GEM CMA helper allocates GEM objects with dma_alloc_writecombine(),
so the objects are cache coherent, and the exporter methods for these
objects can skip cache sync.

To accomplish this a dma atrribute is added to drm_gem_object,
and is used with dma_map_sg_attrs/dma_unmap_sg_attrs in
drm_gem_map_dma_buf/drm_gem_map_detach. GEM CMA sets the attribute
to DMA_ATTR_SKIP_CPU_SYNC when the object is allocated in
drm_gem_cma_create().

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
---
 drivers/gpu/drm/drm_gem.c            | 1 +
 drivers/gpu/drm/drm_gem_cma_helper.c | 2 ++
 drivers/gpu/drm/drm_prime.c          | 7 ++++---
 include/drm/drm_gem.h                | 5 +++++
 4 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 16a1647..f7f4531 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -169,6 +169,7 @@ void drm_gem_private_object_init(struct drm_device *dev,
 	obj->handle_count = 0;
 	obj->size = size;
 	drm_vma_node_reset(&obj->vma_node);
+	init_dma_attrs(&obj->dma_attrs);
 }
 EXPORT_SYMBOL(drm_gem_private_object_init);
 
diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
index bd75f30..73c949a 100644
--- a/drivers/gpu/drm/drm_gem_cma_helper.c
+++ b/drivers/gpu/drm/drm_gem_cma_helper.c
@@ -75,6 +75,8 @@ __drm_gem_cma_create(struct drm_device *drm, size_t size)
 		goto error;
 	}
 
+	dma_set_attr(DMA_ATTR_SKIP_CPU_SYNC, &gem_obj->dma_attrs);
+
 	return cma_obj;
 
 error:
diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 9f935f5..9eaa8d3 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -153,8 +153,8 @@ 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, &obj->dma_attrs);
 		sg_free_table(sgt);
 	}
 
@@ -201,7 +201,8 @@ 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, &obj->dma_attrs)) {
 			sg_free_table(sgt);
 			kfree(sgt);
 			sgt = ERR_PTR(-ENOMEM);
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 7a592d7..f5546bf 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -104,6 +104,11 @@ struct drm_gem_object {
 	struct dma_buf *dma_buf;
 
 	/**
+	 * dma_attrs - DMA attributes for this GEM object
+	 */
+	struct dma_attrs dma_attrs;
+
+	/**
 	 * import_attach - dma buf attachment backing this object
 	 *
 	 * Any foreign dma_buf imported as a gem object has this set to the
-- 
1.9.1

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