dma-buf: start caching of sg_table objects v2

To allow a smooth transition from pinning buffer objects to dynamic
invalidation we first start to cache the sg_table for an attachment.

v2: keep closer to the DRM implementation
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.kernel.org/patch/10943053/

drivers/dma-buf/dma-buf.c

@@ -576,6 +576,7 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
 	list_add(&attach->node, &dmabuf->attachments);
 
 	mutex_unlock(&dmabuf->lock);
+
 	return attach;
 
 err_attach:
@@ -598,6 +599,9 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
 	if (WARN_ON(!dmabuf || !attach))
 		return;
 
+	if (attach->sgt)
+		dmabuf->ops->unmap_dma_buf(attach, attach->sgt, attach->dir);
+
 	mutex_lock(&dmabuf->lock);
 	list_del(&attach->node);
 	if (dmabuf->ops->detach)
@@ -633,10 +637,27 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
 	if (WARN_ON(!attach || !attach->dmabuf))
 		return ERR_PTR(-EINVAL);
 
+	if (attach->sgt) {
+		/*
+		 * Two mappings with different directions for the same
+		 * attachment are not allowed.
+		 */
+		if (attach->dir != direction &&
+		    attach->dir != DMA_BIDIRECTIONAL)
+			return ERR_PTR(-EBUSY);
+
+		return attach->sgt;
+	}
+
 	sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
 	if (!sg_table)
 		sg_table = ERR_PTR(-ENOMEM);
 
+	if (!IS_ERR(sg_table) && attach->dmabuf->ops->cache_sgt_mapping) {
+		attach->sgt = sg_table;
+		attach->dir = direction;
+	}
+
 	return sg_table;
 }
 EXPORT_SYMBOL_GPL(dma_buf_map_attachment);
@@ -660,8 +681,10 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
 	if (WARN_ON(!attach || !attach->dmabuf || !sg_table))
 		return;
 
-	attach->dmabuf->ops->unmap_dma_buf(attach, sg_table,
-						direction);
+	if (attach->sgt == sg_table)
+		return;
+
+	attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);
 }
 EXPORT_SYMBOL_GPL(dma_buf_unmap_attachment);
 

include/linux/dma-buf.h

@@ -44,6 +44,15 @@ struct dma_buf_attachment;
  * @vunmap: [optional] unmaps a vmap from the buffer
  */
 struct dma_buf_ops {
+	/**
+	  * @cache_sgt_mapping:
+	  *
+	  * If true the framework will cache the first mapping made for each
+	  * attachment. This avoids creating mappings for attachments multiple
+	  * times.
+	  */
+	bool cache_sgt_mapping;
+
 	/**
 	 * @attach:
 	 *
@@ -323,6 +332,8 @@ struct dma_buf {
  * @dmabuf: buffer for this attachment.
  * @dev: device attached to the buffer.
  * @node: list of dma_buf_attachment.
+ * @sgt: cached mapping.
+ * @dir: direction of cached mapping.
  * @priv: exporter specific attachment data.
  *
  * This structure holds the attachment information between the dma_buf buffer
@@ -338,6 +349,8 @@ struct dma_buf_attachment {
 	struct dma_buf *dmabuf;
 	struct device *dev;
 	struct list_head node;
+	struct sg_table *sgt;
+	enum dma_data_direction dir;
 	void *priv;
 };