dma-buf: add get_dma_buf()

Works in a similar way to get_file(), and is needed in cases such as
when the exporter needs to also keep a reference to the dmabuf (that
is later released with a dma_buf_put()), and possibly other similar
cases.

Signed-off-by: Rob Clark <rob@ti.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

include/linux/dma-buf.h

@@ -30,6 +30,7 @@
 #include <linux/scatterlist.h>
 #include <linux/list.h>
 #include <linux/dma-mapping.h>
+#include <linux/fs.h>
 
 struct dma_buf;
 struct dma_buf_attachment;
@@ -110,6 +111,20 @@ struct dma_buf_attachment {
 	void *priv;
 };
 
+/**
+ * get_dma_buf - convenience wrapper for get_file.
+ * @dmabuf:	[in]	pointer to dma_buf
+ *
+ * Increments the reference count on the dma-buf, needed in case of drivers
+ * that either need to create additional references to the dmabuf on the
+ * kernel side.  For example, an exporter that needs to keep a dmabuf ptr
+ * so that subsequent exports don't create a new dmabuf.
+ */
+static inline void get_dma_buf(struct dma_buf *dmabuf)
+{
+	get_file(dmabuf->file);
+}
+
 #ifdef CONFIG_DMA_SHARED_BUFFER
 struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
 							struct device *dev);