media: vb2: Keep dma-buf buffers mapped until they are freed

When using vb2 for video decoding, dequeued capture buffers may still
be accessed by the hardware: this is the case when they are used as
reference frames for decoding subsequent frames.

When the buffer is imported with dma-buf, it needs to be mapped before
access. Until now, it was mapped when queuing and unmapped when
dequeuing, which doesn't work for access as a reference frames.

One way to solve this would be to map the buffer again when it is
needed as a reference, but the mapping/unmapping operations can
seriously impact performance. As a result, map the buffer once (when it
is first needed when queued) and keep it mapped until it is freed.

Reviewed-on: https://chromium-review.googlesource.com/334103
[Paul: Updated for mainline and changed commit message]
Signed-off-by: Pawel Osciak <posciak@chromium.org>
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/common/videobuf2/videobuf2-core.c

@@ -1196,6 +1196,9 @@ static int __prepare_dmabuf(struct vb2_buffer *vb)
 	 * userspace knows sooner rather than later if the dma-buf map fails.
 	 */
 	for (plane = 0; plane < vb->num_planes; ++plane) {
+		if (vb->planes[plane].dbuf_mapped)
+			continue;
+
 		ret = call_memop(vb, map_dmabuf, vb->planes[plane].mem_priv);
 		if (ret) {
 			dprintk(1, "failed to map dmabuf for plane %d\n",
@@ -1758,14 +1761,6 @@ static void __vb2_dqbuf(struct vb2_buffer *vb)
 
 	vb->state = VB2_BUF_STATE_DEQUEUED;
 
-	/* unmap DMABUF buffer */
-	if (q->memory == VB2_MEMORY_DMABUF)
-		for (i = 0; i < vb->num_planes; ++i) {
-			if (!vb->planes[i].dbuf_mapped)
-				continue;
-			call_void_memop(vb, unmap_dmabuf, vb->planes[i].mem_priv);
-			vb->planes[i].dbuf_mapped = 0;
-		}
 	call_void_bufop(q, init_buffer, vb);
 }