media: videobuf2-core: dequeue if start_streaming fails

If a vb2_queue sets q->min_buffers_needed then when the number of
queued buffers reaches q->min_buffers_needed, vb2_core_qbuf() will call
the start_streaming() callback. If start_streaming() returns an error,
then that error was just returned by vb2_core_qbuf(), but the buffer
was still queued. However, userspace expects that if VIDIOC_QBUF fails,
the buffer is returned dequeued.

So if start_streaming() fails, then remove the buffer from the queue,
thus avoiding this unwanted side-effect.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Fixes: b3379c62 ("[media] vb2: only call start_streaming if sufficient buffers are queued")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

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

@@ -1573,6 +1573,7 @@ int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb,
 		  struct media_request *req)
 {
 	struct vb2_buffer *vb;
+	enum vb2_buffer_state orig_state;
 	int ret;
 
 	if (q->error) {
@@ -1673,6 +1674,7 @@ int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb,
 	 * Add to the queued buffers list, a buffer will stay on it until
 	 * dequeued in dqbuf.
 	 */
+	orig_state = vb->state;
 	list_add_tail(&vb->queued_entry, &q->queued_list);
 	q->queued_count++;
 	q->waiting_for_buffers = false;
@@ -1703,8 +1705,17 @@ int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb,
 	if (q->streaming && !q->start_streaming_called &&
 	    q->queued_count >= q->min_buffers_needed) {
 		ret = vb2_start_streaming(q);
-		if (ret)
+		if (ret) {
+			/*
+			 * Since vb2_core_qbuf will return with an error,
+			 * we should return it to state DEQUEUED since
+			 * the error indicates that the buffer wasn't queued.
+			 */
+			list_del(&vb->queued_entry);
+			q->queued_count--;
+			vb->state = orig_state;
 			return ret;
+		}
 	}
 
 	dprintk(q, 2, "qbuf of buffer %d succeeded\n", vb->index);