[media] uvcvideo: Implement videobuf2 .wait_prepare and .wait_finish operations

Those optional operations are used to release and reacquire the queue
lock when videobuf2 needs to perform operations that sleep for a long
time, such as waiting for a buffer to be complete. Implement them to
avoid blocking qbuf or streamoff calls when a dqbuf is in progress.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/usb/uvc/uvc_queue.c

@@ -115,11 +115,27 @@ static int uvc_buffer_finish(struct vb2_buffer *vb)
 	return 0;
 }
 
+static void uvc_wait_prepare(struct vb2_queue *vq)
+{
+	struct uvc_video_queue *queue = vb2_get_drv_priv(vq);
+
+	mutex_unlock(&queue->mutex);
+}
+
+static void uvc_wait_finish(struct vb2_queue *vq)
+{
+	struct uvc_video_queue *queue = vb2_get_drv_priv(vq);
+
+	mutex_lock(&queue->mutex);
+}
+
 static struct vb2_ops uvc_queue_qops = {
 	.queue_setup = uvc_queue_setup,
 	.buf_prepare = uvc_buffer_prepare,
 	.buf_queue = uvc_buffer_queue,
 	.buf_finish = uvc_buffer_finish,
+	.wait_prepare = uvc_wait_prepare,
+	.wait_finish = uvc_wait_finish,
 };
 
 int uvc_queue_init(struct uvc_video_queue *queue, enum v4l2_buf_type type,