media: videobuf2-v4l2.c: add vb2_queue_change_type() helper

On some platforms a video device can capture either video data or
metadata. The driver can implement vidioc functions for both video and
metadata, and use a single vb2_queue for the buffers. However, vb2_queue
requires choosing a single buffer type, which conflicts with the idea of
capturing either video or metadata.

The buffer type of vb2_queue can be changed, but it's not obvious how
this should be done in the drivers. To help this, add a new helper
function vb2_queue_change_type() which ensures the correct checks and
documents how it can be used.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/common/videobuf2/videobuf2-v4l2.c

@@ -939,6 +939,20 @@ void vb2_queue_release(struct vb2_queue *q)
 }
 EXPORT_SYMBOL_GPL(vb2_queue_release);
 
+int vb2_queue_change_type(struct vb2_queue *q, unsigned int type)
+{
+	if (type == q->type)
+		return 0;
+
+	if (vb2_is_busy(q))
+		return -EBUSY;
+
+	q->type = type;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(vb2_queue_change_type);
+
 __poll_t vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
 {
 	struct video_device *vfd = video_devdata(file);

include/media/videobuf2-v4l2.h

@@ -261,6 +261,22 @@ int __must_check vb2_queue_init_name(struct vb2_queue *q, const char *name);
  */
 void vb2_queue_release(struct vb2_queue *q);
 
+/**
+ * vb2_queue_change_type() - change the type of an inactive vb2_queue
+ * @q:		pointer to &struct vb2_queue with videobuf2 queue.
+ * @type:	the type to change to (V4L2_BUF_TYPE_VIDEO_*)
+ *
+ * This function changes the type of the vb2_queue. This is only possible
+ * if the queue is not busy (i.e. no buffers have been allocated).
+ *
+ * vb2_queue_change_type() can be used to support multiple buffer types using
+ * the same queue. The driver can implement v4l2_ioctl_ops.vidioc_reqbufs and
+ * v4l2_ioctl_ops.vidioc_create_bufs functions and call vb2_queue_change_type()
+ * before calling vb2_ioctl_reqbufs() or vb2_ioctl_create_bufs(), and thus
+ * "lock" the buffer type until the buffers have been released.
+ */
+int vb2_queue_change_type(struct vb2_queue *q, unsigned int type);
+
 /**
  * vb2_poll() - implements poll userspace operation
  * @q:		pointer to &struct vb2_queue with videobuf2 queue.