[media] V4L: Add v4l2_ctrl_subdev_subscribe_event() helper function

Add a v4l2 core helper function that can be used as the subdev
.subscribe_event handler. This allows to eliminate some boilerplate
from drivers that are only handling the control events.
Signed-off-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/v4l2-core/v4l2-ctrls.c

@@ -2885,6 +2885,15 @@ int v4l2_ctrl_subscribe_event(struct v4l2_fh *fh,
 }
 EXPORT_SYMBOL(v4l2_ctrl_subscribe_event);
 
+int v4l2_ctrl_subdev_subscribe_event(struct v4l2_subdev *sd, struct v4l2_fh *fh,
+				     struct v4l2_event_subscription *sub)
+{
+	if (!sd->ctrl_handler)
+		return -EINVAL;
+	return v4l2_ctrl_subscribe_event(fh, sub);
+}
+EXPORT_SYMBOL(v4l2_ctrl_subdev_subscribe_event);
+
 unsigned int v4l2_ctrl_poll(struct file *file, struct poll_table_struct *wait)
 {
 	struct v4l2_fh *fh = file->private_data;

include/media/v4l2-ctrls.h

@@ -654,4 +654,9 @@ int v4l2_subdev_s_ext_ctrls(struct v4l2_subdev *sd, struct v4l2_ext_controls *cs
 int v4l2_subdev_g_ctrl(struct v4l2_subdev *sd, struct v4l2_control *ctrl);
 int v4l2_subdev_s_ctrl(struct v4l2_subdev *sd, struct v4l2_control *ctrl);
 
+/* Can be used as a subscribe_event function that just subscribes control
+   events. */
+int v4l2_ctrl_subdev_subscribe_event(struct v4l2_subdev *sd, struct v4l2_fh *fh,
+				     struct v4l2_event_subscription *sub);
+
 #endif