[media] v4l2-ctrls: add a notify callback

Sometimes platform/bridge drivers need to be notified when a control from
a sub-device changes value. In order to support this a notify callback was
added.
[dheitmueller@kernellabs.com: fix merge conflict in v4l2-ctrls.c]

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

1 files changed, 14 insertions, 8 deletions

diff --git a/Documentation/video4linux/v4l2-controls.txt b/Documentation/video4linux/v4l2-controls.txt
index cfe52c798d7..676f8736602 100644
--- a/Documentation/video4linux/v4l2-controls.txt
+++ b/Documentation/video4linux/v4l2-controls.txt
@@ -715,14 +715,20 @@ a control of this type whenever the first control belonging to a new control
 class is added.
 
 
-Proposals for Extensions
-========================
+Adding Notify Callbacks
+=======================
+
+Sometimes the platform or bridge driver needs to be notified when a control
+from a sub-device driver changes. You can set a notify callback by calling
+this function:
 
-Some ideas for future extensions to the spec:
+void v4l2_ctrl_notify(struct v4l2_ctrl *ctrl,
+	void (*notify)(struct v4l2_ctrl *ctrl, void *priv), void *priv);
 
-1) Add a V4L2_CTRL_FLAG_HEX to have values shown as hexadecimal instead of
-decimal. Useful for e.g. video_mute_yuv.
+Whenever the give control changes value the notify callback will be called
+with a pointer to the control and the priv pointer that was passed with
+v4l2_ctrl_notify. Note that the control's handler lock is held when the
+notify function is called.
 
-2) It is possible to mark in the controls array which controls have been
-successfully written and which failed by for example adding a bit to the
-control ID. Not sure if it is worth the effort, though.
+There can be only one notify function per control handler. Any attempt
+to set another notify function will cause a WARN_ON.