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Commit 60c07322 authored by Hans Verkuil's avatar Hans Verkuil Committed by Mauro Carvalho Chehab
Browse files

[media] v4l2-ctrls.c: add support for V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK 



Normally no control events will go to the filehandle that called the
VIDIOC_S_CTRL/VIDIOC_S_EXT_CTRLS ioctls. This is to prevent a feedback
loop.

This can now be overridden by setting the new V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK
flag.

Based on suggestions from Mauro Carvalho Chehab <mchehab@redhat.com> and
Laurent Pinchart <laurent.pinchart@ideasonboard.com>.

Signed-off-by: default avatarHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@redhat.com>
parent 3ad57c33

Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml

+29 −7
Original line number Diff line number Diff line
@@ -114,25 +114,28 @@ 
	  <row>

	    <entry><constant>V4L2_EVENT_CTRL</constant></entry>

	    <entry>3</entry>

	    <entry>This event requires that the <structfield>id</structfield>

	    <entry><para>This event requires that the <structfield>id</structfield>

		matches the control ID from which you want to receive events.

		This event is triggered if the control's value changes, if a

		button control is pressed or if the control's flags change.

	    	This event has a &v4l2-event-ctrl; associated with it. This struct

		contains much of the same information as &v4l2-queryctrl; and

		&v4l2-control;.

		&v4l2-control;.</para>



		If the event is generated due to a call to &VIDIOC-S-CTRL; or

		&VIDIOC-S-EXT-CTRLS;, then the event will not be sent to

		<para>If the event is generated due to a call to &VIDIOC-S-CTRL; or

		&VIDIOC-S-EXT-CTRLS;, then the event will <emphasis>not</emphasis> be sent to

		the file handle that called the ioctl function. This prevents

		nasty feedback loops.

		nasty feedback loops. If you <emphasis>do</emphasis> want to get the

		event, then set the <constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant>

		flag.

		</para>



		This event type will ensure that no information is lost when

		<para>This event type will ensure that no information is lost when

		more events are raised than there is room internally. In that

		case the &v4l2-event-ctrl; of the second-oldest event is kept,

		but the <structfield>changes</structfield> field of the

		second-oldest event is ORed with the <structfield>changes</structfield>

		field of the oldest event.

		field of the oldest event.</para>

	    </entry>

	  </row>

	  <row>
@@ -157,6 +160,25 @@ 
		that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.

		Other events will ignore this flag.</entry>

	  </row>

	  <row>

	    <entry><constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant></entry>

	    <entry>0x0002</entry>

	    <entry><para>If set, then events directly caused by an ioctl will also be sent to

		the filehandle that called that ioctl. For example, changing a control using

		&VIDIOC-S-CTRL; will cause a V4L2_EVENT_CTRL to be sent back to that same

		filehandle. Normally such events are suppressed to prevent feedback loops

		where an application changes a control to a one value and then another, and

		then receives an event telling it that that control has changed to the first

		value.</para>



		<para>Since it can't tell whether that event was caused by another application

		or by the &VIDIOC-S-CTRL; call it is hard to decide whether to set the

		control to the value in the event, or ignore it.</para>



		<para>Think carefully when you set this flag so you won't get into situations

		like that.</para>

	    </entry>

	  </row>

	</tbody>

      </tgroup>

    </table>

drivers/media/video/v4l2-ctrls.c

+2 −1
Original line number Diff line number Diff line
@@ -590,7 +590,8 @@ static void send_event(struct v4l2_fh *fh, struct v4l2_ctrl *ctrl, u32 changes) 
	fill_event(&ev, ctrl, changes);



	list_for_each_entry(sev, &ctrl->ev_subs, node)

		if (sev->fh && sev->fh != fh)

		if (sev->fh && (sev->fh != fh ||

				(sev->flags & V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK)))

			v4l2_event_queue_fh(sev->fh, &ev);

}

include/linux/videodev2.h

+2 −1
Original line number Diff line number Diff line
@@ -1833,6 +1833,7 @@ struct v4l2_event { 
};



#define V4L2_EVENT_SUB_FL_SEND_INITIAL		(1 << 0)

#define V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK	(1 << 1)



struct v4l2_event_subscription {

	__u32				type;