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

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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: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Hans Verkuil 2011-06-29 08:56:22 -03:00 • committed by Mauro Carvalho Chehab
commit 60c0732244
3 changed files with 33 additions and 9 deletions
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36
Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml
View file

@ -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>

3
drivers/media/video/v4l2-ctrls.c
View file

@ -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);
}

3
include/linux/videodev2.h
View file

@ -1832,7 +1832,8 @@ struct v4l2_event {
__u32 reserved[8];
};
#define V4L2_EVENT_SUB_FL_SEND_INITIAL (1 << 0)
#define V4L2_EVENT_SUB_FL_SEND_INITIAL (1 << 0)
#define V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK (1 << 1)
struct v4l2_event_subscription {
__u32 type;