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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> <row>
<entry><constant>V4L2_EVENT_CTRL</constant></entry> <entry><constant>V4L2_EVENT_CTRL</constant></entry>
<entry>3</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. matches the control ID from which you want to receive events.
This event is triggered if the control's value changes, if a This event is triggered if the control's value changes, if a
button control is pressed or if the control's flags change. button control is pressed or if the control's flags change.
This event has a &v4l2-event-ctrl; associated with it. This struct This event has a &v4l2-event-ctrl; associated with it. This struct
contains much of the same information as &v4l2-queryctrl; and 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 <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 &VIDIOC-S-EXT-CTRLS;, then the event will <emphasis>not</emphasis> be sent to
the file handle that called the ioctl function. This prevents 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 more events are raised than there is room internally. In that
case the &v4l2-event-ctrl; of the second-oldest event is kept, case the &v4l2-event-ctrl; of the second-oldest event is kept,
but the <structfield>changes</structfield> field of the but the <structfield>changes</structfield> field of the
second-oldest event is ORed with the <structfield>changes</structfield> second-oldest event is ORed with the <structfield>changes</structfield>
field of the oldest event. field of the oldest event.</para>
</entry> </entry>
</row> </row>
<row> <row>
@ -157,6 +160,25 @@
that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>. that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.
Other events will ignore this flag.</entry> Other events will ignore this flag.</entry>
</row> </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> </tbody>
</tgroup> </tgroup>
</table> </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); fill_event(&ev, ctrl, changes);
list_for_each_entry(sev, &ctrl->ev_subs, node) 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); v4l2_event_queue_fh(sev->fh, &ev);
} }

3
include/linux/videodev2.h
View file

@ -1832,7 +1832,8 @@ struct v4l2_event {
__u32 reserved[8]; __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 { struct v4l2_event_subscription {
__u32 type; __u32 type;