path: root/Documentation/DocBook/media/v4l
diff options
authorHans Verkuil <hans.verkuil@cisco.com>2011-06-29 08:56:22 -0300
committerMauro Carvalho Chehab <mchehab@redhat.com>2011-07-27 17:53:34 -0300
commit60c0732244164f14e376cfae493dba368f761514 (patch)
tree104e04777d3239dabb1c4dba20ec17a7b1089c86 /Documentation/DocBook/media/v4l
parent3ad57c33846d884cfeed770238b27f3dcf3742f1 (diff)
[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: Hans Verkuil <hans.verkuil@cisco.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Diffstat (limited to 'Documentation/DocBook/media/v4l')
1 files changed, 29 insertions, 7 deletions
diff --git a/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml b/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml
index 039a9694fd1..25471e8e5da 100644
--- a/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml
@@ -114,25 +114,28 @@
- <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>
@@ -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>
+ <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>