diff options
author | Hans Verkuil <hans.verkuil@cisco.com> | 2011-06-29 08:56:22 -0300 |
---|---|---|
committer | Mauro Carvalho Chehab <mchehab@redhat.com> | 2011-07-27 17:53:34 -0300 |
commit | 60c0732244164f14e376cfae493dba368f761514 (patch) | |
tree | 104e04777d3239dabb1c4dba20ec17a7b1089c86 /Documentation/DocBook | |
parent | 3ad57c33846d884cfeed770238b27f3dcf3742f1 (diff) | |
download | linux-stable-60c0732244164f14e376cfae493dba368f761514.tar.gz linux-stable-60c0732244164f14e376cfae493dba368f761514.tar.bz2 linux-stable-60c0732244164f14e376cfae493dba368f761514.zip |
[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')
-rw-r--r-- | Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml | 36 |
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 039a9694fd1d..25471e8e5da5 100644 --- a/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml +++ b/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml @@ -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> |