diff options
Diffstat (limited to 'Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml')
| -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> |