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

[pandora-kernel.git] / Documentation / DocBook / media / v4l / vidioc-subscribe-event.xml

diff --git a/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml b/Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml
index 039a969..25471e8 100644 (file)
--- 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>