Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml - arm/linux-arm64-legacy - Git at Google

 <refentry id="vidioc-subscribe-event">
   <refmeta>
     <refentrytitle>ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refentrytitle>
     &manvol;
   </refmeta>

   <refnamediv>
     <refname>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refname>
     <refpurpose>Subscribe or unsubscribe event</refpurpose>
   </refnamediv>

   <refsynopsisdiv>
     <funcsynopsis>
       <funcprototype>
 	<funcdef>int <function>ioctl</function></funcdef>
 	<paramdef>int <parameter>fd</parameter></paramdef>
 	<paramdef>int <parameter>request</parameter></paramdef>
 	<paramdef>struct v4l2_event_subscription
 *<parameter>argp</parameter></paramdef>
       </funcprototype>
     </funcsynopsis>
   </refsynopsisdiv>

   <refsect1>
     <title>Arguments</title>

     <variablelist>
       <varlistentry>
 	<term><parameter>fd</parameter></term>
 	<listitem>
 	  <para>&fd;</para>
 	</listitem>
       </varlistentry>
       <varlistentry>
 	<term><parameter>request</parameter></term>
 	<listitem>
 	  <para>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</para>
 	</listitem>
       </varlistentry>
       <varlistentry>
 	<term><parameter>argp</parameter></term>
 	<listitem>
 	  <para></para>
 	</listitem>
       </varlistentry>
     </variablelist>
   </refsect1>

   <refsect1>
     <title>Description</title>

     <para>Subscribe or unsubscribe V4L2 event. Subscribed events are
     dequeued by using the &VIDIOC-DQEVENT; ioctl.</para>

     <table frame="none" pgwide="1" id="v4l2-event-subscription">
       <title>struct <structname>v4l2_event_subscription</structname></title>
       <tgroup cols="3">
 	&cs-str;
 	<tbody valign="top">
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>type</structfield></entry>
 	    <entry>Type of the event.</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>id</structfield></entry>
 	    <entry>ID of the event source. If there is no ID associated with
 		the event source, then set this to 0. Whether or not an event
 		needs an ID depends on the event type.</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>flags</structfield></entry>
 	    <entry>Event flags, see <xref linkend="event-flags" />.</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>reserved</structfield>[5]</entry>
 	    <entry>Reserved for future extensions. Drivers and applications
 	    must set the array to zero.</entry>
 	  </row>
 	</tbody>
       </tgroup>
     </table>

     <table frame="none" pgwide="1" id="event-type">
       <title>Event Types</title>
       <tgroup cols="3">
 	&cs-def;
 	<tbody valign="top">
 	  <row>
 	    <entry><constant>V4L2_EVENT_ALL</constant></entry>
 	    <entry>0</entry>
 	    <entry>All events. V4L2_EVENT_ALL is valid only for
 	    VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
 	    </entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_EVENT_VSYNC</constant></entry>
 	    <entry>1</entry>
 	    <entry>This event is triggered on the vertical sync.
 	    This event has a &v4l2-event-vsync; associated with it.
 	    </entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_EVENT_EOS</constant></entry>
 	    <entry>2</entry>
 	    <entry>This event is triggered when the end of a stream is reached.
 	    This is typically used with MPEG decoders to report to the application
 	    when the last of the MPEG stream has been decoded.
 	    </entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_EVENT_CTRL</constant></entry>
 	    <entry>3</entry>
 	    <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;.</para>

 		<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. If you <emphasis>do</emphasis> want to get the
 		event, then set the <constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant>
 		flag.
 		</para>

 		<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.</para>
 	    </entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_EVENT_FRAME_SYNC</constant></entry>
 	    <entry>4</entry>
 	    <entry>
 	      <para>Triggered immediately when the reception of a
 	      frame has begun. This event has a
 	      &v4l2-event-frame-sync; associated with it.</para>

 	      <para>If the hardware needs to be stopped in the case of a
 	      buffer underrun it might not be able to generate this event.
 	      In such cases the <structfield>frame_sequence</structfield>
 	      field in &v4l2-event-frame-sync; will not be incremented. This
 	      causes two consecutive frame sequence numbers to have n times
 	      frame interval in between them.</para>
 	    </entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_EVENT_SOURCE_CHANGE</constant></entry>
 	    <entry>5</entry>
 	    <entry>
 	      <para>This event is triggered when a source parameter change is
 	       detected during runtime by the video device. It can be a
 	       runtime resolution change triggered by a video decoder or the
 	       format change happening on an input connector.
 	       This event requires that the <structfield>id</structfield>
 	       matches the input index (when used with a video device node)
 	       or the pad index (when used with a subdevice node) from which
 	       you want to receive events.</para>

               <para>This event has a &v4l2-event-src-change; associated
 	      with it. The <structfield>changes</structfield> bitfield denotes
 	      what has changed for the subscribed pad. If multiple events
 	      occurred before application could dequeue them, then the changes
 	      will have the ORed value of all the events generated.</para>
 	    </entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_EVENT_PRIVATE_START</constant></entry>
 	    <entry>0x08000000</entry>
 	    <entry>Base event number for driver-private events.</entry>
 	  </row>
 	</tbody>
       </tgroup>
     </table>

     <table pgwide="1" frame="none" id="event-flags">
       <title>Event Flags</title>
       <tgroup cols="3">
 	&cs-def;
 	<tbody valign="top">
 	  <row>
 	    <entry><constant>V4L2_EVENT_SUB_FL_SEND_INITIAL</constant></entry>
 	    <entry>0x0001</entry>
 	    <entry>When this event is subscribed an initial event will be sent
 		containing the current status. This only makes sense for events
 		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>

   </refsect1>
   <refsect1>
     &return-value;
   </refsect1>
 </refentry>
	<refentry id="vidioc-subscribe-event">
	<refmeta>
	<refentrytitle>ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refentrytitle>
	&manvol;
	</refmeta>

	<refnamediv>
	<refname>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</refname>
	<refpurpose>Subscribe or unsubscribe event</refpurpose>
	</refnamediv>

	<refsynopsisdiv>
	<funcsynopsis>
	<funcprototype>
	<funcdef>int <function>ioctl</function></funcdef>
	<paramdef>int <parameter>fd</parameter></paramdef>
	<paramdef>int <parameter>request</parameter></paramdef>
	<paramdef>struct v4l2_event_subscription
	*<parameter>argp</parameter></paramdef>
	</funcprototype>
	</funcsynopsis>
	</refsynopsisdiv>

	<refsect1>
	<title>Arguments</title>

	<variablelist>
	<varlistentry>
	<term><parameter>fd</parameter></term>
	<listitem>
	<para>&fd;</para>
	</listitem>
	</varlistentry>
	<varlistentry>
	<term><parameter>request</parameter></term>
	<listitem>
	<para>VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT</para>
	</listitem>
	</varlistentry>
	<varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	<para></para>
	</listitem>
	</varlistentry>
	</variablelist>
	</refsect1>

	<refsect1>
	<title>Description</title>

	<para>Subscribe or unsubscribe V4L2 event. Subscribed events are
	dequeued by using the &VIDIOC-DQEVENT; ioctl.</para>

	<table frame="none" pgwide="1" id="v4l2-event-subscription">
	<title>struct <structname>v4l2_event_subscription</structname></title>
	<tgroup cols="3">
	&cs-str;
	<tbody valign="top">
	<row>
	<entry>__u32</entry>
	<entry><structfield>type</structfield></entry>
	<entry>Type of the event.</entry>
	</row>
	<row>
	<entry>__u32</entry>
	<entry><structfield>id</structfield></entry>
	<entry>ID of the event source. If there is no ID associated with
	the event source, then set this to 0. Whether or not an event
	needs an ID depends on the event type.</entry>
	</row>
	<row>
	<entry>__u32</entry>
	<entry><structfield>flags</structfield></entry>
	<entry>Event flags, see <xref linkend="event-flags" />.</entry>
	</row>
	<row>
	<entry>__u32</entry>
	<entry><structfield>reserved</structfield>[5]</entry>
	<entry>Reserved for future extensions. Drivers and applications
	must set the array to zero.</entry>
	</row>
	</tbody>
	</tgroup>
	</table>

	<table frame="none" pgwide="1" id="event-type">
	<title>Event Types</title>
	<tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	<row>
	<entry><constant>V4L2_EVENT_ALL</constant></entry>
	<entry>0</entry>
	<entry>All events. V4L2_EVENT_ALL is valid only for
	VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
	</entry>
	</row>
	<row>
	<entry><constant>V4L2_EVENT_VSYNC</constant></entry>
	<entry>1</entry>
	<entry>This event is triggered on the vertical sync.
	This event has a &v4l2-event-vsync; associated with it.
	</entry>
	</row>
	<row>
	<entry><constant>V4L2_EVENT_EOS</constant></entry>
	<entry>2</entry>
	<entry>This event is triggered when the end of a stream is reached.
	This is typically used with MPEG decoders to report to the application
	when the last of the MPEG stream has been decoded.
	</entry>
	</row>
	<row>
	<entry><constant>V4L2_EVENT_CTRL</constant></entry>
	<entry>3</entry>
	<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;.</para>

	<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. If you <emphasis>do</emphasis> want to get the
	event, then set the <constant>V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK</constant>
	flag.
	</para>

	<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.</para>
	</entry>
	</row>
	<row>
	<entry><constant>V4L2_EVENT_FRAME_SYNC</constant></entry>
	<entry>4</entry>
	<entry>
	<para>Triggered immediately when the reception of a
	frame has begun. This event has a
	&v4l2-event-frame-sync; associated with it.</para>

	<para>If the hardware needs to be stopped in the case of a
	buffer underrun it might not be able to generate this event.
	In such cases the <structfield>frame_sequence</structfield>
	field in &v4l2-event-frame-sync; will not be incremented. This
	causes two consecutive frame sequence numbers to have n times
	frame interval in between them.</para>
	</entry>
	</row>
	<row>
	<entry><constant>V4L2_EVENT_SOURCE_CHANGE</constant></entry>
	<entry>5</entry>
	<entry>
	<para>This event is triggered when a source parameter change is
	detected during runtime by the video device. It can be a
	runtime resolution change triggered by a video decoder or the
	format change happening on an input connector.
	This event requires that the <structfield>id</structfield>
	matches the input index (when used with a video device node)
	or the pad index (when used with a subdevice node) from which
	you want to receive events.</para>

	<para>This event has a &v4l2-event-src-change; associated
	with it. The <structfield>changes</structfield> bitfield denotes
	what has changed for the subscribed pad. If multiple events
	occurred before application could dequeue them, then the changes
	will have the ORed value of all the events generated.</para>
	</entry>
	</row>
	<row>
	<entry><constant>V4L2_EVENT_PRIVATE_START</constant></entry>
	<entry>0x08000000</entry>
	<entry>Base event number for driver-private events.</entry>
	</row>
	</tbody>
	</tgroup>
	</table>

	<table pgwide="1" frame="none" id="event-flags">
	<title>Event Flags</title>
	<tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	<row>
	<entry><constant>V4L2_EVENT_SUB_FL_SEND_INITIAL</constant></entry>
	<entry>0x0001</entry>
	<entry>When this event is subscribed an initial event will be sent
	containing the current status. This only makes sense for events
	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>

	</refsect1>
	<refsect1>
	&return-value;
	</refsect1>
	</refentry>