Commit e9690840 authored by Hans Verkuil's avatar Hans Verkuil Committed by Mauro Carvalho Chehab
Browse files

[media] DocBook: update V4L Event Interface section 



Starting with v3.1 the V4L2 API provides certain guarantees with respect to
events. Document these.

Signed-off-by: default avatarHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@redhat.com>
parent 1de7310a

Documentation/DocBook/media/v4l/dev-event.xml

+25 −5
Original line number Diff line number Diff line 
  <title>Event Interface</title>



  <para>The V4L2 event interface provides means for user to get

  <para>The V4L2 event interface provides a means for a user to get

  immediately notified on certain conditions taking place on a device.

  This might include start of frame or loss of signal events, for

  example.

  example. Changes in the value or state of a V4L2 control can also be

  reported through events.

  </para>



  <para>To receive events, the events the user is interested in first must
@@ -15,13 +16,32 @@ 


  <para>The event subscriptions and event queues are specific to file

  handles. Subscribing an event on one file handle does not affect

  other file handles.

  </para>

  other file handles.</para>



  <para>The information on dequeueable events is obtained by using select or

  poll system calls on video devices. The V4L2 events use POLLPRI events on

  poll system call and exceptions on select system call.</para>



  <para>Starting with kernel 3.1 certain guarantees can be given with

  regards to events:<orderedlist>

	<listitem>

	  <para>Each subscribed event has its own internal dedicated event queue.

This means that flooding of one event type will not interfere with other

event types.</para>

	</listitem>

	<listitem>

	  <para>If the internal event queue for a particular subscribed event

becomes full, then the oldest event in that queue will be dropped.</para>

	</listitem>

	<listitem>

	  <para>Where applicable, certain event types can ensure that the payload

of the oldest event that is about to be dropped will be merged with the payload

of the next oldest event. Thus ensuring that no information is lost, but only an

intermediate step leading up to that information. See the documentation for the

event you want to subscribe to whether this is applicable for that event or not.</para>

	</listitem>

      </orderedlist></para>



  <!--

Local Variables:

mode: sgml

Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml

+11 −4
Original line number Diff line number Diff line
@@ -100,7 +100,7 @@ 
	    <entry><constant>V4L2_EVENT_VSYNC</constant></entry>

	    <entry>1</entry>

	    <entry>This event is triggered on the vertical sync.

	    This event has &v4l2-event-vsync; associated with it.

	    This event has a &v4l2-event-vsync; associated with it.

	    </entry>

	  </row>

	  <row>
@@ -118,7 +118,7 @@ 
		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 &v4l2-event-ctrl; associated with it. This struct

	    	This event has a &v4l2-event-ctrl; associated with it. This struct

		contains much of the same information as &v4l2-queryctrl; and

		&v4l2-control;.
@@ -126,6 +126,13 @@ 
		&VIDIOC-S-EXT-CTRLS;, then the event will not be sent to

		the file handle that called the ioctl function. This prevents

		nasty feedback loops.



		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.

	    </entry>

	  </row>

	  <row>
@@ -147,8 +154,8 @@ 
	    <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. Other events will ignore

		this flag.</entry>

		that are triggered by a status change such as <constant>V4L2_EVENT_CTRL</constant>.

		Other events will ignore this flag.</entry>

	  </row>

	</tbody>

      </tgroup>