path: root/Documentation/DocBook/media/v4l/dev-event.xml
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authorFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
committerFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
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treeb9996006addfd7ae70a39672b76843b49aebc189 /Documentation/DocBook/media/v4l/dev-event.xml
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+ <title>Event Interface</title>
+ <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. 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
+ be subscribed using the &VIDIOC-SUBSCRIBE-EVENT; ioctl. Once an event is
+ subscribed, the events of subscribed types are dequeueable using the
+ &VIDIOC-DQEVENT; ioctl. Events may be unsubscribed using
+ VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event type V4L2_EVENT_ALL may
+ be used to unsubscribe all the events the driver supports.</para>
+ <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>
+ <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>