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Diffstat (limited to 'Documentation/DocBook/media/v4l/dev-event.xml')
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diff --git a/Documentation/DocBook/media/v4l/dev-event.xml b/Documentation/DocBook/media/v4l/dev-event.xml new file mode 100644 index 00000000000..f14ae3fe107 --- /dev/null +++ b/Documentation/DocBook/media/v4l/dev-event.xml @@ -0,0 +1,51 @@ + <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> + + <!-- +Local Variables: +mode: sgml +sgml-parent-document: "v4l2.sgml" +indent-tabs-mode: nil +End: + --> |