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43 lines
2 KiB
XML
43 lines
2 KiB
XML
<title>Event Interface</title>
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<para>The V4L2 event interface provides a means for a user to get
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immediately notified on certain conditions taking place on a device.
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This might include start of frame or loss of signal events, for
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example. Changes in the value or state of a V4L2 control can also be
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reported through events.
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</para>
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<para>To receive events, the events the user is interested in first must
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be subscribed using the &VIDIOC-SUBSCRIBE-EVENT; ioctl. Once an event is
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subscribed, the events of subscribed types are dequeueable using the
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&VIDIOC-DQEVENT; ioctl. Events may be unsubscribed using
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VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event type V4L2_EVENT_ALL may
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be used to unsubscribe all the events the driver supports.</para>
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<para>The event subscriptions and event queues are specific to file
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handles. Subscribing an event on one file handle does not affect
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other file handles.</para>
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<para>The information on dequeueable events is obtained by using select or
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poll system calls on video devices. The V4L2 events use POLLPRI events on
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poll system call and exceptions on select system call.</para>
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<para>Starting with kernel 3.1 certain guarantees can be given with
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regards to events:<orderedlist>
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<listitem>
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<para>Each subscribed event has its own internal dedicated event queue.
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This means that flooding of one event type will not interfere with other
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event types.</para>
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</listitem>
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<listitem>
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<para>If the internal event queue for a particular subscribed event
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becomes full, then the oldest event in that queue will be dropped.</para>
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</listitem>
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<listitem>
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<para>Where applicable, certain event types can ensure that the payload
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of the oldest event that is about to be dropped will be merged with the payload
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of the next oldest event. Thus ensuring that no information is lost, but only an
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intermediate step leading up to that information. See the documentation for the
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event you want to subscribe to whether this is applicable for that event or not.</para>
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</listitem>
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</orderedlist></para>
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