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Diffstat (limited to 'Documentation/DocBook/media/v4l/dev-event.xml')
| -rw-r--r-- | Documentation/DocBook/media/v4l/dev-event.xml | 30 |
1 files changed, 25 insertions, 5 deletions
diff --git a/Documentation/DocBook/media/v4l/dev-event.xml b/Documentation/DocBook/media/v4l/dev-event.xml index be5a98f..f14ae3f 100644 --- a/Documentation/DocBook/media/v4l/dev-event.xml +++ b/Documentation/DocBook/media/v4l/dev-event.xml @@ -1,9 +1,10 @@ <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 @@ -15,12 +16,31 @@ <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> + 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: |
