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[media] DocBook: update V4L Event Interface section

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

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Hans Verkuil 2011-06-18 06:58:29 -03:00 committed by Mauro Carvalho Chehab
parent 1de7310ac9
commit e969084030
2 changed files with 36 additions and 9 deletions
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30
Documentation/DocBook/media/v4l/dev-event.xml
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@ -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:

15
Documentation/DocBook/media/v4l/vidioc-subscribe-event.xml
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@ -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>