1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _event:
4
5***************
6Event Interface
7***************
8
9The V4L2 event interface provides a means for a user to get immediately
10notified on certain conditions taking place on a device. This might
11include start of frame or loss of signal events, for example. Changes in
12the value or state of a V4L2 control can also be reported through
13events.
14
15To receive events, the events the user is interested in first must be
16subscribed using the
17:ref:`VIDIOC_SUBSCRIBE_EVENT` ioctl. Once
18an event is subscribed, the events of subscribed types are dequeueable
19using the :ref:`VIDIOC_DQEVENT` ioctl. Events may be
20unsubscribed using VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event
21type V4L2_EVENT_ALL may be used to unsubscribe all the events the
22driver supports.
23
24The event subscriptions and event queues are specific to file handles.
25Subscribing an event on one file handle does not affect other file
26handles.
27
28The information on dequeueable events is obtained by using select or
29poll system calls on video devices. The V4L2 events use POLLPRI events
30on poll system call and exceptions on select system call.
31
32Starting with kernel 3.1 certain guarantees can be given with regards to
33events:
34
351. Each subscribed event has its own internal dedicated event queue.
36   This means that flooding of one event type will not interfere with
37   other event types.
38
392. If the internal event queue for a particular subscribed event becomes
40   full, then the oldest event in that queue will be dropped.
41
423. Where applicable, certain event types can ensure that the payload of
43   the oldest event that is about to be dropped will be merged with the
44   payload of the next oldest event. Thus ensuring that no information
45   is lost, but only an intermediate step leading up to that
46   information. See the documentation for the event you want to
47   subscribe to whether this is applicable for that event or not.
48