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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+.. _event:
+
+***************
+Event Interface
+***************
+
+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.
+
+To receive events, the events the user is interested in first must be
+subscribed using the
+:ref:`VIDIOC_SUBSCRIBE_EVENT` ioctl. Once
+an event is subscribed, the events of subscribed types are dequeueable
+using the :ref:`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.
+
+The event subscriptions and event queues are specific to file handles.
+Subscribing an event on one file handle does not affect other file
+handles.
+
+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.
+
+Starting with kernel 3.1 certain guarantees can be given with regards to
+events:
+
+1. 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.
+
+2. If the internal event queue for a particular subscribed event becomes
+   full, then the oldest event in that queue will be dropped.
+
+3. 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.