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