1.. Permission is granted to copy, distribute and/or modify this 2.. document under the terms of the GNU Free Documentation License, 3.. Version 1.1 or any later version published by the Free Software 4.. Foundation, with no Invariant Sections, no Front-Cover Texts 5.. and no Back-Cover Texts. A copy of the license is included at 6.. Documentation/userspace-api/media/fdl-appendix.rst. 7.. 8.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections 9 10.. _event: 11 12*************** 13Event Interface 14*************** 15 16The V4L2 event interface provides a means for a user to get immediately 17notified on certain conditions taking place on a device. This might 18include start of frame or loss of signal events, for example. Changes in 19the value or state of a V4L2 control can also be reported through 20events. 21 22To receive events, the events the user is interested in first must be 23subscribed using the 24:ref:`VIDIOC_SUBSCRIBE_EVENT` ioctl. Once 25an event is subscribed, the events of subscribed types are dequeueable 26using the :ref:`VIDIOC_DQEVENT` ioctl. Events may be 27unsubscribed using VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event 28type V4L2_EVENT_ALL may be used to unsubscribe all the events the 29driver supports. 30 31The event subscriptions and event queues are specific to file handles. 32Subscribing an event on one file handle does not affect other file 33handles. 34 35The information on dequeueable events is obtained by using select or 36poll system calls on video devices. The V4L2 events use POLLPRI events 37on poll system call and exceptions on select system call. 38 39Starting with kernel 3.1 certain guarantees can be given with regards to 40events: 41 421. Each subscribed event has its own internal dedicated event queue. 43 This means that flooding of one event type will not interfere with 44 other event types. 45 462. If the internal event queue for a particular subscribed event becomes 47 full, then the oldest event in that queue will be dropped. 48 493. Where applicable, certain event types can ensure that the payload of 50 the oldest event that is about to be dropped will be merged with the 51 payload of the next oldest event. Thus ensuring that no information 52 is lost, but only an intermediate step leading up to that 53 information. See the documentation for the event you want to 54 subscribe to whether this is applicable for that event or not. 55