1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 3 .. _event: 4 5 *************** 6 Event Interface 7 *************** 8 9 The V4L2 event interface provides a means for a user to get immediately 10 notified on certain conditions taking place on a device. This might 11 include start of frame or loss of signal events, for example. Changes in 12 the value or state of a V4L2 control can also be reported through 13 events. 14 15 To receive events, the events the user is interested in first must be 16 subscribed using the 17 :ref:`VIDIOC_SUBSCRIBE_EVENT` ioctl. Once 18 an event is subscribed, the events of subscribed types are dequeueable 19 using the :ref:`VIDIOC_DQEVENT` ioctl. Events may be 20 unsubscribed using VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event 21 type V4L2_EVENT_ALL may be used to unsubscribe all the events the 22 driver supports. 23 24 The event subscriptions and event queues are specific to file handles. 25 Subscribing an event on one file handle does not affect other file 26 handles. 27 28 The information on dequeueable events is obtained by using select or 29 poll system calls on video devices. The V4L2 events use POLLPRI events 30 on poll system call and exceptions on select system call. 31 32 Starting with kernel 3.1 certain guarantees can be given with regards to 33 events: 34 35 1. 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 39 2. If the internal event queue for a particular subscribed event becomes 40 full, then the oldest event in that queue will be dropped. 41 42 3. 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.
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