Orange Pi5 kernel

^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  1) .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  2) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  3) .. _event:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  4) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  5) ***************
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  6) Event Interface
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  7) ***************
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  8) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  9) The V4L2 event interface provides a means for a user to get immediately
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10) notified on certain conditions taking place on a device. This might
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11) include start of frame or loss of signal events, for example. Changes in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12) the value or state of a V4L2 control can also be reported through
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13) events.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15) To receive events, the events the user is interested in first must be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) subscribed using the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17) :ref:`VIDIOC_SUBSCRIBE_EVENT` ioctl. Once
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18) an event is subscribed, the events of subscribed types are dequeueable
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) using the :ref:`VIDIOC_DQEVENT` ioctl. Events may be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20) unsubscribed using VIDIOC_UNSUBSCRIBE_EVENT ioctl. The special event
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21) type V4L2_EVENT_ALL may be used to unsubscribe all the events the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22) driver supports.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) The event subscriptions and event queues are specific to file handles.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25) Subscribing an event on one file handle does not affect other file
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) handles.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28) The information on dequeueable events is obtained by using select or
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29) poll system calls on video devices. The V4L2 events use POLLPRI events
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30) on poll system call and exceptions on select system call.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) Starting with kernel 3.1 certain guarantees can be given with regards to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) events:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) 1. Each subscribed event has its own internal dedicated event queue.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36)    This means that flooding of one event type will not interfere with
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37)    other event types.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39) 2. If the internal event queue for a particular subscribed event becomes
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40)    full, then the oldest event in that queue will be dropped.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42) 3. Where applicable, certain event types can ensure that the payload of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43)    the oldest event that is about to be dropped will be merged with the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44)    payload of the next oldest event. Thus ensuring that no information
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45)    is lost, but only an intermediate step leading up to that
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46)    information. See the documentation for the event you want to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47)    subscribe to whether this is applicable for that event or not.