From 2c3c1048746a4622d8c89a29670120dc8fab93c4 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 7 Apr 2024 20:49:45 +0200 Subject: Adding upstream version 6.1.76. Signed-off-by: Daniel Baumann --- .../userspace-api/media/v4l/dev-event.rst | 47 ++++++++++++++++++++++ 1 file changed, 47 insertions(+) create mode 100644 Documentation/userspace-api/media/v4l/dev-event.rst (limited to 'Documentation/userspace-api/media/v4l/dev-event.rst') diff --git a/Documentation/userspace-api/media/v4l/dev-event.rst b/Documentation/userspace-api/media/v4l/dev-event.rst new file mode 100644 index 000000000..f34f9cf6c --- /dev/null +++ b/Documentation/userspace-api/media/v4l/dev-event.rst @@ -0,0 +1,47 @@ +.. 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. -- cgit v1.2.3