1.. SPDX-License-Identifier: GPL-2.0
2
3V4L2 events
4-----------
5
6The V4L2 events provide a generic way to pass events to user space.
7The driver must use :c:type:`v4l2_fh` to be able to support V4L2 events.
8
9Events are subscribed per-filehandle. An event specification consists of a
10``type`` and is optionally associated with an object identified through the
11``id`` field. If unused, then the ``id`` is 0. So an event is uniquely
12identified by the ``(type, id)`` tuple.
13
14The :c:type:`v4l2_fh` struct has a list of subscribed events on its
15``subscribed`` field.
16
17When the user subscribes to an event, a :c:type:`v4l2_subscribed_event`
18struct is added to :c:type:`v4l2_fh`\ ``.subscribed``, one for every
19subscribed event.
20
21Each :c:type:`v4l2_subscribed_event` struct ends with a
22:c:type:`v4l2_kevent` ringbuffer, with the size given by the caller
23of :c:func:`v4l2_event_subscribe`. This ringbuffer is used to store any events
24raised by the driver.
25
26So every ``(type, ID)`` event tuple will have its own
27:c:type:`v4l2_kevent` ringbuffer. This guarantees that if a driver is
28generating lots of events of one type in a short time, then that will
29not overwrite events of another type.
30
31But if you get more events of one type than the size of the
32:c:type:`v4l2_kevent` ringbuffer, then the oldest event will be dropped
33and the new one added.
34
35The :c:type:`v4l2_kevent` struct links into the ``available``
36list of the :c:type:`v4l2_fh` struct so :ref:`VIDIOC_DQEVENT` will
37know which event to dequeue first.
38
39Finally, if the event subscription is associated with a particular object
40such as a V4L2 control, then that object needs to know about that as well
41so that an event can be raised by that object. So the ``node`` field can
42be used to link the :c:type:`v4l2_subscribed_event` struct into a list of
43such objects.
44
45So to summarize:
46
47- struct v4l2_fh has two lists: one of the ``subscribed`` events,
48  and one of the ``available`` events.
49
50- struct v4l2_subscribed_event has a ringbuffer of raised
51  (pending) events of that particular type.
52
53- If struct v4l2_subscribed_event is associated with a specific
54  object, then that object will have an internal list of
55  struct v4l2_subscribed_event so it knows who subscribed an
56  event to that object.
57
58Furthermore, the internal struct v4l2_subscribed_event has
59``merge()`` and ``replace()`` callbacks which drivers can set. These
60callbacks are called when a new event is raised and there is no more room.
61
62The ``replace()`` callback allows you to replace the payload of the old event
63with that of the new event, merging any relevant data from the old payload
64into the new payload that replaces it. It is called when this event type has
65a ringbuffer with size is one, i.e. only one event can be stored in the
66ringbuffer.
67
68The ``merge()`` callback allows you to merge the oldest event payload into
69that of the second-oldest event payload. It is called when
70the ringbuffer has size is greater than one.
71
72This way no status information is lost, just the intermediate steps leading
73up to that state.
74
75A good example of these ``replace``/``merge`` callbacks is in v4l2-event.c:
76``ctrls_replace()`` and ``ctrls_merge()`` callbacks for the control event.
77
78.. note::
79	these callbacks can be called from interrupt context, so they must
80	be fast.
81
82In order to queue events to video device, drivers should call:
83
84	:c:func:`v4l2_event_queue <v4l2_event_queue>`
85	(:c:type:`vdev <video_device>`, :c:type:`ev <v4l2_event>`)
86
87The driver's only responsibility is to fill in the type and the data fields.
88The other fields will be filled in by V4L2.
89
90Event subscription
91~~~~~~~~~~~~~~~~~~
92
93Subscribing to an event is via:
94
95	:c:func:`v4l2_event_subscribe <v4l2_event_subscribe>`
96	(:c:type:`fh <v4l2_fh>`, :c:type:`sub <v4l2_event_subscription>` ,
97	elems, :c:type:`ops <v4l2_subscribed_event_ops>`)
98
99
100This function is used to implement :c:type:`video_device`->
101:c:type:`ioctl_ops <v4l2_ioctl_ops>`-> ``vidioc_subscribe_event``,
102but the driver must check first if the driver is able to produce events
103with specified event id, and then should call
104:c:func:`v4l2_event_subscribe` to subscribe the event.
105
106The elems argument is the size of the event queue for this event. If it is 0,
107then the framework will fill in a default value (this depends on the event
108type).
109
110The ops argument allows the driver to specify a number of callbacks:
111
112.. tabularcolumns:: |p{1.5cm}|p{16.0cm}|
113
114======== ==============================================================
115Callback Description
116======== ==============================================================
117add      called when a new listener gets added (subscribing to the same
118	 event twice will only cause this callback to get called once)
119del      called when a listener stops listening
120replace  replace event 'old' with event 'new'.
121merge    merge event 'old' into event 'new'.
122======== ==============================================================
123
124All 4 callbacks are optional, if you don't want to specify any callbacks
125the ops argument itself maybe ``NULL``.
126
127Unsubscribing an event
128~~~~~~~~~~~~~~~~~~~~~~
129
130Unsubscribing to an event is via:
131
132	:c:func:`v4l2_event_unsubscribe <v4l2_event_unsubscribe>`
133	(:c:type:`fh <v4l2_fh>`, :c:type:`sub <v4l2_event_subscription>`)
134
135This function is used to implement :c:type:`video_device`->
136:c:type:`ioctl_ops <v4l2_ioctl_ops>`-> ``vidioc_unsubscribe_event``.
137A driver may call :c:func:`v4l2_event_unsubscribe` directly unless it
138wants to be involved in unsubscription process.
139
140The special type ``V4L2_EVENT_ALL`` may be used to unsubscribe all events. The
141drivers may want to handle this in a special way.
142
143Check if there's a pending event
144~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
145
146Checking if there's a pending event is via:
147
148	:c:func:`v4l2_event_pending <v4l2_event_pending>`
149	(:c:type:`fh <v4l2_fh>`)
150
151
152This function returns the number of pending events. Useful when implementing
153poll.
154
155How events work
156~~~~~~~~~~~~~~~
157
158Events are delivered to user space through the poll system call. The driver
159can use :c:type:`v4l2_fh`->wait (a wait_queue_head_t) as the argument for
160``poll_wait()``.
161
162There are standard and private events. New standard events must use the
163smallest available event type. The drivers must allocate their events from
164their own class starting from class base. Class base is
165``V4L2_EVENT_PRIVATE_START`` + n * 1000 where n is the lowest available number.
166The first event type in the class is reserved for future use, so the first
167available event type is 'class base + 1'.
168
169An example on how the V4L2 events may be used can be found in the OMAP
1703 ISP driver (``drivers/media/platform/omap3isp``).
171
172A subdev can directly send an event to the :c:type:`v4l2_device` notify
173function with ``V4L2_DEVICE_NOTIFY_EVENT``. This allows the bridge to map
174the subdev that sends the event to the video node(s) associated with the
175subdev that need to be informed about such an event.
176
177V4L2 event functions and data structures
178^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
179
180.. kernel-doc:: include/media/v4l2-event.h
181
182