V4L2 events

.. SPDX-License-Identifier: GPL-2.0

The V4L2 events provide a generic way to pass events to user space. The driver must use :c:type:v4l2_fh to be able to support V4L2 events.

Events are subscribed per-filehandle. An event specification consists of a type and is optionally associated with an object identified through the id field. If unused, then the id is 0. So an event is uniquely identified by the (type, id) tuple.

The :c:type:v4l2_fh struct has a list of subscribed events on its subscribed field.

When the user subscribes to an event, a :c:type:v4l2_subscribed_event struct is added to :c:type:v4l2_fh\ .subscribed, one for every subscribed event.

Each :c:type:v4l2_subscribed_event struct ends with a :c:type:v4l2_kevent ringbuffer, with the size given by the caller of :c:func:v4l2_event_subscribe. This ringbuffer is used to store any events raised by the driver.

So every (type, ID) event tuple will have its own :c:type:v4l2_kevent ringbuffer. This guarantees that if a driver is generating lots of events of one type in a short time, then that will not overwrite events of another type.

But if you get more events of one type than the size of the :c:type:v4l2_kevent ringbuffer, then the oldest event will be dropped and the new one added.

The :c:type:v4l2_kevent struct links into the available list of the :c:type:v4l2_fh struct so :ref:VIDIOC_DQEVENT will know which event to dequeue first.

Finally, if the event subscription is associated with a particular object such as a V4L2 control, then that object needs to know about that as well so that an event can be raised by that object. So the node field can be used to link the :c:type:v4l2_subscribed_event struct into a list of such objects.

So to summarize:

struct v4l2_fh has two lists: one of the subscribed events, and one of the available events.
struct v4l2_subscribed_event has a ringbuffer of raised (pending) events of that particular type.
If struct v4l2_subscribed_event is associated with a specific object, then that object will have an internal list of struct v4l2_subscribed_event so it knows who subscribed an event to that object.

Furthermore, the internal struct v4l2_subscribed_event has merge() and replace() callbacks which drivers can set. These callbacks are called when a new event is raised and there is no more room.

The replace() callback allows you to replace the payload of the old event with that of the new event, merging any relevant data from the old payload into the new payload that replaces it. It is called when this event type has a ringbuffer with size is one, i.e. only one event can be stored in the ringbuffer.

The merge() callback allows you to merge the oldest event payload into that of the second-oldest event payload. It is called when the ringbuffer has size is greater than one.

This way no status information is lost, just the intermediate steps leading up to that state.

A good example of these replace/merge callbacks is in v4l2-event.c: ctrls_replace() and ctrls_merge() callbacks for the control event.

.. note:: these callbacks can be called from interrupt context, so they must be fast.

In order to queue events to video device, drivers should call:

:c:func:`v4l2_event_queue <v4l2_event_queue>`
(:c:type:`vdev <video_device>`, :c:type:`ev <v4l2_event>`)

The driver's only responsibility is to fill in the type and the data fields. The other fields will be filled in by V4L2.

Event subscription


Subscribing to an event is via:

	:c:func:`v4l2_event_subscribe <v4l2_event_subscribe>`
	(:c:type:`fh <v4l2_fh>`, :c:type:`sub <v4l2_event_subscription>` ,
	elems, :c:type:`ops <v4l2_subscribed_event_ops>`)


This function is used to implement :c:type:`video_device`->
:c:type:`ioctl_ops <v4l2_ioctl_ops>`-> ``vidioc_subscribe_event``,
but the driver must check first if the driver is able to produce events
with specified event id, and then should call
:c:func:`v4l2_event_subscribe` to subscribe the event.

The elems argument is the size of the event queue for this event. If it is 0,
then the framework will fill in a default value (this depends on the event
type).

The ops argument allows the driver to specify a number of callbacks:

.. tabularcolumns:: |p{1.5cm}|p{16.0cm}|

======== ==============================================================
Callback Description
======== ==============================================================
add      called when a new listener gets added (subscribing to the same
	 event twice will only cause this callback to get called once)
del      called when a listener stops listening
replace  replace event 'old' with event 'new'.
merge    merge event 'old' into event 'new'.
======== ==============================================================

All 4 callbacks are optional, if you don't want to specify any callbacks
the ops argument itself maybe ``NULL``.

Unsubscribing an event

Unsubscribing to an event is via:

:c:func:`v4l2_event_unsubscribe <v4l2_event_unsubscribe>`
(:c:type:`fh <v4l2_fh>`, :c:type:`sub <v4l2_event_subscription>`)

This function is used to implement :c:type:video_device-> :c:type:ioctl_ops <v4l2_ioctl_ops>-> vidioc_unsubscribe_event. A driver may call :c:func:v4l2_event_unsubscribe directly unless it wants to be involved in unsubscription process.

The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The drivers may want to handle this in a special way.

Check if there's a pending event


Checking if there's a pending event is via:

	:c:func:`v4l2_event_pending <v4l2_event_pending>`
	(:c:type:`fh <v4l2_fh>`)


This function returns the number of pending events. Useful when implementing
poll.

How events work
~~~~~~~~~~~~~~~

Events are delivered to user space through the poll system call. The driver
can use :c:type:`v4l2_fh`->wait (a wait_queue_head_t) as the argument for
``poll_wait()``.

There are standard and private events. New standard events must use the
smallest available event type. The drivers must allocate their events from
their own class starting from class base. Class base is
``V4L2_EVENT_PRIVATE_START`` + n * 1000 where n is the lowest available number.
The first event type in the class is reserved for future use, so the first
available event type is 'class base + 1'.

An example on how the V4L2 events may be used can be found in the OMAP
3 ISP driver (``drivers/media/platform/ti/omap3isp``).

A subdev can directly send an event to the :c:type:`v4l2_device` notify
function with ``V4L2_DEVICE_NOTIFY_EVENT``. This allows the bridge to map
the subdev that sends the event to the video node(s) associated with the
subdev that need to be informed about such an event.

V4L2 event functions and data structures
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. kernel-doc:: include/media/v4l2-event.h