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+.. SPDX-License-Identifier: GPL-2.0
+
+V4L2 events
+-----------
+
+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 :c:type:`v4l2_fh` has two lists: one of the ``subscribed`` events,
+  and one of the ``available`` events.
+
+- struct :c:type:`v4l2_subscribed_event` has a ringbuffer of raised
+  (pending) events of that particular type.
+
+- If struct :c:type:`v4l2_subscribed_event` is associated with a specific
+  object, then that object will have an internal list of
+  struct :c:type:`v4l2_subscribed_event` so it knows who subscribed an
+  event to that object.
+
+Furthermore, the internal struct :c:type:`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/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
+