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V4L2 events
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-----------
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The V4L2 events provide a generic way to pass events to user space.
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The driver must use :c:type:`v4l2_fh` to be able to support V4L2 events.
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Events are subscribed per-filehandle. An event specification consists of a
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``type`` and is optionally associated with an object identified through the
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``id`` field. If unused, then the ``id`` is 0. So an event is uniquely
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identified by the ``(type, id)`` tuple.
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The :c:type:`v4l2_fh` struct has a list of subscribed events on its
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``subscribed`` field.
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When the user subscribes to an event, a :c:type:`v4l2_subscribed_event`
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struct is added to :c:type:`v4l2_fh`\ ``.subscribed``, one for every
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subscribed event.
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Each :c:type:`v4l2_subscribed_event` struct ends with a
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:c:type:`v4l2_kevent` ringbuffer, with the size given by the caller
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of :c:func:`v4l2_event_subscribe`. This ringbuffer is used to store any events
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raised by the driver.
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So every ``(type, ID)`` event tuple will have its own
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:c:type:`v4l2_kevent` ringbuffer. This guarantees that if a driver is
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generating lots of events of one type in a short time, then that will
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not overwrite events of another type.
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But if you get more events of one type than the size of the
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:c:type:`v4l2_kevent` ringbuffer, then the oldest event will be dropped
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and the new one added.
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The :c:type:`v4l2_kevent` struct links into the ``available``
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list of the :c:type:`v4l2_fh` struct so :ref:`VIDIOC_DQEVENT` will
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know which event to dequeue first.
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Finally, if the event subscription is associated with a particular object
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such as a V4L2 control, then that object needs to know about that as well
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so that an event can be raised by that object. So the ``node`` field can
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be used to link the :c:type:`v4l2_subscribed_event` struct into a list of
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such objects.
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So to summarize:
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- struct :c:type:`v4l2_fh` has two lists: one of the ``subscribed`` events,
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and one of the ``available`` events.
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- struct :c:type:`v4l2_subscribed_event` has a ringbuffer of raised
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(pending) events of that particular type.
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- If struct :c:type:`v4l2_subscribed_event` is associated with a specific
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object, then that object will have an internal list of
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struct :c:type:`v4l2_subscribed_event` so it knows who subscribed an
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event to that object.
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Furthermore, the internal struct :c:type:`v4l2_subscribed_event` has
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``merge()`` and ``replace()`` callbacks which drivers can set. These
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callbacks are called when a new event is raised and there is no more room.
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The ``replace()`` callback allows you to replace the payload of the old event
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with that of the new event, merging any relevant data from the old payload
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into the new payload that replaces it. It is called when this event type has
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a ringbuffer with size is one, i.e. only one event can be stored in the
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ringbuffer.
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The ``merge()`` callback allows you to merge the oldest event payload into
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that of the second-oldest event payload. It is called when
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the ringbuffer has size is greater than one.
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This way no status information is lost, just the intermediate steps leading
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up to that state.
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A good example of these ``replace``/``merge`` callbacks is in v4l2-event.c:
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``ctrls_replace()`` and ``ctrls_merge()`` callbacks for the control event.
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.. note::
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these callbacks can be called from interrupt context, so they must
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be fast.
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In order to queue events to video device, drivers should call:
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:c:func:`v4l2_event_queue <v4l2_event_queue>`
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(:c:type:`vdev <video_device>`, :c:type:`ev <v4l2_event>`)
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The driver's only responsibility is to fill in the type and the data fields.
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The other fields will be filled in by V4L2.
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Event subscription
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~~~~~~~~~~~~~~~~~~
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Subscribing to an event is via:
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:c:func:`v4l2_event_subscribe <v4l2_event_subscribe>`
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(:c:type:`fh <v4l2_fh>`, :c:type:`sub <v4l2_event_subscription>` ,
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elems, :c:type:`ops <v4l2_subscribed_event_ops>`)
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This function is used to implement :c:type:`video_device`->
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:c:type:`ioctl_ops <v4l2_ioctl_ops>`-> ``vidioc_subscribe_event``,
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but the driver must check first if the driver is able to produce events
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with specified event id, and then should call
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:c:func:`v4l2_event_subscribe` to subscribe the event.
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The elems argument is the size of the event queue for this event. If it is 0,
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then the framework will fill in a default value (this depends on the event
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type).
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The ops argument allows the driver to specify a number of callbacks:
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.. tabularcolumns:: |p{1.5cm}|p{16.0cm}|
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======== ==============================================================
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Callback Description
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======== ==============================================================
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add called when a new listener gets added (subscribing to the same
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event twice will only cause this callback to get called once)
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del called when a listener stops listening
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replace replace event 'old' with event 'new'.
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merge merge event 'old' into event 'new'.
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======== ==============================================================
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All 4 callbacks are optional, if you don't want to specify any callbacks
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the ops argument itself maybe ``NULL``.
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Unsubscribing an event
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~~~~~~~~~~~~~~~~~~~~~~
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Unsubscribing to an event is via:
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:c:func:`v4l2_event_unsubscribe <v4l2_event_unsubscribe>`
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(:c:type:`fh <v4l2_fh>`, :c:type:`sub <v4l2_event_subscription>`)
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This function is used to implement :c:type:`video_device`->
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:c:type:`ioctl_ops <v4l2_ioctl_ops>`-> ``vidioc_unsubscribe_event``.
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A driver may call :c:func:`v4l2_event_unsubscribe` directly unless it
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wants to be involved in unsubscription process.
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The special type ``V4L2_EVENT_ALL`` may be used to unsubscribe all events. The
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drivers may want to handle this in a special way.
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Check if there's a pending event
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Checking if there's a pending event is via:
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:c:func:`v4l2_event_pending <v4l2_event_pending>`
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(:c:type:`fh <v4l2_fh>`)
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This function returns the number of pending events. Useful when implementing
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poll.
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How events work
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~~~~~~~~~~~~~~~
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Events are delivered to user space through the poll system call. The driver
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can use :c:type:`v4l2_fh`->wait (a wait_queue_head_t) as the argument for
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``poll_wait()``.
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There are standard and private events. New standard events must use the
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smallest available event type. The drivers must allocate their events from
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their own class starting from class base. Class base is
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``V4L2_EVENT_PRIVATE_START`` + n * 1000 where n is the lowest available number.
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The first event type in the class is reserved for future use, so the first
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available event type is 'class base + 1'.
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An example on how the V4L2 events may be used can be found in the OMAP
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3 ISP driver (``drivers/media/platform/omap3isp``).
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A subdev can directly send an event to the :c:type:`v4l2_device` notify
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function with ``V4L2_DEVICE_NOTIFY_EVENT``. This allows the bridge to map
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the subdev that sends the event to the video node(s) associated with the
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subdev that need to be informed about such an event.
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V4L2 event functions and data structures
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. kernel-doc:: include/media/v4l2-event.h
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