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V4L2 events
-----------
The V4L2 events provide a generic way to pass events to user space.
The driver must use v4l2_fh to be able to support V4L2 events.
Events are defined by a type and an optional ID. The ID may refer to a V4L2
object such as a control ID. If unused, then the ID is 0.
When the user subscribes to an event the driver will allocate a number of
kevent structs for that event. So every (type, ID) event tuple will have
its own set of kevent structs. 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 number of kevents that were
reserved, then the oldest event will be dropped and the new one added.
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 only one kevent struct
allocated. The merge() callback allows you to merge the oldest event payload
into that of the second-oldest event payload. It is called when there are two
or more kevent structs allocated.
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.
Useful functions:
.. code-block:: none
void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev)
Queue events to video device. The driver's only responsibility is to fill
in the type and the data fields. The other fields will be filled in by
V4L2.
.. code-block:: none
int v4l2_event_subscribe(struct v4l2_fh *fh,
struct v4l2_event_subscription *sub, unsigned elems,
const struct v4l2_subscribed_event_ops *ops)
The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
is able to produce events with specified event id. Then it calls
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:
* 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.
.. code-block:: none
int v4l2_event_unsubscribe(struct v4l2_fh *fh,
struct v4l2_event_subscription *sub)
vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
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.
.. code-block:: none
int v4l2_event_pending(struct v4l2_fh *fh)
Returns the number of pending events. Useful when implementing poll.
Events are delivered to user space through the poll system call. The driver
can use 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 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 kAPI
^^^^^^^^^^^^^^^
.. kernel-doc:: include/media/v4l2-event.h
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