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Diffstat (limited to 'Documentation/media/kapi/v4l2-event.rst')
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diff --git a/Documentation/media/kapi/v4l2-event.rst b/Documentation/media/kapi/v4l2-event.rst index b54bd915fff6..d40aa60b8bae 100644 --- a/Documentation/media/kapi/v4l2-event.rst +++ b/Documentation/media/kapi/v4l2-event.rst @@ -1,3 +1,110 @@ + +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 ^^^^^^^^^^^^^^^ |