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.
In order to queue events to video device, drivers should call:
The driver’s only responsibility is to fill in the type and the data fields. The other fields will be filled in by V4L2.
Subscribing to an event is via:
This function is used to implement video_device-> 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 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:
| 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 to an event is via:
This function is used to implement video_device-> ioctl_ops-> vidioc_unsubscribe_event. A driver may call 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.
Checking if there’s a pending event is via:
This function 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.
Internal kernel event struct.
Definition
struct v4l2_kevent {
struct list_head list;
struct v4l2_subscribed_event * sev;
struct v4l2_event event;
};
Members
Subscribed event operations.
Definition
struct v4l2_subscribed_event_ops {
int (* add) (struct v4l2_subscribed_event *sev, unsigned int elems);
void (* del) (struct v4l2_subscribed_event *sev);
void (* replace) (struct v4l2_event *old, const struct v4l2_event *new);
void (* merge) (const struct v4l2_event *old, struct v4l2_event *new);
};
Members
Internal struct representing a subscribed event.
Definition
struct v4l2_subscribed_event {
struct list_head list;
u32 type;
u32 id;
u32 flags;
struct v4l2_fh * fh;
struct list_head node;
const struct v4l2_subscribed_event_ops * ops;
unsigned int elems;
unsigned int first;
unsigned int in_use;
struct v4l2_kevent events[];
};
Members
Dequeue events from video device.
Parameters
Queue events to video device.
Parameters
Description
The event will be queued for all struct v4l2_fh file handlers.
Note
The driver’s only responsibility is to fill in the type and the data fields.The other fields will be filled in by V4L2.
Queue events to video device.
Parameters
Description
The event will be queued only for the specified struct v4l2_fh file handler.
Note
The driver’s only responsibility is to fill in the type and the data fields.The other fields will be filled in by V4L2.
Parameters
Description
Returns the number of pending events.
Subscribes to an event
Parameters
Description
Note
if elems is zero, the framework will fill in a default value, with is currently 1 element.
Unsubscribes to an event
Parameters
Parameters
Subdev variant of v4l2_event_unsubscribe()
Parameters
Description
Note
This function should be used for the struct v4l2_subdev_core_ops unsubscribe_event field.
helper function that calls v4l2_event_subscribe() if the event is V4L2_EVENT_SOURCE_CHANGE.
Parameters
Variant of v4l2_event_subscribe(), meant to subscribe only events of the type V4L2_EVENT_SOURCE_CHANGE.
Parameters