[RFC] Video events, version 2.2

[Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>]

Hi,


Here's the version 2.2 of the video events RFC. It's based on Laurent
Pinchart's original RFC and versions 2 and 2.1 which I wrote. The old 
RFC is available here:

<URL:http://www.spinics.net/lists/linux-media/msg11056.html>

Added Mauro to Cc.

Changes to version 2.1
--------------------

V4L2_EVENT_ALL is now 0 instead 0x07ffffff.

V4L2_EVENT_RESERVED is gone. A note will be added not to use four 
topmost bits.

It's V4L2_EVENT_PRIVATE_START, not V4L2_EVENT_PRIVATE.

Interface description
---------------------

Event type is either a standard event or private event. Standard events
will be defined in videodev2.h. Private event types begin from
V4L2_EVENT_PRIVATE_START. The four topmost bits of the type should not 
be used for the moment.

#define V4L2_EVENT_ALL			0
#define V4L2_EVENT_PRIVATE_START	0x08000000

VIDIOC_DQEVENT is used to get events. count is number of pending events
after the current one. sequence is the event type sequence number and
the data is specific to event type.

The user will get the information that there's an event through
exception file descriptors by using select(2). When an event is
available the poll handler sets POLLPRI which wakes up select. -EINVAL
will be returned if there are no pending events.

VIDIOC_SUBSCRIBE_EVENT and VIDIOC_UNSUBSCRIBE_EVENT are used to
subscribe and unsubscribe from events. The argument is struct
v4l2_event_subscription which now only contains the type field for the
event type. Every event can be subscribed or unsubscribed by one ioctl
by using special type V4L2_EVENT_ALL.


struct v4l2_event {
	__u32		count;
	__u32		type;
	__u32		sequence;
	struct timeval	timestamp;
	__u32		reserved[8];
	__u8		data[64];
};

struct v4l2_event_subscription {
	__u32		type;
	__u32		reserved[8];
};

#define VIDIOC_DQEVENT		_IOR('V', 84, struct v4l2_event)
#define VIDIOC_SUBSCRIBE_EVENT	_IOW('V', 85, struct
				     v4l2_event_subscription)
#define VIDIOC_UNSUBSCRIBE_EVENT _IOW('V', 86, struct
				      v4l2_event_subscription)


The size of the event queue is decided by the driver. Which events will
be discarded on queue overflow depends on the implementation.


Questions
---------

None on my side.

Comments and questions are still very very welcome.

-- 
Sakari Ailus
sakari.ailus@maxwell.research.nokia.com