Hi,
Here's the version 2.1 of the video events RFC. It's based on Laurent
Pinchart's original RFC and version 2 which I wrote some time ago. This
time the changes are done based on discussion on the list. The old RFC
is available here:
<URL:http://www.spinics.net/lists/linux-media/msg10971.html>
(Cc:d to Mike Krufky and Devin Heitmueller, too.)
Changes to version 2
--------------------
#define V4L2_EVENT_ALL
VIDIOC_G_EVENT -> VIDIOC_DQEVENT
Event enumeration is gone.
Reserved fields moved before data in v4l2_event and now there are 8 of
them instead of 4.
Event (un)subscription argument is now v4l2_event_subscription.
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. Some high order bits will be reserved for future use.
#define V4L2_EVENT_ALL 0x07ffffff
#define V4L2_EVENT_PRIVATE_START 0x08000000
#define V4L2_EVENT_RESERVED 0x10000000
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
---------
One more question I have is that there can be situations that the
application wants to know something has happened but does not want an
explicit notification from that. So it gets an event from VIDIOC_DQEVENT
but does not want to get woken up for that reason. I guess one flag in
event subscription should do that. Perhaps that is something that should
be implemented when needed, though.
Are there enough reserved fields now? How about the event type high
order bits split?
What should we really call v4l2_event_subscription? A better name for
the structure would be perhaps favourable.
Comments and questions are still very very welcome.
--
Sakari Ailus
sakari.ailus@xxxxxxxxxxxxxxxxxxxxxxxxxx
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