Hans Verkuil wrote:
[clip]
#define V4L2_EVENT_ALL 0x07ffffff
I suggest using 0 instead of 0x07ffffff. Yes, 0 is still a magic number, but
somehow it feels a lot less magic :-)
Okay.
#define V4L2_EVENT_PRIVATE_START 0x08000000
#define V4L2_EVENT_RESERVED 0x10000000
Rather than calling this RESERVED turn this into a mask:
#define V4L2_EVENT_MASK 0x0fffffff
Ok.
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)
Perhaps we should use just one ioctl and use a flag in the
event_subscription struct to tell whether to subscribe or unsubscribe? Just
brainstorming here.
Having two ioctls would be equivalent to STREAMON and STREAMOFF, that's
why I originally picked that. I can't immediately figure a way it could
be done nicely by using a flag.
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.
Yeah, lets implement this only when needed.
Are there enough reserved fields now?
Personally I think 4 reserved fields for the event_subscription is enough. 8
reserved fields for that seems overkill to me.
struct v4l2_format is IMO a good example of having enough unused fields. ;)
I see that 8 reserved fields might make sense at least for v4l2_event. I
wouldn't mind if we had that many in v4l2_event_subscription as well.
There is already proposed use for three of them:
- flags (e.g. notification / no notification)
- entity
- number of pending events
The two first ones might make sense in v4l2_event_subscription as well.
That would leave just two reserved fields afterwards.
The entity field would fit to v4l2_event_subscription for the same
reasons than to v4l2_event; if there are several entities the event
could be coming from we could limit it to just some. Perhaps a bit
far-fetched but still...
And I wouldn't be surprised if a need appeared to something like
priority as Tomasz suggested. After all that we'd be left with just one
reserved field if we decided to use all 32 bits for priority.
The basic event delivery problem is IMO very well understood but there
are just so many ideas on extensions (many of which sound quite
reasonable) already at this point that I'm slightly worried about the
future if we just have a few reserved fields. Unnecessary bloat still
must be kept away, of course.
How about the event type high
order bits split?
Yes, what's the purpose of that? I don't see a good reason for that.
Me neither. Although even if we don't see use for them now it doesn't
mean there couldn't be any in future. We can always say that the
reserved bits are no more reserved but not the other way around.
I originally though those few bits could be used for flags that now are
part of the structure.
Or we could just drop the reserved bits, I'm not against that.
--
Sakari Ailus
sakari.ai...@maxwell.research.nokia.com
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