> Em Wed, 11 Nov 2009 18:59:09 +0100
> Hans Verkuil <hverkuil@xxxxxxxxx> escreveu:
>
>> On Wednesday 11 November 2009 18:29:52 Sakari Ailus wrote:
>> > Hans Verkuil wrote:
>> > > On Saturday 24 October 2009 23:56:24 Sakari Ailus wrote:
>> > >> Ivan T. Ivanov wrote:
>> > >>> Hi Sakari,
>> > >> Hi,
>> > >>
>> > >>> On Fri, 2009-10-23 at 13:18 +0300, Sakari Ailus wrote:
>> > >> [clip]
>> > >>>> struct v4l2_event {
>> > >>>> __u32 count;
>> > >>>> __u32 type;
>> > >>>> __u32 sequence;
>> > >>>> struct timeval timestamp;
>> > >>> Can we use 'struct timespec' here. This will force actual
>> > >>> implementation to use high-resolution source if possible,
>> > >>> and remove hundreds gettimeofday() in user space, which
>> > >>> should be used for event synchronization, with more
>> > >>> power friendly clock_getres(CLOCK_MONOTONIC).
>> > >> Good point. I originally picked timeval since it was used in
>> > >> v4l2_buffer. The spec tells to use gettimeofday() for system time
>> but
>> > >> clock skewing is causes problems in video encoding.
>> > >> clock_getres(CLOCK_MONOTONIC) is free of clock skewing and thus
>> should
>> > >> be more suitable for this kind of use.
>> > >>
>> > >> I also propose to use timespec instead of timeval.
>> > >>
>> > >
>> > > Hi Sakari,
>> > >
>> > > What is that status of the event API? It is my impression that it is
>> pretty
>> > > much finished. Sakari, can you make a final 2.3 RFC? Then Guru can
>> take over
>> > > and start the implementation.
>> >
>> > Ah.
>> >
>> > One thing that I was still wondering was that are there use cases
>> where
>> > other kind of time stamps might be useful? I guess that when the V4L2
>> > was designed no-one though of the need for time stamps of different
>> > type. So are there use cases where gettimeofday() style stamps would
>> > still be better?
>>
>> If you ever need to relate an event to a specific captured frame, then
>> that
>> might well be useful. But I can't think of an actual use case, though.
>>
>> > In that case we might choose to leave it driver's decision to decide
>> > what kind of timestamps to use and in that case application would just
>> > have to know. The alternative would be to use union and a flag telling
>> > what's in there.
>> >
>>
>> Let's go with timespec. If we need to add an event that has to relate to
>> a specific captured frame then it is always possible to add a struct
>> timeval
>> as part of the event data for that particular event.
>
> I don't agree. It is better to use the same timestamp type used by the
> streaming
> interface. Having two different ways to represent it for the same devices
> is
> confusing, and changing it later doesn't make sense. I foresee some cases
> where
> correlating the two timestamps would be a need.
No, I don't mean that we change it later. The timespec will be used as the
timestamp for all events. But if we need to add an event type that needs
to be correlated with a buffer, then it can add the timeval as part of the
event-specific data. Actually, if you need to relate it to a buffer, then
you might be better off by specifying the buffer index directly.
Normally I would prefer to go with just a single timeval as well, but
gettimeofday() is quite expensive (I've noticed that before), and for all
the events that I can think of right now it is not needed.
Does gettimeofday() also have additional penalties on embedded systems? If
I understand the original comment correctly, then gettimeofday() is less
power-friendly than clock_getres. Why is that? Is that just because
gettimeofday() needs more CPU cycles or are there additional reasons?
Regards,
Hans
>> Regards,
>>
>> Hans
>>
>
> Cheers,
> Mauro
>
--
Hans Verkuil - video4linux developer - sponsored by TANDBERG Telecom
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