Re: [Xenomai-core] Realtime gettimeofday()

[Jan Kiszka <jan.kiszka@domain.hid>]

Gilles Chanteperdrix wrote:
> Wolfgang Mauerer wrote:
>> Hi,
>>
>> we'd like to implement a gettimeofday() mechanism that works
>> in realtime context, both from user- and kernelland. Most importantly,
>> the correctins made to the wall time by the NTP protcol on Linux
>> must be transferred into the Xenomai domain.
> 
> Yes, the real issue is NTP, because other than that, you can simply
> implement gettimeofday in terms of clock_gettime(CLOCK_REALTIME).

Not truly as Linux processes that tune the host clock do not affect the
Xenomai world.

> 
> This issue has been discussed several times, but never a lot. We have a
> simple solution: starting with 2.4 (if I remember correctly) xenomai
> provides clock_settime, so you can simply rely on clock_gettime, and
> call clock_settime from time to time to resync the Xenomai clock with
> Linux NTP-enhanced clock. This is not pretty, you get a drift increasing
> over time, then suddenly been reset. But I guess this has been enough
> for current users, until now. You control the maximum drift by how often
> you call clock_settime.
> 
> Would not it be enough for your use of xenomai?

Nope for the reasons you mentioned.

Moreover, reading the Xenomai real-time clock requires a syscall,
gettimeofday can work without that penalty (given non-broken TSC).

Next issue is that the different clock bases of
clock_gettime(CLOCK_REALTIME) and (real) gettimeofday + the switch-back
or even lock-up side-effects of the latter have been quite a PITA in our
project.

And finally, the vision is to have NTP-accurate (or whatever the sync
source is, eg. IEEE1588) absolute timing also for the nucleus one day.
Having an RT-safe way to obtain the NTP parameters from any context is
the first step.

Jan

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