| Re: [chrony-users] PPS time |
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On Sat, Jun 29, 2019 at 10:48:41AM -0400, Steve Summit wrote:
> * Does chrony have anything like ntpd's tos minsane parameter?
> (According to the documentation for ntp's driver22, with
> minsane = 0, "if no sources are available, the system clock
> continues to be disciplined by the PPS driver on an indefinite
> basis".)
If I understand that description correctly, that's the default
behavior of chronyd when a PPS refclock is not "locked" to a non-PPS
refclock.
> * Is there a good way to readjust chrony's time sources on the
> fly, to achieve our desired configuration of "use GPS initially,
> but then after the first synch, don't"?
There is no chronyc command to stop a reference clock (yet?). So you
would need to stop the time source itself (GPS, gpsd, ...).
> * If dynamic reconfiguration is a bad idea, is there any way to
> get chrony to just listen to PPS only, to steer the system
> clock to PPS without worrying about what time it is? (We'd
> be perfectly happy to set the Linux clock once, at power up,
> "by hand", from our code, using date(1) or settimeofday(2).)
>
> * Is there a way to configure chrony to *use the system clock*
> as a source for second-level timing information, using PPS only
> for subsecond steering? (Yes, I know this sounds terribly
> circular, but is it completely insane?)
I think those two questions have the same answer. With the local
directive in the config file, chronyd will synchronize to a PPS
refclock without any other timesource, using the system time as a
reference.
> * Should I be looking into a way to get second-level as well as
> subsecond timing information out of our reference clock and
> into chrony, so that there's never any ambiguity on this score
> at all?
It shouldn't be necessary, if you can make sure chronyd is started
only when the clock is already close to the true time.
> As a point of departure, the refclock sources we're using in
> chrony.conf today (that do not reflect answers to these questions
> and do not really meet our requirements) are:
>
> refclock SHM 0 refid GPS precision 1e-1 filter 1000 noselect
> refclock SHM 1 refid PPS precision 1e-9 trust
If you are planning to use the "PPS" SHM refclock with gpsd, then
those questions need to be asked for gpsd, not chrony.
--
Miroslav Lichvar
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