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

> * 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

> * 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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