Re: [chrony-users] PPS time

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On Wed, 17 Jul 2019, Steve Summit wrote:

John Burton wrote:
Okay, my understanding is that the primary reason for PPS in CHRONY is to
get a more accurate time synchronization from a given source. The PPS
signal is sent out to mark the top of the GPS second in the time message
being sent over the serial port from the GPS...

That's true, and that's the usual case, but I'm trying to do
something a bit different.

We've established, earlier in the thread, that chrony does
support synchronization (or "time maintenance" or "steering" if
you wish) using PPS only, which is precisely what I need to do.

Be careful. PPS cannot give you the second number (ie the seconds since
epoch). It will assume that that the seconds as listed by the internal clock
are correct. Ie, you must make sure that the system clock is no more than 1/2
a second out (or rather that the second number when the pulse arrives is
within about 1/3 of a second of the true time).

How you accomplish that is up to you-- nmea from the gps, ntp from external
clocks, you setting the system clock from your wristwatch,....)

The question now concerns the chrony directive for doing that,

	refclock PPS /dev/pps0

This works quite well, the only drawback being that in practice
the name "/dev/pps0" may not be as stable as it needs to be given
that it's to be baked into /etc/chrony/chrony.conf as a constant.

When the pps? is opened, it must use some name is that is not already open. If
you rerun the serial pps module, if pps0 is in use, then it it cannot be used.
So you might want to make sure that it is closed before trying to reopen it. (How to do that I am not sure, so will not give any comments on that).

And while this is probably mostly a question for the LinuxPPS
folks, I was hoping someone here might have experience with this,
and have some ideas about either making the name "/dev/pps0" more
stable, or somehow making the chrony configuration resilient in
the face of pps0 maybe spontaneously renaming itself to pps1.
(And in fact Olivier Delbeke's reply up above has given me a few
ideas on this.  Thanks, Olivier.)

So what is happening on your system that it is trying to reopen the pps device
more than once?

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