Re: [chrony-users] PPS time

[scs@xxxxxxxxxx (Steve Summit)]

Bill Unruh wrote:
> On Wed, 17 Jul 2019, Steve Summit wrote:
> > ...chrony does support synchronization [...] using PPS only,
> > which is precisely what I need to do.
>
> Be careful. PPS cannot give you the second number...

Of course.

> ...you must make sure that the system clock is no more than 1/2
> a second out... How you accomplish that is up to you-- nmea from
> the gps, ntp from external clocks...

Taken care of, thanks.

> 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.

Miroslav Lichvar made a similar comment.  It must have something
to do with chrony keeping it open.  I'll pursue that.

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

Well, I'm not entirely sure, and in the end it may not be
anything.  But (at the moment) pps0 is being created by gpsd.
And at various times, I've noticed that gpsd had restarted or
wasn't running (meaning that I had to restart it).  So gpsd
restart is one of the "use cases" I've been worried about, and
if a gpsd restart causes me to lose pps0, I can't accept that.

Since a process has to run continuously to keep a fd open to
keep the PPS ldisc attached to it, and since gpsd has to run
continuously and knows how to do the ldisc attachment, at first
I didn't want to set up a second process (e.g. ldattach) just
to do that, but that's probably what I'll end up doing.

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