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Re: Questions about GPSD as a time source with Chrony

From: Gary E. Miller
Subject: Re: Questions about GPSD as a time source with Chrony
Date: Tue, 5 Apr 2022 11:13:30 -0700

Yo Dominick!

On Mon, 4 Apr 2022 22:27:38 -0400
"Dominick C. Pastore" <> wrote:

> >> refclock SHM 0 refid GPSH
> >> refclock SHM 1 refid PPSH
> >> refclock SOCK /var/run/chrony.gpsd0.sock refid GPSO  
> > 
> > The above 3 a godd.  
> Pardon? (My best guess is that line was supposed to read, "The above
> 3 are good," but I don't want to assume.)

Yes, good.

> >> refclock SOCK /var/run/chrony.pps0.sock refid PPSO  
> > 
> > What said to do that?  chonry.gpsd0.sock is obviously your PPS,
> > note the good 120 ns jitter.  That is what you want.  All done.  
> The GPSD Time Service HOWTO did. Specifically, it said this:
> "To get chronyd to connect to gpsd using the socket method add the
> following lines your chrony.conf file. Except, replace XXXX with the
> basename of your device’s serial port, often ttyS0, ttyACM0, or
> ttyAMA0. Replace YYYY with the basename of your PPS device, usually
> pps0.

Yes, except when you have the MAGIC_HAT option on, as you do.  The
doc needs to be clearer.

> > So you have both SHM and the socket file working.  I don't see any
> > problems.  
> Then I don't have any problems, but it seems the GPSD Time Service
> HOWTO might. It sounds like the suggested configuration for the
> socket method is incorrect, unless I am misunderstanding something.

It is incorrect for your configuration, but correct for most

> >> The GPSD Time Service HOWTO seems to suggest using the socket
> >> interface for Chrony over the shared memory interface.  
> > 
> > Yes.
> >   
> >> Is there some
> >> advantage to the socket interface?  
> > 
> > No.  Just the opposite.  
> If by "just the opposite" you mean there is a disadvantage to the
> socket interface (as opposed to merely no advantage),


> that begs the
> questions: 1) Why is the SHM interface better,

It is more general (works with ntpd and chrinyd).  It avoids the
stupid effects of SELinux, AppArmore, etc.  It avoids file permission

> and 2) should the
> HOWTO reflect that reality?

No, too much details turns off readers.

> I appreciate the responses.

Be sure to pass on your knowledge to the next newbie.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

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