Re: [Linuxptp-users] How to use PTP for accurate relative time accuracy (when doing local timestamping) instead of absolute time accuracy?

[Miroslav L. <mli...@re...>]

On Sun, Mar 03, 2019 at 02:10:27AM +0000, C. Devereaux wrote:
> #x PTP2                          0   2   177     5   +98.0s[ +98.0s] +/-   82us
> 
> I believe I am doing several thing wrong - for example, I should set the PHC using the system time instead of doing the opposite, but I have not found how to do that with timemaster. I did some more reading, but I could not find anything close to what I need except maybe https://github.com/not1337/pps-stuff which use a GPS to serve the time by PTP. Based on that, I run instead of systemd phc2sys@eth1 a "sys2phc" script on server3: phc2sys -s CLOCK_REALTIME -c eth1 -O 0 -R 10 -N 2 -E linreg -L 50000000 -n 0 -q -m
> 
> Then server1 has time jumps back and forth on the PTP source, for ex:

I think that means there are two processes (phc2sys and ptp4l) trying
to control the same clock at the same time.

It's not possible to use timemaster to run a PTP master. The PTP clock
will not be synchronized to the system clock. The NTPSHM servo can
work only in one direction. With timemaster and ntpd/chrony it's
always PHC->system clock.

My suggestion would be to dedicate a node in the cluster to work as a
PTP master for the other nodes, configured manually with phc2sys and
ptp4l. The PTP clock on the master node can be synchronized to the
system clock using a weak PI servo, which can be synchronized with
NTP.

-- 
Miroslav Lichvar