Re: [chrony-users] PPS-only PHC refclock

On Wed, Feb 2, 2022 at 1:31 AM Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:

On Tue, Feb 01, 2022 at 02:37:20PM -0700, Joe Williams wrote:
> In this test set up chronyc sources shows the PHC PPS-only refclock as a
> falseticker and synchronizes fine with the FB time server. Which is why I
> was wondering if it needed to be disciplined in some way to make it useful.

It needs to be synchronized. chronyd cannot use a time source for
frequency only.

It cannot be synchronized to the system clock as that would create a
positive feedback loop.

One way to synchronize it would be to run a separate chronyd instance
using the following patch to control the PHC instead of the system
clock:
https://listengine.tuxfamily.org/chrony.tuxfamily.org/chrony-users/2019/04/msg00009.html

It could be started like this:

chronyd.phc -c /dev/ptp0 \
'pidfile /run/chrony/chronyd.phc.pid' \
'cmdport 11323' 'bindcmdaddress /' \
'hwtimestamp eth0' \
'server time.example.com maxpoll 12 minsamples 64'

This assumes eth0 is the NIC timestamping the NTP packets and it's the
NIC which has the /dev/ptp0 PHC. It's not possible to mix timestamps
from two different HW clocks, or with the system clock. You might need
to change the maxpoll setting and/or change corrtimeratio to get best
results corresponding to the network jitter to the NTP server.

The instance synchronizing the system clock would have just one
refclock:

chronyd 'refclock PHC /dev/ptp0'

--
Miroslav Lichvar

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