Re: [Linuxptp-users] Does PTP require NTP?

[Richard C. <ric...@gm...>]

On Fri, Jun 12, 2015 at 05:18:57PM -0500, Robb wrote:
> I have two freshly installed machines that I am trying to sync the
> CLOCK_REALTIME between the two.  Do I need to have them get close in
> time using ntp before I can use PTP?

No.

> How far apart can the clocks be and still have PTP work?

You can correct any offset by letting ptp4l reset the clock (which is
the default setting).

> They currently have a difference of 36 seconds but ptp4l and phc2sys
> are both telling me <5uS.

The 36 second offset [1] is the result of a configuration error.  Your
system time is UTC but PTP uses the TAI time scale.  See the section
titled "TIME SCALE USAGE" in the phc2sys man page.

> Machine1 (master):
> sudo ptp4l -i eth1 -m -2 -q
> sudo phc2sys -s CLOCK_REALTIME -c eth1 -O 0 -u 60
> 
> Machine2:
> sudo ptp4l -i eth0 -m -2 -q -s
> sudo phc2sys -s eth0 -c CLOCK_REALTIME -w -u 60

If your network is not connected to the outside world WRT time, then
the TAI offset is not so important.  Just leave off the the "-O 0" on
the master node, since that is wrong in any case.

Thanks,
Richard

1. Actually I would expect 35 and not 36.