Hi,
thank you guys for the explanation.
My master is another ptp4l running device. What you're saying would then
mean that ptp4l in master mode sends its time in UTC. Fair enough.
But it doesn't make much sense if it sends UTC time on one side and
interprets it as TAI on the other side, does it?
Do you have maybe some hints how the master needs to be configured?
I don't need an absolute synchronization, I just need that the few
devices are accurately in sync with each other.
Thanks
Petr
On 14/07/17 09:42, Longworth, Gethyn wrote:
Hi,
We had a similar issue where our time source was sending out ARB time rather
than PTP time. The Hardware timestamped node was always out by the TAI / UTC
offset (36 seconds at the moment) from the software timestamped node.
When it sends ARB (there is a flag in the PTP packet), it ends up sending the
UTC time which appears to get interpreted by the hardware timestamping system
as TAI, whereas the software system correctly picks it as UTC. When it sends
PTP time, it has to be TAI, so both interpret it correctly.
Once we managed to get the timesource to output PTP time (a Symmetricom source
that wouldn't send PTP time unless it has a valid GPS lock) the problem went
away.
Cheers,
Gethyn
-----Original Message-----
From: Miroslav Lichvar [mailto:mlich...@redhat.com]
Sent: 14 July 2017 08:05
To: Petr Kulhavy
Cc: linuxptp-users@lists.sourceforge.net
Subject: Re: [Linuxptp-users] UTC offset
On Fri, Jul 14, 2017 at 12:32:58AM +0200, Petr Kulhavy wrote:
I have two devices running linuxptp as slaves with the same
configuration, the only difference is that one uses SW timestamping
and the other HW timestamping.
Both report the same GM in `pmc GET TIME_STATUS_NP`, but they have a
different time, approximately the 36 seconds in difference.
Generally I would expect both slaves to behave the same if they have
the same configuration and use the same GM.
What is wrong? What do I have to do in order to make them both have
the same time as the master?
As you observed, the difference is in the timestamping. With HW timestamping
ptp4l keeps the PTP clock in TAI and it's up to phc2sys to convert it to UTC
when it synchronizes the system clock. With SW timestamping ptp4l synchronizes
the system clock, so it must convert it to UTC.
I guess it would be possible to implement a mode where ptp4l keeps the PTP
clock in UTC. I'm not sure how useful that would be.
--
Miroslav Lichvar
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