Re: [Linuxptp-users] PTP master slewing to other network time

[Dave C. <dav...@un...>]

On Wed, Aug 21, 2013 at 1:03 PM, Miroslav Lichvar <mli...@re...>wrote:

> On Wed, Aug 21, 2013 at 12:46:01PM +0100, Dave Craig wrote:
> > On Wed, Aug 21, 2013 at 12:29 PM, Miroslav Lichvar <mli...@re...
> >wrote:
> > > You can check if the local PTP port is in the master state with
> > > pmc -u -b 0 'GET PORT_DATA_SET'.
> >
> > Although I can get the master state and then choose to set the system
> clock
> > based on that, it means that I'd be writing very similar code to the
> servo
> > code of PTP inside my NTP process. It currently just calls settimeofday,
> > but I'd rather it slewed to the new time more gracefully. I wondered if
> > having that work done inside ptp4l would be tidier? In the master state
> the
> > clock servo would be accepting changes over pmc and in the slave state
> > ignoring it.
>
> You could use the adjtime system call instead of settimeofday to
> adjust the clock slowly and not in one step.
>
That's probably the approach I'll take. I was concerned about a race
between calling pmc to find out if the local device was the master and
actually setting the time, but it's by far the simplest solution and the
chances of a race are small and it's effects fairly minor.


> I guess there could be a private pmc command to feed the servo
> manually, but it would have to be called periodically in the sync
> interval as the servo can't slew the clock by a specified offset with
> just one point. It's a feedback loop.
>
The NTP process gets the time fairly regularly so this wouldn't be a
problem.


> Why not use ntpdate or a regular NTP daemon instead?
>
Unfortunate legacy reasons :-(

Thanks again for your advice,

Dave