Re: [gpsd-dev] Clarifications needed for the time-service HOWTO

[Andy Walls]

On Sat, 2013-10-19 at 21:16 -0700, Hal Murray wrote:
> address@hidden said:
[snip]
> >    Those hotplug devices will, however, may be able to use plain,
> >    non-kernel PPS. gpsd tries to automatically fall back to this when
> >    absence of root permissions makes KPPS unavailable. This fallback is
> >    complicated by the fact that gpsd needs to communicate to ntpd in
> >    a different way in root and non-root mode.  This complicates the
> >    configuration in ways beyond the scope of this document and is strongly
> >    discouraged in practice. 
> 
> I can't quite figure out what that's trying to say.
> 
> What do you mean by "KPPS"?
> 
> RFC-whatever has two parts.  One is to just grab the time stamp when the 
> modem control signal changes, and the API to read it.  The other is to push 
> the whole timekeeping phase locked loop into the kernel.  Linux doesn't 
> implement the second part.

Really?  Upon cursory examination, it looks like it does to me:

http://lxr.free-electrons.com/source/kernel/time/ntp.c#L80
http://lxr.free-electrons.com/source/kernel/time/ntp.c#L243

Although, the median filter is not implemented from what I can see:
http://lxr.free-electrons.com/source/kernel/time/ntp.c#L724

Regards,
Andy