[pkg-ntp-maintainers] Bug#691672:

[Douglas Calvert]

>> Do you know if the ppsapi has been added to the standard kernel, and that
>> it's enabled in Debian by default?

> PPS cannot be properly used with the default Debian kernels because they do
> not enable CONFIG_NTP_PPS. But this requires enabling CONFIG_NO_HZ_COMMON
> and CONFIG_RCU_FAST_NO_HZ, which is why CONFIG_NTP_PPS probably won't be a
> default.

> However, the NTP package should still build-depend on pps-tools. Then that's
> just one fewer step to making a PPS-enabled stratum 1 NTP server on Debian,
> and there's no harm if you're not running PPS (you're already building
> support for refclocks in the Debian NTP package, so it doesn't make sense to
> leave out PPS support). It's pretty easy to build a custom kernel nowadays
> with "make deb-pkg" in the standard kernel source.


Kenyon is correct that the default debian kernels do not enable
CONFIG_NTP_PPS, aka pps kernel discipline. *However,* he is incorrect that
without this option a default debian kernel+ntpd+pps-tools would not
function as a stratum 1 server using the PPS pulse.

ntpd provides two options for pps discipline, kernel and ntp. In fact ntpd
defaults to using ntp pps discipline over kernel discipline. The
choice of which pps  discipline to use is governed by flag3 in the
NMEA and the PPS ref drivers:


flag3 0 | 1

    Controls the kernel PPS discipline: 0 for disable (default), 1 for
    enable. Not used under Windows - if the special serialpps.sys
    serial port driver is used then kernel PPS will be available and used.

    See: https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html
         https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver22.html


I have never seen a clear answer regarding wether to use kernel
discipline over ntpd disciplne in linux. The freebsd kernel discpline is
certainly superior to the ntpd discipline, but freebsd means phk so that is
not surprising. Even if things are better with kernel pps discipline, ntp
pps discipline is a lot better than no pps at all.

Last night I rebuilt the latest ntpd in debian unstable from source with
pps-tools installed. After restarting ntpd the rebuilt ntpd was able to use
the pps signal from my sure gps.

As it is I see no reason not to add pps-tools to the build-deps for ntpd.

This could also help to close:

#790973 ntp: does not do PPS sync
#241548 Request PPS refclock support (Oncore)