PPS documentation

[Fred Wright]

On Sun, 26 Feb 2017, Gary E. Miller wrote:
> On Sun, 26 Feb 2017 14:29:59 +0100
> Achim Gratz <Stromeko at nexgo.de> wrote:
>
> > It should probably be mentioned that hardpps is not available even
> > with the PPS API present on most distributions since it is
> > incompatible with a tickless kernel, something that became standard
> > somewhere in the 3.x line of Linux kernels ca. 2012.
>
> Ah, no.  KPPS works better on kernels with a tick, but not incompatible
> with tickless.  And most tickless kernels distributed by distros have
> the option at boot time to change the tick configuration.  Certainly
> RasPian is that way.
>
> Can you be more specific on a kernel that does not work?

>From drivers/pps/Kconfig:

---------------------------------------------------------------------
config NTP_PPS
	bool "PPS kernel consumer support"
	depends on !NO_HZ_COMMON
	help
	  This option adds support for direct in-kernel time
	  synchronization using an external PPS signal.

	  It doesn't work on tickless systems at the moment.
---------------------------------------------------------------------

There's no reason in principle why it can't work on tickless kernels, but
the current implementation is deficient.

Fred Wright