PPS documentation

[Gary E. Miller]

Yo Fred!

On Sun, 26 Feb 2017 12:42:37 -0800 (PST)
Fred Wright <fw at fwright.net> wrote:

> 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.
> ---------------------------------------------------------------------

Oh, that interface.  NTP does not use that interface at all.  If you are
going to connect your PPS directly to the kernel then you do not use ntpd
at all.

Image if the kernel and ntpd where both trying to steer the clock with 
slightly different input data and control loops...

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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