[time-nuts] Re: PPS latency? User vs kernel mode

[David Taylor]

On 13/12/2021 21:22, Trent Piepho wrote:
> And finally we can have the kernel act on the PPS timestamps itself.
> But NTP network traffic might be a bridge too far.  Mills describes
> this as a "hardpps()".  This was added to Linux in 2011 by Alexander
> Gordeev, based on Mills' work, but with a different implementation.
> This is a "kernel consumer" of PPS timestamps, which can be used to
> steer the PLL entirely inside kernel code.  It's enabled with the
> kernel config option NTP_PPS.  One can easily see if this is present
> in the kernel via the usual methods.  It is often NOT enabled because
> it doesn't work with tickless kernels.  I do now know if current ntpd
> supports this or what it takes to use it with ntpd.  I have used it,
> but I wrote my own code from scratch to integrate it with a GPS module
> to perform time sync without installing either ntpd nor chrony.

Trent,

Thanks for that!  I was only aware of a limited subset of that as I came later 
to NTP on Linux (it was mostly Windows before), and I understood even less at 
the time!  Your notes helped clarify things, thanks.

David
-- 
SatSignal Software - Quality software for you
Web: https://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk
Twitter: @gm8arv