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

[David Taylor]

On 13/12/2021 04:17, Adam Space wrote:
>> What do you mean by "kernel mode"?
> I am referring to this guide
> <http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html>  that another user here
> recommended to me a bit ago. I am not too sure myself since I am relatively
> new to this.

That document is now historic background.

It refers to the early "kernel mode" when (AIUI) part of the OS rather than NTP 
did the fine clock adjustment.  I recall that early Raspberry PI OS kernels did 
not include all the functions required for the most accurate timekeeping, and 
that a kernel recompile was required.

Fortunately that is no longer required, and recent OS versions can directly 
accept PPS pulses, although they no longer include NTP!

You may find my current version less confusing:

   https://www.satsignal.eu/ntp/Raspberry-Pi-quickstart.html

although the confusion is now within the different RPi models, mapping the 
different physical serial ports to the virtual ports, and choosing the ports 
which don't have a variable clock speed!

In Windows we do need to distinguish user/kernel modes, because we used to have 
kernel mode as a modification of the device driver but it's no longer 
accessible, and a workaround runs in user space, hence user mode.

Cheers,
David
-- 
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