[ntp:questions] serialPPS+gpsd+ntpd large offset & jitter

Mike S mikes at flatsurface.com
Fri Jan 14 23:34:02 UTC 2011


At 05:51 PM 1/14/2011, Evandro Menezes wrote...
>On Jan 14, 3:14 pm, Evandro Menezes <evan... at mailinator.com> wrote:
> > On Jan 13, 7:38 pm, mi... at flatsurface.com (Mike S) wrote:
> > > I was able to force the kernel to use the 8259 by adding
> > > "clocksource=acpi_pm"  to the boot options. The available clock 
> sources
> > > can be found with "cat
> > > 
> /sys/devices/system/clocksource/clocksource0/available_clocksource".
> >
> > FWIW, the 8259 is the PIT, not the ACPI timer.
>
>Ahem, the 8254 is the PIT, not the ACPI timer.

Yep, and if I'm not mistaken, the ACPI timer interrupt is routed 
through the APIC, which is basically a fancy 8259 PIC. Of course, there 
is no 8254 or 8259 or APIC on any modern motherboard, just subsets of 
devices (e.g. south bridge chip) which provide equivalent 
functionality. One might also say that the PIT is not an 8254, and the 
PIC is not an 8259, etc. and complain about that, too.

In any case, pedanticism aside, the whole point was to avoid using the 
HPET (which - trying to satisfy the pedants again - is not a thing, but 
a function), because it gets set up inconsistently. acpi_pm worked for 
me. The choices I had available were tsc, hpet, and acpi_pm (pit was 
not a choice). I didn't even bother trying tsc, since I understand 
there's a strong likelihood of issues due to processor frequency 
scaling.




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