GPS rollover in HP Driver
Eric S. Raymond
esr at thyrsus.com
Tue Aug 22 22:01:46 UTC 2017
Hal Murray via devel <devel at ntpsec.org>:
> I've got a box sending bogus time, but the system seems to be running happily.
> I assume there is some GPS rollover fixup code someplace, but I can't find
> it. Anybody know where it is?
> There are two likely places. On is in the HP driver itself,
> The other is in ntpd/ntp_refclock.c where it might fixup many drivers
> I can't find "week", ""roll", or 1024 in either file.
I am certain the generic refclock code is not doing anything explicit
to compensate for GPS rollover. For one, I just re-skimmed it to check.
For another, if that were possible there wouldn't be ad-hoc rollover
code scattered through multiple drivers.
Looking at the hpgps driver...I don't see any rollover handling there, either.
The only other possibility I can see is that the core sync algorithms simply
ignore any date that isn't plausibly within a certain delta of the system
clock date, using only its low-order bits and assuming the the ae is correct.
This should actually be a decent heuristic except near midnight.
But I'm not sure how to check whether this is happening.
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