Yo Paul! On Mon, 15 Dec 2025 19:00:31 +0000 Paul Tretiakov <[email protected]> wrote:
> We are using Linux kernel API that implements RFC2783 for PPS edge
> detection, but the GPSd implementation has an issue for our use case:
> it uses timestamps from realtime clock provided by the kernel,
Uh. No. gpsd drives NTP to drive the kernel time. Not the other
way around.
> this
> means that if the clock is step back into the past, GPSd loses a
> single PPS, which is critical for our system.
If missing one PPS is a problem, then you are doing things wrong. It
is very host dependent, but only using every 10th, or every 64th PPS
greatly improves performance.
> Why doesn't GPSd use
> sequence numbers to detect edge changes
Because they are often wrong, or unvailable. Many systems do not
support RFC2783.
> this means that if the clock is step back into the past, GPSd loses
> a single PPS,
Entirely intentional. Ain't broke, don't fix it. gpsd only passes on
PPS that it is 100% certain that it knows the proper timestamp for.
When the clock, from the NMEA, jumps araound, we do NOT want anything to
do with it.
Much better a dropped clock than a wrong clock. PPS is to trim your
PLLs, not as a primmary time source.
> like we suggest in the
> following diff:
That would work in some cases, but be fatal in many others.
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RGDS
GARY
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