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

[Adam Space]

I have a PPS with Raspberry Pi setup going. Of course, in terms of
precision, it's quite good (for me it's as good as I'd ever care for), and
for accuracy it's quite good too. Although I guess the problem is, I don't
really know how good the accuracy is, nor am I sure how I would go about
finding out.

The only factor here would be a delay between the PPS signal and the
processing in the Pi. I am not using kernel mode right now, which I assume
makes this even worse. Nonetheless, it is certainly no more than one or two
ms. When I run ntp with several other peers, you can't tell much of a
difference. The accuracy limits of NTP are greater than the potential delay
introduced by the processing of the PPS signal, so even running NTP with a
lot of peers and averaging over long periods of time doesn't give much info
on this. (Perhaps if I ran it long enough I could get the mean offset, but
I think the uncertainty on this value would be larger than the value
itself).

I am curious what other folks with this type of set-up have done. Just
leave it at that and accept that there's some unknown latency? Or perhaps
the latency is on the order of microseconds, in which case it's not really
a big deal for me.

With regards to kernel mode, I was looking to give it a try. However, the
guides I've found on this, some posted several years ago on this list, are
pretty outdated. If someone has a suggestion for this, I'm interested in
hearing it. Also, my Raspberry Pi is relatively new, running Bullseye, if
that matters.

Adam