Steering the same PHC with phc2sys as chronyd is using for HW timestamping is not the best approach as that creates a feedback loop (instability). It would be better to leave the PHC running free and just compare the sys<->PHC with PHC<->PPS offsets.
> So your 100m->1G link for example will already introduce over 1 us of error (to accuracy!), but NTP will never show you this
That doesn't apply to NTP to such an extent as PTP because it timestamps end of the reception (HW RX timestamps are transposed to the end of the packet), so the asymmetries in transmission lengths due to different link speeds should cancel out (unless the switches are cutting through in the faster->slower link direction, but that seems to be rare).
The author has other posts in the series where he tried to measure the accuracy relative to the PHC (not system clock) using PPS: https://scottstuff.net/posts/2025/06/02/measuring-ntp-accura...
Steering the same PHC with phc2sys as chronyd is using for HW timestamping is not the best approach as that creates a feedback loop (instability). It would be better to leave the PHC running free and just compare the sys<->PHC with PHC<->PPS offsets.
> So your 100m->1G link for example will already introduce over 1 us of error (to accuracy!), but NTP will never show you this
That doesn't apply to NTP to such an extent as PTP because it timestamps end of the reception (HW RX timestamps are transposed to the end of the packet), so the asymmetries in transmission lengths due to different link speeds should cancel out (unless the switches are cutting through in the faster->slower link direction, but that seems to be rare).