I'm trying to understand the use of hardware timestamps in Chrony and I feel like I'm missing something. I understand how user space timestamps and kernel software timestamps will have errors relative to the time the packet leaves the NIC due to thread scheduling, interrupt coalescing, and whatnot, and why it would be desirable to avoid those errors. But it seems to me that when you hardware timestamp a packet, that timestamp will reflect the state of the NIC's clock, not the state of the system clock, which would seem to be a problem if your goal is to synchronize the system clock.
With PTP, you control the relationship explicitly, with ptp4l syncing the NIC clock to the PTP master clock, and phc2sys syncing the system clock to the NIC clock.
I haven't found any mention of something similar in the Chrony docs, so I'm wondering what if anything Chrony does to NIC clocks.
Thanks,
Scott