Just one slight trapdoor I triggered in my travails. I don't know whether PTP does PLL slaving but while NTP is also for wall time, it has an effect on clock_gettime(CLOCK_MONOTONIC) so if you need a local clock without external (network, I mean) influence, CLOCK_MONOTONIC_RAW is there for you. And luckily, on recent kernel it has been vDSO'd.