Alan Cox wrote:

quoted

It implies clock tuning in userspace for a potential sub microsecond
accurate clock. The clock accuracy will be limited by user space
latencies and noise. You wont be able to discipline the system clock
accurately.

Noise matters, latency doesn't. 

Well put! That's why we need hardware support for PTP timestamping to 
reduce the noise, but get along well with the clock servo that is 
steering the PHC in user space.

Christian