On Thu, 23 Sep 2010, Christian Riesch wrote:

quoted

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.

Even if I buy into the catch phrase above: User space is subject to noise
that the in kernel code is not. If you do the tuning over long intervals
then it hopefully averages out but it still causes jitter effects that
affects the degree of accuracy (or sync) that you can reach. And the noise
varies with the load on the system.