Re: [PATCH v6 0/8] ptp: IEEE 1588 hardware clock support

There is no other scalable time source available for fast timer access
than the time stamp counter in the cpu. Other time source require
memory accesses which is inherently slower.

On what hardware ?

An accurate other time source is used to adjust this clock. NTP does that
via the clock interfaces from user space which has its problems with
accuracy. PTP can provide the network synced time access
that would a more accurate calibration of the time.

Accuracy does not require speed of access. Accuracy requires predictible
latency of access.

Userland code would introduce latencies that would make sub microsecond
time sync very difficult.

You can take a multiple micro-second I/O stall or SMI trap on a PC so you
already lost the battle on the platform you seem to be discussing.

You dont need the user APIs if you directly use the PTP time source to
steer the system clock. In fact I think you have to do it in kernel space
since user space latencies will degrade accuracy otherwise.

PTP is not a 'time source' it is one or more source of time. The
distinction is rather important.

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. And the kernel is getting more and more
real time support all the time.