On Thu, Jul 19, 2012 at 04:50:51PM +0100, Ben Hutchings wrote:

On Thu, 2012-07-19 at 16:29 +0100, Stuart Hodgson wrote:

quoted

On 19/07/12 15:25, Richard Cochran wrote:

[...]

quoted

quoted

I am trying to purge the whole SYS thing (only blackfin is left)
because there is a much better way to go about this, namely
synchronizing the system time to the PHC time via an internal PPS
signal.

This may be possible in future. But leads us to another problem
where the PPS event that is generated by the PHC subsystem to the 
PPS subsystem is stamped with the current system_time. That may
be fine for a PPS signal generated from an interrupt but not when
the internal PPS event has implicit jitter from the handler/event_queue
that we have in the driver.

[...]

We can certainly take a timestamp in the hard interrupt handler; in fact
that's what I originally expected we would do since we have a separate
MSI-X vector for PTP.  But even hard interrupt handling can be subject
to substantial jitter.

What kind of jitter do you see or expect?

I did a study of synching system to PHC on a PowerPC system, where the
PPS timestamps varied from about 10 usec (on average under light load)
to over 30 usec (under heavy load).

Even so, it was easy to synchronize the system clock to within about a
microsecond under light load, with heavy load producing about an
additional 6 usec error.

Thanks,
Richard