Re: [Linuxptp-users] clock_nanosleep on /dev/ptpX

[Richard C. <ric...@gm...>]

On Wed, Feb 19, 2014 at 02:58:40PM +0000, Ledda William EXT wrote:
> > I might do it one day, but so far I haven't had a really compelling reason to do so. Probably using the Linux system clock (and phc2sys) will be good enough 
> > most of the time. It would be interesting to find out whether that is true for your own application.
> 
> Richard, 
> Think about this "simple" but very interesting problem. System time is not monotonic (assuming it is in UTC), PTP time yes (assuming it is TAI). In a real time control system you could have the need to make a "wait_until" or to execute some functions in a very well-defined time in spite of any clock adjustment made to recover some UTC leap second event. This could be a valid reason to implement these features on a PHC?

Well, now that we have CLOCK_TAI in Linux, that takes of the leap
second issue.

I agree that it would be nice to have the PHC timers, but considering
the scheduling latency on typical Linux systems (even RT), I do think
using the system CLOCK_REALTIME or CLOCK_TAI will be good enough.

In fact, timers built off of PHC devices which are PCIe cards will
probably have *worse* latency than using system timers. I would expect
that only register based SoC devices (like the gianfar) would bring
any benefit at all.

Thanks,
Richard