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

[Ledda W. E. <Wil...@it...>]

Richard, 

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

Which is the kernel version that include CLOCK_TAI?

Thanks

William


-----Original Message-----
From: Richard Cochran [mailto:ric...@gm...] 
Sent: 19 February 2014 16:08
To: Ledda William EXT
Cc: Koehrer Mathias (ETAS/ESW5); lin...@li...
Subject: Re: [Linuxptp-users] clock_nanosleep on /dev/ptpX

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