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Date: Wed, 24 Jun 2009 10:29:15 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: john stultz <johnstul@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>, Miroslav Lichvar <mlichvar@redhat.com>,
       Thomas Gleixner <tglx@linutronix.de>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>,
       Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [GIT pull] ntp updates for 2.6.31
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> On Tue, 2009-06-23 at 15:36 +0200, Ingo Molnar wrote:
> > The PPS patches i've seen just export IRQ timestamps to user-space.

Correct. They improve the sampling information quality and help cut down
on jitter between the IRQ sampling and using the timestamp. The jitter is
what matters most here and NTP can figure out constant latencies rather
well.

> At some point that stops being NTP. NTP has quite a bit of userland
> policy for filtering and managing a number of different network clocks
> (other ntp servers, PPS sources, etc).
> 
> >From what you're describing (direct offset from a hardware time device
> used to steer the clock directly in kernel), you might want to look at
> the STP code in s390 (stp_sync_clock).

And also hardware distributed timing systems like those that distribute a
clock with ethernet signals.

Alan