linux-kernel - Re: [GIT pull] ntp updates for 2.6.31

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Date:	Tue, 23 Jun 2009 14:41:56 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Miroslav Lichvar <mlichvar@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [GIT pull] ntp updates for 2.6.31

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.
> 
> That is not very robust in my opinion when it comes to do time 
> approximations - to get quick, low-latency action and precise 
> measurements it's best to keep the critical path as short as 
> possible, and within a single source code repository: i.e. within 
> the kernel.
> 
> There's little policy really, other than setting some general 
> parameters. NTPd can still provide the raw _network time_ 
> timestamps, as that is probably best fetched by user-space and fed 
> to the kernel.

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).

thanks
-john

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