linux-kernel - Re: [PATCH 1/1] LinuxPPS core support.

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Message-ID: <49A1BC28.9020402@zytor.com>
Date:	Sun, 22 Feb 2009 12:57:12 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Rodolfo Giometti <giometti@...ux.it>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Dave Jones <davej@...hat.com>, Sam Ravnborg <sam@...nborg.org>,
	Greg KH <greg@...ah.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Kay Sievers <kay.sievers@...y.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [PATCH 1/1] LinuxPPS core support.

Ingo Molnar wrote:
>>
>> Common use is the combination of the NTPD as userland program 
>> with a GPS receiver as PPS source to obtain a wallclock-time 
>> with sub-millisecond synchronisation to UTC.
> 
> Hm. I was looking at this stuff with the prospect of adding it 
> to the timer tree, but i'm really struggling with a few 
> fundamental questions.
> 
> The most basic one is: why do we need this?
> 
> The main purpose of your current patchset seems to be to deliver 
> interrupt timestamps to user-space, where it will in essence be 
> used to feed new adjtimex adjustments via ntpd.
> 
> I.e. the whole thing comes around in a circle in the end, but 
> via user-space, where jitter will only increase.
> 
> Why not cut out the jittery middle man and add some intelligent 
> API to register PPS interrupt sources straight with the NTP 
> code, and let those IRQ timestamps be fed _directly_ into our 
> time adjustment code?
> 

Well, let's be fair here... the kernel-user space time model involving
ntpd has been very carefully developed over a period of over a decade.
It's known to work.  The userspace involvement isn't just about feeding
the data to the local clock, but also -- or perhaps primarily so -- to
keep the timing inside ntpd calibrated, as that is the time that will be
provided to the outside world.

There is a real benefit to using the model designed for and expected by
ntpd, too.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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