linux-kernel - Re: Linux 2.6.29-rc6

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Date:	Thu, 26 Feb 2009 14:24:25 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	john stultz <johnstul@...ibm.com>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>
Subject: Re: Linux 2.6.29-rc6

On Thu, 26 Feb 2009, Thomas Gleixner wrote:
> 
> Is the delta anything NTP might get upset about:
> 
> 2.6.26: time.c: Detected 2311.847 MHz processor.
> 2.6.29: Detected 2310.029 MHz processor.
> 
> If yes, then we need to fix NTP not the calibration code :)

Well, that _is_ about 500ppm difference, and we claim that we _should_ 
have reached 150ppm with the 15ms delay. We clearly don't seem to have 
done that. I'm not quite sure why - we _should_ be finding the edge of the 
PIT events to within roughly a microsecond (assuming that's about as long 
as an "inb" takes), and that should give us a pretty good fast 
calibration, but maybe I'm overlooking something.

Or - and this may be more likely - there are chipsets that aren't very 
good at reading the PIT in a tight loop. That may explain why it's a 
problem on Jesper's hardware, but we haven't gotten tons of reports of 
this from others.

I see that it's a SunFire X2200, which I think uses an nVidia HT 
southbridge. I assume it's an nForce4 thing. There shouldn't be anything 
odd there, and the PIT read shouldn't be taking any longer than on 
anything else, but who knows? 

		Linus
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