linux-kernel - Re: Linux 2.6.29-rc6

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Date:	Sun, 15 Mar 2009 16:44:28 +0100
From:	Jesper Krogh <jesper@...gh.cc>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	john stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>
Subject: Re: Linux 2.6.29-rc6

Linus Torvalds wrote:
> 
> Jesper, here's a patch that actually tries to take teh TSC error really 
> into account, and which I suspect will result (on your machine) in failing 
> the fast PIT calibration. 
> 
> It also has a few extra printk's for debugging, and to see just what the 
> values are on your machine.
> 
> The idea behind the patch is to just keep track of how big the difference 
> was in TSC values between two successive reads of the PIT timer. We only 
> really care about the difference when the MSB turns around, and we only 
> really care about the two end points. The maximum error in TSC estimation 
> will simply be the sum of the differences at those points (d1 and d2).
> 
> We can then compare the maximum error with the actual TSC differences 
> between those points, and see if the max error is within 500 ppm. That 
> _should_ mean that it all works - assuming that the PIT itself is running 
> at the correct frequency, of course!
> 
> Regardless of whether is succeeds or not, it will print out some debug 
> messages, which will be interesting to see.


[    0.000000] Fast TSC delta=34227730, error=6223+6219=12442
[    0.000000] Fast TSC calibration using PIT
[    0.000000] Detected 2312.045 MHz processor.

Using "ntpq -c peers" .. the offset steadily grows as time goes.

Full dmesg: http://krogh.cc/~jesper/dmesg-linux-2.6.29-rc8-linus1.txt

jk@...d11:~$ ntpdc -c kerninfo
pll offset:           0.085167 s
pll frequency:        -18.722 ppm
maximum error:        0.137231 s
estimated error:      0.008823 s
status:               0001  pll
pll time constant:    6
precision:            1e-06 s
frequency tolerance:  500 ppm



-- 
Jesper


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