linux-kernel - Re: [RFC patch 0/4] TSC calibration improvements

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Date:	Sun, 7 Sep 2008 00:40:16 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Alok Kataria <akataria@...are.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Veen <arjan@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dan Hecht <dhecht@...are.com>,
	Garrett Smith <garrett@...are.com>
Subject: Re: [RFC patch 0/4] TSC calibration improvements


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> "expect" is an unsigned char. There are absolutely _zero_ issues with 
> overflow, underflow, random phases of the moon, madness levels or 
> anything else. But yes, it does look like Ingo screwed up when adding 
> that final check, since expect was already decremented at the end of 
> the loop.
> 
> Ingo? Did you actually test it?

hm, yes, that's my brown paper bag fault, sorry.

I did that addition in tip/x86/tsc and posted it to you and i did test 
it immediately - and i noticed that i never saw the fast-calibration 
message i expected to see. I even pasted the boot log over irc yesterday 
and i still have it:

 *> [  0.000] TSC: PIT calibration deviates from PMTIMER: 738839 846296.
 *> [  0.000] TSC: Using PIT calibration value
 *> [  0.000] Detected 738.839 MHz processor.
 *> does not seem to trigger anywhere

i wanted to debug that problem straight after i worked down my 800+ 
mails post-vacation mbox :-/ Which state i reached about 2 hours ago so 
i'm now free - the fix is below.

i _think_ that the quality of calibration should now be pretty OK with 
latest -git. The clever fast calibration stuff could be .28 material. 

And/or we could change the 5x 50msec calibration to 3x 30msec right now, 
the precision is still plenty and the 90 msec is then replaced with your 
fast-calibrate method anyway on proper boxes. Hm?

	Ingo

------------------->
>From 5df45515512436a808d3476a90e83f2efb022422 Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@...e.hu>
Date: Sat, 6 Sep 2008 23:55:40 +0200
Subject: [PATCH] x86, tsc calibration: fix

my brown paperbag day ...

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 arch/x86/kernel/tsc.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 6dab90f..4847a92 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -319,7 +319,7 @@ static unsigned long quick_pit_calibrate(void)
 		/*
 		 * Make sure we can rely on the second TSC timestamp:
 		 */
-		if (!pit_expect_msb(--expect))
+		if (!pit_expect_msb(expect))
 			goto failed;
 
 		/*
--
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