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

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Date:	Sat, 6 Sep 2008 22:58:02 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	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

On Sat, 6 Sep 2008, Linus Torvalds wrote:

> 
> 
> On Sat, 6 Sep 2008, Thomas Gleixner wrote:
> > 
> > Just checked. The -tip version still has the expect-- in the for()
> > which might lead to stupid results depending on the gcc madness level.
> 
> Umm. What? You're on some odd drugs.

Just straight forward german beer :)
 
 
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 6dab90f..3bfe083 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -310,8 +310,8 @@ static unsigned long quick_pit_calibrate(void)
 		unsigned char expect = 0xfe;
 
 		t1 = get_cycles();
		for (i = 0; i < QUICK_PIT_ITERATIONS; i++, expect--) {
			if (!pit_expect_msb(expect))
 				goto failed;
 		}
 		t2 = get_cycles();

 		/*
 		 * Make sure we can rely on the second TSC timestamp:
 		 */
		if (!pit_expect_msb(--expect))
 			goto failed;
 
Where is a guarantee, that excpect is not decremented before we break
out of the loop ? 

the "expect--" can be done _BEFORE_ the i < QUICK_PIT_ITERATIONS
evaluation. Not likely, but ...

This version works always

 		t1 = get_cycles();
		for (i = 0; i < QUICK_PIT_ITERATIONS; i++) {
			if (!pit_expect_msb(expect--))
 				goto failed;
 		}
 		t2 = get_cycles();

 		/*
 		 * Make sure we can rely on the second TSC timestamp:
 		 */

		if (!pit_expect_msb(expect))
 			goto failed;

Thanks,

	tglx
--
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