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Message-ID: <alpine.LFD.1.10.0809062253030.3243@apollo.tec.linutronix.de>
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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