linux-kernel - Re: Regression in 2.6.27 caused by commit bfc0f59

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Date:	Mon, 1 Sep 2008 11:42:38 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Larry Finger <Larry.Finger@...inger.net>
cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alok Kataria <akataria@...are.com>,
	Michael Buesch <mb@...sch.de>
Subject: Re: Regression in 2.6.27 caused by commit bfc0f59

On Mon, 1 Sep 2008, Larry Finger wrote:
>
> TSC calibrated against PIT
> Detected 428.823 MHz processor.

Ok, Thomas, that means that the PIT is reliable (not surprising), and the 
PM_TIMER isn't (again, I'm not horribly surprised). And HPET isn't 
available, of course.

The old x86-32 code never even bothered with the PM_TIMER for calibration. 
I don't understand why the x86-64 code bothers with it either. Why not 
just drop that whole broken thing, and just depend on the PIT if there is 
no HPET?

I would also like to point out that the 32-bit code actually had a much 
nicer PIT setup, using the much better documented mach_prepare_counter() 
and mach_countup() helper functions. I'm unhappy to note that the new 
"common" code uses what appears to be the inferior code.

Also, note that this is _not_ a new issue. See "verify_pmtmr_rate()" in 
drivers/clocksource/acpi_pm.c, along with all the code to check that the 
reads are stable in "init_acpi_pm_clocksource()".

IOW, the PM_TIMER has been found to be broken before. Depending on it for 
calibration is broken.

			Linus
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