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Subject: Re: [PATCH] i386: trust the PM-Timer calibration of the local APIC
	timer
From: Thomas Gleixner <tglx@linutronix.de>
Reply-To: tglx@linutronix.de
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>, linux-kernel@vger.kernel.org,
       Ingo Molnar <mingo@elte.hu>, Adrian Bunk <bunk@stusta.de>,
       Arjan van de Ven <arjan@infradead.org>, Len Brown <lenb@kernel.org>
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Date: Sun, 18 Mar 2007 09:45:26 +0100
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On Sun, 2007-03-18 at 00:12 -0800, Andrew Morton wrote:
> On Sat, 17 Mar 2007 01:04:56 +0100 Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > When PM-Timer is available for local APIC timer calibration we can skip
> > the verification of the calibrated time value. The resulting error is
> > quite small on a bunch of evaluated platforms and is less harming than
> > the observed false positives.
> > 
> > We need to keep the verification on systems, which have no PM-Timer to
> > avoid bogus local APIC timer calibrations in the range of factor 2-10,
> > which can be observed when swicthing off the PM-timer support in the
> > kernel configuration.
> > 
> > The wrong calibration values are probably caused by SMM code trying to
> > emulate a PS/2 keyboard from a (maybe connected or not) USB keyboard.
> > This prohibits the accurate delivery of PIT interrupts, which are used
> > to calibrate the local APIC timer. Unfortunately we have no way to
> > disable this BIOS misfeature in the early boot process.
> > 
> > Add also the dropped cpu_relax() back to the wait loops.
> 
> Is this a for-2.6.21 thing?

Yes please. The false positives of the original calibration are
annoying.

	tglx