linux-kernel - Re: [PATCH] i386: trust the PM-Timer calibration of the local APIC timer

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Message-Id: <1174207526.13341.465.camel@localhost.localdomain>
Date:	Sun, 18 Mar 2007 09:45:26 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Adrian Bunk <bunk@...sta.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH] i386: trust the PM-Timer calibration of the local APIC
	timer

On Sun, 2007-03-18 at 00:12 -0800, Andrew Morton wrote:
> On Sat, 17 Mar 2007 01:04:56 +0100 Thomas Gleixner <tglx@...utronix.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


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