linux-kernel - Re: PMTMR running too fast

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Message-Id: <200612061744.47249.ak@suse.de>
Date:	Wed, 6 Dec 2006 17:44:47 +0100
From:	Andi Kleen <ak@...e.de>
To:	john stultz <johnstul@...ibm.com>
Cc:	Ian Campbell <ijc@...lion.org.uk>, linux-kernel@...r.kernel.org
Subject: Re: PMTMR running too fast


> 
> > Is there a specific reason the check was removed (I couldn't see on in
> > the archives) or was it simply overlooked? Without it I need to pass
> > clocksource=tsc to have 2.6.18 work correctly on an older K6 system with
> > an Aladdin chipset (will dig out the precise details if required). Would
> > a patch to reintroduce the check be acceptable or would some sort of
> > blacklist based solution be more acceptable?
> 
> If I recall correctly, it was pulled because there was some question as
> to if it was actually needed (x86_64 didn't need it) and it slows down
> the boot time (although not by much). 
> 
> I'm fine just re-adding it. Although if the number of affected systems
> are small we could just blacklist it (Ian, mind sending dmidecode
> output?).
> 
> Andi, your thoughts?

Doing a check at boot time is fine for me. Just I don't want the
"read pmtmr three times at runtime" code anywhere near x86-64

I don't think the boot time check needs DMI guarding

But BTW the check is not necessarily enough -- there is at least one
NF3 machine around where the PIT timer ticks at a wrong frequency.
Safer would be probably to calibrate against RTC which is afaik used
by Windows too (so it's likely to be ok) 

-Andi
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