linux-kernel - Re: System clock runs too fast after 2.6.27 -> 2.6.28.1 upgrade

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Date:	Fri, 23 Jan 2009 12:04:39 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	john stultz <johnstul@...ibm.com>
Cc:	Miklos Vajna <vmiklos@...galware.org>,
	Florian Fainelli <florian.fainelli@...ecomint.eu>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Gabriel C <nix.or.die@...glemail.com>,
	CSÉCSY László <boobaa@...galware.org>
Subject: Re: System clock runs too fast after 2.6.27 -> 2.6.28.1 upgrade


* john stultz <johnstul@...ibm.com> wrote:

> On Fri, 2009-01-23 at 00:57 +0100, Miklos Vajna wrote:
> > On Thu, Jan 22, 2009 at 12:03:14PM -0800, john stultz 
> > > CONFIG_X86_VISWS: If I recall, this is support for SGI workstations that
> > > had different apics then most systems, and might be related.
> > > 
> > I just did a new build with CONFIG_X86_VISWS and CONFIG_X86_RDC321X
> > disabled, but I kept CONFIG_PARAVIRT_GUEST enabled and that works fine
> > as well. So CONFIG_PARAVIRT_GUEST seems to be unrelated.
> 
> Ah. CONFIG_X86_RDC321X is indeed the issue.
> 
> >From arch/x86/include/asm/timex.h:
> 
> #ifdef CONFIG_X86_ELAN
> #  define PIT_TICK_RATE 1189200 /* AMD Elan has different frequency! */
> #elif defined(CONFIG_X86_RDC321X)
> #  define PIT_TICK_RATE 1041667 /* Underlying HZ for R8610 */
> #else
> #  define PIT_TICK_RATE 1193182 /* Underlying HZ */
> #endif
> 
> This gives the kernel an incorrect slower frequency for the PIT hardware 
> on your system and is why the TSC calibration is off and why the ACPI PM 
> gets thrown out.
> 
> I'd think this would ideally be more of a subarch in the config 
> selection like the AMD Elan is, so folks know it really is not just a 
> extra-support feature and in fact limits what hardware you can use the 
> resulting kernel on.

a better solution is that if RDC is set [it is unset by default] we should 
turn PIT_TICK_RATE into a variable, defaulting it to 1193182. If a real 
RDC or ELAN system is running, that variable is changed during early 
bootup, in the init functions for those platforms.

	Ingo
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