Re: 2.6.2: P4 ClockMod speed

[Pavel Machek <pavel@ucw.cz>]

On Po 15-03-04 14:59:27, Dominik Brodowski wrote:
> On Mon, Mar 15, 2004 at 01:44:06PM +0100, Pavel Machek wrote:
> > > On Sun, Mar 14, 2004 at 10:23:04PM +0100, Pavel Machek wrote:
> > > > Hi!
> > > > 
> > > > > > Hey, that's ugly. Values should be real.
> > > > > 
> > > > > Indeed. But if they're not known (and cpu_khz _is_ unreliable), what should
> > > > > be done?
> > > > 
> > > > Is it possible to measure cpu_khz with arbitrary precision if we make
> > > > measurement slower? I think so; if cpu_khz is too unreliable, perhaps
> > > > we can make measurement slower but more precise?
> > > 
> > > cpu_khz may be zero if the PIT is used as primary time source.
> > 
> > But that's a bug to be fixed, right?
> 
> Not necessarily: if the PIT is used and the TSC is disabled [there are some such
> systems, even modern ones...], you can't use the current cpu_khz detection
> routine. And please don't try to deduce it from BogoMIPS...

Ahha, I see. notsc. But, even if TSC is disabled for normal
timekeeping, it should be okay to use it to determine cpu_khz.

I believe that every CPU supported by cpufreq has TSC working
good-enough to measuring cpu_khz. While measuring cpu_khz, you are not
issuing halt, not changing cpu frequency, and you do not have problems
with unsynchronizes TSCs.
								Pavel
-- 
When do you have a heart between your knees?
[Johanka's followup: and *two* hearts?]