Re: writing a cpufreq driver

[Bruno Ducrot <ducrot@poupinou.org>]

On Fri, Sep 22, 2006 at 09:39:54AM -0500, Ryan Underwood wrote:
> 
> On Fri, Sep 22, 2006 at 09:13:18AM -0500, Ryan Underwood wrote:
> > 
> > I have a question, how critical is it that I have exactly the correct
> > CPU MHz for the scaling to work properly?  Unfortunately, for the slow
> > speed there is no way for me to know what it actually is on a particular
> > system due to limitations of the firmware, so the best I could do is a
> > RDTSC timing to try to figure it out.  I wonder if skew between the
> > actual CPU speed and what cpufreq thinks it is would be the source of
> > the problem.
> 
> Hmm, this might have been the problem.  On my machine, the fast and slow
> MHz (as measured by my TSC routine) are the same.  However, the machine
> is noticeably slower at the 'slow' firmware setting.  I thought maybe
> they were disabling L1 cache, but that is not the case.  The CPU is a
> Pentium VRT 120MHZ so I'm not sure what is going on.... maybe down
> clocking the system bus or memory or inserting wait states somewhere.
> 
> In any case, since the fast and slow MHZ are the same, cpufreq does
> nothing.  But cpufreq should still do "something" in this case, since
> something else that my driver does is to disable the L2 cache when going
> to the slow state, which is a decent power savings even on my laptop
> that doesn't have an actual slow MHZ.  But since the fast and slow mhz
> are the same, cpufreq performs no actions.
> 

I've seen toshiba laptop using throttling when HCI is used.
If you have the datasheet of the southbridge I think we should be
able to verify this hypothesis.

-- 
Bruno Ducrot

--  Which is worse:  ignorance or apathy?
--  Don't know.  Don't care.