Re: speedstep-centrino on dothan

[Nikolay Pelov <Nikolay.Pelov@cs.kuleuven.ac.be>]

On Thu, 7 Jul 2005 st3@riseup.net wrote:

> 
> int main() {
> unsigned int index, frequency, voltage
> 
> index = (((frequency)/100) << 8) | ((voltage - 700) / 16);
> printf ("%u\n", index);
> }
> 
> frequency is expressed in MHz, voltage in mV, index is the value for
> centrino_model[cpu]->op_points[y].index 

Few days ago I discussed on the cpufreq mailing list that the above formula
is not the correct way to compute the index. Instead of frequency, one
should use a multiplier, i.e.:

  index = (multiplier << 8) | ((voltage - 700) / 16);

and the multiplier is computed as multiplier = frequency/fsb_speed.
On older systems fsb_speed = 100 MHz and that's why the original formula
was working. But for the Sonoma (Centrino 2) Pentium M processors
fsb_speed = 133 MHz. 

To give an example, take a CPU speed of 800MHz and a front side bus of
133MHz. Using the original formula we will use the value

  frequency/100 = 800/100 = 8

which is wrong. The correct value is:

  multiplier = frequency/fsb_speed = 800/133 = 6.

Nikolay

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