Re: Intel Core2Duo mobile - how does the VID get set?

[Wes Felter <wesley@felter.org>]

Brian Morrison wrote:
> After a fair bit of Googling and reading around, I'm none the wiser 
> about exactly how Linux 2.6.x sets the processor VID (or for that matter 
> how it decides the FID settings) when using the ondemand governor and 
> cpufreq stuff.
> 
> Can anyone tell me a) whether this is obtained from the BIOS, something 
> in the MSR of the processor or elsewhere and b) whether there is an 
> interface in /proc or /sys where one can find out what is set and modify 
> it?

There is an ACPI table created by the BIOS that lists all the P-states; 
a P-state is a <frequency, opaque value> tuple where the opaque value 
contains the FID and VID but Intel pretends it is magic. The 
acpi-cpufreq driver reads this table and uses it to change states by 
writing the opaque value into the appropriate MSR.

The Intel 64 and IA-32 Architectures Software Developer’s Manual, Volume 
3A, chapter 13 describes the hardware (vaguely).

If you want to see what is going on under the hood, try Linux-PHC: 
http://phc.athousandnights.de/

(Setting followup to cpufreq list.)

Wes Felter - wesley@felter.org