[ltp] Processor speed changes on Thinkpad 240X

[Matthew Garrett]

I have a 500MHz 240X. I'm pretty convinced that it's not Speedstep, so 
I've been trying to figure out how the processor speed change works. 
I've noticed that PCI register 47 in the 82440MX power management 
controller gets set to f1 when I hold down fn+f8 and back to ff if I do 
the same again. This is accompanied by a change in speed. However, 
simply changing the value of this register myself doesn't result in the 
speed changing.

The speed changes produced by the Fn+F8 thing (or switching from AC to 
battery) also show up as changes in the ACPI throttling status. But if I 
set the ACPI throttling status myself, as well as the speed change I get 
a high pitched whining noise from the laptop. So there seems to be 
something slightly different going on there.

So, does anyone have any idea how the throttling is actually being 
controlled and what this mysterious PCI register is up to? I'm also 
confused about what "Automatic medium" and co refer to in the BIOS menu 
relating to this stuff - does this just mean that it'll switch between 
high speed and medium speed depending on the power status?

It'd also be nice to know how I could change the PCI bus power level 
without having to switch to battery power. While it's in its normal 
state, there's a lot of interference on the headphone out, but at low 
power it's much better.

-- 
Matthew Garrett | mjg59@srcf.ucam.org