[ltp] trick: use cpu state C4 on AC

[Damien Challet]

Dear All,

This may be well known, but I have found a way to cool down my t41p by 4-5 
degrees when idle:

with kubuntu 6.10, suspend2 kernel

boot without the battery

insert it,

hibernate

remove it

boot and enjoy 35C when idle instead of 39C (fglrx, no dri and powerstate=1, 
ath_pci rmmoded). The C4 state survives further hibernations.  The fan 
(almost?) never kicks in under gentle use. What is bizarre is that its stays 
off even if the CPU reaches ~42C and then goes off.

A contrario, when planning to use the laptop away from a power 
socket (without having used this trick), one should insert the battery before 
hibernating.

Now the obvious question is: does this come from the bios, or from the kernel 
(or from kde-power-manager)? 

I think that it is due to the kernel that decides to make the C4 state 
available. So it seems possible to activate the C4 state without using the 
above trick. Is there any kernel guru that could 
comment on that?

Yours

Damien