CPU off power consumption

[Benjamin Herrenschmidt]

On Mon, 2005-11-28 at 13:15 +1100, Paul Mackerras wrote:
> Giuliano Pochini writes:
> 
> > Out of curiosity, what's the difference between a cpu that has never been
> > enabled and one that has been disabled with echo 0>/sys/.../online ? It
> > happens that when I boot with maxcpus=0 the temperature always stays low
> > enoung that the fan never spins up. If I enable and then I immediately
> > disable the 2nd cpu, the temperature goes a few degrees up. I have a dual
> > G4-MDD.
> 
> Interesting.  A cpu that has been disabled will be in sleep mode with
> interrupts disabled and its caches flushed.  One that has never been
> started may possibly be held in the reset state.  The way to check
> would be to check the state of the GPIO register that controls the
> soft reset line of the second CPU.

No, I think CPUs that have not been started are held in a similar sleep
loop in ROM. I don't see right away why there would be any power
consumption difference unless some bug causing us to never actually call
the sleep loop ....

Ben.