Re: PowerPC cpufreq using ICTC

[Paul Mackerras <paulus@samba.org>]

Matt Sealey writes:

> I am investigating more generic power management (cpufreq) for PowerPC, both
> in terms of DFS and the G3 dual-PLL feature (which is already implemented in
> the PowerMac driver) and also using the G3/G4 "ICTC" register which stalls
> the instruction fetch for 1-255 clocks in order to allow the execution units to
> go into dynamic power management (when DPM is enabled). This should, by all
> documentation, reduce the power consumption of the CPU.

But does it reduce the energy consumed per instruction executed, or
increase it?  I would expect that you wouldn't see any improvement in
energy per instruction unless you can reduce the CPU core Vdd voltage.
If you can't reduce the energy per instruction then you might as well
"race to idle", which is effectively what we do currently.

Paul.