Re: G4 (7447A) CPU and dynamic frequency scaling

On Jan 6, 2009, at 11:17 AM, Joel Carnat wrote:

I read here
(http://en.wikipedia.org/wiki/PowerPC_G4#PowerPC_7447.2F7457_.22Apollo_7.22)
that my MacMini G4 should support "dynamic frequency scaling". From what I
understood this should enable to CPU frequency to be modified by software
(like Intel Speedstep does).

Most Macs have ways to mess with CPU speed, none of them are well documented. I implemented the one used by my iBook G4 ( a gpio is used to switch bus speed between 133MHz and something around 100MHz ), others use PMU commands.

I need to read the manual about this frequecy scaling stuff, IIRC it happens completely automatic, you just need to set a bit in some SPR which we probably already do.

NetBSD 4.99.72/macppc says:
cpu0 at mainbus0: 7447A (Revision 1.5), ID 0 (primary)
cpu0: HID0 8450c0bc<EMCP,EICE,NAP,DPM,ICE,DCE,SGE,BTIC,FBIOB,ABE,BHT>,
powersave: 1
cpu0: 1500.00 MHz, 512KB L2 cache no parity parity enabled

On /i386 and /amd64, I use "sysctl machdep" to check/set to speed settings
but there's nothing on my Mini/macppc.

As I said, we can do that on a select few models...

My questions are :
- do the 7447A really support CPU frequency scaling (at software level) ?

Yes and probably not.

- do NetBSD support this ?

Not sure.

- what does "powersave: 1" mean ?

It means that we put the CPU to sleep when it's idle.

have fun

Michael