iMac G5 frequency switching

Markus Demleitner msdemlei at
Tue Sep 13 02:34:53 EST 2005

On Sun, Sep 11, 2005 at 07:58:18AM +1000, Benjamin Herrenschmidt wrote:
> On Fri, 2005-09-09 at 13:43 +0200, Markus Demleitner wrote:
> > I've figured out how to switch frequencies of the iMac G5 CPU (at
> > least for my 1600 MHz models) and at that opportunity found that OF
> > I don't do any voltage stuff myself, mainly because OF doesn't
> > either.  If anyone knows if I should, please let me know.
> OF does set voltage no ? You definitely should as this is where you save
> the most power. It can be done by getting the target voltage for the
Well, let me put it this way: The OF words smu-powertune-hi and
smu-powertune-low don't touch the voltage.

There are, indeed, some words dealing with voltage, most prominently
slew-voltage.  Unfortunately, it takes three arguments and is only
called in slew-(high|low)-pulsar.  These words, in turn, aren't used
at all but have to take to arguments, since they call slew-voltage
like 5 slew-voltage.  Since they're not called, I can't find out what
arguments they take.

On the other hand, seeing the definition
['] drop to set-frequency
: slew-high-pulsar
   5 slew-voltage slew-wait 5 set-frequency slew-wait  ;
(which means that no frequency change on the pulsar chip -- I'm told
it's some kind of master clock in the machine -- takes place anyway)
I get the strange feeling that the whole thing is broken in my
version of OF (which is 5.2.2f1-2004-08-18).  In short: I'm stumped.

If some kind soul has a newer iMac G5 and would let me telnet into
OF, I'd be willing to try my luck and see if there's something
different in there, otherwise someone would have to dive into
whatever OS X does.  That someone is unlikely to be me, to be
entirely frank -- I probably can't spend much more time (which I'd
need to get up to speed reading ppc64 machine code) on this until
some time in March 2006.  Sorry.

I *would* have time to explain my findings so far, as poor as they
are, to anyone wanting to take up the voltage control issue on the
imac G5s.  So, if anyone coming across this while googling for imac G5
support in Linux feels like kernel work, drop me a mail...



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