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Subject: Re: [PATCH] powermac: support G5 CPU hotplug
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Segher Boessenkool <segher@kernel.crashing.org>
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Date: Thu, 03 May 2007 10:24:21 +1000
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Cc: linuxppc-dev@ozlabs.org, Johannes Berg <johannes@sipsolutions.net>,
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On Thu, 2007-05-03 at 10:21 +1000, Benjamin Herrenschmidt wrote:
> > The difference between doze and nap is that in nap mode
> > the bus interface unit is switched off; on the 970 and
> > 970FX this means that the L2 cache has to be flushed
> > before entering nap mode (or otherwise snoops will go
> > missing in action); 
> 
> BZZZT... incorrect... the 970FX and MP behave the same here. There is
> only one state you can "manually" enter and it's NAP. 

Actually, I'm not totally correct here neither, that's what 745x does...
the 970 _do_ give manual access to DOZE. But the rest is still true, it
will transition automatically to do snoops.

> Transitions
> between DOZE and NAP are automatic to handle snooping in both cases.
> Cache flushing is never needed ... unless in our case the CPU gets
> actually powered down for suspend-to-ram.
> 
> > the 970MP automatically transitions
> > from nap to doze mode if snoops are requested (and back
> > to nap again), so no L2 flush is required.  The 970MP
> > also has a "deep nap" mode which saves more power by
> > automatically clocking the CPU down.
> 
> Also deep nap only really works on 970MP DD3 and later I think
> 
> > One more thing -- perhaps keeping the CPU in reset
> > state saves even more power?
> 
> Dunno... Somebody can try... but I don't think we have access to the
> reset line on 970's..
> 
> Ben.
> 
>