NAP mode on powerpc 970

[Jake Moilanen]

> > Linux does not support NAP mode for the 970's.  Going to this mode
> > powers off the caches (thus killing the caches).  There are some other
> > potential issues (thermal, and power supply).
> >
> > This should not be done in Linux in the first place.  The correct place
> > is in the FW.
>
> No, no ... I do it on the G5 without problem :)

On the G5, how does the OS have knowledge if it needs to go into NAP
mode or not?  From my understanding, going in and out of NAP mode causes
power spikes and there are some unknown thermal implications on the
CPU.

> The north bridge will get the CPU out of NAP mode for snooping, the
> cache aren't powered off.

You are correct.  After getting a second opinion from a different FW
guy, I was told that the cache's are not powered off.

> Then HV could do it as well... Actually, on the js20, it could just
> leave HID0:NAP set permanently and let the kernel use MSR:POW from
> its idle loop...

When I talked to the HV team, they do not want to do this solution.
They are leaning towards another alternative, or having it done in FW so
the change does not need to be done in multiple places (e.g. Different
linux distros, and AIX).

Thanks,
Jake

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