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Date: Mon, 31 Mar 2008 13:05:18 -0500
From: Josh Boyer <jwboyer@linux.vnet.ibm.com>
To: Josh Boyer <jwboyer@gmail.com>
Subject: Re: [PATCH] Add idle power save for ppc 4xx
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On Mon, 31 Mar 2008 12:07:17 -0500
Josh Boyer <jwboyer@gmail.com> wrote:

> On Mon, 2008-03-31 at 08:12 -0500, Jerone Young wrote:
> > # HG changeset patch
> > # User Jerone Young <jyoung5@us.ibm.com>
> > # Date 1206969060 18000
> > # Node ID 10aea37177130bbe5de7bee6ec06d9010bc5da1f
> > # Parent  1506aa38ddabb0bf73fff3ac3f3db5f9ef6458cc
> > Add idle power save for ppc 4xx
> > 
> > This patch sets the wait state MSR when power_save is called in cpu_idle loop for ppc4xx. This is mainly to help out virtualization solutions such as KVM. This way the virtualization soultions are able to tell if the guest kernel is idle.
> > 
> > I have tested this on hardware & KVM virtual guest.
> 
> I'm not overly thrilled with adding this to all of 4xx.  It doesn't
> actually save much power at all (1% on a project that actually measured
> it with an amp meter recently) and there's really no other benefit to
> doing it outside of the virtual guest case.
> 
> I'm assuming you pass a dtb to the virtual guest when you start it up.
> Could you define a property in the CPU node there that can be parsed to
> use the power_save function instead of always making it the default?

Actually, you probably don't want this as a property in the device
tree.  It doesn't describe hardware.  A Kconfig option might be
warranted though.

josh