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Subject: Re: [v6 PATCH 0/7]: cpuidle/x86/POWER: Cleanup idle power
	management code in x86, cleanup drivers/cpuidle/cpuidle.c and introduce
	cpuidle to POWER.
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
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Date: Fri, 25 Sep 2009 10:54:24 +0200
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Cc: Shaohua Li <shaohua.li@intel.com>, Gautham R Shenoy <ego@in.ibm.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>,
	linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org,
	Paul Mackerras <paulus@samba.org>, arun@linux.vnet.ibm.com,
	Ingo Molnar <mingo@elte.hu>, Arjan van de Ven <arjan@infradead.org>,
	linuxppc-dev@lists.ozlabs.org, Len Brown <lenb@kernel.org>
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On Fri, 2009-09-25 at 12:36 +0530, Vaidyanathan Srinivasan wrote:
> * Arjan van de Ven <arjan@infradead.org> [2009-09-24 14:22:28]:
> 
> > On Thu, 24 Sep 2009 10:42:41 +0530
> > Arun R Bharadwaj <arun@linux.vnet.ibm.com> wrote:
> > 
> > > * Arun R Bharadwaj <arun@linux.vnet.ibm.com> [2009-09-22 16:55:27]:
> > > 
> > > Hi Len, (or other acpi folks),
> > > 
> > > I had a question regarding ACPI-cpuidle interaction in the current
> > > implementation.
> > > 
> > > Currently, every cpu (i.e. acpi_processor) registers to cpuidle as
> > > a cpuidle_device. So every cpu has to go through the process of
> > > setting up the idle states and then registering as a cpuidle device.
> > > 
> > > What exactly is the reason behind this?
> > > 
> > 
> > technically a BIOS can opt to give you C states via ACPI on some cpus,
> > but not on others.
> > 
> > in practice when this happens it tends to be a bug.. but it's
> > technically a valid configuration
> 
> So we will need to keep the per-cpu registration as of now because we
> may have such buggy BIOS in the field and we don't want the cpuidle
> framework to malfunction there.

If the BIOS doesn't mention a certain C state on a cpu, and you try to
set it anyway, does that go boom?

This whole per-cpu registration thing is horridly ugly, can't you have a
per-cpu C state exception mask and leave it at that -- if its really
needed?