linux-kernel - Re: [RFC] Add Arm cpu topology definition

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Date:	Wed, 22 Jun 2011 13:19:02 +0300
From:	Amit Kucheria <amit.kucheria@...aro.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Stephen Boyd <sboyd@...eaurora.org>, linaro-dev@...ts.linaro.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [RFC] Add Arm cpu topology definition

On 11 Jun 22, Catalin Marinas wrote:
> On Tue, Jun 21, 2011 at 01:36:15PM -0700, Stephen Boyd wrote:
> > On 06/16/2011 11:54 PM, Vincent Guittot wrote:
> > > On 16 June 2011 21:40, Stephen Boyd <sboyd@...eaurora.org> wrote:
> > >> The ARM ARM says these fields are IMPLEMENTATION DEFINED meaning that
> > >> different vendors may attribute different meaning to these fields if
> > >> they wish. Does that mean this should be a platform_*() function?
> > >>
> > > The ARM ARM also provides a recommended use of the fields of this
> > > register and the TRM of each Cortex adds some details. On the cortex
> > > A9, each platform can only set the value of the Cluster ID with the
> > > CLUSTERID pins. I have tried to consolidate the value of MPIDR  across
> > > several platforms and they all match with the description.
> > >
> > > Have you got an example of a MPIDR register which doesn't match with
> > > the implementation ?
> > 
> > Not that I know of. I'm more concerned with how the ARM ARM has two
> > recommended usages for these fields depending on virtualization or not.
> > I suppose we can handle that issue when it arises (or does your
> > implementation already handle that?)
> 
> According to the ARM ARM:
> 
> 	MPIDR provides a mechanism with up to three levels of affinity
> 	information, but the meaning of those levels of affinity is
> 	entirely IMPLEMENTATION DEFINED.
> 
> So we can't really tell the meaning of the affinity bits. There are two
> recommended ways indeed (with or without virtualisation) which are not
> that different with regards to the topology (just introducing another
> level for virtual CPUs).
> 
> But I think a more general solution would be for the CPU topology to be
> provided via the FDT.

Agreed. That will be the next step.

We decided on doing it this way to allow non-DT-enabled platforms to be able
to use the feature and to allow DT-enabled platforms to settle down in the
mean time.

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