linux-kernel - Re: [PATCH v2 RESEND 2/2] ARM: local timers: add timer support using IO mapped register

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Date:	Tue, 2 Oct 2012 16:03:33 +0100
From:	Dave Martin <dave.martin@...aro.org>
To:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:	Mark Rutland <Mark.Rutland@....com>,
	Russell King <linux@....linux.org.uk>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	Will Deacon <Will.Deacon@....com>,
	Rohit Vaswani <rvaswani@...eaurora.org>,
	Rob Herring <rob.herring@...xeda.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Bryan Huntsman <bryanh@...eaurora.org>,
	Rob Landley <rob@...dley.net>,
	Daniel Walker <dwalker@...o99.com>,
	David Brown <davidb@...eaurora.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 RESEND 2/2] ARM: local timers: add timer support using
 IO mapped register

On Tue, Oct 02, 2012 at 02:44:44PM +0100, Lorenzo Pieralisi wrote:
> On Tue, Oct 02, 2012 at 12:27:04PM +0100, Dave Martin wrote:
> > On Fri, Sep 28, 2012 at 06:15:53PM +0100, Lorenzo Pieralisi wrote:
> > > On Fri, Sep 28, 2012 at 04:57:46PM +0100, Dave Martin wrote:
> 
> [...]
> 
> > > There must be a common way for all devices to link to the topology, though.
> > > 
> > > The topology must be descriptive enough to cater for all required cases
> > > and that's what Mark with PMU and all of us are trying to come up with, a solid
> > > way to represent with DT the topology of current and future ARM systems.
> > > 
> > > First idea I implemented and related LAK posting:
> > > 
> > > http://lists.infradead.org/pipermail/linux-arm-kernel/2012-January/080873.html
> > > 
> > > Are "cluster" nodes really needed or "cpu" nodes are enough ? I do not
> > > know, let's get this discussion started, that's all I need.
> > 
> > One thing which now occurs to me on this point it that if we want to describe
> > the CCI properly in the DT (yes) then we need a way to describe the mapping
> > between clusters and CCI slave ports.  Currently that knowledge just has to
> > be a hard-coded hack somewhere: it's not probeable at all.
> 
> That's definitely a good point. We can still define CCI ports as belonging
> to a range of CPUs, but that's a bit of a stretch IMHO.
> 
> > I'm not sure how we do that, or how we describe the cache topology, without
> > the clusters being explicit in the DT
> > 
> > ...unless you already have ideas ?
> 
> Either we define the cluster node explicitly or we can always see it as a
> collection of CPUs, ie phandles to "cpu" nodes. That's what the decision
> we have to make is all about. I think that describing it explicitly make
> sense, but we need to check all possible use cases to see if that's
> worthwhile.

How is the cache topology described today (forgive my laziness in not
answering this question for myself)?  The issues are somewhat similar.

I still have some misgivings about describing clusters in terms of sets of
CPUs.  For example, when we boot up a cluster, we have to set up ... the
cluster.  This is a distinct thing which we must set up in addition to
any of the actual CPUs.

There is a strict child/parent relationship between clusters and CPUs, so
a tree of nodes does seem the most natural description ... but I'm not
aware of all the background to this discussion.

Cheers
---Dave
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