linux-kernel - Re: [PATCH v4 2/8] Documentation: arm: define DT cpu capacity bindings

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Date:	Mon, 21 Mar 2016 10:53:30 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Sai Gurrappadi <sgurrappadi@...dia.com>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	peterz@...radead.org, vincent.guittot@...aro.org,
	robh+dt@...nel.org, mark.rutland@....com, linux@....linux.org.uk,
	sudeep.holla@....com, lorenzo.pieralisi@....com,
	catalin.marinas@....com, will.deacon@....com,
	morten.rasmussen@....com, dietmar.eggemann@....com,
	broonie@...nel.org, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Olof Johansson <olof@...om.net>,
	Gregory CLEMENT <gregory.clement@...e-electrons.com>,
	Paul Walmsley <paul@...an.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Chen-Yu Tsai <wens@...e.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	pboonstoppel@...dia.com
Subject: Re: [PATCH v4 2/8] Documentation: arm: define DT cpu capacity
 bindings

Hi Sai,

On 18/03/16 10:49, Sai Gurrappadi wrote:
> Hi Juri,
> 
> On 03/18/2016 07:24 AM, Juri Lelli wrote:
> 
> <snip>
> 
> > +
> > +==========================================
> > +2 - CPU capacity definition
> > +==========================================
> > +
> > +CPU capacity is a number that provides the scheduler information about CPUs
> > +heterogeneity. Such heterogeneity can come from micro-architectural differences
> > +(e.g., ARM big.LITTLE systems) or maximum frequency at which CPUs can run
> > +(e.g., SMP systems with multiple frequency domains). Heterogeneity in this
> > +context is about differing performance characteristics; this binding tries to
> > +capture a first-order approximation of the relative performance of CPUs.
> 
> Any reason why this capacity number is not dynamically generated based on the
> max frequency for each CPU? The DT property would then instead specify just
> the micro-architectural differences between the CPU types.
> 

I'm not sure I clearly understand your question, so I'll try to
reiterate it.

Are you asking why we don't dynamically profile the system, at boot for
example, to get this number? Or do you ask why this number couldn't be
only describing micro-arch differences (so, if I get it right, we should
then multiply it by max freq to get the capacity of a CPU)?

We already played with the first option (please refer to v2 and v3), but
we ended up agreeing that dynamic profiling adds overhead to the boot
process (while a DT approach can provide information to speed up boot)
and it is in general not repeatable/reliable (as numbers can vary from
boot to boot for different reasons).

The second option I think can be feasible, but I'm not sure what we gain
in practice. We will still need to specify a per-platform number, right?

Best,

- Juri