linux-kernel - Re: [PATCH v6 1/8] Documentation: arm: define DT cpu capacity-dmips-mhz bindings

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Date:	Thu, 21 Jul 2016 12:43:38 +0100
From:	Juri Lelli <juri.lelli@....com>
To:	Rob Herring <robh@...nel.org>
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,
	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>
Subject: Re: [PATCH v6 1/8] Documentation: arm: define DT cpu
 capacity-dmips-mhz bindings

Hi Rob,

On 20/07/16 13:56, Rob Herring wrote:
> On Tue, Jul 19, 2016 at 01:40:41PM +0100, Juri Lelli wrote:
> > ARM systems may be configured to have cpus with different power/performance
> > characteristics within the same chip. In this case, additional information
> > has to be made available to the kernel (the scheduler in particular) for it
> > to be aware of such differences and take decisions accordingly.
> > 
> > Therefore, this patch aims at standardizing cpu capacities device tree
> > bindings for ARM platforms. Bindings define cpu capacity-dmips-mhz
> > parameter, to allow operating systems to retrieve such information from
> > the device tree and initialize related kernel structures, paving the way
> > for common code in the kernel to deal with heterogeneity.
> > 
> > Cc: Rob Herring <robh+dt@...nel.org>
> > Cc: Pawel Moll <pawel.moll@....com>
> > Cc: Mark Rutland <mark.rutland@....com>
> > Cc: Ian Campbell <ijc+devicetree@...lion.org.uk>
> > Cc: Kumar Gala <galak@...eaurora.org>
> > Cc: Maxime Ripard <maxime.ripard@...e-electrons.com>
> > Cc: Olof Johansson <olof@...om.net>
> > Cc: Gregory CLEMENT <gregory.clement@...e-electrons.com>
> > Cc: Paul Walmsley <paul@...an.com>
> > Cc: Linus Walleij <linus.walleij@...aro.org>
> > Cc: Chen-Yu Tsai <wens@...e.org>
> > Cc: Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
> > Cc: devicetree@...r.kernel.org
> > Signed-off-by: Juri Lelli <juri.lelli@....com>
> > ---
> > 
> > Changes from v1:
> >  - removed section regarding capacity-scale
> >  - added information regarding normalization
> > 
> > Changes from v4:
> >  - binding changed to capacity-dmips-mhz
> >  - sections and changelod updated accordingly
> > 
> > Changes from v5:
> >  - addressed Mark and Vincent comments
> > ---
> >  .../devicetree/bindings/arm/cpu-capacity.txt       | 236 +++++++++++++++++++++
> >  Documentation/devicetree/bindings/arm/cpus.txt     |  10 +
> >  2 files changed, 246 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/arm/cpu-capacity.txt
> 
> I guess I'm okay with the scaled values, so:
> 
> Acked-by: Rob Herring <robh@...nel.org>
> 

Thanks!

> [...]
> 
> > +Example 2 (ARM 32-bit, 4-cpu system, two clusters,
> > +	   cpus 0,1@...z, cpus 2,3@...MHz):
> > +capacities-dmips-mhz are scaled w.r.t. 2 (cpu@0 and cpu@1), this means that first
> > +cpu@0 and cpu@1 are twice fast than cpu@2 and cpu@3 (at the same frequency)
> 
> This example is a bit confusing with both the capacity and frequency 
> being half. I also find it a bit unrealistic to have a 2x performance 
> difference on the same micro arch. But it is all just an example...
> 

Right, it's a fake example just to highlight how the values are used.

Best,

- Juri