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

[Juri Lelli <juri.lelli@arm.com>]

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@kernel.org>
> > Cc: Pawel Moll <pawel.moll@arm.com>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
> > Cc: Kumar Gala <galak@codeaurora.org>
> > Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
> > Cc: Olof Johansson <olof@lixom.net>
> > Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
> > Cc: Paul Walmsley <paul@pwsan.com>
> > Cc: Linus Walleij <linus.walleij@linaro.org>
> > Cc: Chen-Yu Tsai <wens@csie.org>
> > Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
> > Cc: devicetree@vger.kernel.org
> > Signed-off-by: Juri Lelli <juri.lelli@arm.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@kernel.org>
> 

Thanks!

> [...]
> 
> > +Example 2 (ARM 32-bit, 4-cpu system, two clusters,
> > +	   cpus 0,1@1GHz, cpus 2,3@500MHz):
> > +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