linux-kernel - Re: [PATCH 2/4] arm: dts: exynos: add exynos5420 cpu capacity-dmips-mhz information

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Message-ID: <20170903195614.iwe7v6g3xsg7p6nf@kozik-lap>
Date:   Sun, 3 Sep 2017 21:56:14 +0200
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        Russell King <linux@...linux.org.uk>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Kukjin Kim <kgene@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@....com>
Subject: Re: [PATCH 2/4] arm: dts: exynos: add exynos5420 cpu
 capacity-dmips-mhz information

On Thu, Aug 31, 2017 at 11:36:07AM +0100, Dietmar Eggemann wrote:
> On 30/08/17 21:26, Krzysztof Kozlowski wrote:
> > On Wed, Aug 30, 2017 at 03:41:18PM +0100, Dietmar Eggemann wrote:
> >> The following 'capacity-dmips-mhz' dt property values are used:
> >>
> >> Cortex-A15: 1024, Cortex-A7: 539
> >>
> >> They have been derived from the cpu_efficiency values:
> >>
> >> Cortex-A15: 3891, Cortex-A7: 2048
> >>
> >> by scaling them so that the Cortex-A15s (big cores) use 1024.
> >>
> >> The cpu_efficiency values were originally derived from the "Big.LITTLE
> >> Processing with ARM Cortex™-A15 & Cortex-A7" white paper
> >> (http://www.cl.cam.ac.uk/~rdm34/big.LITTLE.pdf). Table 1 lists 1.9x
> >> (3891/2048) as the Cortex-A15 vs Cortex-A7 performance ratio for the
> >> Dhrystone benchmark.
> >>
> >> The following platforms are affected once cpu-invariant accounting
> >> support is re-connected to the task scheduler:
> >>
> >> arndale-octa, peach-pi, peach-pit, smdk5420
> >>
> >> The patch has been tested on Samsung Chromebook 2 13" (peach-pi, Exynos
> >> 5800).
> >>
> >> $ cat /sys/devices/system/cpu/cpu*/cpu_capacity
> >> 1024
> >> 1024
> >> 1024
> >> 1024
> >> 389
> >> 389
> >> 389
> >> 389
> > 
> > I am missing something... shouldn't this be 539? Or is it scaled with
> > the clock-frequency (1 GHz) value?
> 
> Yeah, the capacity-dmips-mhz dt value of 539 for the little cpus is
> scaled by 1.3/1.8 (max cpu capacity/ system wide max cpu capacity):
> 
> 539 * 1.3/1.8 = 389
> 
> This max cpu capacity scaling is part of both solutions, the 'cpu
> capacity-dmips-mhz' and the 'cpu_efficiency/clock-frequency dt property'
> one.
> 
> The (original*) cpu capacity on a heterogeneous platform expresses uArch
> and max cpu frequency differences between the (logical) cpus of the
> system.
> 
> * not further reduced by rt and/or irq pressure.
> 
> [...]

Thanks for explanation, looks fine for me. I'll take it after merge
window.

Best regards,
Krzysztof