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

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Date:   Thu, 31 Aug 2017 11:36:07 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
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 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.

[...]