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Date:   Sun, 17 Sep 2017 09:37:05 +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 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
> 
> The Cortex-A15 vs Cortex-A7 performance ratio is 1024/389 = 2.63.
> 
> The values derived with the 'cpu_efficiency/clock-frequency dt property'
> solution are:
> 
> $ cat /sys/devices/system/cpu/cpu*/cpu_capacity
> 1535
> 1535
> 1535
> 1535
> 448
> 448
> 448
> 448
> 
> The Cortex-A15 vs Cortex-A7 performance ratio is 1535/448 = 3.43.
> 
> The discrepancy between 2.63 and 3.43 is due to the false assumption
> when using the 'cpu_efficiency/clock-frequency dt property' solution
> that the max cpu frequency of the little cpus is 1 GHZ and not 1.3 GHz.
> The Cortex-A7 cluster runs with a max cpu frequency of 1.3 GHZ whereas
> the 'clock-frequency' property value is set to 1 GHz.
> 
> 3.43/1.3 = 2.64
> 
> $ cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_max_freq
> 1800000
> 1800000
> 1800000
> 1800000
> 1300000 <-- max cpu frequency of the Cortex-A7s (little cores)
> 1300000
> 1300000
> 1300000
> 
> Running another benchmark (single-threaded sysbench affine to the
> individual cpus) with performance cpufreq governor on the Samsung
> Chromebook 2 13" showed the following numbers:
> 
> $ for i in `seq 0 7`; do taskset -c $i sysbench --test=cpu
>   --num-threads=1 --max-time=10 run | grep "total number of events:";
>   done
> 
> total number of events: 1083
> total number of events: 1085
> total number of events: 1085
> total number of events: 1085
> total number of events: 454
> total number of events: 454
> total number of events: 454
> total number of events: 454
> 
> The Cortex-A15 vs Cortex-A7 performance ratio is 2.39, i.e. very close
> to the one derived from the Dhrystone based one of the "Big.LITTLE
> Processing with ARM Cortex™-A15 & Cortex-A7" white paper (2.63).
> 
> We don't aim for exact values for the cpu capacity values. Besides the
> CPI (Cycles Per Instruction), the instruction mix and whether the system
> runs cpu-bound or memory-bound has an impact on the cpu capacity values
> derived from these benchmark results.
> 
> Cc: Rob Herring <robh+dt@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Kukjin Kim <kgene@...nel.org>
> Cc: Krzysztof Kozlowski <krzk@...nel.org>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> ---
>  arch/arm/boot/dts/exynos5420-cpus.dtsi | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 

Thanks, applied (with s/arm/ARM/ change in subject).

Best regards,
Krzysztof

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