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Message-ID: <9b4bd8b5-d5d9-7f75-128a-87b727ad0bd7@linaro.org>
Date:   Thu, 7 Jul 2022 10:31:09 +0200
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Subject: Re: [PATCH v2 1/4] PM: EM: convert power field to micro-Watts
 precision and align drivers
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To:     Lukasz Luba <lukasz.luba@arm.com>, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org
Cc:     amitk@kernel.org, rui.zhang@intel.com, viresh.kumar@linaro.org,
        rafael@kernel.org, dietmar.eggemann@arm.com, nm@ti.com,
        sboyd@kernel.org, sudeep.holla@arm.com, cristian.marussi@arm.com,
        matthias.bgg@gmail.com, linux-mediatek@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org
References: <20220707071555.10085-1-lukasz.luba@arm.com>
 <20220707071555.10085-2-lukasz.luba@arm.com>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
In-Reply-To: <20220707071555.10085-2-lukasz.luba@arm.com>
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Precedence: bulk

On 07/07/2022 09:15, Lukasz Luba wrote:
> The milli-Watts precision causes rounding errors while calculating
> efficiency cost for each OPP. This is especially visible in the 'simple'
> Energy Model (EM), where the power for each OPP is provided from OPP
> framework. This can cause some OPPs to be marked inefficient, while
> using micro-Watts precision that might not happen.
> 
> Update all EM users which access 'power' field and assume the value is
> in milli-Watts.
> 
> Solve also an issue with potential overflow in calculation of energy
> estimation on 32bit machine. It's needed now since the power value
> (thus the 'cost' as well) are higher.
> 
> Example calculation which shows the rounding error and impact:
> 
> power = 'dyn-power-coeff' * volt_mV * volt_mV * freq_MHz
> 
> power_a_uW = (100 * 600mW * 600mW * 500MHz) / 10^6 = 18000
> power_a_mW = (100 * 600mW * 600mW * 500MHz) / 10^9 = 18
> 
> power_b_uW = (100 * 605mW * 605mW * 600MHz) / 10^6 = 21961
> power_b_mW = (100 * 605mW * 605mW * 600MHz) / 10^9 = 21
> 
> max_freq = 2000MHz
> 
> cost_a_mW = 18 * 2000MHz/500MHz = 72
> cost_a_uW = 18000 * 2000MHz/500MHz = 72000
> 
> cost_b_mW = 21 * 2000MHz/600MHz = 70 // <- artificially better
> cost_b_uW = 21961 * 2000MHz/600MHz = 73203
> 
> The 'cost_b_mW' (which is based on old milli-Watts) is misleadingly
> better that the 'cost_b_uW' (this patch uses micro-Watts) and such
> would have impact on the 'inefficient OPPs' information in the Cpufreq
> framework. This patch set removes the rounding issue.
> 
> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>


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