From mboxrd@z Thu Jan 1 00:00:00 1970 From: Punit Agrawal Subject: Re: [PATCH v2] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit Date: Thu, 30 Aug 2018 09:46:18 +0100 Message-ID: <87y3co8co5.fsf@e105922-lin.cambridge.arm.com> References: <1535526518-29022-1-git-send-email-vincent.guittot@linaro.org> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <1535526518-29022-1-git-send-email-vincent.guittot@linaro.org> (Vincent Guittot's message of "Wed, 29 Aug 2018 09:08:38 +0200") Sender: linux-kernel-owner@vger.kernel.org To: Vincent Guittot Cc: robh+dt@kernel.org, mark.rutland@arm.com, devicetree@vger.kernel.org, edubezval@gmail.com, viresh.kumar@linaro.org, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org List-Id: devicetree@vger.kernel.org Hi Vincent, Vincent Guittot writes: > The unit of dynamic-power-coefficient is described as mW/MHz/uV^2 whereas > its usage in the code assumes that unit is uW/MHz/V^2 > > In drivers/thermal/cpu_cooling.c, the code is : > > power = (u64)capacitance * freq_mhz * voltage_mv * voltage_mv; > do_div(power, 1000000000); > > which can be summarized as : > power (mW) = capacitance * freq_mhz/1000 * (voltage_mv/1000)^2 > or > power (mW) = (capacitance * freq_mhz * (voltage_mv/1000)^2) / 1000 > then > power (mW) = power (uW) / 1000 > so > power (uW) = capacitance * freq_mhz * (voltage_mv/1000)^2 > > Furthermore, if we test basic values like : > voltage_mv = 1000mV = 1V > freq_mhz = 1000Mhz > > The minimum possible power, when dynamic-power-coefficient equals 1, will > be with current unit: > min power = 1 * 1000 * (1000000)^2 = 10^15 mW > which is not realistic > > With the unit used by the code, the min power is > min power = 1 * 1000 * 1^2 = 1000uW = 1mW which is far more realistic > > Signed-off-by: Vincent Guittot Acked-by: Punit Agrawal Thanks for fixing the mismatch. > --- > Documentation/devicetree/bindings/arm/cpus.txt | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt > index 29e1dc5..71d8cd0 100644 > --- a/Documentation/devicetree/bindings/arm/cpus.txt > +++ b/Documentation/devicetree/bindings/arm/cpus.txt > @@ -274,7 +274,7 @@ described below. > Usage: optional > Value type: > Definition: A u32 value that represents the running time dynamic > - power coefficient in units of mW/MHz/uV^2. The > + power coefficient in units of uW/MHz/V^2. The > coefficient can either be calculated from power > measurements or derived by analysis. > > @@ -285,7 +285,7 @@ described below. > > Pdyn = dynamic-power-coefficient * V^2 * f > > - where voltage is in uV, frequency is in MHz. > + where voltage is in V, frequency is in MHz. > > Example 1 (dual-cluster big.LITTLE system 32-bit):