Re: [Eas-dev] Computing the dynamic-power-coefficient on Exynos5422

Hi Russel and Oliver,

On Fri, 20 Jul 2018 at 17:36, Russell King - ARM Linux linux@armlinux.org.uk wrote:

On Fri, Jul 20, 2018 at 04:15:30PM +0200, Oliver Effland wrote:

...

Hello everyone,

I hope this is the right place to ask, otherwise please just point me in the right direction.

I'm currently testing the EAS patches [v4] on an ODROID-XU3 board, which has an Exynos5422 SoC. However, the corresponding DT is missing the "dynamic-power-coefficient" that is needed for an appropriate EM. So I'm trying to compute the dynamic-power-coefficient according to the formula:

Pdyn = dynamic-power-coefficient * V^2 * f

The frequency f is given by the DT.The actual Voltage and Power are determined by means of the on-chip sensors (returns the values for the specific cluster). When using the Voltage given in the DT, the differences for the d-p-coefficient are negligible.

So I'm calculating the d-p-coefficient (mW/MHz/uV^2) by reading out the following SoC sensor values.

Well, let's work this forward. Let's take a simple, reasonable case.

V=1V, f = 200MHz. Now let's select the smallest value that dynamic-power-coefficient can be in DT, which, as it's an integer, is 1. We're told that V is in uV, and f is in MHz. So, let's plug the figures in:

Pdyn = 1 * (1000000)^2 * 200

That is 2 * 10^14mW, or 200GW.

Somehow, I think the binding documentation is very wrong.

...

For the little cluster (A7): frequency(MHz) Voltage(V) Power(mW) Dynamic-power-coefficient 200 0.9175 49.470 ~2.938*10^-13 400 0.9165 91.892 ~2.736*10^-13 600 0.9638 149.454 ~2.682*10^-13 800 1.0263 223.453 ~2.652*10^-13 1000 1.1000 327.707 ~2.708*10^-13 1200 1.1725 445.899 ~2.703*10^-13 1400 1.2713 627.010 ~2.771*10^-13

For the big cluster (A15): frequency(MHz) Voltage(V) Power(mW) Dynamic-power-coefficient 200 0.9162 159.676 ~9.510*10^-13 500 0.9138 325.480 ~7.797*10^-13 800 0.9288 511.360 ~7.410*10^-13 1100 1.0063 828.020 ~7.434*10^-13 1400 1.0713 1209.774 ~7.530*10^-13 1700 1.1750 1835.784 ~7.822*10^-13 2000 1.2700 2661.849 ~8.252*10^-13

But those values are way off for the DT, unless I multiply the d-p-coefficient with 10^15.

Assuming the Power value needs to be subtracted by a static component (power usage when idle), the results change as follows:

For the little cluster (A7): frequency(MHz) Voltage(V) P-dyn(mW) Dynamic-power-coefficient 200 0.9175 36,381 ~2,160*10^-13 400 0.9165 73,438 ~2,186*10^-13 600 0.9638 122,470 ~2,197*10^-13 800 1.0263 184,135 ~2,185*10^-13 1000 1.1000 270,425 ~2,234*10^-13 1200 1.1725 367,121 ~2,225*10^-13 1400 1.2713 513,989 ~2,271*10^-13

For the big cluster (A15): frequency(MHz) Voltage(V) P-dyn(mW) Dynamic-power-coefficient 200 0.9162 99,834 ~5,945*10^-13 500 0.9138 244,651 ~5,860*10^-13 800 0.9288 401,986 ~5,825*10^-13 1100 1.0063 664,565 ~5,966*10^-13 1400 1.0713 985,268 ~6,132*10^-13 1700 1.1750 1494,173 ~6,366*10^-13 2000 1.2700 2086,273 ~6,467*10^-13

While looking for examples in the kernel that calculate the dynamic-power-coefficient, I found this patch [1] by Caesar Wang that introduced this value for the rk3399 big cluster. Unfortunately, I'm unable to reproduce the same results for the coefficient with the given values. My results are also with a 10^-13 factor and even when I scale them with 10^15, my results have a certain margin of error:

...

frequency(MHz) Voltage(V) Current(mA) Dynamic-power-coefficient (My) d-p-coefficient 24 0.8 15 48 0.8 23 ~417 ~598 96 0.8 40 ~443 ~520 216 0.8 82 ~438 ~474 312 0.8 115 ~430 ~460 408 0.8 150 ~455 ~459

Hmm. Let's take your 48MHz values.

Pdyn (mW) = 0.8 (V) * 23 (mA) = 18.4mW

dynamic-power-coefficient = Pdyn / (V^2 * f) = 18.4 (mW) / (800000 * 800000 * 48) = 5.99 * 10^-13

Seems, again, to me that there is something very wrong with the binding documentation.

Now, let's dig into the code:

• dev_pm_opp_get_voltage() - Gets the voltage corresponding to an opp
• Return: voltage in micro volt corresponding to the opp, else
• return 0

So this returns uV.

            voltage_mv = dev_pm_opp_get_voltage(opp) / 1000;

This is mV.

            u32 freq_mhz = freq_table[i].frequency / 1000;

MHz.

            power = (u64)capacitance * freq_mhz * voltage_mv * voltage_mv;
            do_div(power, 1000000000);

            /* power is stored in mW */
            freq_table[i].power = power;

So we have:

    power(mW) = dynamic-power-coefficient * f(MHz) * V(mV)^2 / 10^9

That 10^9 can't come from the fact that the calculation is using mV instead of uV. Let's see what happens if we use this:

    dynamic-power-coefficient = power(mW) * 10^9 / (f(MHz) * V(mV)^2)
            = 18.4 * 10^9 / (48 * 800 * 800)
            = 599

which, given that dynamic-power-coefficient is an integer, is a damn sight more sensible. This is equivalent to:

    dynamic-power-coefficient = power(mW) * 10^15 / (f(MHz) * V(uV)^2)

which means the binding documentation is wrong.

I came to similar conclusion on another thread: https://patchwork.kernel.org/patch/10493615 the right unit of dynamic-power-coefficient is mW/V^2/Ghz

Regards, Vincent

Now, one of the things to bear in mind is that if you're measuring the voltage and current, and wanting "dynamic power" then you don't get that from multiplying the voltage and current - that includes the _static_ consumption.

    total power = static power + dynamic power
                = V * (Ileakage + Idynamic)

and Idynamic will be based upon the frequency.

Taking your table:

...

...

frequency(MHz) Voltage(V) Current(mA) dpc Pdyn Ptot Pstatic 24 0.8 15 48 0.8 23 ~417 12.8 18.4 5.6 96 0.8 40 ~443 27.2 32 4.8 216 0.8 82 ~438 60.5 65.6 5.1 312 0.8 115 ~430 85.9 92 6.1 408 0.8 150 ~455 119 120 1

Pdyn comes from the dynamic-power-coefficient calculation, Ptot from the voltage and current figures, and Pstatic being the difference between Ptot and Pdyn.

Apart from the last, it looks like there's a static power of an average 5.4mW. Plugging that back in:

    dynamic-power-coefficient = power(mW) * 10^9 / (f(MHz) * V(mV)^2)
            = (18.4 - 5.4) * 10^9 / (48 * 800 * 800)  = 423
            = (32 - 5.4) * 10^9 / (96 * 800 * 800)    = 433
            = (65.6 - 5.4) * 10^9 / (216 * 800 * 800) = 435
            = (92 - 5.4) * 10^9 / (312 * 800 * 800)   = 434
            = (120 - 5.4) * 10^9 / (408 * 800 * 800)  = 439

which is a bit more believable (even compared to the original post of the table.)

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