[PATCH 2/3] ARM: dts: exynos: Add Exynos5422 CPU dynamic-power-coefficient information

[lukasz . luba]

From: Lukasz Luba <lukasz.luba@xxxxxxx>

Add dynamic power coefficient into CPU nodes which let CPUFreq subsystem
register the Energy Model (EM) for the CPUs.

The 'dynamic-power-coefficient' is used for calculating the dynamic power
according to the equation in documentation [1].  The Energy Model (EM)
framework relies on calculated power and cost for each OPP. The OPP power
values come from CPUFreq driver, which registered required callback
function. The simple implementation of a CPUFREQ driver, like cpufreq-dt,
uses 'dev_pm_opp_of_register_em()' which relay on
'dynamic-power-coefficient' to calculate the power of requested OPP for the
EM [2].

The calculated values might be checked in
/sys/kernel/debug/energy_model/pd*/

$ grep . /sys/kernel/debug/energy_model/pd1/cs*/*
/sys/kernel/debug/energy_model/pd1/cs:1000000/cost:558
/sys/kernel/debug/energy_model/pd1/cs:1000000/frequency:1000000
/sys/kernel/debug/energy_model/pd1/cs:1000000/power:310
/sys/kernel/debug/energy_model/pd1/cs:1100000/cost:558
/sys/kernel/debug/energy_model/pd1/cs:1100000/frequency:1100000
/sys/kernel/debug/energy_model/pd1/cs:1100000/power:341
/sys/kernel/debug/energy_model/pd1/cs:1200000/cost:558
/sys/kernel/debug/energy_model/pd1/cs:1200000/frequency:1200000
/sys/kernel/debug/energy_model/pd1/cs:1200000/power:372
/sys/kernel/debug/energy_model/pd1/cs:1300000/cost:674
/sys/kernel/debug/energy_model/pd1/cs:1300000/frequency:1300000
/sys/kernel/debug/energy_model/pd1/cs:1300000/power:487
/sys/kernel/debug/energy_model/pd1/cs:1400000/cost:675 ...

$ grep . /sys/kernel/debug/energy_model/pd0/cs*/*
/sys/kernel/debug/energy_model/pd0/cs:1000000/cost:200
/sys/kernel/debug/energy_model/pd0/cs:1000000/frequency:1000000
/sys/kernel/debug/energy_model/pd0/cs:1000000/power:154
/sys/kernel/debug/energy_model/pd0/cs:1100000/cost:260
/sys/kernel/debug/energy_model/pd0/cs:1100000/frequency:1100000
/sys/kernel/debug/energy_model/pd0/cs:1100000/power:220
/sys/kernel/debug/energy_model/pd0/cs:1200000/cost:260
/sys/kernel/debug/energy_model/pd0/cs:1200000/frequency:1200000
/sys/kernel/debug/energy_model/pd0/cs:1200000/power:240
/sys/kernel/debug/energy_model/pd0/cs:1300000/cost:260
/sys/kernel/debug/energy_model/pd0/cs:1300000/frequency:1300000
/sys/kernel/debug/energy_model/pd0/cs:1300000/power:260
/sys/kernel/debug/energy_model/pd0/cs:200000/cost:130 ...

To provide a proper value of the 'dynamic-power-coefficient' the real power
can be measured using a dedicated hardware, i.e. INA2xx. The Odroid-XU3
hwmon sensors have been used to capture the power value during a sysbench
test running on single core and at each possible OPP. The measured values
were divided by 2, since the dynamic power is typically half of the
consumed power (the second half is static power). Next, the approximation
was made and the power model derived, showing the 'C' value of routhly X.
Check the example equations in drivers/opp/of.c [2].
Thus, i.e. the power = 1.0Watt at 1GHz => 0.5W dynamic power =>
dynamic-power-coefficient = 400

Using this simple technique we can provide and needed coefficient.  The
approximation does not have to be super precised. The proportion is
important and the difference between power consumed by different CPUs
running at the same frequency, which is then used in Energy Aware Scheduler
algorithms. An example power values on Odroid-XU3:

(LITTLE CPU)
/sys/kernel/debug/energy_model/pd0/cs:1000000/frequency:1000000
/sys/kernel/debug/energy_model/pd0/cs:1000000/power:154
(big CPU)
/sys/kernel/debug/energy_model/pd1/cs:1000000/frequency:1000000
/sys/kernel/debug/energy_model/pd1/cs:1000000/power:310

In Odroid-XU3 case the derived coefficient value for 'big' CPU has:
dynamic-power-coefficient = <310>;
while the 'LITTLE':
dynamic-power-coefficient = <128>;

[1] Documentation/devicetree/bindings/arm/cpus.yaml
[2] https://elixir.bootlin.com/linux/v5.4/source/drivers/opp/of.c#L1044

Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
---
 arch/arm/boot/dts/exynos5422-cpus.dtsi | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/arm/boot/dts/exynos5422-cpus.dtsi b/arch/arm/boot/dts/exynos5422-cpus.dtsi
index 1b8605cf2407..c9a0dc99d2fb 100644
--- a/arch/arm/boot/dts/exynos5422-cpus.dtsi
+++ b/arch/arm/boot/dts/exynos5422-cpus.dtsi
@@ -31,6 +31,7 @@
 			operating-points-v2 = <&cluster_a7_opp_table>;
 			#cooling-cells = <2>; /* min followed by max */
 			capacity-dmips-mhz = <539>;
+			dynamic-power-coefficient = <128>;
 		};
 
 		cpu1: cpu@101 {
@@ -43,6 +44,7 @@
 			operating-points-v2 = <&cluster_a7_opp_table>;
 			#cooling-cells = <2>; /* min followed by max */
 			capacity-dmips-mhz = <539>;
+			dynamic-power-coefficient = <128>;
 		};
 
 		cpu2: cpu@102 {
@@ -55,6 +57,7 @@
 			operating-points-v2 = <&cluster_a7_opp_table>;
 			#cooling-cells = <2>; /* min followed by max */
 			capacity-dmips-mhz = <539>;
+			dynamic-power-coefficient = <128>;
 		};
 
 		cpu3: cpu@103 {
@@ -67,6 +70,7 @@
 			operating-points-v2 = <&cluster_a7_opp_table>;
 			#cooling-cells = <2>; /* min followed by max */
 			capacity-dmips-mhz = <539>;
+			dynamic-power-coefficient = <128>;
 		};
 
 		cpu4: cpu@0 {
@@ -79,6 +83,7 @@
 			operating-points-v2 = <&cluster_a15_opp_table>;
 			#cooling-cells = <2>; /* min followed by max */
 			capacity-dmips-mhz = <1024>;
+			dynamic-power-coefficient = <310>;
 		};
 
 		cpu5: cpu@1 {
@@ -91,6 +96,7 @@
 			operating-points-v2 = <&cluster_a15_opp_table>;
 			#cooling-cells = <2>; /* min followed by max */
 			capacity-dmips-mhz = <1024>;
+			dynamic-power-coefficient = <310>;
 		};
 
 		cpu6: cpu@2 {
@@ -103,6 +109,7 @@
 			operating-points-v2 = <&cluster_a15_opp_table>;
 			#cooling-cells = <2>; /* min followed by max */
 			capacity-dmips-mhz = <1024>;
+			dynamic-power-coefficient = <310>;
 		};
 
 		cpu7: cpu@3 {
@@ -115,6 +122,7 @@
 			operating-points-v2 = <&cluster_a15_opp_table>;
 			#cooling-cells = <2>; /* min followed by max */
 			capacity-dmips-mhz = <1024>;
+			dynamic-power-coefficient = <310>;
 		};
 	};
 };
-- 
2.17.1