[PATCH 2/4] arm: dts: exynos: add exynos5420 cpu capacity-dmips-mhz information

[Dietmar Eggemann]

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 at kernel.org>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Russell King <linux at armlinux.org.uk>
Cc: Kukjin Kim <kgene at kernel.org>
Cc: Krzysztof Kozlowski <krzk at kernel.org>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann at arm.com>
---
 arch/arm/boot/dts/exynos5420-cpus.dtsi | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/arm/boot/dts/exynos5420-cpus.dtsi b/arch/arm/boot/dts/exynos5420-cpus.dtsi
index 5c052d7ff554..d7d703aa1699 100644
--- a/arch/arm/boot/dts/exynos5420-cpus.dtsi
+++ b/arch/arm/boot/dts/exynos5420-cpus.dtsi
@@ -36,6 +36,7 @@
 			cooling-min-level = <0>;
 			cooling-max-level = <11>;
 			#cooling-cells = <2>; /* min followed by max */
+			capacity-dmips-mhz = <1024>;
 		};
 
 		cpu1: cpu at 1 {
@@ -48,6 +49,7 @@
 			cooling-min-level = <0>;
 			cooling-max-level = <11>;
 			#cooling-cells = <2>; /* min followed by max */
+			capacity-dmips-mhz = <1024>;
 		};
 
 		cpu2: cpu at 2 {
@@ -60,6 +62,7 @@
 			cooling-min-level = <0>;
 			cooling-max-level = <11>;
 			#cooling-cells = <2>; /* min followed by max */
+			capacity-dmips-mhz = <1024>;
 		};
 
 		cpu3: cpu at 3 {
@@ -72,6 +75,7 @@
 			cooling-min-level = <0>;
 			cooling-max-level = <11>;
 			#cooling-cells = <2>; /* min followed by max */
+			capacity-dmips-mhz = <1024>;
 		};
 
 		cpu4: cpu at 100 {
@@ -85,6 +89,7 @@
 			cooling-min-level = <0>;
 			cooling-max-level = <7>;
 			#cooling-cells = <2>; /* min followed by max */
+			capacity-dmips-mhz = <539>;
 		};
 
 		cpu5: cpu at 101 {
@@ -97,6 +102,7 @@
 			cooling-min-level = <0>;
 			cooling-max-level = <7>;
 			#cooling-cells = <2>; /* min followed by max */
+			capacity-dmips-mhz = <539>;
 		};
 
 		cpu6: cpu at 102 {
@@ -109,6 +115,7 @@
 			cooling-min-level = <0>;
 			cooling-max-level = <7>;
 			#cooling-cells = <2>; /* min followed by max */
+			capacity-dmips-mhz = <539>;
 		};
 
 		cpu7: cpu at 103 {
@@ -121,6 +128,7 @@
 			cooling-min-level = <0>;
 			cooling-max-level = <7>;
 			#cooling-cells = <2>; /* min followed by max */
+			capacity-dmips-mhz = <539>;
 		};
 	};
 };
-- 
2.11.0