[RFC PATCH 2/8] Documentation: arm: define DT cpu capacity bindings

[Juri Lelli <juri.lelli@arm.com>]

ARM systems may be configured to have cpus with different power/performance
characteristics within the same chip. In this case, additional information
has to be made available to the kernel (the scheduler in particular) for it
to be aware of such differences and take decisions accordingly.

Therefore, this patch aims at standardizing cpu capacities device tree
bindings for ARM platforms. Bindings define cpu capacity parameter, to
allow operating systems to retrieve such information from the device tree
and initialize related kernel structures, paving the way for common code in
the kernel to deal with heterogeneity.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Olof Johansson <olof@lixom.net>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
---
 .../devicetree/bindings/arm/cpu-capacity.txt       | 227 +++++++++++++++++++++
 Documentation/devicetree/bindings/arm/cpus.txt     |  17 ++
 2 files changed, 244 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/arm/cpu-capacity.txt

diff --git a/Documentation/devicetree/bindings/arm/cpu-capacity.txt b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
new file mode 100644
index 0000000..2a00af0
--- /dev/null
+++ b/Documentation/devicetree/bindings/arm/cpu-capacity.txt
@@ -0,0 +1,227 @@
+==========================================
+ARM CPUs capacity bindings
+==========================================
+
+==========================================
+1 - Introduction
+==========================================
+
+ARM systems may be configured to have cpus with different power/performance
+characteristics within the same chip. In this case, additional information
+has to be made available to the kernel (the scheduler in particular) for
+it to be aware of such differences and take decisions accordingly.
+
+==========================================
+2 - CPU capacity definition
+==========================================
+
+CPU capacity is a number that provides the scheduler information about CPUs
+heterogeneity. Such heterogeneity can come from micro-architectural differences
+(e.g., ARM big.LITTLE systems) or maximum frequency at which CPUs can run
+(e.g., SMP systems with multiple frequency domains). Heterogeneity in this
+context is about differing performance characteristics; this binding tries to
+capture a first-order approximation of the relative performance of CPUs.
+
+One simple way to estimate CPU capacities is to iteratively run a well-known
+CPU user space benchmark (e.g, sysbench, dhrystone, etc.) on each CPU at
+maximum frequency and then normalize values w.r.t.  the best performing CPU.
+One can also do a statistically significant study of a wide collection of
+benchmarks, but pros of such an approach are not really evident at the time of
+writing.
+
+==========================================
+3 - capacity-scale
+==========================================
+
+CPUs capacities are defined with respect to capacity-scale property in the cpus
+node [1]. The property is optional; if not defined a 1024 capacity-scale is
+assumed. This property defines both the highest CPU capacity present in the
+system and granularity of CPU capacity values.
+
+==========================================
+4 - capacity
+==========================================
+
+capacity is an optional cpu node [1] property: u32 value representing CPU
+capacity, relative to capacity-scale. It is required and enforced that capacity
+<= capacity-scale.
+
+===========================================
+5 - Examples
+===========================================
+
+Example 1 (ARM 64-bit, 6-cpu system, two clusters):
+capacity-scale is not defined, so it is assumed to be 1024
+
+cpus {
+	#address-cells = <2>;
+	#size-cells = <0>;
+
+	cpu-map {
+		cluster0 {
+			core0 {
+				cpu = <&A57_0>;
+			};
+			core1 {
+				cpu = <&A57_1>;
+			};
+		};
+
+		cluster1 {
+			core0 {
+				cpu = <&A53_0>;
+			};
+			core1 {
+				cpu = <&A53_1>;
+			};
+			core2 {
+				cpu = <&A53_2>;
+			};
+			core3 {
+				cpu = <&A53_3>;
+			};
+		};
+	};
+
+	idle-states {
+		entry-method = "arm,psci";
+
+		CPU_SLEEP_0: cpu-sleep-0 {
+			compatible = "arm,idle-state";
+			arm,psci-suspend-param = <0x0010000>;
+			local-timer-stop;
+			entry-latency-us = <100>;
+			exit-latency-us = <250>;
+			min-residency-us = <150>;
+		};
+
+		CLUSTER_SLEEP_0: cluster-sleep-0 {
+			compatible = "arm,idle-state";
+			arm,psci-suspend-param = <0x1010000>;
+			local-timer-stop;
+			entry-latency-us = <800>;
+			exit-latency-us = <700>;
+			min-residency-us = <2500>;
+		};
+	};
+
+	A57_0: cpu@0 {
+		compatible = "arm,cortex-a57","arm,armv8";
+		reg = <0x0 0x0>;
+		device_type = "cpu";
+		enable-method = "psci";
+		next-level-cache = <&A57_L2>;
+		clocks = <&scpi_dvfs 0>;
+		cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
+		capacity = <1024>;
+	};
+
+	A57_1: cpu@1 {
+		compatible = "arm,cortex-a57","arm,armv8";
+		reg = <0x0 0x1>;
+		device_type = "cpu";
+		enable-method = "psci";
+		next-level-cache = <&A57_L2>;
+		clocks = <&scpi_dvfs 0>;
+		cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
+		capacity = <1024>;
+	};
+
+	A53_0: cpu@100 {
+		compatible = "arm,cortex-a53","arm,armv8";
+		reg = <0x0 0x100>;
+		device_type = "cpu";
+		enable-method = "psci";
+		next-level-cache = <&A53_L2>;
+		clocks = <&scpi_dvfs 1>;
+		cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
+		capacity = <447>;
+	};
+
+	A53_1: cpu@101 {
+		compatible = "arm,cortex-a53","arm,armv8";
+		reg = <0x0 0x101>;
+		device_type = "cpu";
+		enable-method = "psci";
+		next-level-cache = <&A53_L2>;
+		clocks = <&scpi_dvfs 1>;
+		cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
+		capacity = <447>;
+	};
+
+	A53_2: cpu@102 {
+		compatible = "arm,cortex-a53","arm,armv8";
+		reg = <0x0 0x102>;
+		device_type = "cpu";
+		enable-method = "psci";
+		next-level-cache = <&A53_L2>;
+		clocks = <&scpi_dvfs 1>;
+		cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
+		capacity = <447>;
+	};
+
+	A53_3: cpu@103 {
+		compatible = "arm,cortex-a53","arm,armv8";
+		reg = <0x0 0x103>;
+		device_type = "cpu";
+		enable-method = "psci";
+		next-level-cache = <&A53_L2>;
+		clocks = <&scpi_dvfs 1>;
+		cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
+		capacity = <447>;
+	};
+
+	A57_L2: l2-cache0 {
+		compatible = "cache";
+	};
+
+	A53_L2: l2-cache1 {
+		compatible = "cache";
+	};
+};
+
+Example 2 (ARM 32-bit, 4-cpu system, two clusters,
+	   cpus 0,1@1GHz, cpus 2,3@500MHz):
+capacity-scale is equal to 2, so first cluster is twice faster than second
+cluster (which matches with clock frequencies)
+
+cpus {
+	#address-cells = <1>;
+	#size-cells = <0>;
+	capacity-scale = <2>;
+
+	cpu0: cpu@0 {
+		device_type = "cpu";
+		compatible = "arm,cortex-a15";
+		reg = <0>;
+		capacity = <2>;
+	};
+
+	cpu1: cpu@1 {
+		device_type = "cpu";
+		compatible = "arm,cortex-a15";
+		reg = <1>;
+		capacity = <2>;
+	};
+
+	cpu2: cpu@2 {
+		device_type = "cpu";
+		compatible = "arm,cortex-a15";
+		reg = <0x100>;
+		capacity = <1>;
+	};
+
+	cpu3: cpu@3 {
+		device_type = "cpu";
+		compatible = "arm,cortex-a15";
+		reg = <0x101>;
+		capacity = <1>;
+	};
+};
+
+===========================================
+6 - References
+===========================================
+
+[1] ARM Linux Kernel documentation - CPUs bindings
+    Documentation/devicetree/bindings/arm/cpus.txt
diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
index 91e6e5c..7593584 100644
--- a/Documentation/devicetree/bindings/arm/cpus.txt
+++ b/Documentation/devicetree/bindings/arm/cpus.txt
@@ -62,6 +62,14 @@ nodes to be present and contain the properties described below.
 		Value type: <u32>
 		Definition: must be set to 0
 
+	A cpus node may also define the following optional property:
+
+	- capacity-scale
+		Usage: optional
+		Value type: <u32>
+		Definition: value used as a reference for CPU capacity [3]
+			    (see below).
+
 - cpu node
 
 	Description: Describes a CPU in an ARM based system
@@ -231,6 +239,13 @@ nodes to be present and contain the properties described below.
 			# List of phandles to idle state nodes supported
 			  by this cpu [3].
 
+	- capacity
+		Usage: Optional
+		Value type: <u32>
+		Definition:
+			# u32 value representing CPU capacity [3], relative to
+			  capacity-scale (see above).
+
 	- rockchip,pmu
 		Usage: optional for systems that have an "enable-method"
 		       property value of "rockchip,rk3066-smp"
@@ -437,3 +452,5 @@ cpus {
 [2] arm/msm/qcom,kpss-acc.txt
 [3] ARM Linux kernel documentation - idle states bindings
     Documentation/devicetree/bindings/arm/idle-states.txt
+[3] ARM Linux kernel documentation - cpu capacity bindings
+    Documentation/devicetree/bindings/arm/cpu-capacity.txt
-- 
2.2.2