Documentation/devicetree/bindings/arm/idle-states.txt - linux - Git at Google

 ==========================================
 ARM idle states binding description
 ==========================================

 ==========================================
 1 - Introduction
 ==========================================

 ARM systems contain HW capable of managing power consumption dynamically,
 where cores can be put in different low-power states (ranging from simple
 wfi to power gating) according to OS PM policies. The CPU states representing
 the range of dynamic idle states that a processor can enter at run-time, can be
 specified through device tree bindings representing the parameters required
 to enter/exit specific idle states on a given processor.

 According to the Server Base System Architecture document (SBSA, [3]), the
 power states an ARM CPU can be put into are identified by the following list:

 - Running
 - Idle_standby
 - Idle_retention
 - Sleep
 - Off

 The power states described in the SBSA document define the basic CPU states on
 top of which ARM platforms implement power management schemes that allow an OS
 PM implementation to put the processor in different idle states (which include
 states listed above; "off" state is not an idle state since it does not have
 wake-up capabilities, hence it is not considered in this document).

 Idle state parameters (eg entry latency) are platform specific and need to be
 characterized with bindings that provide the required information to OS PM
 code so that it can build the required tables and use them at runtime.

 The device tree binding definition for ARM idle states is the subject of this
 document.

 ===========================================
 2 - idle-states definitions
 ===========================================

 Idle states are characterized for a specific system through a set of
 timing and energy related properties, that underline the HW behaviour
 triggered upon idle states entry and exit.

 The following diagram depicts the CPU execution phases and related timing
 properties required to enter and exit an idle state:

 ..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]__..
 	    |          |           |          |          |

 	    |<------ entry ------->|
 	    |       latency        |
 					      |<- exit ->|
 					      |  latency |
 	    |<-------- min-residency -------->|
 		       |<-------  wakeup-latency ------->|

 		Diagram 1: CPU idle state execution phases

 EXEC:	Normal CPU execution.

 PREP:	Preparation phase before committing the hardware to idle mode
 	like cache flushing. This is abortable on pending wake-up
 	event conditions. The abort latency is assumed to be negligible
 	(i.e. less than the ENTRY + EXIT duration). If aborted, CPU
 	goes back to EXEC. This phase is optional. If not abortable,
 	this should be included in the ENTRY phase instead.

 ENTRY:	The hardware is committed to idle mode. This period must run
 	to completion up to IDLE before anything else can happen.

 IDLE:	This is the actual energy-saving idle period. This may last
 	between 0 and infinite time, until a wake-up event occurs.

 EXIT:	Period during which the CPU is brought back to operational
 	mode (EXEC).

 entry-latency: Worst case latency required to enter the idle state. The
 exit-latency may be guaranteed only after entry-latency has passed.

 min-residency: Minimum period, including preparation and entry, for a given
 idle state to be worthwhile energywise.

 wakeup-latency: Maximum delay between the signaling of a wake-up event and the
 CPU being able to execute normal code again. If not specified, this is assumed
 to be entry-latency + exit-latency.

 These timing parameters can be used by an OS in different circumstances.

 An idle CPU requires the expected min-residency time to select the most
 appropriate idle state based on the expected expiry time of the next IRQ
 (ie wake-up) that causes the CPU to return to the EXEC phase.

 An operating system scheduler may need to compute the shortest wake-up delay
 for CPUs in the system by detecting how long will it take to get a CPU out
 of an idle state, eg:

 wakeup-delay = exit-latency + max(entry-latency - (now - entry-timestamp), 0)

 In other words, the scheduler can make its scheduling decision by selecting
 (eg waking-up) the CPU with the shortest wake-up latency.
 The wake-up latency must take into account the entry latency if that period
 has not expired. The abortable nature of the PREP period can be ignored
 if it cannot be relied upon (e.g. the PREP deadline may occur much sooner than
 the worst case since it depends on the CPU operating conditions, ie caches
 state).

 An OS has to reliably probe the wakeup-latency since some devices can enforce
 latency constraints guarantees to work properly, so the OS has to detect the
 worst case wake-up latency it can incur if a CPU is allowed to enter an
 idle state, and possibly to prevent that to guarantee reliable device
 functioning.

 The min-residency time parameter deserves further explanation since it is
 expressed in time units but must factor in energy consumption coefficients.

 The energy consumption of a cpu when it enters a power state can be roughly
 characterised by the following graph:

                |
                |
                |
            e   |
            n   |                                      /---
            e   |                               /------
            r   |                        /------
            g   |                  /-----
            y   |           /------
                |       ----
                |      /|
                |     / |
                |    /  |
                |   /   |
                |  /    |
                | /     |
                |/      |
           -----|-------+----------------------------------
               0|       1                              time(ms)

 		Graph 1: Energy vs time example

 The graph is split in two parts delimited by time 1ms on the X-axis.
 The graph curve with X-axis values = { x | 0 < x < 1ms } has a steep slope
 and denotes the energy costs incurred while entering and leaving the idle
 state.
 The graph curve in the area delimited by X-axis values = {x | x > 1ms } has
 shallower slope and essentially represents the energy consumption of the idle
 state.

 min-residency is defined for a given idle state as the minimum expected
 residency time for a state (inclusive of preparation and entry) after
 which choosing that state become the most energy efficient option. A good
 way to visualise this, is by taking the same graph above and comparing some
 states energy consumptions plots.

 For sake of simplicity, let's consider a system with two idle states IDLE1,
 and IDLE2:

           |
           |
           |
           |                                                  /-- IDLE1
        e  |                                              /---
        n  |                                         /----
        e  |                                     /---
        r  |                                /-----/--------- IDLE2
        g  |                    /-------/---------
        y  |        ------------    /---|
           |       /           /----    |
           |      /        /---         |
           |     /    /----             |
           |    / /---                  |
           |   ---                      |
           |  /                         |
           | /                          |
           |/                           |                  time
        ---/----------------------------+------------------------
           |IDLE1-energy < IDLE2-energy | IDLE2-energy < IDLE1-energy
                                        |
                                 IDLE2-min-residency

 		Graph 2: idle states min-residency example

 In graph 2 above, that takes into account idle states entry/exit energy
 costs, it is clear that if the idle state residency time (ie time till next
 wake-up IRQ) is less than IDLE2-min-residency, IDLE1 is the better idle state
 choice energywise.

 This is mainly down to the fact that IDLE1 entry/exit energy costs are lower
 than IDLE2.

 However, the lower power consumption (ie shallower energy curve slope) of idle
 state IDLE2 implies that after a suitable time, IDLE2 becomes more energy
 efficient.

 The time at which IDLE2 becomes more energy efficient than IDLE1 (and other
 shallower states in a system with multiple idle states) is defined
 IDLE2-min-residency and corresponds to the time when energy consumption of
 IDLE1 and IDLE2 states breaks even.

 The definitions provided in this section underpin the idle states
 properties specification that is the subject of the following sections.

 ===========================================
 3 - idle-states node
 ===========================================

 ARM processor idle states are defined within the idle-states node, which is
 a direct child of the cpus node [1] and provides a container where the
 processor idle states, defined as device tree nodes, are listed.

 - idle-states node

 	Usage: Optional - On ARM systems, it is a container of processor idle
 			  states nodes. If the system does not provide CPU
 			  power management capabilities or the processor just
 			  supports idle_standby an idle-states node is not
 			  required.

 	Description: idle-states node is a container node, where its
 		     subnodes describe the CPU idle states.

 	Node name must be "idle-states".

 	The idle-states node's parent node must be the cpus node.

 	The idle-states node's child nodes can be:

 	- one or more state nodes

 	Any other configuration is considered invalid.

 	An idle-states node defines the following properties:

 	- entry-method
 		Value type: <stringlist>
 		Usage and definition depend on ARM architecture version.
 			# On ARM v8 64-bit this property is required and must
 			  be:
 			   - "psci"
 			# On ARM 32-bit systems this property is optional

 The nodes describing the idle states (state) can only be defined within the
 idle-states node, any other configuration is considered invalid and therefore
 must be ignored.

 ===========================================
 4 - state node
 ===========================================

 A state node represents an idle state description and must be defined as
 follows:

 - state node

 	Description: must be child of the idle-states node

 	The state node name shall follow standard device tree naming
 	rules ([5], 2.2.1 "Node names"), in particular state nodes which
 	are siblings within a single common parent must be given a unique name.

 	The idle state entered by executing the wfi instruction (idle_standby
 	SBSA,[3][4]) is considered standard on all ARM platforms and therefore
 	must not be listed.

 	With the definitions provided above, the following list represents
 	the valid properties for a state node:

 	- compatible
 		Usage: Required
 		Value type: <stringlist>
 		Definition: Must be "arm,idle-state".

 	- local-timer-stop
 		Usage: See definition
 		Value type: <none>
 		Definition: if present the CPU local timer control logic is
 			    lost on state entry, otherwise it is retained.

 	- entry-latency-us
 		Usage: Required
 		Value type: <prop-encoded-array>
 		Definition: u32 value representing worst case latency in
 			    microseconds required to enter the idle state.
 			    The exit-latency-us duration may be guaranteed
 			    only after entry-latency-us has passed.

 	- exit-latency-us
 		Usage: Required
 		Value type: <prop-encoded-array>
 		Definition: u32 value representing worst case latency
 			    in microseconds required to exit the idle state.

 	- min-residency-us
 		Usage: Required
 		Value type: <prop-encoded-array>
 		Definition: u32 value representing minimum residency duration
 			    in microseconds, inclusive of preparation and
 			    entry, for this idle state to be considered
 			    worthwhile energy wise (refer to section 2 of
 			    this document for a complete description).

 	- wakeup-latency-us:
 		Usage: Optional
 		Value type: <prop-encoded-array>
 		Definition: u32 value representing maximum delay between the
 			    signaling of a wake-up event and the CPU being
 			    able to execute normal code again. If omitted,
 			    this is assumed to be equal to:

 				entry-latency-us + exit-latency-us

 			    It is important to supply this value on systems
 			    where the duration of PREP phase (see diagram 1,
 			    section 2) is non-neglibigle.
 			    In such systems entry-latency-us + exit-latency-us
 			    will exceed wakeup-latency-us by this duration.

 	- status:
 		Usage: Optional
 		Value type: <string>
 		Definition: A standard device tree property [5] that indicates
 			    the operational status of an idle-state.
 			    If present, it shall be:
 			    "okay": to indicate that the idle state is
 				    operational.
 			    "disabled": to indicate that the idle state has
 					been disabled in firmware so it is not
 					operational.
 			    If the property is not present the idle-state must
 			    be considered operational.

 	- idle-state-name:
 		Usage: Optional
 		Value type: <string>
 		Definition: A string used as a descriptive name for the idle
 			    state.

 	In addition to the properties listed above, a state node may require
 	additional properties specifics to the entry-method defined in the
 	idle-states node, please refer to the entry-method bindings
 	documentation for properties definitions.

 ===========================================
 4 - Examples
 ===========================================

 Example 1 (ARM 64-bit, 16-cpu system, PSCI enable-method):

 cpus {
 	#size-cells = <0>;
 	#address-cells = <2>;

 	CPU0: cpu@0 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a57";
 		reg = <0x0 0x0>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
 				   &CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU1: cpu@1 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a57";
 		reg = <0x0 0x1>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
 				   &CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU2: cpu@100 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a57";
 		reg = <0x0 0x100>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
 				   &CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU3: cpu@101 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a57";
 		reg = <0x0 0x101>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
 				   &CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU4: cpu@10000 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a57";
 		reg = <0x0 0x10000>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
 				   &CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU5: cpu@10001 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a57";
 		reg = <0x0 0x10001>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
 				   &CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU6: cpu@10100 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a57";
 		reg = <0x0 0x10100>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
 				   &CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU7: cpu@10101 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a57";
 		reg = <0x0 0x10101>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
 				   &CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU8: cpu@100000000 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a53";
 		reg = <0x1 0x0>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
 				   &CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
 	};

 	CPU9: cpu@100000001 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a53";
 		reg = <0x1 0x1>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
 				   &CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
 	};

 	CPU10: cpu@100000100 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a53";
 		reg = <0x1 0x100>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
 				   &CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
 	};

 	CPU11: cpu@100000101 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a53";
 		reg = <0x1 0x101>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
 				   &CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
 	};

 	CPU12: cpu@100010000 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a53";
 		reg = <0x1 0x10000>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
 				   &CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
 	};

 	CPU13: cpu@100010001 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a53";
 		reg = <0x1 0x10001>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
 				   &CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
 	};

 	CPU14: cpu@100010100 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a53";
 		reg = <0x1 0x10100>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
 				   &CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
 	};

 	CPU15: cpu@100010101 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a53";
 		reg = <0x1 0x10101>;
 		enable-method = "psci";
 		cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
 				   &CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
 	};

 	idle-states {
 		entry-method = "psci";

 		CPU_RETENTION_0_0: cpu-retention-0-0 {
 			compatible = "arm,idle-state";
 			arm,psci-suspend-param = <0x0010000>;
 			entry-latency-us = <20>;
 			exit-latency-us = <40>;
 			min-residency-us = <80>;
 		};

 		CLUSTER_RETENTION_0: cluster-retention-0 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			arm,psci-suspend-param = <0x1010000>;
 			entry-latency-us = <50>;
 			exit-latency-us = <100>;
 			min-residency-us = <250>;
 			wakeup-latency-us = <130>;
 		};

 		CPU_SLEEP_0_0: cpu-sleep-0-0 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			arm,psci-suspend-param = <0x0010000>;
 			entry-latency-us = <250>;
 			exit-latency-us = <500>;
 			min-residency-us = <950>;
 		};

 		CLUSTER_SLEEP_0: cluster-sleep-0 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			arm,psci-suspend-param = <0x1010000>;
 			entry-latency-us = <600>;
 			exit-latency-us = <1100>;
 			min-residency-us = <2700>;
 			wakeup-latency-us = <1500>;
 		};

 		CPU_RETENTION_1_0: cpu-retention-1-0 {
 			compatible = "arm,idle-state";
 			arm,psci-suspend-param = <0x0010000>;
 			entry-latency-us = <20>;
 			exit-latency-us = <40>;
 			min-residency-us = <90>;
 		};

 		CLUSTER_RETENTION_1: cluster-retention-1 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			arm,psci-suspend-param = <0x1010000>;
 			entry-latency-us = <50>;
 			exit-latency-us = <100>;
 			min-residency-us = <270>;
 			wakeup-latency-us = <100>;
 		};

 		CPU_SLEEP_1_0: cpu-sleep-1-0 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			arm,psci-suspend-param = <0x0010000>;
 			entry-latency-us = <70>;
 			exit-latency-us = <100>;
 			min-residency-us = <300>;
 			wakeup-latency-us = <150>;
 		};

 		CLUSTER_SLEEP_1: cluster-sleep-1 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			arm,psci-suspend-param = <0x1010000>;
 			entry-latency-us = <500>;
 			exit-latency-us = <1200>;
 			min-residency-us = <3500>;
 			wakeup-latency-us = <1300>;
 		};
 	};

 };

 Example 2 (ARM 32-bit, 8-cpu system, two clusters):

 cpus {
 	#size-cells = <0>;
 	#address-cells = <1>;

 	CPU0: cpu@0 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a15";
 		reg = <0x0>;
 		cpu-idle-states = <&CPU_SLEEP_0_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU1: cpu@1 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a15";
 		reg = <0x1>;
 		cpu-idle-states = <&CPU_SLEEP_0_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU2: cpu@2 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a15";
 		reg = <0x2>;
 		cpu-idle-states = <&CPU_SLEEP_0_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU3: cpu@3 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a15";
 		reg = <0x3>;
 		cpu-idle-states = <&CPU_SLEEP_0_0 &CLUSTER_SLEEP_0>;
 	};

 	CPU4: cpu@100 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a7";
 		reg = <0x100>;
 		cpu-idle-states = <&CPU_SLEEP_1_0 &CLUSTER_SLEEP_1>;
 	};

 	CPU5: cpu@101 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a7";
 		reg = <0x101>;
 		cpu-idle-states = <&CPU_SLEEP_1_0 &CLUSTER_SLEEP_1>;
 	};

 	CPU6: cpu@102 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a7";
 		reg = <0x102>;
 		cpu-idle-states = <&CPU_SLEEP_1_0 &CLUSTER_SLEEP_1>;
 	};

 	CPU7: cpu@103 {
 		device_type = "cpu";
 		compatible = "arm,cortex-a7";
 		reg = <0x103>;
 		cpu-idle-states = <&CPU_SLEEP_1_0 &CLUSTER_SLEEP_1>;
 	};

 	idle-states {
 		CPU_SLEEP_0_0: cpu-sleep-0-0 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			entry-latency-us = <200>;
 			exit-latency-us = <100>;
 			min-residency-us = <400>;
 			wakeup-latency-us = <250>;
 		};

 		CLUSTER_SLEEP_0: cluster-sleep-0 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			entry-latency-us = <500>;
 			exit-latency-us = <1500>;
 			min-residency-us = <2500>;
 			wakeup-latency-us = <1700>;
 		};

 		CPU_SLEEP_1_0: cpu-sleep-1-0 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			entry-latency-us = <300>;
 			exit-latency-us = <500>;
 			min-residency-us = <900>;
 			wakeup-latency-us = <600>;
 		};

 		CLUSTER_SLEEP_1: cluster-sleep-1 {
 			compatible = "arm,idle-state";
 			local-timer-stop;
 			entry-latency-us = <800>;
 			exit-latency-us = <2000>;
 			min-residency-us = <6500>;
 			wakeup-latency-us = <2300>;
 		};
 	};

 };

 ===========================================
 5 - References
 ===========================================

 [1] ARM Linux Kernel documentation - CPUs bindings
     Documentation/devicetree/bindings/arm/cpus.yaml

 [2] ARM Linux Kernel documentation - PSCI bindings
     Documentation/devicetree/bindings/arm/psci.txt

 [3] ARM Server Base System Architecture (SBSA)
     http://infocenter.arm.com/help/index.jsp

 [4] ARM Architecture Reference Manuals
     http://infocenter.arm.com/help/index.jsp

 [5] Devicetree Specification
     https://www.devicetree.org/specifications/
	==========================================
	ARM idle states binding description
	==========================================

	==========================================
	1 - Introduction
	==========================================

	ARM systems contain HW capable of managing power consumption dynamically,
	where cores can be put in different low-power states (ranging from simple
	wfi to power gating) according to OS PM policies. The CPU states representing
	the range of dynamic idle states that a processor can enter at run-time, can be
	specified through device tree bindings representing the parameters required
	to enter/exit specific idle states on a given processor.

	According to the Server Base System Architecture document (SBSA, [3]), the
	power states an ARM CPU can be put into are identified by the following list:

	- Running
	- Idle_standby
	- Idle_retention
	- Sleep
	- Off

	The power states described in the SBSA document define the basic CPU states on
	top of which ARM platforms implement power management schemes that allow an OS
	PM implementation to put the processor in different idle states (which include
	states listed above; "off" state is not an idle state since it does not have
	wake-up capabilities, hence it is not considered in this document).

	Idle state parameters (eg entry latency) are platform specific and need to be
	characterized with bindings that provide the required information to OS PM
	code so that it can build the required tables and use them at runtime.

	The device tree binding definition for ARM idle states is the subject of this
	document.

	===========================================
	2 - idle-states definitions
	===========================================

	Idle states are characterized for a specific system through a set of
	timing and energy related properties, that underline the HW behaviour
	triggered upon idle states entry and exit.

	The following diagram depicts the CPU execution phases and related timing
	properties required to enter and exit an idle state:

	..__[EXEC]__\|__[PREP]__\|__[ENTRY]__\|__[IDLE]__\|__[EXIT]__\|__[EXEC]__..
	\| \| \| \| \|

	\|<------ entry ------->\|
	\| latency \|
	\|<- exit ->\|
	\| latency \|
	\|<-------- min-residency -------->\|
	\|<------- wakeup-latency ------->\|

	Diagram 1: CPU idle state execution phases

	EXEC: Normal CPU execution.

	PREP: Preparation phase before committing the hardware to idle mode
	like cache flushing. This is abortable on pending wake-up
	event conditions. The abort latency is assumed to be negligible
	(i.e. less than the ENTRY + EXIT duration). If aborted, CPU
	goes back to EXEC. This phase is optional. If not abortable,
	this should be included in the ENTRY phase instead.

	ENTRY: The hardware is committed to idle mode. This period must run
	to completion up to IDLE before anything else can happen.

	IDLE: This is the actual energy-saving idle period. This may last
	between 0 and infinite time, until a wake-up event occurs.

	EXIT: Period during which the CPU is brought back to operational
	mode (EXEC).

	entry-latency: Worst case latency required to enter the idle state. The
	exit-latency may be guaranteed only after entry-latency has passed.

	min-residency: Minimum period, including preparation and entry, for a given
	idle state to be worthwhile energywise.

	wakeup-latency: Maximum delay between the signaling of a wake-up event and the
	CPU being able to execute normal code again. If not specified, this is assumed
	to be entry-latency + exit-latency.

	These timing parameters can be used by an OS in different circumstances.

	An idle CPU requires the expected min-residency time to select the most
	appropriate idle state based on the expected expiry time of the next IRQ
	(ie wake-up) that causes the CPU to return to the EXEC phase.

	An operating system scheduler may need to compute the shortest wake-up delay
	for CPUs in the system by detecting how long will it take to get a CPU out
	of an idle state, eg:

	wakeup-delay = exit-latency + max(entry-latency - (now - entry-timestamp), 0)

	In other words, the scheduler can make its scheduling decision by selecting
	(eg waking-up) the CPU with the shortest wake-up latency.
	The wake-up latency must take into account the entry latency if that period
	has not expired. The abortable nature of the PREP period can be ignored
	if it cannot be relied upon (e.g. the PREP deadline may occur much sooner than
	the worst case since it depends on the CPU operating conditions, ie caches
	state).

	An OS has to reliably probe the wakeup-latency since some devices can enforce
	latency constraints guarantees to work properly, so the OS has to detect the
	worst case wake-up latency it can incur if a CPU is allowed to enter an
	idle state, and possibly to prevent that to guarantee reliable device
	functioning.

	The min-residency time parameter deserves further explanation since it is
	expressed in time units but must factor in energy consumption coefficients.

	The energy consumption of a cpu when it enters a power state can be roughly
	characterised by the following graph:

	\|
	\|
	\|
	e \|
	n \| /---
	e \| /------
	r \| /------
	g \| /-----
	y \| /------
	\| ----
	\| /\|
	\| / \|
	\| / \|
	\| / \|
	\| / \|
	\| / \|
	\|/ \|
	-----\|-------+----------------------------------
	0\| 1 time(ms)

	Graph 1: Energy vs time example

	The graph is split in two parts delimited by time 1ms on the X-axis.
	The graph curve with X-axis values = { x \| 0 < x < 1ms } has a steep slope
	and denotes the energy costs incurred while entering and leaving the idle
	state.
	The graph curve in the area delimited by X-axis values = {x \| x > 1ms } has
	shallower slope and essentially represents the energy consumption of the idle
	state.

	min-residency is defined for a given idle state as the minimum expected
	residency time for a state (inclusive of preparation and entry) after
	which choosing that state become the most energy efficient option. A good
	way to visualise this, is by taking the same graph above and comparing some
	states energy consumptions plots.

	For sake of simplicity, let's consider a system with two idle states IDLE1,
	and IDLE2:

	\|
	\|
	\|
	\| /-- IDLE1
	e \| /---
	n \| /----
	e \| /---
	r \| /-----/--------- IDLE2
	g \| /-------/---------
	y \| ------------ /---\|
	\| / /---- \|
	\| / /--- \|
	\| / /---- \|
	\| / /--- \|
	\| --- \|
	\| / \|
	\| / \|
	\|/ \| time
	---/----------------------------+------------------------
	\|IDLE1-energy < IDLE2-energy \| IDLE2-energy < IDLE1-energy
	\|
	IDLE2-min-residency

	Graph 2: idle states min-residency example

	In graph 2 above, that takes into account idle states entry/exit energy
	costs, it is clear that if the idle state residency time (ie time till next
	wake-up IRQ) is less than IDLE2-min-residency, IDLE1 is the better idle state
	choice energywise.

	This is mainly down to the fact that IDLE1 entry/exit energy costs are lower
	than IDLE2.

	However, the lower power consumption (ie shallower energy curve slope) of idle
	state IDLE2 implies that after a suitable time, IDLE2 becomes more energy
	efficient.

	The time at which IDLE2 becomes more energy efficient than IDLE1 (and other
	shallower states in a system with multiple idle states) is defined
	IDLE2-min-residency and corresponds to the time when energy consumption of
	IDLE1 and IDLE2 states breaks even.

	The definitions provided in this section underpin the idle states
	properties specification that is the subject of the following sections.

	===========================================
	3 - idle-states node
	===========================================

	ARM processor idle states are defined within the idle-states node, which is
	a direct child of the cpus node [1] and provides a container where the
	processor idle states, defined as device tree nodes, are listed.

	- idle-states node

	Usage: Optional - On ARM systems, it is a container of processor idle
	states nodes. If the system does not provide CPU
	power management capabilities or the processor just
	supports idle_standby an idle-states node is not
	required.

	Description: idle-states node is a container node, where its
	subnodes describe the CPU idle states.

	Node name must be "idle-states".

	The idle-states node's parent node must be the cpus node.

	The idle-states node's child nodes can be:

	- one or more state nodes

	Any other configuration is considered invalid.

	An idle-states node defines the following properties:

	- entry-method
	Value type: <stringlist>
	Usage and definition depend on ARM architecture version.
	# On ARM v8 64-bit this property is required and must
	be:
	- "psci"
	# On ARM 32-bit systems this property is optional

	The nodes describing the idle states (state) can only be defined within the
	idle-states node, any other configuration is considered invalid and therefore
	must be ignored.

	===========================================
	4 - state node
	===========================================

	A state node represents an idle state description and must be defined as
	follows:

	- state node

	Description: must be child of the idle-states node

	The state node name shall follow standard device tree naming
	rules ([5], 2.2.1 "Node names"), in particular state nodes which
	are siblings within a single common parent must be given a unique name.

	The idle state entered by executing the wfi instruction (idle_standby
	SBSA,[3][4]) is considered standard on all ARM platforms and therefore
	must not be listed.

	With the definitions provided above, the following list represents
	the valid properties for a state node:

	- compatible
	Usage: Required
	Value type: <stringlist>
	Definition: Must be "arm,idle-state".

	- local-timer-stop
	Usage: See definition
	Value type: <none>
	Definition: if present the CPU local timer control logic is
	lost on state entry, otherwise it is retained.

	- entry-latency-us
	Usage: Required
	Value type: <prop-encoded-array>
	Definition: u32 value representing worst case latency in
	microseconds required to enter the idle state.
	The exit-latency-us duration may be guaranteed
	only after entry-latency-us has passed.

	- exit-latency-us
	Usage: Required
	Value type: <prop-encoded-array>
	Definition: u32 value representing worst case latency
	in microseconds required to exit the idle state.

	- min-residency-us
	Usage: Required
	Value type: <prop-encoded-array>
	Definition: u32 value representing minimum residency duration
	in microseconds, inclusive of preparation and
	entry, for this idle state to be considered
	worthwhile energy wise (refer to section 2 of
	this document for a complete description).

	- wakeup-latency-us:
	Usage: Optional
	Value type: <prop-encoded-array>
	Definition: u32 value representing maximum delay between the
	signaling of a wake-up event and the CPU being
	able to execute normal code again. If omitted,
	this is assumed to be equal to:

	entry-latency-us + exit-latency-us

	It is important to supply this value on systems
	where the duration of PREP phase (see diagram 1,
	section 2) is non-neglibigle.
	In such systems entry-latency-us + exit-latency-us
	will exceed wakeup-latency-us by this duration.

	- status:
	Usage: Optional
	Value type: <string>
	Definition: A standard device tree property [5] that indicates
	the operational status of an idle-state.
	If present, it shall be:
	"okay": to indicate that the idle state is
	operational.
	"disabled": to indicate that the idle state has
	been disabled in firmware so it is not
	operational.
	If the property is not present the idle-state must
	be considered operational.

	- idle-state-name:
	Usage: Optional
	Value type: <string>
	Definition: A string used as a descriptive name for the idle
	state.

	In addition to the properties listed above, a state node may require
	additional properties specifics to the entry-method defined in the
	idle-states node, please refer to the entry-method bindings
	documentation for properties definitions.

	===========================================
	4 - Examples
	===========================================

	Example 1 (ARM 64-bit, 16-cpu system, PSCI enable-method):

	cpus {
	#size-cells = <0>;
	#address-cells = <2>;

	CPU0: cpu@0 {
	device_type = "cpu";
	compatible = "arm,cortex-a57";
	reg = <0x0 0x0>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
	&CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
	};

	CPU1: cpu@1 {
	device_type = "cpu";
	compatible = "arm,cortex-a57";
	reg = <0x0 0x1>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
	&CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
	};

	CPU2: cpu@100 {
	device_type = "cpu";
	compatible = "arm,cortex-a57";
	reg = <0x0 0x100>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
	&CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
	};

	CPU3: cpu@101 {
	device_type = "cpu";
	compatible = "arm,cortex-a57";
	reg = <0x0 0x101>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
	&CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
	};

	CPU4: cpu@10000 {
	device_type = "cpu";
	compatible = "arm,cortex-a57";
	reg = <0x0 0x10000>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
	&CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
	};

	CPU5: cpu@10001 {
	device_type = "cpu";
	compatible = "arm,cortex-a57";
	reg = <0x0 0x10001>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
	&CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
	};

	CPU6: cpu@10100 {
	device_type = "cpu";
	compatible = "arm,cortex-a57";
	reg = <0x0 0x10100>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
	&CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
	};

	CPU7: cpu@10101 {
	device_type = "cpu";
	compatible = "arm,cortex-a57";
	reg = <0x0 0x10101>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_0_0 &CPU_SLEEP_0_0
	&CLUSTER_RETENTION_0 &CLUSTER_SLEEP_0>;
	};

	CPU8: cpu@100000000 {
	device_type = "cpu";
	compatible = "arm,cortex-a53";
	reg = <0x1 0x0>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
	&CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
	};

	CPU9: cpu@100000001 {
	device_type = "cpu";
	compatible = "arm,cortex-a53";
	reg = <0x1 0x1>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
	&CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
	};

	CPU10: cpu@100000100 {
	device_type = "cpu";
	compatible = "arm,cortex-a53";
	reg = <0x1 0x100>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
	&CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
	};

	CPU11: cpu@100000101 {
	device_type = "cpu";
	compatible = "arm,cortex-a53";
	reg = <0x1 0x101>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
	&CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
	};

	CPU12: cpu@100010000 {
	device_type = "cpu";
	compatible = "arm,cortex-a53";
	reg = <0x1 0x10000>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
	&CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
	};

	CPU13: cpu@100010001 {
	device_type = "cpu";
	compatible = "arm,cortex-a53";
	reg = <0x1 0x10001>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
	&CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
	};

	CPU14: cpu@100010100 {
	device_type = "cpu";
	compatible = "arm,cortex-a53";
	reg = <0x1 0x10100>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
	&CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
	};

	CPU15: cpu@100010101 {
	device_type = "cpu";
	compatible = "arm,cortex-a53";
	reg = <0x1 0x10101>;
	enable-method = "psci";
	cpu-idle-states = <&CPU_RETENTION_1_0 &CPU_SLEEP_1_0
	&CLUSTER_RETENTION_1 &CLUSTER_SLEEP_1>;
	};

	idle-states {
	entry-method = "psci";

	CPU_RETENTION_0_0: cpu-retention-0-0 {
	compatible = "arm,idle-state";
	arm,psci-suspend-param = <0x0010000>;
	entry-latency-us = <20>;
	exit-latency-us = <40>;
	min-residency-us = <80>;
	};

	CLUSTER_RETENTION_0: cluster-retention-0 {
	compatible = "arm,idle-state";
	local-timer-stop;
	arm,psci-suspend-param = <0x1010000>;
	entry-latency-us = <50>;
	exit-latency-us = <100>;
	min-residency-us = <250>;
	wakeup-latency-us = <130>;
	};

	CPU_SLEEP_0_0: cpu-sleep-0-0 {
	compatible = "arm,idle-state";
	local-timer-stop;
	arm,psci-suspend-param = <0x0010000>;
	entry-latency-us = <250>;
	exit-latency-us = <500>;
	min-residency-us = <950>;
	};

	CLUSTER_SLEEP_0: cluster-sleep-0 {
	compatible = "arm,idle-state";
	local-timer-stop;
	arm,psci-suspend-param = <0x1010000>;
	entry-latency-us = <600>;
	exit-latency-us = <1100>;
	min-residency-us = <2700>;
	wakeup-latency-us = <1500>;
	};

	CPU_RETENTION_1_0: cpu-retention-1-0 {
	compatible = "arm,idle-state";
	arm,psci-suspend-param = <0x0010000>;
	entry-latency-us = <20>;
	exit-latency-us = <40>;
	min-residency-us = <90>;
	};

	CLUSTER_RETENTION_1: cluster-retention-1 {
	compatible = "arm,idle-state";
	local-timer-stop;
	arm,psci-suspend-param = <0x1010000>;
	entry-latency-us = <50>;
	exit-latency-us = <100>;
	min-residency-us = <270>;
	wakeup-latency-us = <100>;
	};

	CPU_SLEEP_1_0: cpu-sleep-1-0 {
	compatible = "arm,idle-state";
	local-timer-stop;
	arm,psci-suspend-param = <0x0010000>;
	entry-latency-us = <70>;
	exit-latency-us = <100>;
	min-residency-us = <300>;
	wakeup-latency-us = <150>;
	};

	CLUSTER_SLEEP_1: cluster-sleep-1 {
	compatible = "arm,idle-state";
	local-timer-stop;
	arm,psci-suspend-param = <0x1010000>;
	entry-latency-us = <500>;
	exit-latency-us = <1200>;
	min-residency-us = <3500>;
	wakeup-latency-us = <1300>;
	};
	};

	};

	Example 2 (ARM 32-bit, 8-cpu system, two clusters):

	cpus {
	#size-cells = <0>;
	#address-cells = <1>;

	CPU0: cpu@0 {
	device_type = "cpu";
	compatible = "arm,cortex-a15";
	reg = <0x0>;
	cpu-idle-states = <&CPU_SLEEP_0_0 &CLUSTER_SLEEP_0>;
	};

	CPU1: cpu@1 {
	device_type = "cpu";
	compatible = "arm,cortex-a15";
	reg = <0x1>;
	cpu-idle-states = <&CPU_SLEEP_0_0 &CLUSTER_SLEEP_0>;
	};

	CPU2: cpu@2 {
	device_type = "cpu";
	compatible = "arm,cortex-a15";
	reg = <0x2>;
	cpu-idle-states = <&CPU_SLEEP_0_0 &CLUSTER_SLEEP_0>;
	};

	CPU3: cpu@3 {
	device_type = "cpu";
	compatible = "arm,cortex-a15";
	reg = <0x3>;
	cpu-idle-states = <&CPU_SLEEP_0_0 &CLUSTER_SLEEP_0>;
	};

	CPU4: cpu@100 {
	device_type = "cpu";
	compatible = "arm,cortex-a7";
	reg = <0x100>;
	cpu-idle-states = <&CPU_SLEEP_1_0 &CLUSTER_SLEEP_1>;
	};

	CPU5: cpu@101 {
	device_type = "cpu";
	compatible = "arm,cortex-a7";
	reg = <0x101>;
	cpu-idle-states = <&CPU_SLEEP_1_0 &CLUSTER_SLEEP_1>;
	};

	CPU6: cpu@102 {
	device_type = "cpu";
	compatible = "arm,cortex-a7";
	reg = <0x102>;
	cpu-idle-states = <&CPU_SLEEP_1_0 &CLUSTER_SLEEP_1>;
	};

	CPU7: cpu@103 {
	device_type = "cpu";
	compatible = "arm,cortex-a7";
	reg = <0x103>;
	cpu-idle-states = <&CPU_SLEEP_1_0 &CLUSTER_SLEEP_1>;
	};

	idle-states {
	CPU_SLEEP_0_0: cpu-sleep-0-0 {
	compatible = "arm,idle-state";
	local-timer-stop;
	entry-latency-us = <200>;
	exit-latency-us = <100>;
	min-residency-us = <400>;
	wakeup-latency-us = <250>;
	};

	CLUSTER_SLEEP_0: cluster-sleep-0 {
	compatible = "arm,idle-state";
	local-timer-stop;
	entry-latency-us = <500>;
	exit-latency-us = <1500>;
	min-residency-us = <2500>;
	wakeup-latency-us = <1700>;
	};

	CPU_SLEEP_1_0: cpu-sleep-1-0 {
	compatible = "arm,idle-state";
	local-timer-stop;
	entry-latency-us = <300>;
	exit-latency-us = <500>;
	min-residency-us = <900>;
	wakeup-latency-us = <600>;
	};

	CLUSTER_SLEEP_1: cluster-sleep-1 {
	compatible = "arm,idle-state";
	local-timer-stop;
	entry-latency-us = <800>;
	exit-latency-us = <2000>;
	min-residency-us = <6500>;
	wakeup-latency-us = <2300>;
	};
	};

	};

	===========================================
	5 - References
	===========================================

	[1] ARM Linux Kernel documentation - CPUs bindings
	Documentation/devicetree/bindings/arm/cpus.yaml

	[2] ARM Linux Kernel documentation - PSCI bindings
	Documentation/devicetree/bindings/arm/psci.txt

	[3] ARM Server Base System Architecture (SBSA)
	http://infocenter.arm.com/help/index.jsp

	[4] ARM Architecture Reference Manuals
	http://infocenter.arm.com/help/index.jsp

	[5] Devicetree Specification
	https://www.devicetree.org/specifications/