path: root/Documentation/devicetree/bindings/arm/idle-states.yaml

# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
%YAML 1.2
---
$id: http://devicetree.org/schemas/arm/idle-states.yaml#
$schema: http://devicetree.org/meta-schemas/core.yaml#

title: ARM idle states binding description

maintainers:
  - Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>

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 (e.g. 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
  (i.e. 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, e.g.:

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

  In other words, the scheduler can make its scheduling decision by selecting
  (e.g. waking-up) the CPU with the shortest wake-up delay.
  The wake-up delay 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, i.e. caches
  state).

  An OS has to reliably probe the wakeup-latency since some devices can enforce
  latency constraint 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 (i.e. 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 (i.e. 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.

  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.

  ===========================================
  4 - 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.yaml

  [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

  [6] ARM Linux Kernel documentation - Booting AArch64 Linux
      Documentation/arm64/booting.rst

properties:
  $nodename:
    const: idle-states

  entry-method:
    description: |
      Usage and definition depend on ARM architecture version.

      On ARM v8 64-bit this property is required.
      On ARM 32-bit systems this property is optional

      This assumes that the "enable-method" property is set to "psci" in the cpu
      node[6] that is responsible for setting up CPU idle management in the OS
      implementation.
    const: psci

patternProperties:
  "^(cpu|cluster)-":
    type: object
    description: |
      Each state node represents an idle state description and must be defined
      as follows.

      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.

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

    properties:
      compatible:
        const: arm,idle-state

      local-timer-stop:
        description:
          If present the CPU local timer control logic is
             lost on state entry, otherwise it is retained.
        type: boolean

      entry-latency-us:
        description:
          Worst case latency in microseconds required to enter the idle state.

      exit-latency-us:
        description:
          Worst case latency in microseconds required to exit the idle state.
          The exit-latency-us duration may be guaranteed only after
          entry-latency-us has passed.

      min-residency-us:
        description:
          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:
        description: |
          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.

      idle-state-name:
        $ref: /schemas/types.yaml#definitions/string
        description:
          A string used as a descriptive name for the idle state.

    required:
      - compatible
      - entry-latency-us
      - exit-latency-us
      - min-residency-us

additionalProperties: false

examples:
  - |

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

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;
        };

        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>;

        cpu@0 {
            device_type = "cpu";
            compatible = "arm,cortex-a15";
            reg = <0x0>;
            cpu-idle-states = <&cpu_sleep_0_0 &cluster_sleep_0>;
        };

        cpu@1 {
            device_type = "cpu";
            compatible = "arm,cortex-a15";
            reg = <0x1>;
            cpu-idle-states = <&cpu_sleep_0_0 &cluster_sleep_0>;
        };

        cpu@2 {
            device_type = "cpu";
            compatible = "arm,cortex-a15";
            reg = <0x2>;
            cpu-idle-states = <&cpu_sleep_0_0 &cluster_sleep_0>;
        };

        cpu@3 {
            device_type = "cpu";
            compatible = "arm,cortex-a15";
            reg = <0x3>;
            cpu-idle-states = <&cpu_sleep_0_0 &cluster_sleep_0>;
        };

        cpu@100 {
            device_type = "cpu";
            compatible = "arm,cortex-a7";
            reg = <0x100>;
            cpu-idle-states = <&cpu_sleep_1_0 &cluster_sleep_1>;
        };

        cpu@101 {
            device_type = "cpu";
            compatible = "arm,cortex-a7";
            reg = <0x101>;
            cpu-idle-states = <&cpu_sleep_1_0 &cluster_sleep_1>;
        };

        cpu@102 {
            device_type = "cpu";
            compatible = "arm,cortex-a7";
            reg = <0x102>;
            cpu-idle-states = <&cpu_sleep_1_0 &cluster_sleep_1>;
        };

        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>;
            };
        };
    };

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