Commit baac82fe authored by Rob Herring's avatar Rob Herring
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dt-bindings: arm: Convert arm,idle-state binding to DT schema 



Convert the Arm idle-state binding to a DT schema.

Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: default avatarRob Herring <robh@kernel.org>
parent 95306805

Documentation/devicetree/bindings/arm/idle-states.txt

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==========================================

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 (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.



- 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



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.



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.



	- 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.

			    The exit-latency-us duration may be guaranteed

			    only after entry-latency-us has passed.



	- 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 specific 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.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



[5] Devicetree Specification

    https://www.devicetree.org/specifications/



[6] ARM Linux Kernel documentation - Booting AArch64 Linux

    Documentation/arm64/booting.rst

Documentation/devicetree/bindings/arm/idle-states.yaml

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