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diff --git a/Documentation/devicetree/bindings/arm/idle-states.txt b/Documentation/devicetree/bindings/arm/idle-states.txt new file mode 100644 index 000000000..2c7384749 --- /dev/null +++ b/Documentation/devicetree/bindings/arm/idle-states.txt @@ -0,0 +1,699 @@ +========================================== +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 whilst 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.txt + +[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/ |