STM32MP2 power overview

Applicable for

STM32MP25x lines

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This article gives information about the support power management in OpenSTlinux for STM32MP2 series.

1. Framework purpose

The purpose of this article is to explain how OpenSTlinux components handle the STM32MP2 series power management:

• Software overview
• Peripheral power support
• Low-power modes available on the device

See also the page How to define your low-power strategy.

2. Software overview

The system power request is treated with the PSCI support with TF-A.

The power requests associated to each device (clock, regulator, operation point) are consolidated by the Linux frameworks and can be treated by Linux driver or by OP-TEE through SCMI requests.

2.1. Component description

The OpenSTLinux components for low power are:

Non secure world (Linux):

• Power management frameworks: See Power overview for details
  • CpuIdle
  • PM runtime
  • power domain (GenPD)
  • Clock
  • Regulator
• PSCI library: this is a set of standardized functions to request a low-power service to the secure monitor
• SCMI drivers: driver for system resources (clock, regulators, power domain,...), exposed by OP-TEE with SCMI protocol
• Device driver: any peripheral driver which needs to control power
  • RCC driver for clock and reset managed by non secure world

Secure world components (OP-TEE):

• SCMI protocol: see SCMI overview for details
• PM framework: call the low-power callback of each device on the call of TF-A BL31 PM hooks.
• OP-TEE drivers: driver of device for system resources
  • RCC driver for clock and reset managed by secure world
  • PWR driver for SoC regulators managed by secure world
  • PMIC driver for external regulators

Secure monitor components (TF-A BL31):

• PSCI libray: generic PSCI stack
• PM manage PSCI topology and modes with RCC and PWR registers and DDR self refresh
• DDR driver: DDR driver

STM32 peripherals (Hardware):

• RCC
• PWR
• DDR
• PMIC hardware components

2.2. API description

The OpenSTLinux power management support is based on Arm^® interface specifications:

• Power state coordination interface (PSCI) defines many messages (see identifiers in TF-A include/lib/psci/psci.h or in Linux include/kvm/arm_psci.h ), for examples:
  • CPU_SUSPEND : used to CPU suspend (freeze/s2idle), including OS initiated state support for CpuIdle
  • CPU_ON/CPU_OFF : used for hotplug feature
  • SYSTEM_OFF : used for power off
  • SYSTEM_RESET : used for system reset
  • SYSTEM_SUSPEND : used to system suspend (deep)
  • PSCI_VERSION : return the supported version of the PSCI specication, v1.1 is supported by TF-A
• System Control and Management Interface (SCMI), see SCMI overview for details

3. Configuration

The objective of this chapter is to explain how to configure OpenSTLinux for low power modes.

See the page power overview for kernel configuration.

The SoC device tree describes the PSCI topology in sub-nodes of cpus/domain-idle-states:

• for Linux in arch/arm64/boot/dts/st/stm32mp251.dtsi , one node for each PSCI_CPU_SUSPEND supported parameter (only the Stop1 modes):

domain-idle-states {
	STOP1: domain-stop1 {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x00000011>;
	};
	LP_STOP1: domain-lp-stop1 {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x0000021>;
	};
	LPLV_STOP1: domain-lplv-stop1 {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x00000211>;
	};
};

• for TF-A BL31 secure monitor, in fdts/stm32mp251.dtsi , one node by state supported in PSCI topology:

domain-idle-states {
	stop1 {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x00000011>;
	};
	lp-stop1 {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x00000021>;
	};
	lplv-stop1 {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x00000211>;
	};
	stop2 {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x40001333>;
	};
	lp-stop2 {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x40002333>;
	};
	lplv-stop2 {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x40023333>;
	};
	standby {
		compatible = "domain-idle-state";
		arm,psci-suspend-param = <0x40033333>;
	};
};

The unsupported low power modes and associated references in domain-idle-states power domain nodes must be deleted from TF-A BL31 and Linux board device-tree files.

For example on board without STPMIC25, if the PWR_LP pins control the reduced voltage mode of the V_DDCORE and V_DDCPU regulators, then only LPLV-StopX modes can be reached. LP-StopX modes must be deleted.

• in TF-A BL31 device tree

/ {
	cpus {
		domain-idle-states {
			/delete-node/ lp-stop1;
			/delete-node/ lp-stop2;
		};
	};
};

• in Linux device tree (CLUSTER_PD domain-idle-states = <&STOP1>, <&LP_STOP1>; for CLUSTER_PD in arch/arm64/boot/dts/st/stm32mp251.dts ):

/ {
	cpus {
		domain-idle-states {
			/delete-node/ domain-lp-stop1;
		};
	};
};

&CLUSTER_PD {
	domain-idle-states = <&STOP1>;
};

The selected PSCI power level is indicated to OP-TEE with Secure-EL1 Payload Dispatcher (SPD) PSCI hooks. And this power level is indicated with pm_hint to each OP-TEE drivers to save/restore the device configuration (see core/include/kernel/pm.h and core/arch/arm/plat-stm32mp2/stm32mp_pm.h for details).

This hook is used by the STPMIC25 driver in OP-TEE to configure each regulators according to the selected low power mode as described in device tree by subnodes. Please refer to PMIC OP-TEE page for details for PMIC configuration in OP-TEE for each power mode and to application note AN5727 for configurations.

See OP-TEE device tree in core/arch/arm/dts/stm32mp257f-ev1.dts for configuration example:

	vddcore: buck2 {
		regulator-name = "vddcore";
		regulator-min-microvolt = <820000>;
		regulator-max-microvolt = <820000>;
		regulator-always-on;
		st,pwrctrl-sel = <1>;
		st,pwrctrl-enable;

		default {
			regulator-on-in-suspend;
			regulator-suspend-microvolt = <820000>;
		};
		lplv {
			regulator-on-in-suspend;
			regulator-suspend-microvolt = <670000>;
		};
		standby {
			regulator-off-in-suspend;
		};
		off {
			regulator-off-in-suspend;
		};
	};

4. How to use the framework

4.1. Low-power modes

Refer to STM32MP25 reference manuals for the full description of low-power modes.

The AN5726 Guidelines for using low-power modes on STM32MP2 MPU also gives much more information on these modes and on associated the wake-up sources.

Warning

freeze (s2idle) is not fully functional in OpenSTLinux v5.1 see next chapter for details)

System mode	Linux Command	PSCI request	Power consumption	Wake-up time	VDDCPU	VDDCORE	DDR state
Stop1	freeze (s2idle)	PSCI_CPU_SUSPEND	Medium	Low	ON	ON	SR, VTT on
LP-Stop1			Medium	Low			SR, VTT off
LPLV-Stop1			Medium	Medium	ON (reduced)	ON (reduced)
Stop2	mem (deep)	PSCI_SYSTEM_SUSPEND	Low	Low	OFF	ON	SR, VTT off
LP-Stop2			Low	Medium
LPLV-Stop2			Low	Medium		ON (reduced)
Standby1			Low	High		OFF
Standby2	shutdown	PSCI_SYSTEM_OFF	Very-Low	High			Off
VBAT			Very-Low	Very-High

OpenSTLinux sets the DDR in Self Refresh mode (SR) in all the low-power modes except power off (Standby2 and V_BAT). For LP-Stop1 and LP-Stop2 modes, the DDR V_TT supply is also switched off on STMicroelectronics reference design thought the PWR_LP signal connected to PWRCTRL1 pin of STPMIC25.

This list of Wake-up capability peripheral for each mode is defined in the Table 94. Functionalities depending on system operating mode of the STM32MP25 reference manuals.

The following tables give the list of wake-up sources available in each mode.

Plaform mode	Available wake-up sources
Stop1/2 and LP-Stop1/2	Group1 : Group2 + HPDMAx (x = 1, 2, 3), HSI frequency monitoring, USB, UCPD1, ETHx (x = 1,2), USARTx (x = 1 to 9), I2Cx (x = 1 to 7), I3Cx (x = 1 to 3), SPIx (x = 1 to 7), DTS, LPTIMx (x = 1, 2)
LPLV-Stop1/2	Group2 : Group3 + PVD, PVM, GPIOs
Standby1	Group3 : Group4 + CPU3, DBG, LPDMA, LPUART1, I2C8, I3C4, SPI8, ADF1, LPTIMy (y = 3, 4, 5), WWDG2, MBOX2, HSEM, GPIOZ
Standby2	Group4 : Group5 + 6 x WKUP pins
VBAT	Group5 : BOR, VBATH/VBATL, TEMPH/TEMPL, LSE CSS, RTC/auto wake-up, tamper pins, IWDGx

4.2. PSCI topology

The low power modes for STM32MP2 use the PSCI stack in TF-A BL31, including the PSCI OSI mode (v2.9).

The PSCI topology is defined in TF-A PM driver: plat/st/stm32mp2/stm32mp2_pm.c .

Each low power more is associated to a PSCI State-id, which uses the ARM Recommended StateID Encoding with OS initiated support, based on the PSCI state system topology defined in plat/st/stm32mp2/include/platform_def.h with:

• PLAT_MAX_PWR_LVL = 4, for Core(0), D1(1), D1LPLV(2), D2(3), D2_LPLV(3)
• PLAT_MIN_SUSPEND_PWR_LVL =2, limit level for PSCI_CPU_SUSPEND support
• PLAT_NUM_PWR_DOMAINS = 6 (2 cores and 4 power level)

and with the power domain states:

• STM32MP_LOCAL_STATE_RUN(0)
• STM32MP_LOCAL_STATE_RET(1)
• STM32MP_LOCAL_STATE_LP(2)
• STM32MP_LOCAL_STATE_OFF(3)

For PSCI suspend requests, STM32MP25 supports 5 power levels in the PSCI topology to handle the regulators configuration done in OP-TEE for STPMIC25, in particular to differentiate the voltages for LP and the LPLV modes.

The composite PSCI State-id are used in PSCI_CPU_SUSPEND requests and in device tree (for S2Idle and for OS initiated support) and for used internal in TF-A BL31 for PSCI_CPU_SUSPEND request (Deep).

This State-id encoding use the extended format and a simple additive composition method, with one nibble as a local state index for each power level (bits 27:0) and StateType(bit 30):

PSCI request	Name	State-id	StateType	D2_LPLV	D2	D1_LPLV	D1	Core
PSCI_CPU_SUSPEND	Sleep (WFI)	0x00000001	STANDBY	RUN	RUN	RUN	RUN	RET
	Stop1	0x00000011	STANDBY	RUN	RUN	RUN	RET	RET
	LP-Stop1	0x00000021	STANDBY	RUN	RUN	RUN	LP	RET
	LPLV-Stop1	0x00000211	STANDBY	RUN	RUN	LP	RET	RET
PSCI_SYSTEM_SUSPEND	Stop2	0x40001333	POWERDOWN	RUN	RET	OFF	OFF	OFF
	LP-Stop2	0x40002333	POWERDOWN	RUN	LP	OFF	OFF	OFF
	LPLV-Stop2	0x40023333	POWERDOWN	LP	OFF	OFF	OFF	OFF
	Standby	0x40002333	POWERDOWN	OFF	OFF	OFF	OFF	OFF

4.3. PSCI in Linux (PSCI_CPU_SUSPEND)

In Linux the power domain and associated low power modes for OS initiated mode are described in SoC device tree (in arch/arm64/boot/dts/st/stm32mp251.dtsi and in arch/arm64/boot/dts/st/stm32mp253.dtsi ) using the hierarchical model as described in Documentation/devicetree/bindings/arm/psci.yaml .

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

			CPU_PWRDN: cpu-power-down {
				compatible = "arm,idle-state";
				arm,psci-suspend-param = <0x00000001>;
			};
		};

		domain-idle-states {
			STOP1: domain-stop1 {
				compatible = "domain-idle-state";
				arm,psci-suspend-param = <0x00000011>;
			};

			LP_STOP1: domain-lp-stop1 {
				compatible = "domain-idle-state";
				arm,psci-suspend-param = <0x0000021>;
			};

			LPLV_STOP1: domain-lplv-stop1 {
				compatible = "domain-idle-state";
				arm,psci-suspend-param = <0x00000211>;
			};
		};
	};

	psci {
		compatible = "arm,psci-1.0";
		method = "smc";
		CPU_PD0: power-domain-cpu0 {
			#power-domain-cells = <0>;
			domain-idle-states = <&CPU_PWRDN>;
			power-domains = <&CLUSTER_PD>;
		};
		CPU_PD1: power-domain-cpu1 {
			#power-domain-cells = <0>;
			domain-idle-states =  <&CPU_PWRDN>;
			power-domains = <&CLUSTER_PD>;
		};
		CLUSTER_PD: power-domain-cluster {
			#power-domain-cells = <0>;
			domain-idle-states = <&STOP1>, <&LP_STOP1>;
			power-domains = <&RET_PD>;
		};
		RET_PD: power-domain-retention {
			#power-domain-cells = <0>;
			domain-idle-states = <&LPLV_STOP1>;
		};
	};

This power domain hierarchy is used in generic power domain (GenPD) for S2IDLE request and for dynamic power management, based PSCI OS initiated mode PM runtime: Linux kernel selects the lowest possible low power mode with the associated PSCI State Id, according the state of each power domain (device in the power domain is running, wake-up source is activated) and which respects the OS constraint (wake-up latency in OS initiated mode for example).

The devices are assigned to a domain in Soc device tree according the Table 94. Functionalities depending on system operating mode of the reference manuals (autonomous mode is not managed in OpenSTLinux).

• CLUSTER_PD for peripherals not functional in "Stop1/2 and LP-Stop1/2"
• RET_PD for peripherals not functional in "LPLV-Stop1/2"

The EXTI1 driver manages also the activated wake up source: LPLV-Stop1 mode is not not allowed for Group2 wake up source.

The dynamic power management is based on driver activity, with device PM runtime and GenPD, so you need to stop ALL the STM32MP25 peripherals activity, including console to allow low power modes.

Warning

freeze, S2Idle and OS initiated mode is not fully functional in OpenSTLinux v5.1; it should be used only for power measurements with the known restrictions: - the check of activated wake-up sources in EXTI1 is not functional - Linux console doesn't support pm runtime so CLUSTER_PD is never stopped when console is used - the GPU power domain is not included in CLUSTER_PD, so GPU is stopped in Idle without protection - the Cortex-M33 firmware state is not checked (when it is running in DDR)

4.3.1. PSCI in TF-A (PSCI_SYSTEM_SUSPEND)

For deep request the PSCI_SYSTEM_SUSPEND is sent to T-FA when the secondary core is stopped and all the process are frozen. The activated wake-up are consolidated in TF-A BL31 to select the lowest POWERDOWN supported mode in stm32_get_sys_suspend_power_state() .

If a low power mode is not supported by a board design, it must be removed in TF-A BL31 device tree.

You can limit the maximum supported mode with wake-up sources, but you can't force Stop2 mode by Linux command if LP-Stop2 is supported (same functionalities, same supported wake-up sources).

5. Source code location

Below are listed the software frameworks and drivers managing the OpenSTLinux power management.

• Linux^®: Linux power management

• TF-A:
  • the STM32MP2 PSCI support: plat/st/stm32mp2/stm32mp2_pm.c
  • the low power context: plat/st/stm32mp2/stm32mp2_context.c
  • the PSCI stack: lib/psci/ and include/lib/psci/psci.h

• OP-TEE:
  • TF-A hook for STM32MP2 platform: core/arch/arm/plat-stm32mp2/pm/pm_stm32mp25.c and core/arch/arm/plat-stm32mp2/stm32mp_pm.h
  • PM framework: core/kernel/pm.c and core/include/kernel/pm.h
  • For each device driver, the PM function registered by register_pm_core_service_cb()

• And drivers for each internal peripheral or external component are described in the associated pages:
  • PWR
  • PMIC
  • RCC
  • DDR

6. How to trace and debug

See How to debug TF-A BL2 and How_to_debug_OP-TEE to activate the traces.

In TF-A, you can force the selected mode PSCI_SYSTEM_SUSPEND for your board in stm32_get_sys_suspend_power_state(), for example:

/* Search the max supported POWERDOWN modes  <= max_pwr_state */
for (i = ARRAY_SIZE(stm32mp_supported_pwr_states) - 1U; i > 0U; i--) {
	pwr_state = stm32mp_supported_pwr_states[i];
	if ((pwr_state != 0U) && (pwr_state <= max_pwr_state) &&
		(psci_get_pstate_type(pwr_state) == PSTATE_TYPE_POWERDOWN)) {
	break;
	}
}

pwr_state = PWRSTATE_STOP2;

state_id = psci_get_pstate_id(pwr_state);

or remove some states in TF-A device tree , for example remove LP-Stop2 mode to allow Stop2 selection:

/ {
	cpus {
		domain-idle-states {
			/delete-node/ lp-stop2;
		};
	};
};

7. To go further

Refer to reference manual for a detailed description of low-power modes and peripheral wakeup sources:

• STM32MP25 reference manuals

The next application notes gives additional information on the hardware settings used for low-power management:

• AN5726 - Guidelines for using low-power modes on STM32MP2 MPU
• AN5727 - How to use STPMIC25 for a wall adapter powered application on STM32MP25x lines MPUs

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