Power overview

Applicable for

STM32MP13x lines, STM32MP15x lines, STM32MP25x lines

1. Framework purpose[edit source]

The purpose of this article is to explain how to handle the STM32 Arm^® Cortex^® MPUs low-power modes:

Low-power modes available on the device
Linux software overview for Power Management (on/off support, Suspend / Resume, Idle behavior, policy and control)
How to enter and exit a platform low-power mode

2. Software overview[edit source]

The Linux^® Power Management (PM) is spitted in:

Static PM or System-Wide Power Management
- power off
- the suspend framework is used to trigger a low-power mode entry/exit sequence.
Dynamic PM or Working-State Power Management or Active Power Management based on other frameworks:
- CPUfreq, Devfreq
- OPP with link to thermal framework
- Genpd (performance states)
- Runtime PM: dynamically disable the resources of the peripherals (clocks and power supply when applicable) in case of inactivity (see power/runtime_pm.html).
- Clocks
- Regulators

Refer to power and in admin-guide/pm for more details.

On STM32MP1 series, the "System-Wide Power Management" is supported:

Power mgt.png

The user application issues a suspend request to the kernel. This request is handled by the suspend Framework, which notifies all the device drivers to prepare for low-power entry. It then calls the PSCI service.

On STM32MP2 series:

STM32MP2 Power mgt.png

The suspend framework use the PSCI suspend support and the associated generic power domains (GenPD) and PSCI CPUIdle for OS initiated low power more with S2IDLE and CPUIdle.

2.1. Component description[edit source]

Kernel components for low power:

Dynamic PM: CPU Idle and device Idle with Runtime PM
CPUFreq: DVFS for CPU (see the page How to change the CPU frequency for details)
Suspend framework: this framework schedules the overall sequence by stopping all the ongoing tasks
GenPD: domain framework, based on runtime PM
- GenPD driver: this driver is used for low-power mode selection according to the activated wakeup sources.
PSCI library: this is a set of standardized functions to request a low-power service to the secure monitor
Clock framework
- RCC driver: this driver handles the circuit non-secure clocks
Regulator framework
SCMI framework: including SCMI drivers for secure resource as clock, regulators, reset, performance domain

Secure monitor components (TF-A BL31 on AArch64 platforms, OP-TEE on AArch32 platforms) :

PSCI library: this is a set of standardized functions handling the low-power services
Low power driver: the role of this driver is to choose the low-power mode according to the programmed wakeup source(s)
PWR driver: this driver is responsible for configuring the low-power mode
RCC driver: this driver handles the circuit secure clocks

Secure world components (OP-TEE):

SCMI stack: SCMI stack with access to secure resources, clock with RCC driver, regu with PWR driver and STPMIC driver

2.2. API description[edit source]

The suspend process is triggered from the user space through standard commands.

The system sleep control file is the state file, located under: /sys/power/

Further details can be found in admin-guide/pm/sleep-states.html and in power/interface.html.

The sysfs ABI are described in ABI/testing/sysfs-power and ABI/testing/sysfs-devices-power.

The device power API are described in driver-api/pm/index.html, in particular in driver-api/pm/devices.html.

2.2.1. Modes for STM32MP1 series[edit source]

Only the 'mem' command is supported for STM32MP1 series: the whole system activity is stopped and a low-power mode is entered. The software selects the deepest mode according to the activated wakeup source(s).

Example:

  echo mem > /sys/power/state

STM32MP1 series STMicroelectronics deliveries propose a default mapping of the low-power modes for each type of board, this default mapping can be changed thanks to the device tree as explain in the next paragraph.

2.2.2. Modes for STM32MP2 series[edit source]

For each mode the PSCI stack in TF-A BL31 selects the deepest mode according to the activated wake-up source(s).

Suspend-to-RAM (S2RAM)¶

  echo mem > /sys/power/state

The mode 'deep' for S2RAM is default at boot, you can force it with:

  echo deep > /sys/power/mem_sleep
  echo mem > /sys/power/state

Suspend-to-Idle (S2Idle)

  echo freeze > /sys/power/state

or

  echo s2idle > /sys/power/mem_sleep
  echo mem > /sys/power/state

Power-Off

  shutdown -P now
  systemctl poweroff

3. Configuration[edit source]

The objective of this chapter is to explain how to configure the Linux kernel and device tree to have the Power Management (PM) framework activated.

3.1. Kernel configuration[edit source]

The Power Management framework is activated by default in ST deliveries.

It can be deactivated through the kernel menuconfig using Power management options/Suspend to RAM and standby: Menuconfig or how to configure kernel .

3.2. Device tree configuration[edit source]

3.2.1. STM32MP1 series[edit source]

The default system low-power mode mapping can be modified through the secure monitor device tree, as described OP-TEE page.

Below an example in OP-TEE core/arch/arm/dts/stm32mp135f-dk.dts

 &pwr_regulators {
 	system_suspend_supported_soc_modes = <
 		STM32_PM_CSLEEP_RUN
 		STM32_PM_CSTOP_ALLOW_LP_STOP
 		STM32_PM_CSTOP_ALLOW_LPLV_STOP
 		STM32_PM_CSTOP_ALLOW_LPLV_STOP2
 		STM32_PM_CSTOP_ALLOW_STANDBY_DDR_SR
 	>;
 
 	system_off_soc_mode = <STM32_PM_SHUTDOWN>;
 };

3.2.2. STM32MP2 series[edit source]

At Soc level of TF-A BL31 device tree, all the low power mode are defined in node cpus/domain-idle-states.

fdts/stm32mp251.dtsi

The unsupported low power modes must be deleted in your board device-tree.

For example on board without PMIC, if the used regulator controlled by PWR_LP pins force the reduced voltage on V_DDCORE and V_DDCPU, the LP-Stop1 and LP-Stop2 modes are not supported.

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

4. How to use the framework[edit source]

4.1. Low-power modes available on the device[edit source]

Refer to STM32MP13 reference manuals, STM32MP15 reference manuals or STM32MP25 reference manuals for the full description of low-power modes.
The modes are handled by the RCC and the PWR peripherals.

4.1.1. On STM32MP13x lines [edit source]

The table below summarizes the device hardware states corresponding to each low-power mode.

MPU mode	Platform mode	V_DDCORE state	V_DDCPU state	Clocks state
CRun	Run	On	On	On
CStop	Stop/LPLV-Stop	On/Retention	On/Retention	Off/Off
CStandby	Stop/LPLV-Stop/LPLVL-STOP2/Standby	On/Retention/Retention/Off	On/Retention/Off/Off	Off/Off/Off/Off

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

Platform mode	Available wake-up sources
Stop	BOR, PVD, AVD, Vbat mon, Temp mon, HSE CSS, LSE CSS, RTC, TAMP, USBH, OTG, ETH, USART, I2C, SPI, DTS, LPTIM, IWDG, GPIO, Wakeup pins (from PWR)
LPLV-Stop LPLV-Stop2	BOR, PVD, AVD, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, USART, I2C, SPI, DTS, LPTIM, IWDG, GPIO, Wakeup pins (from PWR)
Standby	BOR, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, IWDG, Wakeup pins (from PWR)

4.1.2. On STM32MP15x lines [edit source]

The AN5109 low-power application note also gives much more information on these modes, including:

the detailed description of the operating modes,
the low-power mode entry and exit sequences,
the low-power mode control registers.
the wake-up sources and the software mechanism that ensures the consistency between the low-power mode and the activated wake-up source,

The table below summarizes the device hardware states corresponding to each low-power mode.

The term "subsystem" either refers to Arm^® Cortex^®-A7 (also called MPU) or to Arm^® Cortex^®-M4 (also called MCU). A mode prefixed by 'C' corresponds to a subsystem mode.

Level	Mode	V_DDCORE state	Subsystem Clocks state
MPU Subsystem	MPU CRun	on	on
	MPU CStop	on	off
	MPU CStandby	on	off
MCU Subsystem	MCU CRun	on	on
	MCU CStop	on	off

A platform mode is the combination of MPU and MCU modes.

MPU mode	MCU mode	Platform mode	V_DDCORE state	Clocks state
CRun/CStop/CStandby	CRun	Run	On	On
CStop/CStandby	CStop	Stop (LPDS = 0)	On	Off
CStop/CStandby	CStop	LP-Stop or LPLV-Stop (LPDS = 1)	Retention	Off
CStandby	CStop MCU PDDS = 1	Standby	Off	Off

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

Plaform mode	Available wake-up sources
Stop	BOR, PVD, AVD, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, USB, CEC, ETH, USART, I²C, SPI, LPTIM, IWDG, GPIO, Wakeup pins (from PWR)
LPLV-Stop	BOR, PVD, AVD, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, IWDG, GPIO, Wakeup pins (from PWR)
Standby	BOR, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, IWDG, Wakeup pins (from PWR)

This list of Wake-up capability peripheral for each mode is defined in the Table 35. Functionalities depending on system operating mode of the STM32MP15 reference manuals.

4.1.3. On STM32MP25x lines [edit source]

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.

System mode	Linux Command	Power consumption	Wake-up time	V_DDCPU	V_DDCORE	DDR state
Stop1	freeze (s2idle)	Medium	Medium	ON	ON	SR, V_TT on
LP-Stop1		Medium	Medium	ON	ON	SR, V_TT off
LPLV-Stop1		Low	Medium	ON (reduced)	ON (reduced)
Stop2	mem (deep)	Low	Medium	OFF	ON
LP-Stop2		Low	Medium		ON
LPLV-Stop2		Low	Medium		ON (reduced)
Standby1		Low	Medium		OFF
Standby2	shutdown	Low	Low			Off
V_BAT	shutdown	Low	Low			Off

For LP-Stop1 and LP-Stop2 modes, the DDR V_TT supply is switched off on STMicroelectronics reference design with PWR_LP signal connected to PWRCTRL1 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^{[note 1]}, I2C8^{[note 1]}, I3C4^{[note 1]}, SPI8^{[note 1]}, ADF1^{[note 1]}, 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

↑ ^1.0 ^1.1 ^1.2 ^1.3 ^1.4 The wake-up associated to autonomous mode is not managed in OpenSTLinux.

4.2. How to enter and exit low-power modes[edit source]

4.2.1. System-Wide Power Management[edit source]

The platform selects the allowed modes depending on the required wakeup source and the coprocessor state.

So you need:

Activate the MCU needed wakeup source(s) and call the expected low-power mode for MCU side
or stop the MCU firmware
Activate the MPU needed wakeup source(s) and call the expected low-power mode for MPU side

Information

You must be sure your wakeup device is able to provide the wake up signal during low-power mode.

E.g. supply, clock, reset, etc... must be present, if any

4.2.1.1. MCU side[edit source]

Please refer to Coprocessor power management for Arm^® Cortex^®-M commands or the MCU firmware can also be stopped before entering in low power (it is mandatory if this firmware is running in DDR).

4.2.1.2. MPU side[edit source]

The wakeup source(s) activation (device dependent) is done in the corresponding Linux driver (see the 'wakeup' attribute in sysfs) and it is possible to check the state of each wakeup source by displaying this attribute.

Then the low-power mode request is done by issuing the following command:

  echo mem > /sys/power/state

When systemd^[1] is used, for example on the Weston OpenSTLinux configuration, the low-power mode is safely entered with the systemctlcommand:

  systemctl suspend

In this case, the configuration file /etc/systemd/sleep.conf ^[2] map the Weston power state to the kernel power state (in /sys/power/state). The OpenSTlinux configuration is:

[Sleep]
SuspendMode=
HibernateMode=
HybridSleepMode=
SuspendState=mem
HibernateState=mem
HybridSleepState=mem

4.2.1.3. Example[edit source]

entering/exiting MPU CStop mode

Enable at least one wakeup source from table 2.1 in CStop category, for example USART4 on STM32MP15:

 echo enabled > /sys/devices/platform/soc/40010000.serial/tty/ttySTM0/power/wakeup
 echo enabled > /sys/devices/platform/soc/40010000.serial/power/wakeup

Call the low-power entry:

 echo mem > /sys/power/state

or systemd function:

  systemctl suspend

The MPU is now in CStop mode, and can be woken up by sending a character to the console.

5. Source code location[edit source]

6. How to trace and debug[edit source]

The suspend/resume process execution is logged in the MPU console. It gives useful information on the platform state (sleeping or active).

 root@stm32mp1:~# echo mem > /sys/power/state
 [ 1072.267571] PM: suspend entry (deep)
 [ 1072.269687] PM: Syncing filesystems ... done.
 [ 1072.279114] Freezing user space processes ... (elapsed 0.008 seconds) done.
 [ 1072.292835] OOM killer disabled.
 [ 1072.296046] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
 [ 1072.303431] Suspending console(s) (use no_console_suspend to debug)
 [ 1072.332520] dwc2 49000000.usb-otg: suspending usb gadget configfs-gadget
 [ 1072.332537] dwc2 49000000.usb-otg: dwc2_hsotg_ep_disable: called for ep0
 [ 1072.332546] dwc2 49000000.usb-otg: dwc2_hsotg_ep_disable: called for ep0
 [ 1072.468536] Disabling non-boot CPUs ...
 [ 1072.507876] CPU1 killed.
 [ 1072.509635] Enabling non-boot CPUs ...
 [ 1072.510508] CPU1 is up
 [ 1072.527553] dwmac4: Master AXI performs any burst length
 [ 1072.527583] stm32-dwmac 5800a000.ethernet eth0: No Safety Features support found
 [ 1072.527621] stm32-dwmac 5800a000.ethernet eth0: ERROR failed to create debugfs directory
 [ 1072.527631] stm32-dwmac 5800a000.ethernet eth0: stmmac_hw_setup: failed debugFS registration
 [ 1072.588234] dwc2 49000000.usb-otg: resuming usb gadget configfs-gadget
 [ 1072.738469] OOM killer enabled.
 [ 1072.741575] Restarting tasks ... done.
 [ 1072.752596] PM: suspend exit

Get more debug information from the console with the following commands:

 echo N > /sys/module/printk/parameters/console_suspend 
 echo "func pm_dev_dbg +p" > /sys/kernel/debug/dynamic_debug/control

It is also possible to monitor the hardware signals related to the system low-power modes thanks to the HDP internal peripheral.
Please refer to HDP Linux driver for its configuration.

7. To go further[edit source]

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

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

[autonomous-1] 1.0 ^1.1 ^1.2 ^1.3 ^1.4 The wake-up associated to autonomous mode is not managed in OpenSTLinux.

[2] systemd: https://systemd.io/

[3] ttps://manpages.debian.org/jessie/systemd/systemd-sleep.conf.5.en.html

[note 1]

[1]

[2]