The Linux kernel uses several power management strategies:

  • Dynamic voltage frequency scaling (DVFS) adjusts the CPU frequency and the different voltages depending on the system’s load. The combination of a CPU frequency and a set of voltages for that frequency is called an operating point.

  • Suspend to memory allows for the system to sleep while waiting for an event. On suspend, most system devices, including CPU and memory, enter low power mode. On resume, the system will continue from the same state it was in before it suspended.

  • Power off, which brings the system to a halt until an event wakes the system. On power off the system power remains enabled and the system is placed on its lowest consumption mode. On wakeup, the bootloader starts up again and the system is initialized.

Dynamic Voltage Frequency Scaling

Dynamic Voltage Frequency Scaling is enabled by default, so the CPU frequency and voltages of a running system will adapt to the system’s load. The DVFS subsystem is controlled through the /sys/devices/system/cpu/cpuN/cpufreq/ sysfs path, where N corresponds to the core number:

Sysfs attributes of CPU frequency scaling
# ls /sys/devices/system/cpu/cpu0/cpufreq/
affected_cpus                  related_cpus                   scaling_governor
cpuinfo_cur_freq               scaling_available_frequencies  scaling_max_freq
cpuinfo_max_freq               scaling_available_governors    scaling_min_freq
cpuinfo_min_freq               scaling_cur_freq               scaling_setspeed
cpuinfo_transition_latency     scaling_driver                 stats

The scaling_cur_freq file shows the current frequency of the core:

# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq

The available frequencies are shown in scaling_available_frequencies.

# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies

The DVFS subsystem can be configured with different governors that control its behavior. The available governors are listed in scaling_available_governors.

# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors
conservative userspace powersave ondemand performance schedutil
Actual output may differ depending on kernel version/configuration.

The current governor is shown in scaling_governor, and by default is set to the ondemand governor.

  • ondemand: This governor does a periodic polling to adjust the operating mode to the system load.

  • conservative: Similar to the on-demand governor, the conservative governor performs graceful frequency and voltage changes that are better suited to battery-powered devices.

  • powersave: This governor uses the lowest available frequency without dynamic adaptation.

  • userspace: This governor allows for user space to select the operating frequencies.

  • performance: This governor uses the highest available frequency without dynamic adaptation.

  • schedutil: This governor uses a scheduler-driven CPU frequency selection.

Manually select a working CPU frequency
# echo userspace > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies
500000 900000 1404000 1800000
# echo 1800000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed
# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
1800000

Sample application

Example applications called apix-cpu-example and apix-pm-sample are included in the dey-examples-digiapix recipe (part of dey-examples package) of the meta-digi layer. These applications show how to access and manage power-management-related features using the Digi APIx library on the ConnectCore 95 platform.

Go to GitHub to see applications instructions and source code.

See Power management API for more information about the Power management APIx.

Suspend to memory

The Linux kernel can perform a suspend-to-memory or suspend-to-RAM operation. When entering this low-power mode, the system state is kept in self-refreshing RAM while the system enters a low-power-consumption mode. The system resumes when a previously selected interrupt is received, restores the previous state, and continues running from where it left off. There is often a trade-off between the depth of the low-power mode and the speed at which the system can resume.

Entering suspend mode

To enter suspend mode:

  • Run /bin/standby from the command line, or

  • Briefly (less than two seconds) press the power key

Resume events

The system can resume from any interrupt-generating event, including:

  • Internal RTC alarm

  • External RTC alarm

  • Power key event

  • Wake on LAN (if supported by the driver)

  • Wake on wireless LAN (if supported by the driver)

The following wake-up sources are enabled by default:

  • Power key

  • Internal RTC alarm

See Configuring wake-up sources for additional details.

Power off

The Linux kernel can perform a power-off operation that places the Power Management IC (PMIC) in power off mode, disabling all power sources that are not needed for wake up.

Entering power off

To enter power off mode you can do one of the following:

  • Run the command poweroff from the command line to perform a controlled software power-off sequence.

  • Press the power key for longer than six seconds to perform a controlled software power-off sequence.

Wake up events

You can wake up the target from power-off mode with an interrupt event to the PMIC, but not to the CPU as it will not be powered. These include:

  • Power key event

  • Internal RTC alarm

  • External RTC alarm

Press the power key for one second or longer to wake the system from power-off.

Configuring wake-up sources

GPIO resume from suspend

CPU GPIOs as wake-up source

CPU GPIOs can only act as wake-up sources if they have been configured to send an input key event to the system via a driver such as gpio-keys. To do so, add an entry like this to the device tree:

#include <dt-bindings/gpio/gpio.h>
#include <dt-bindings/input/input.h>

/ {
	...

	gpio-keys {
		compatible = "gpio-keys";
		power {
			label = "Power Button";
			gpios = <&gpio2 20 GPIO_ACTIVE_LOW>;
			linux,code = <KEY_POWER>;
			gpio-key,wakeup;
		};
	};

	...
};
Triggering the GPIO during suspend wakes the system up. Triggering the GPIO when the system is running also sends the KEY_POWER event, which powers the system off.

RTC alarm resume

To enable the RTC wake alarm to trigger in 60 seconds:

  • Enable the RTC device as a wake-up source:

    # echo enabled > /sys/class/rtc/rtc0/device/power/wakeup

  • Program the time when the alarm should trigger an interrupt (format is seconds since the epoch or, if there’s a leading +, seconds in the future or, if there’s a leading +=, seconds ahead of the current alarm):

    # echo +60 > /sys/class/rtc/rtc0/wakealarm
For more complex setups or information about other wakeup options, visit https://www.nxp.com/docs/en/application-note/AN14465.pdf.