Devices operating on battery power need effective power management: anything you can do to reduce power usage will increase battery life. Even for devices running on mains power, better power managements has benefits in reducing the need for cooling and lower energy costs. This presentation describes the four principles of power management: don't rush if you don't have to; don't be ashamed of being idle; turn off things you are not using; and sleep when there is nothing else to do. Each of these has a counterpart in the Linux kernel.

1. Linux power management: are you doing it
right?
Chris Simmonds
Embedded World 2018
Linux power management: are you doing it right? 1 Copyright © 2011-2018, 2net Ltd

2. License
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3. About Chris Simmonds
• Consultant and trainer
• Author of Mastering Embedded Linux Programming
• Working with embedded Linux since 1999
• Android since 2009
• Speaker at many conferences and workshops
"Looking after the Inner Penguin" blog at http://2net.co.uk/
https://uk.linkedin.com/in/chrisdsimmonds/
https://google.com/+chrissimmonds
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4. Overview
• Reducing power usage is important, especially for battery-powered
devices
• In this talk I will introduce
• The rules of power management
• The major Linux systems that implement power management
• Some practical measures you can take
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5. The rules of power management
1. Don’t rush if you don’t have to
2. Take a break when things are quiet
3. Turn things off when they are not in use
4. Have a snooze when you expect things to be quite for a while
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6. Rule 1: Don’t rush
• Reducing CPU frequency saves power?
• P = CfV2
• Reducing voltage, not frequency, saves power
• But, lowering frequency allows for lowering the voltage, and therefore
saves power
• The tuple of frequency and voltage is called an Operating Performance
Point (OPP)
• The ability to change OPP at run-time is called Dynamic Voltage and
Frequency Scaling (DVFS)
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7. OPP definitions in device tree
Example from am33xx.dtsi
cpu0_opp_table: opp-table {
compatible = "operating-points-v2-ti-cpu";
syscon = <&scm_conf>;
opp50@300000000 {
opp-hz = /bits/ 64 <300000000>;
opp-microvolt = <950000 931000 969000>;
opp-supported-hw = <0x06 0x0010>;
opp-suspend;
};
opp100@600000000 {
opp-hz = /bits/ 64 <600000000>;
opp-microvolt = <1100000 1078000 1122000>;
opp-supported-hw = <0x06 0x0040>;
};
[...]
};
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8. CPUFreq
• In Linux, selecting the best OPP is done by the CPUFreq driver
• Enable with kernel option CONFIG_CPU_FREQ=y
• Part of the Linux support for each SoC
• Control is per CPU, via directory /sys/devices/system/cpu/cpuN/cpufreq
• List of possible frequencies (corresponding to OPPs):
# cd /sys/devices/system/cpu/cpu0/cpufreq
# cat scaling_available_frequencies
300000 600000 720000 800000
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9. CPUFreq Governors
• The policy for changing the OPP is controlled by a governor
• There are a small number of governors coded into Linux:
# cd /sys/devices/system/cpu/cpu0/cpufreq
# cat scaling_available_governors
conservative ondemand userspace powersave performance
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10. Governors
• powersave: Always selects the lowest frequency
• performance: Always selects the highest frequency
• ondemand: Changes frequency based on the CPU utilization. If the
CPU is idle less than 20% of the time, it sets the frequency to the
maximum; if it is idle more than 30% of the time, it decrements the
frequency by 5%
• conservative: As ondemand , but switches to higher frequencies in
5% steps rather than going immediately to the maximum
• userspace: Frequency is set by a user space program
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11. Example of the potential power saving
• BeagleBone Black running a constant load
• No governor: using fixed frequencies
OPP Freq MHz CPU use, % Power, mW
OPP50 300 94 320
OPP100 600 48 345
OPP120 720 40 370
Turbo 800 34 370
Nitro 1000 28 370
15% saving by running the job at 300 MHz instead of 1 GHz
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12. Rule 2: take a break
• When the CPU has nothing to do, it enters an idle state
• Usually, there are several idle states to choose from
• Sometimes known as C-states
• Deeper idle states take longer to return to full operation
• Example: the CPU L1 cache may be powered down, so it takes time to
re-load the cache when restarting
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13. CPUIdle
• In Linux, the transition between idle states is controlled by the CPUIdle
driver
• Enable with kernel option CONFIG_CPU_IDLE
• Information about idle state C for CPU N is in
/sys/devices/system/cpu/cpuN/cpuidle/stateC
• For example the TI AM335x SoC has only two states
• State0: WFI (Wait For Interrupt)
• State1: C1
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14. CPUIdle governors
• The policy for selecting idle states is controlled by the CPUIdle
governor
• Cannot be changed at run-time
• Only two options
• ladder: steps through the idle states one at a time depending on the
time spent in the last idle period. Works well with a regular timer tick
• menu: selects an idle state based on the expected idle time. Works well
with tickless systems
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15. Tickless operation
• Linux was written assuming a constant tick, typically 100Hz
• On an idle system, this is a waste of CPU power; stops the CU
entering deeper sleep states
• For tickless operation, configure your kernel with CONFIG_NO_HZ_IDLE=y
(on older kernels, CONFIG_NO_HZ=y)
• Schedules timer interrupts for the next event, skipping any intermediate
ticks
• Potential power saving on a device that is mostly idle: up to 70%
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16. Rule 3: turn things off
• Managing the power usage of peripherals is also important
• In Linux, the runtime power management (pm runtime) tracks the
state of pm-aware device drivers
• Each device driver can register callback functions such as
runtime_suspend and runtime_resume
• Kernel configuration option CONFIG_PM
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17. Controlling pm runtime
• PM runtime is exposed via directory power for each device that
supports it
• Files in that directory include:
• control: "auto": kernel will select state; "on": device always on (pm
runtime disabled)
• runtime_status: current state of the device: "active", "suspended", or
"unsupported"
• autosuspend_delay_ms: minimum time before suspending device
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18. Runtime PM
• For example, the MMC controller on TI AM335x
# cd /sys/devices/platform/ocp/481d8000.mmc/mmc_host/mmc1/mmc1:0001/power
# grep "" *
async:disabled
autosuspend_delay_ms:3000
control:auto
runtime_active_kids:0
runtime_active_time:164440
runtime_enabled:enabled
runtime_status:suspended
runtime_suspended_time:139270
runtime_usage:0
• Note that autosuspend_delay_ms is 3 seconds
• That runtime_status is "suspended"
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19. Rule 4: Have a snooze
• The techniques so far will optimize power usage of a running system
• Largest power saving comes from entering a system sleep mode
• Example: when closing the lid of a laptop
• Example: Android device suspending after activity timeout (default 60
seconds)
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20. Power states
Sleep state S-state Description
freeze Stops (freezes) all activity in user space
standby S1 Powers down all CPUs except the boot CPU
mem S3 Powers down the system, puts RAM into self-refresh
disk S4 Saves memory to disk, followed by complete power down
Linux sleep states, together with equivalent ACPI S-state
• mem (suspend to RAM) typically consumes a few milliamps
(dependent on board design and BSP)
• disk (suspend to disk) typically consumes no power at all
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21. Controlling power state
• In Linux, macro power management is via by the power management
subsystem
• Enable with kernel option CONFIG_PM
• You can view the sleep states implemented in BSP via /sys/power/state
# cat /sys/power/state
freeze standby mem disk
• mem is the one most often used in embedded systems
• disk (aka hibernation) is usually too slow and takes too much storage
when using flash memory
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22. Changing power state
• Simply, write the desired state to /sys/power/state
• For example, to enter suspend mode:
# echo mem > /sys/power/state
[ 1646.158274] PM: Syncing filesystems ...done.
[ 1646.178387] Freezing user space processes ...(elapsed 0.001 seconds)
done.
[ 1646.188098] Freezing remaining freezable tasks ...
(elapsed 0.001 seconds) done.
[ 1646.197017] Suspending console(s) (use
no_console_suspend to debug)
[ 1646.338657] PM: suspend of devices complete
after 134.322 msecs
[ 1646.343428] PM: late suspend of devices
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23. Waking up
• CPU is woken by interrupt or other input
• Wakeup sources need to be powered while the main CPU is powered
down
• Wakeup sources may include
• Real-time clock (RTC)
• Certain GPIO pins
• UART
• Usually managed by a dedicated Power Management chip, PMIC
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24. Wakeup sources
• Devices that can act as wakeup sources have
/sys/devices/.../power/wakeup set to "enabled"
• For example, GPIOs 0 to 7 on AM335x
# cat /sys/devices/platform/gpio_keys/power/wakeup
enabled
• Hence, pressing the power button wakes the device
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25. Conclusion
• A well configured Linux device will save power
• Linux features considered here:
• CPUFreq: selects OPP depending on load
• CPUIdle: selects idle state based on predicted load
• Tickless: eliminates unnecessary timer interrupts
• Runtime power management: powers off unused peripherals
• Suspend to RAM: puts system in minimum power usage state
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26. Questions?
Further reading: Chapter 11 of Mastering Embedded Linux Programming
Contact:
Web: www.2net.co.uk
Email: training@2net.co.uk
LinkedIn: https://uk.linkedin.com/in/chrisdsimmonds
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