CPU Frequency and DVFS (Linux)
Introduction
The Linux Kernel supports dynamic voltage and frequency switching (DVFS) in order to minimize power usage. Generally, the feature should remain enabled, however, if power consumption and heat dissipation aren't an issue and low latency is required, it might make sense to disable frequency scaling.
This article complies with the Typographic Conventions for Torizon Documentation.
Maximum Frequency by SoMs
The maximum frequency to be used depends on the SoC present in the module, see the table below:
| Module | Maximum frequency (kHz) |
|---|---|
| Colibri iMX6S/DL | 996000 |
| Colibri iMX6 IT | 792000 |
| Colibri IMX6ULL | 792000 |
| Colibri iMX7S | 792000 |
| Colibri iMX7DL | 996000 |
| Colibri iMX8QXP | 1200000 |
| Colibri T20/T20 IT | 996000 |
| Colibri T30 | 1400000 |
| Colibri T30 IT | 1000000 |
| Apalis iMX6 | 996000 |
| Apalis iMX6 IT | 792000 |
| Apalis iMX8QM/QP | 1596000 (A72) 1200000 (A53) |
| Apalis iMX8QXP | 1200000 |
| Apalis T30 | 1400000 |
| Apalis TK1 | 2070000 |
| Verdin iMX8M Mini (IT) | 1600000 |
| Verdin iMX8M Mini | 1800000 |
| Verdin iMX8M Plus (IT) | 1600000 |
| Verdin iMX8M Plus | 1800000 |
| Verdin AM62 Solo | 800000 |
| Verdin AM62 Solo (IT) | 1000000 |
| Verdin AM62 Dual/Quad | 1400000 |
| Verdin iMX95 | 1800000 (A55) 800000 (M7) 333000 (M33) |
| Aquila AM69 | 2000000 (A72) 1000000 (R5F) |
Tegra modules can either have one or all cores enabled. This is controlled by the Linux kernel based on the CPU load. For instance, when the first CPU load goes to 100%, the other cores are enabled.
For the Apalis T30 and Colibri T30 (non-IT), the maximum CPU frequency depends on the number of cores enabled: 1 core 1.4GHz; 4 cores 1.3GHz. For the Colibri T30 IT, the maximum frequency is 1 GHz independent of the number of cores enabled.
For the Verdin iMX8M Mini/Plus (non-IT), the maximum CPU frequency is 1.8GHz, for IT SoC revisions the maximum CPU frequency is 1.6GHz.
CPU Governor
CPU governors can be viewed as preconfigured power settings for the CPU, for detailed information about governors consult this article.
Available governors can be seen through scaling_available_governors file:
# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors
conservative userspace powersave ondemand performance
Change CPU frequency behavior by using an appropriate governor:
# echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
CPU Frequency
Available frequencies values can be seen through scaling_available_frequencies file:
# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies
1200000 1600000
Change CPU frequency value by setting the CPU frequency explicitly:
# echo 1600000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed
The userspace governor must be set to change the frequency value.
The system will adjust to an appropriate voltage according to frequency. Please note that depending on the board/die temperature, thermal throttling might limit the current frequency in use.
How to change the CPU power profile on boot
By default, even if you change the CPU power profile using the CPU Governor, that profile reverts to the default after restarting the SoM. To address this, a service can be added to systemd to run during boot.
Looking for this, create this archive on this specific directory:
# sudo touch /etc/systemd/system/cpu-governor.service
Now, copy this configuration to the created archive /etc/systemd/system/cpu-governor.service:
[Unit]
Description=Performance CPU frequency governor
[Service]
Type=oneshot
RemainAfterExit=true
ExecStart=/bin/sh -c '\
echo powersave > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor'
ExecStop=/bin/sh -c '\
echo ondemand > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor'
[Install]
WantedBy=multi-user.target
To activate the service, run:
# sudo systemctl enable performance-cpu-governor.service
CPUPower Userspace Tools
The cpupower userspace tools can be used to achieve the same results as above.
Check the current frequency information:
# cpupower frequency-info
analyzing CPU 0:
driver: cpufreq-dt
CPUs which run at the same hardware frequency: 0 1 2 3
CPUs which need to have their frequency coordinated by software: 0 1 2 3
maximum transition latency: 182 us
hardware limits: 1.20 GHz - 1.60 GHz
available frequency steps: 1.20 GHz, 1.60 GHz
available cpufreq governors: conservative ondemand userspace powersave performance schedutil
current policy: frequency should be within 1.20 GHz and 1.60 GHz.
The governor "ondemand" may decide which speed to use
within this range.
current CPU frequency: Unable to call hardware
current CPU frequency: 1.20 GHz (asserted by call to kernel)
Change CPU frequency behavior to userspace:
# sudo cpupower frequency-set -g userspace
Change CPU frequency value by setting the CPU frequency explicitly:
# sudo cpupower frequency-set -f 1600000