CPU frequency scaling
Raspbian fixes cpu frequency to 700MHz by default. It would not be reasonable to keep max cpu freq for the usage which doesn't requires full cpu power at all times.
Following instruction explains how to custermize your raspberry pi to be lower cpu freq at idle.
GPU setting
/boot/config.txt is read by GPU for hardware configuration before CPU is up. That includes the setting of cpu freq.
Please add following lines in the text.
# Meaningful word. This means 'enable cpu freq scaling'
force_turbo=0
# Set lowest cpu freq. By default, it's 700.
arm_freq_min=100
Reboot is required to reflect them.
Refer following page for more detailed information about config.txt: http://elinux.org/RPiconfig
Linux kernel configuration
By GPU setting, cpu gets capability for freq scaling. But, Linux kernel has its own mechanism to control it and default would fix cpu freq to 100MHz.
To check current/min/max cpu freq please use following commands on shell.
Current cpu freq:
# cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freqMax cpu freq:
# cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freqMin cpu freq:
# cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_min_freqLinux kernel has driver which manages cpu freq policy. Following option is available for that. Rasbian set powersave by default.
performance: always use max cpu freq.
powersave: always use min cpu freq.
ondemand: change cpu freq depending on cpu load (On rasbian, it just switches min and max).
conservative: smoothly change cpu freq depending on cpu load.
uesrspace: allow user space daemon to control cpufreq.
We can change the setting from shell at runtime via sysfs:
# echo ondemand > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor To keep this setting over reboot, There are 2 options.
CONFIG_CPU_FREQ_DEFAULT_GOV_ONDEMAND
CONFIG_CPU_FREQ_DEFAULT_GOV_CONSERVATIVE
Though 2nd way requires kernel build, we can use full cpu power during boot.