Linux Dynamic Frequency Scaling

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Introduction

Dynamic Frequency Scaling aka DFS is only available on APF27 and APF51 based systems.

Linux configuration

CPU Power Management  --->
    [*] CPU Frequency scaling
    [*]   Enable CPUfreq debugging
    <*>   CPU frequency translation statistics
    [ ]     CPU frequency translation statistics details
          Default CPUFreq governor (performance)  --->
    -*-   'performance' governor
    <M>   'powersave' governor
    <M>   'userspace' governor for userspace frequency scaling
    <M>   'ondemand' cpufreq policy governor
    <M>   'conservative' cpufreq governor
    <M>   CPUfreq driver for i.MX CPUs

frequency scaling handling is done in arch/arm/plat-mxc/cpufreq.c or arch/arm/mach-mx2/cpufreq_imx27.c

To know if driver is launched

# dmesg | grep freq
i.MXC CPU frequency driver

Usage (sysfs)

• all is here:

# cd /sys/devices/system/cpu/cpu0/cpufreq/
# ls

• get current governor:

# cat scaling_governor
performance

• list available governors:

# cat scaling_available_governors
performance

• eventually load some more:

# modprobe cpufreq_powersave
# modprobe cpufreq_conservative
# cat scaling_available_governors
conservative powersave performance

• change governor:

# echo powersave > scaling_governor

• change frequency manually (here 133MHz). Requires userspace governor ! :

# echo 133000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed

• To know current CPU frequency (here 160MHz):

# cat cpuinfo_cur_freq 
160000

• To know how much time CPU has spent in its different state (here 160MHz and 800MHz):

# cat stats/time_in_state 
160000 243595
800000 585

• To Be Continued...

Links

• http://lxr.linux.no/linux+v2.6.37/Documentation/cpu-freq/user-guide.txt