Linux Dynamic Frequency Scaling

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Introduction

Dynamic Frequency Scaling aka DFS is available on APF27, APF51, APF28 and APF6 based systems.

You need to have a kernel with the CPUfreq drivers built-in (done by default when available):

$ make linux-menuconfig

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
    <*>   'powersave' governor
    <*>   'userspace' governor for userspace frequency scaling
    <*>   'ondemand' cpufreq policy governor
    <*>   'conservative' cpufreq governor
    <*>   CPUfreq driver for i.MX CPUs  or  <*> i.MX27 frequency driver

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

To know whether the driver is working

APF27 / APF28 / APF51	APF6
# dmesg | grep freq i.MXC CPU frequency driver	Should be activated/working by default :-)

Usage (sysfs)

• all is here:

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

• identify the current governor:

# cat scaling_governor
performance

• list available governors:

# cat scaling_available_governors
conservative ondemand userspace powersave performance

• change governor:

# echo powersave > scaling_governor

• 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

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

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

Effect on current consumption

When there is little activity, the CPU frequency has nearly no visible effect on the current consumed.

This is caused by the "doze mode" : when all processes are sleeping, the kernel turns off the CPU clock until the next IO or timer event.

When there is a 100% CPU activity, the effect of the CPU frequency can be observed with a ammeter.

The % CPU activity per process can be checked with the top command, which is supported by Busybox (but it may be necessary to configure and rebuild busybox to have it)

Note: If you make current measurements using a development board like APF27dev or APF51dev, take care of making all the measurement with the same supply voltage. Unlike linear regulators, the switching regulator of the board reduces the current consumed when the input voltage is increased (to keep the power constant)

• To Be Continued...

Links

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