Acer TravelMate 4001WLMi

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For Fedora Core 5 speed stepping works out of the box.

The rest of this section applies to my Fedora Core 2 installation.

Clock Scaling allows you to change the clock speed of the CPUs on the fly. This is a nice method to save battery power, because the lower the clock speed is, the less power the CPU consumes.

There are several techniques for Clock Scaling. Modern Centrino CPUs have a speed stepping feature. Alternatively, the CPU freqency can be controlled using the ACPI.

It turned out that this laptop does not have a sufficient ACPI implementation to use the Centrino SpeedStep feature, so I went for the acpi-cpufreq alternative. To enable this, I needed at least the following settings in my kernel config:

Power management options (ACPI, APM)  --->

  CPU Frequency scaling  --->

    [*] CPU Frequency scaling

    Default CPUfreq governor (userspace) --->

    [M] ACPI Processor P-States driver

Under /sys/devices/system/cpu/cpu0/cpufreq there are all (pseudo)files related to frequency scaling. To name a few:

scaling_cur_freq: Shows the current frequency the CPU ist running at. Note that the frequency is in kilohertz. So 600000 means 600Mhz.
scaling_available_frequencies: The list of supported frequencies
scaling_setspeed: Writing a value to this file sets the CPU speed accordingly.

Fedora comes with a nice tool called cpuspeed to toggle frequency automaticly depending on the system load. To have it automatically started upon boot, change the configuration file /etc/cpuspeed.conf to contain the line

MODULE=acpi-cpufreq