The majority of modern CPUs that support CPU scaling now use a common driver (acpi-cpufreq). Judging by the search queries that hit my blog, and the amount of mail I get on the subject, there is a failure mode of this driver that many people are hitting, that there isn’t a great deal of information on.

The failure mode looks like this:

$ modprobe acpi-cpufreq
FATAL: Error inserting (/lib/modules/…/kernel/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.ko): No such device

Not particularly informative. We don’t spit out anything helpful to dmesg either. So what is the cause of this problem?
In many cases, /proc/cpuinfo shows the cpu supports speedstep (the ‘est’ flag). The answer in nearly all of these cases is.. The BIOS. The ACPI tables in the BIOS list which P-states a particular CPU supports. If your CPU was manufactured after your BIOS was written, you’re probably going to be out of luck. Sometimes, there are BIOS updates on the motherboard manufacturers website that will add support for newer processors. Sometimes we aren’t so lucky. In these cases, we’re out of luck, there’s nothing we can do.

There is another possibility for the error message above: kernel bugs. We have introduced bugs in the ACPI interpretor in the past which have broken parsing of the P-states on some platforms. These kinds of bugs tend to get noticed very quickly, and fixed in equally short time, but it’s worth making a point that it’s important to be running on the last kernel version before reporting bugs.