CPU frequency and voltage scaling code in the Linux(TM) kernel
L i n u x C P U F r e q
C P U F r e q C o r e
Dominik Brodowski <linux@brodo.de>
David Kimdon <dwhedon@debian.org>
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, the less power the CPU consumes.
Contents:
---------
1. CPUFreq core and interfaces
2. CPUFreq notifiers
1. General Information
=======================
The CPUFreq core code is located in linux/kernel/cpufreq.c. This
cpufreq code offers a standardized interface for the CPUFreq
architecture drivers (those pieces of code that do actual
frequency transitions), as well as to "notifiers". These are device
drivers or other part of the kernel that need to be informed of
policy changes (ex. thermal modules like ACPI) or of all
frequency changes (ex. timing code) or even need to force certain
speed limits (like LCD drivers on ARM architecture). Additionally, the
kernel "constant" loops_per_jiffy is updated on frequency changes
here.
Reference counting is done by cpufreq_get_cpu and cpufreq_put_cpu,
which make sure that the cpufreq processor driver is correctly
registered with the core, and will not be unloaded until
cpufreq_put_cpu is called.
2. CPUFreq notifiers
====================
CPUFreq notifiers conform to the standard kernel notifier interface.
See linux/include/linux/notifier.h for details on notifiers.
There are two different CPUFreq notifiers - policy notifiers and
transition notifiers.
2.1 CPUFreq policy notifiers
----------------------------
These are notified when a new policy is intended to be set. Each
CPUFreq policy notifier is called three times for a policy transition:
1.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if
they see a need for this - may it be thermal considerations or
hardware limitations.
2.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid
hardware failure.
3.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
- if two hardware drivers failed to agree on a new policy before this
stage, the incompatible hardware shall be shut down, and the user
informed of this.
The phase is specified in the second argument to the notifier.
The third argument, a void *pointer, points to a struct cpufreq_policy
consisting of five values: cpu, min, max, policy and max_cpu_freq. min
and max are the lower and upper frequencies (in kHz) of the new
policy, policy the new policy, cpu the number of the affected CPU or
CPUFREQ_ALL_CPUS for all CPUs; and max_cpu_freq the maximum supported
CPU frequency. This value is given for informational purposes only.
2.2 CPUFreq transition notifiers
--------------------------------
These are notified twice when the CPUfreq driver switches the CPU core
frequency and this change has any external implications.
The second argument specifies the phase - CPUFREQ_PRECHANGE or
CPUFREQ_POSTCHANGE.
The third argument is a struct cpufreq_freqs with the following
values:
cpu - number of the affected CPU or CPUFREQ_ALL_CPUS
old - old frequency
new - new frequency
Download Driver Pack
After your driver has been downloaded, follow these simple steps to install it.
Expand the archive file (if the download file is in zip or rar format).
If the expanded file has an .exe extension, double click it and follow the installation instructions.
Otherwise, open Device Manager by right-clicking the Start menu and selecting Device Manager.
Find the device and model you want to update in the device list.
Double-click on it to open the Properties dialog box.
From the Properties dialog box, select the Driver tab.
Click the Update Driver button, then follow the instructions.
Very important: You must reboot your system to ensure that any driver updates have taken effect.
For more help, visit our Driver Support section for step-by-step videos on how to install drivers for every file type.