1 CPU frequency and voltage scaling code in the Linux(TM) kernel
4 L i n u x C P U F r e q
9 Dominik Brodowski <linux@brodo.de>
13 Clock scaling allows you to change the clock speed of the CPUs on the
14 fly. This is a nice method to save battery power, because the lower
15 the clock speed, the less power the CPU consumes.
20 1. Supported Architectures and Processors
27 2. "Policy" / "Governor"?
31 3. How to change the CPU cpufreq policy and/or speed
32 3.1 Preferred interface: sysfs
33 3.2 Deprecated interfaces
37 1. Supported Architectures and Processors
38 =========================================
43 The following ARM processors are supported by cpufreq:
53 The following processors for the x86 architecture are supported by cpufreq:
55 AMD Elan - SC400, SC410
63 Intel mobile PIII and Intel mobile PIII-M on certain chipsets
64 Intel Pentium 4, Intel Xeon
65 Intel Pentium M (Centrino)
66 National Semiconductors Geode GX
69 various processors on some ACPI 2.0-compatible systems [*]
71 [*] Only if "ACPI Processor Performance States" are available
72 to the ACPI<->BIOS interface.
78 The following processors for the sparc64 architecture are supported by
87 Several "PowerBook" and "iBook2" notebooks are supported.
93 The following SuperH processors are supported by cpufreq:
99 2. "Policy" / "Governor" ?
100 ==========================
102 Some CPU frequency scaling-capable processor switch between various
103 frequencies and operating voltages "on the fly" without any kernel or
104 user involvement. This guarantees very fast switching to a frequency
105 which is high enough to serve the user's needs, but low enough to save
112 On these systems, all you can do is select the lower and upper
113 frequency limit as well as whether you want more aggressive
114 power-saving or more instantly available processing power.
120 On all other cpufreq implementations, these boundaries still need to
121 be set. Then, a "governor" must be selected. Such a "governor" decides
122 what speed the processor shall run within the boundaries. One such
123 "governor" is the "userspace" governor. This one allows the user - or
124 a yet-to-implement userspace program - to decide what specific speed
125 the processor shall run at.
128 3. How to change the CPU cpufreq policy and/or speed
129 ====================================================
131 3.1 Preferred Interface: sysfs
132 ------------------------------
134 The preferred interface is located in the sysfs filesystem. If you
135 mounted it at /sys, the cpufreq interface is located in a subdirectory
136 "cpufreq" within the cpu-device directory
137 (e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).
139 cpuinfo_min_freq : this file shows the minimum operating
140 frequency the processor can run at(in kHz)
141 cpuinfo_max_freq : this file shows the maximum operating
142 frequency the processor can run at(in kHz)
143 scaling_driver : this file shows what cpufreq driver is
144 used to set the frequency on this CPU
146 scaling_available_governors : this file shows the CPUfreq governors
147 available in this kernel. You can see the
148 currently activated governor in
150 scaling_governor, and by "echoing" the name of another
151 governor you can change it. Please note
152 that some governors won't load - they only
153 work on some specific architectures or
156 scaling_max_freq show the current "policy limits" (in
157 kHz). By echoing new values into these
158 files, you can change these limits.
161 If you have selected the "userspace" governor which allows you to
162 set the CPU operating frequency to a specific value, you can read out
163 the current frequency in
165 scaling_setspeed. By "echoing" a new frequency into this
166 you can change the speed of the CPU,
167 but only within the limits of
168 scaling_min_freq and scaling_max_freq.
171 3.2 Deprecated Interfaces
172 -------------------------
174 Depending on your kernel configuration, you might find the following
175 cpufreq-related files:
177 /proc/sys/cpu/*/speed
178 /proc/sys/cpu/*/speed-min
179 /proc/sys/cpu/*/speed-max
181 These are files for deprecated interfaces to cpufreq, which offer far
182 less functionality. Because of this, these interfaces aren't described