1 CPU frequency and voltage scaling code in the Linux(TM) kernel
2
3
4 L i n u x C P U F r e q
5
6 U S E R G U I D E
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8
9 Dominik Brodowski <linux@brodo.de>
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11
12
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.
16
17
18 Contents:
19 ---------
20 1. Supported Architectures and Processors
21 1.1 ARM
22 1.2 x86
23 1.3 sparc64
24 1.4 ppc
25 1.5 SuperH
26
27 2. "Policy" / "Governor"?
28 2.1 Policy
29 2.2 Governor
30
31 3. How to change the CPU cpufreq policy and/or speed
32 3.1 Preferred interface: sysfs
33 3.2 Deprecated interfaces
34
35
36
37 1. Supported Architectures and Processors
38 =========================================
39
40 1.1 ARM
41 -------
42
43 The following ARM processors are supported by cpufreq:
44
45 ARM Integrator
46 ARM-SA1100
47 ARM-SA1110
48
49
50 1.2 x86
51 -------
52
53 The following processors for the x86 architecture are supported by cpufreq:
54
55 AMD Elan - SC400, SC410
56 AMD mobile K6-2+
57 AMD mobile K6-3+
58 AMD mobile Duron
59 AMD mobile Athlon
60 AMD Opteron
61 AMD Athlon 64
62 Cyrix Media GXm
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
67 Transmeta Crusoe
68 Transmeta Efficeon
69 VIA Cyrix 3 / C3
70 various processors on some ACPI 2.0-compatible systems [*]
71
72 [*] Only if "ACPI Processor Performance States" are available
73 to the ACPI<->BIOS interface.
74
75
76 1.3 sparc64
77 -----------
78
79 The following processors for the sparc64 architecture are supported by
80 cpufreq:
81
82 UltraSPARC-III
83
84
85 1.4 ppc
86 -------
87
88 Several "PowerBook" and "iBook2" notebooks are supported.
89
90
91 1.5 SuperH
92 ----------
93
94 The following SuperH processors are supported by cpufreq:
95
96 SH-3
97 SH-4
98
99
100 2. "Policy" / "Governor" ?
101 ==========================
102
103 Some CPU frequency scaling-capable processor switch between various
104 frequencies and operating voltages "on the fly" without any kernel or
105 user involvement. This guarantees very fast switching to a frequency
106 which is high enough to serve the user's needs, but low enough to save
107 power.
108
109
110 2.1 Policy
111 ----------
112
113 On these systems, all you can do is select the lower and upper
114 frequency limit as well as whether you want more aggressive
115 power-saving or more instantly available processing power.
116
117
118 2.2 Governor
119 ------------
120
121 On all other cpufreq implementations, these boundaries still need to
122 be set. Then, a "governor" must be selected. Such a "governor" decides
123 what speed the processor shall run within the boundaries. One such
124 "governor" is the "userspace" governor. This one allows the user - or
125 a yet-to-implement userspace program - to decide what specific speed
126 the processor shall run at.
127
128
129 3. How to change the CPU cpufreq policy and/or speed
130 ====================================================
131
132 3.1 Preferred Interface: sysfs
133 ------------------------------
134
135 The preferred interface is located in the sysfs filesystem. If you
136 mounted it at /sys, the cpufreq interface is located in a subdirectory
137 "cpufreq" within the cpu-device directory
138 (e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).
139
140 cpuinfo_min_freq : this file shows the minimum operating
141 frequency the processor can run at(in kHz)
142 cpuinfo_max_freq : this file shows the maximum operating
143 frequency the processor can run at(in kHz)
144 scaling_driver : this file shows what cpufreq driver is
145 used to set the frequency on this CPU
146
147 scaling_available_governors : this file shows the CPUfreq governors
148 available in this kernel. You can see the
149 currently activated governor in
150
151 scaling_governor, and by "echoing" the name of another
152 governor you can change it. Please note
153 that some governors won't load - they only
154 work on some specific architectures or
155 processors.
156 scaling_min_freq and
157 scaling_max_freq show the current "policy limits" (in
158 kHz). By echoing new values into these
159 files, you can change these limits.
160
161
162 If you have selected the "userspace" governor which allows you to
163 set the CPU operating frequency to a specific value, you can read out
164 the current frequency in
165
166 scaling_setspeed. By "echoing" a new frequency into this
167 you can change the speed of the CPU,
168 but only within the limits of
169 scaling_min_freq and scaling_max_freq.
170
171
172 3.2 Deprecated Interfaces
173 -------------------------
174
175 Depending on your kernel configuration, you might find the following
176 cpufreq-related files:
177 /proc/cpufreq
178 /proc/sys/cpu/*/speed
179 /proc/sys/cpu/*/speed-min
180 /proc/sys/cpu/*/speed-max
181
182 These are files for deprecated interfaces to cpufreq, which offer far
183 less functionality. Because of this, these interfaces aren't described
184 here.
185
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