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1da177e4 LT |
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 | |
7 | ||
8 | ||
9 | Dominik Brodowski <linux@brodo.de> | |
10 | ||
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 | |
9e2697ff | 48 | Intel PXA |
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49 | |
50 | ||
51 | 1.2 x86 | |
52 | ------- | |
53 | ||
54 | The following processors for the x86 architecture are supported by cpufreq: | |
55 | ||
56 | AMD Elan - SC400, SC410 | |
57 | AMD mobile K6-2+ | |
58 | AMD mobile K6-3+ | |
59 | AMD mobile Duron | |
60 | AMD mobile Athlon | |
61 | AMD Opteron | |
62 | AMD Athlon 64 | |
63 | Cyrix Media GXm | |
64 | Intel mobile PIII and Intel mobile PIII-M on certain chipsets | |
65 | Intel Pentium 4, Intel Xeon | |
66 | Intel Pentium M (Centrino) | |
67 | National Semiconductors Geode GX | |
68 | Transmeta Crusoe | |
69 | Transmeta Efficeon | |
70 | VIA Cyrix 3 / C3 | |
71 | various processors on some ACPI 2.0-compatible systems [*] | |
72 | ||
73 | [*] Only if "ACPI Processor Performance States" are available | |
74 | to the ACPI<->BIOS interface. | |
75 | ||
76 | ||
77 | 1.3 sparc64 | |
78 | ----------- | |
79 | ||
80 | The following processors for the sparc64 architecture are supported by | |
81 | cpufreq: | |
82 | ||
83 | UltraSPARC-III | |
84 | ||
85 | ||
86 | 1.4 ppc | |
87 | ------- | |
88 | ||
89 | Several "PowerBook" and "iBook2" notebooks are supported. | |
90 | ||
91 | ||
92 | 1.5 SuperH | |
93 | ---------- | |
94 | ||
95 | The following SuperH processors are supported by cpufreq: | |
96 | ||
97 | SH-3 | |
98 | SH-4 | |
99 | ||
100 | ||
101 | 2. "Policy" / "Governor" ? | |
102 | ========================== | |
103 | ||
104 | Some CPU frequency scaling-capable processor switch between various | |
105 | frequencies and operating voltages "on the fly" without any kernel or | |
106 | user involvement. This guarantees very fast switching to a frequency | |
107 | which is high enough to serve the user's needs, but low enough to save | |
108 | power. | |
109 | ||
110 | ||
111 | 2.1 Policy | |
112 | ---------- | |
113 | ||
114 | On these systems, all you can do is select the lower and upper | |
115 | frequency limit as well as whether you want more aggressive | |
116 | power-saving or more instantly available processing power. | |
117 | ||
118 | ||
119 | 2.2 Governor | |
120 | ------------ | |
121 | ||
122 | On all other cpufreq implementations, these boundaries still need to | |
123 | be set. Then, a "governor" must be selected. Such a "governor" decides | |
124 | what speed the processor shall run within the boundaries. One such | |
125 | "governor" is the "userspace" governor. This one allows the user - or | |
126 | a yet-to-implement userspace program - to decide what specific speed | |
127 | the processor shall run at. | |
128 | ||
129 | ||
130 | 3. How to change the CPU cpufreq policy and/or speed | |
131 | ==================================================== | |
132 | ||
133 | 3.1 Preferred Interface: sysfs | |
134 | ------------------------------ | |
135 | ||
136 | The preferred interface is located in the sysfs filesystem. If you | |
137 | mounted it at /sys, the cpufreq interface is located in a subdirectory | |
138 | "cpufreq" within the cpu-device directory | |
139 | (e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU). | |
140 | ||
141 | cpuinfo_min_freq : this file shows the minimum operating | |
142 | frequency the processor can run at(in kHz) | |
143 | cpuinfo_max_freq : this file shows the maximum operating | |
144 | frequency the processor can run at(in kHz) | |
145 | scaling_driver : this file shows what cpufreq driver is | |
146 | used to set the frequency on this CPU | |
147 | ||
148 | scaling_available_governors : this file shows the CPUfreq governors | |
149 | available in this kernel. You can see the | |
150 | currently activated governor in | |
151 | ||
152 | scaling_governor, and by "echoing" the name of another | |
153 | governor you can change it. Please note | |
154 | that some governors won't load - they only | |
155 | work on some specific architectures or | |
156 | processors. | |
9c9a43ed | 157 | scaling_min_freq and |
1da177e4 LT |
158 | scaling_max_freq show the current "policy limits" (in |
159 | kHz). By echoing new values into these | |
160 | files, you can change these limits. | |
9c9a43ed MD |
161 | NOTE: when setting a policy you need to |
162 | first set scaling_max_freq, then | |
163 | scaling_min_freq. | |
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164 | |
165 | ||
166 | If you have selected the "userspace" governor which allows you to | |
167 | set the CPU operating frequency to a specific value, you can read out | |
168 | the current frequency in | |
169 | ||
170 | scaling_setspeed. By "echoing" a new frequency into this | |
171 | you can change the speed of the CPU, | |
172 | but only within the limits of | |
173 | scaling_min_freq and scaling_max_freq. | |
174 | ||
175 | ||
176 | 3.2 Deprecated Interfaces | |
177 | ------------------------- | |
178 | ||
179 | Depending on your kernel configuration, you might find the following | |
180 | cpufreq-related files: | |
181 | /proc/cpufreq | |
182 | /proc/sys/cpu/*/speed | |
183 | /proc/sys/cpu/*/speed-min | |
184 | /proc/sys/cpu/*/speed-max | |
185 | ||
186 | These are files for deprecated interfaces to cpufreq, which offer far | |
187 | less functionality. Because of this, these interfaces aren't described | |
188 | here. | |
189 |