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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 | |
121fe86b | 26 | 1.6 Blackfin |
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27 | |
28 | 2. "Policy" / "Governor"? | |
29 | 2.1 Policy | |
30 | 2.2 Governor | |
31 | ||
32 | 3. How to change the CPU cpufreq policy and/or speed | |
33 | 3.1 Preferred interface: sysfs | |
34 | 3.2 Deprecated interfaces | |
35 | ||
36 | ||
37 | ||
38 | 1. Supported Architectures and Processors | |
39 | ========================================= | |
40 | ||
41 | 1.1 ARM | |
42 | ------- | |
43 | ||
44 | The following ARM processors are supported by cpufreq: | |
45 | ||
46 | ARM Integrator | |
47 | ARM-SA1100 | |
48 | ARM-SA1110 | |
9e2697ff | 49 | Intel PXA |
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50 | |
51 | ||
52 | 1.2 x86 | |
53 | ------- | |
54 | ||
55 | The following processors for the x86 architecture are supported by cpufreq: | |
56 | ||
57 | AMD Elan - SC400, SC410 | |
58 | AMD mobile K6-2+ | |
59 | AMD mobile K6-3+ | |
60 | AMD mobile Duron | |
61 | AMD mobile Athlon | |
62 | AMD Opteron | |
63 | AMD Athlon 64 | |
64 | Cyrix Media GXm | |
65 | Intel mobile PIII and Intel mobile PIII-M on certain chipsets | |
66 | Intel Pentium 4, Intel Xeon | |
67 | Intel Pentium M (Centrino) | |
68 | National Semiconductors Geode GX | |
69 | Transmeta Crusoe | |
70 | Transmeta Efficeon | |
71 | VIA Cyrix 3 / C3 | |
72 | various processors on some ACPI 2.0-compatible systems [*] | |
73 | ||
74 | [*] Only if "ACPI Processor Performance States" are available | |
75 | to the ACPI<->BIOS interface. | |
76 | ||
77 | ||
78 | 1.3 sparc64 | |
79 | ----------- | |
80 | ||
81 | The following processors for the sparc64 architecture are supported by | |
82 | cpufreq: | |
83 | ||
84 | UltraSPARC-III | |
85 | ||
86 | ||
87 | 1.4 ppc | |
88 | ------- | |
89 | ||
90 | Several "PowerBook" and "iBook2" notebooks are supported. | |
91 | ||
92 | ||
93 | 1.5 SuperH | |
94 | ---------- | |
95 | ||
8a655053 PM |
96 | All SuperH processors supporting rate rounding through the clock |
97 | framework are supported by cpufreq. | |
1da177e4 | 98 | |
121fe86b RG |
99 | 1.6 Blackfin |
100 | ------------ | |
101 | ||
102 | The following Blackfin processors are supported by cpufreq: | |
103 | ||
104 | BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher | |
105 | BF531, BF532, BF533, Rev 0.3 or higher | |
106 | BF534, BF536, BF537, Rev 0.2 or higher | |
107 | BF561, Rev 0.3 or higher | |
108 | BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher | |
109 | ||
1da177e4 LT |
110 | |
111 | 2. "Policy" / "Governor" ? | |
112 | ========================== | |
113 | ||
114 | Some CPU frequency scaling-capable processor switch between various | |
115 | frequencies and operating voltages "on the fly" without any kernel or | |
116 | user involvement. This guarantees very fast switching to a frequency | |
117 | which is high enough to serve the user's needs, but low enough to save | |
118 | power. | |
119 | ||
120 | ||
121 | 2.1 Policy | |
122 | ---------- | |
123 | ||
124 | On these systems, all you can do is select the lower and upper | |
125 | frequency limit as well as whether you want more aggressive | |
126 | power-saving or more instantly available processing power. | |
127 | ||
128 | ||
129 | 2.2 Governor | |
130 | ------------ | |
131 | ||
132 | On all other cpufreq implementations, these boundaries still need to | |
133 | be set. Then, a "governor" must be selected. Such a "governor" decides | |
134 | what speed the processor shall run within the boundaries. One such | |
135 | "governor" is the "userspace" governor. This one allows the user - or | |
136 | a yet-to-implement userspace program - to decide what specific speed | |
137 | the processor shall run at. | |
138 | ||
139 | ||
140 | 3. How to change the CPU cpufreq policy and/or speed | |
141 | ==================================================== | |
142 | ||
143 | 3.1 Preferred Interface: sysfs | |
144 | ------------------------------ | |
145 | ||
146 | The preferred interface is located in the sysfs filesystem. If you | |
147 | mounted it at /sys, the cpufreq interface is located in a subdirectory | |
148 | "cpufreq" within the cpu-device directory | |
149 | (e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU). | |
150 | ||
151 | cpuinfo_min_freq : this file shows the minimum operating | |
152 | frequency the processor can run at(in kHz) | |
153 | cpuinfo_max_freq : this file shows the maximum operating | |
154 | frequency the processor can run at(in kHz) | |
ed129784 TR |
155 | cpuinfo_transition_latency The time it takes on this CPU to |
156 | switch between two frequencies in nano | |
157 | seconds. If unknown or known to be | |
158 | that high that the driver does not | |
159 | work with the ondemand governor, -1 | |
160 | (CPUFREQ_ETERNAL) will be returned. | |
161 | Using this information can be useful | |
162 | to choose an appropriate polling | |
163 | frequency for a kernel governor or | |
164 | userspace daemon. Make sure to not | |
165 | switch the frequency too often | |
166 | resulting in performance loss. | |
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167 | scaling_driver : this file shows what cpufreq driver is |
168 | used to set the frequency on this CPU | |
169 | ||
170 | scaling_available_governors : this file shows the CPUfreq governors | |
171 | available in this kernel. You can see the | |
172 | currently activated governor in | |
173 | ||
174 | scaling_governor, and by "echoing" the name of another | |
175 | governor you can change it. Please note | |
176 | that some governors won't load - they only | |
177 | work on some specific architectures or | |
178 | processors. | |
605400a8 DW |
179 | |
180 | cpuinfo_cur_freq : Current speed of the CPU, in KHz. | |
181 | ||
182 | scaling_available_frequencies : List of available frequencies, in KHz. | |
183 | ||
9c9a43ed | 184 | scaling_min_freq and |
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185 | scaling_max_freq show the current "policy limits" (in |
186 | kHz). By echoing new values into these | |
187 | files, you can change these limits. | |
9c9a43ed MD |
188 | NOTE: when setting a policy you need to |
189 | first set scaling_max_freq, then | |
190 | scaling_min_freq. | |
1da177e4 | 191 | |
605400a8 DW |
192 | affected_cpus : List of CPUs that require software coordination |
193 | of frequency. | |
194 | ||
195 | related_cpus : List of CPUs that need some sort of frequency | |
196 | coordination, whether software or hardware. | |
197 | ||
198 | scaling_driver : Hardware driver for cpufreq. | |
199 | ||
200 | scaling_cur_freq : Current frequency of the CPU, in KHz. | |
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201 | |
202 | If you have selected the "userspace" governor which allows you to | |
203 | set the CPU operating frequency to a specific value, you can read out | |
204 | the current frequency in | |
205 | ||
206 | scaling_setspeed. By "echoing" a new frequency into this | |
207 | you can change the speed of the CPU, | |
208 | but only within the limits of | |
209 | scaling_min_freq and scaling_max_freq. |