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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 | |
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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 | ||
8a655053 PM |
95 | All SuperH processors supporting rate rounding through the clock |
96 | framework are supported by cpufreq. | |
1da177e4 | 97 | |
121fe86b RG |
98 | 1.6 Blackfin |
99 | ------------ | |
100 | ||
101 | The following Blackfin processors are supported by cpufreq: | |
102 | ||
103 | BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher | |
104 | BF531, BF532, BF533, Rev 0.3 or higher | |
105 | BF534, BF536, BF537, Rev 0.2 or higher | |
106 | BF561, Rev 0.3 or higher | |
107 | BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher | |
108 | ||
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109 | |
110 | 2. "Policy" / "Governor" ? | |
111 | ========================== | |
112 | ||
113 | Some CPU frequency scaling-capable processor switch between various | |
114 | frequencies and operating voltages "on the fly" without any kernel or | |
115 | user involvement. This guarantees very fast switching to a frequency | |
116 | which is high enough to serve the user's needs, but low enough to save | |
117 | power. | |
118 | ||
119 | ||
120 | 2.1 Policy | |
121 | ---------- | |
122 | ||
123 | On these systems, all you can do is select the lower and upper | |
124 | frequency limit as well as whether you want more aggressive | |
125 | power-saving or more instantly available processing power. | |
126 | ||
127 | ||
128 | 2.2 Governor | |
129 | ------------ | |
130 | ||
131 | On all other cpufreq implementations, these boundaries still need to | |
132 | be set. Then, a "governor" must be selected. Such a "governor" decides | |
133 | what speed the processor shall run within the boundaries. One such | |
134 | "governor" is the "userspace" governor. This one allows the user - or | |
135 | a yet-to-implement userspace program - to decide what specific speed | |
136 | the processor shall run at. | |
137 | ||
138 | ||
139 | 3. How to change the CPU cpufreq policy and/or speed | |
140 | ==================================================== | |
141 | ||
142 | 3.1 Preferred Interface: sysfs | |
143 | ------------------------------ | |
144 | ||
145 | The preferred interface is located in the sysfs filesystem. If you | |
146 | mounted it at /sys, the cpufreq interface is located in a subdirectory | |
147 | "cpufreq" within the cpu-device directory | |
148 | (e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU). | |
149 | ||
150 | cpuinfo_min_freq : this file shows the minimum operating | |
151 | frequency the processor can run at(in kHz) | |
152 | cpuinfo_max_freq : this file shows the maximum operating | |
153 | frequency the processor can run at(in kHz) | |
ed129784 TR |
154 | cpuinfo_transition_latency The time it takes on this CPU to |
155 | switch between two frequencies in nano | |
156 | seconds. If unknown or known to be | |
157 | that high that the driver does not | |
158 | work with the ondemand governor, -1 | |
159 | (CPUFREQ_ETERNAL) will be returned. | |
160 | Using this information can be useful | |
161 | to choose an appropriate polling | |
162 | frequency for a kernel governor or | |
163 | userspace daemon. Make sure to not | |
164 | switch the frequency too often | |
165 | resulting in performance loss. | |
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166 | scaling_driver : this file shows what cpufreq driver is |
167 | used to set the frequency on this CPU | |
168 | ||
169 | scaling_available_governors : this file shows the CPUfreq governors | |
170 | available in this kernel. You can see the | |
171 | currently activated governor in | |
172 | ||
173 | scaling_governor, and by "echoing" the name of another | |
174 | governor you can change it. Please note | |
175 | that some governors won't load - they only | |
176 | work on some specific architectures or | |
177 | processors. | |
605400a8 | 178 | |
da470db1 NC |
179 | cpuinfo_cur_freq : Current frequency of the CPU as obtained from |
180 | the hardware, in KHz. This is the frequency | |
181 | the CPU actually runs at. | |
605400a8 DW |
182 | |
183 | scaling_available_frequencies : List of available frequencies, in KHz. | |
184 | ||
9c9a43ed | 185 | scaling_min_freq and |
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186 | scaling_max_freq show the current "policy limits" (in |
187 | kHz). By echoing new values into these | |
188 | files, you can change these limits. | |
9c9a43ed MD |
189 | NOTE: when setting a policy you need to |
190 | first set scaling_max_freq, then | |
191 | scaling_min_freq. | |
1da177e4 | 192 | |
605400a8 DW |
193 | affected_cpus : List of CPUs that require software coordination |
194 | of frequency. | |
195 | ||
196 | related_cpus : List of CPUs that need some sort of frequency | |
197 | coordination, whether software or hardware. | |
198 | ||
199 | scaling_driver : Hardware driver for cpufreq. | |
200 | ||
da470db1 NC |
201 | scaling_cur_freq : Current frequency of the CPU as determined by |
202 | the governor and cpufreq core, in KHz. This is | |
203 | the frequency the kernel thinks the CPU runs | |
204 | at. | |
1da177e4 | 205 | |
e2f74f35 TR |
206 | bios_limit : If the BIOS tells the OS to limit a CPU to |
207 | lower frequencies, the user can read out the | |
208 | maximum available frequency from this file. | |
209 | This typically can happen through (often not | |
210 | intended) BIOS settings, restrictions | |
211 | triggered through a service processor or other | |
212 | BIOS/HW based implementations. | |
213 | This does not cover thermal ACPI limitations | |
214 | which can be detected through the generic | |
215 | thermal driver. | |
216 | ||
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217 | If you have selected the "userspace" governor which allows you to |
218 | set the CPU operating frequency to a specific value, you can read out | |
219 | the current frequency in | |
220 | ||
221 | scaling_setspeed. By "echoing" a new frequency into this | |
222 | you can change the speed of the CPU, | |
223 | but only within the limits of | |
224 | scaling_min_freq and scaling_max_freq. |