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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 | CPU frequency and voltage scaling code in the Linux(TM) kernel L i n u x C P U F r e q U S E R G U I D E Dominik Brodowski <linux@brodo.de> Clock scaling allows you to change the clock speed of the CPUs on the fly. This is a nice method to save battery power, because the lower the clock speed, the less power the CPU consumes. Contents: --------- 1. Supported Architectures and Processors 1.1 ARM 1.2 x86 1.3 sparc64 1.4 ppc 1.5 SuperH 1.6 Blackfin 2. "Policy" / "Governor"? 2.1 Policy 2.2 Governor 3. How to change the CPU cpufreq policy and/or speed 3.1 Preferred interface: sysfs 1. Supported Architectures and Processors ========================================= 1.1 ARM ------- The following ARM processors are supported by cpufreq: ARM Integrator ARM-SA1100 ARM-SA1110 Intel PXA 1.2 x86 ------- The following processors for the x86 architecture are supported by cpufreq: AMD Elan - SC400, SC410 AMD mobile K6-2+ AMD mobile K6-3+ AMD mobile Duron AMD mobile Athlon AMD Opteron AMD Athlon 64 Cyrix Media GXm Intel mobile PIII and Intel mobile PIII-M on certain chipsets Intel Pentium 4, Intel Xeon Intel Pentium M (Centrino) National Semiconductors Geode GX Transmeta Crusoe Transmeta Efficeon VIA Cyrix 3 / C3 various processors on some ACPI 2.0-compatible systems [*] [*] Only if "ACPI Processor Performance States" are available to the ACPI<->BIOS interface. 1.3 sparc64 ----------- The following processors for the sparc64 architecture are supported by cpufreq: UltraSPARC-III 1.4 ppc ------- Several "PowerBook" and "iBook2" notebooks are supported. 1.5 SuperH ---------- All SuperH processors supporting rate rounding through the clock framework are supported by cpufreq. 1.6 Blackfin ------------ The following Blackfin processors are supported by cpufreq: BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher BF531, BF532, BF533, Rev 0.3 or higher BF534, BF536, BF537, Rev 0.2 or higher BF561, Rev 0.3 or higher BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher 2. "Policy" / "Governor" ? ========================== Some CPU frequency scaling-capable processor switch between various frequencies and operating voltages "on the fly" without any kernel or user involvement. This guarantees very fast switching to a frequency which is high enough to serve the user's needs, but low enough to save power. 2.1 Policy ---------- On these systems, all you can do is select the lower and upper frequency limit as well as whether you want more aggressive power-saving or more instantly available processing power. 2.2 Governor ------------ On all other cpufreq implementations, these boundaries still need to be set. Then, a "governor" must be selected. Such a "governor" decides what speed the processor shall run within the boundaries. One such "governor" is the "userspace" governor. This one allows the user - or a yet-to-implement userspace program - to decide what specific speed the processor shall run at. 3. How to change the CPU cpufreq policy and/or speed ==================================================== 3.1 Preferred Interface: sysfs ------------------------------ The preferred interface is located in the sysfs filesystem. If you mounted it at /sys, the cpufreq interface is located in a subdirectory "cpufreq" within the cpu-device directory (e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU). cpuinfo_min_freq : this file shows the minimum operating frequency the processor can run at(in kHz) cpuinfo_max_freq : this file shows the maximum operating frequency the processor can run at(in kHz) cpuinfo_transition_latency The time it takes on this CPU to switch between two frequencies in nano seconds. If unknown or known to be that high that the driver does not work with the ondemand governor, -1 (CPUFREQ_ETERNAL) will be returned. Using this information can be useful to choose an appropriate polling frequency for a kernel governor or userspace daemon. Make sure to not switch the frequency too often resulting in performance loss. scaling_driver : this file shows what cpufreq driver is used to set the frequency on this CPU scaling_available_governors : this file shows the CPUfreq governors available in this kernel. You can see the currently activated governor in scaling_governor, and by "echoing" the name of another governor you can change it. Please note that some governors won't load - they only work on some specific architectures or processors. cpuinfo_cur_freq : Current frequency of the CPU as obtained from the hardware, in KHz. This is the frequency the CPU actually runs at. scaling_available_frequencies : List of available frequencies, in KHz. scaling_min_freq and scaling_max_freq show the current "policy limits" (in kHz). By echoing new values into these files, you can change these limits. NOTE: when setting a policy you need to first set scaling_max_freq, then scaling_min_freq. affected_cpus : List of Online CPUs that require software coordination of frequency. related_cpus : List of Online + Offline CPUs that need software coordination of frequency. scaling_driver : Hardware driver for cpufreq. scaling_cur_freq : Current frequency of the CPU as determined by the governor and cpufreq core, in KHz. This is the frequency the kernel thinks the CPU runs at. bios_limit : If the BIOS tells the OS to limit a CPU to lower frequencies, the user can read out the maximum available frequency from this file. This typically can happen through (often not intended) BIOS settings, restrictions triggered through a service processor or other BIOS/HW based implementations. This does not cover thermal ACPI limitations which can be detected through the generic thermal driver. If you have selected the "userspace" governor which allows you to set the CPU operating frequency to a specific value, you can read out the current frequency in scaling_setspeed. By "echoing" a new frequency into this you can change the speed of the CPU, but only within the limits of scaling_min_freq and scaling_max_freq. |