| CPU frequency and voltage scaling code in the Linux(TM) kernel |
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| L i n u x C P U F r e q |
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| U S E R G U I D E |
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| Dominik Brodowski <linux@brodo.de> |
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| 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. |
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| Contents: |
| --------- |
| 1. Supported Architectures and Processors |
| 1.1 ARM |
| 1.2 x86 |
| 1.3 sparc64 |
| 1.4 ppc |
| 1.5 SuperH |
| 1.6 Blackfin |
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| 2. "Policy" / "Governor"? |
| 2.1 Policy |
| 2.2 Governor |
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| 3. How to change the CPU cpufreq policy and/or speed |
| 3.1 Preferred interface: sysfs |
| 3.2 Deprecated interfaces |
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| 1. Supported Architectures and Processors |
| ========================================= |
| |
| 1.1 ARM |
| ------- |
| |
| The following ARM processors are supported by cpufreq: |
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| ARM Integrator |
| ARM-SA1100 |
| ARM-SA1110 |
| Intel PXA |
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| 1.2 x86 |
| ------- |
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| The following processors for the x86 architecture are supported by cpufreq: |
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| 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 [*] |
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| [*] Only if "ACPI Processor Performance States" are available |
| to the ACPI<->BIOS interface. |
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| 1.3 sparc64 |
| ----------- |
| |
| The following processors for the sparc64 architecture are supported by |
| cpufreq: |
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| UltraSPARC-III |
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| 1.4 ppc |
| ------- |
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| Several "PowerBook" and "iBook2" notebooks are supported. |
| |
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| 1.5 SuperH |
| ---------- |
| |
| All SuperH processors supporting rate rounding through the clock |
| framework are supported by cpufreq. |
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| 1.6 Blackfin |
| ------------ |
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| The following Blackfin processors are supported by cpufreq: |
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| 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 |
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| 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. |
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| 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. |
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| 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) |
| 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. |
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| cpuinfo_cur_freq : Current speed of the CPU, in KHz. |
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| scaling_available_frequencies : List of available frequencies, in KHz. |
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| 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. |
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| affected_cpus : List of CPUs that require software coordination |
| of frequency. |
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| related_cpus : List of CPUs that need some sort of frequency |
| coordination, whether software or hardware. |
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| scaling_driver : Hardware driver for cpufreq. |
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| scaling_cur_freq : Current frequency of the CPU, in KHz. |
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| 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 |
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| 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. |