Documentation/cpu-freq/user-guide.txt

   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
  48 Intel PXA
  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.
 157
 158 cpuinfo_cur_freq :              Current speed of the CPU, in KHz.
 159
 160 scaling_available_frequencies : List of available frequencies, in KHz.
 161
 162 scaling_min_freq and
 163 scaling_max_freq                show the current "policy limits" (in
 164                                 kHz). By echoing new values into these
 165                                 files, you can change these limits.
 166                                 NOTE: when setting a policy you need to
 167                                 first set scaling_max_freq, then
 168                                 scaling_min_freq.
 169
 170 affected_cpus :                 List of CPUs that require software coordination
 171                                 of frequency.
 172
 173 related_cpus :                  List of CPUs that need some sort of frequency
 174                                 coordination, whether software or hardware.
 175
 176 scaling_driver :                Hardware driver for cpufreq.
 177
 178 scaling_cur_freq :              Current frequency of the CPU, in KHz.
 179
 180 If you have selected the "userspace" governor which allows you to
 181 set the CPU operating frequency to a specific value, you can read out
 182 the current frequency in
 183
 184 scaling_setspeed.               By "echoing" a new frequency into this
 185                                 you can change the speed of the CPU,
 186                                 but only within the limits of
 187                                 scaling_min_freq and scaling_max_freq.
 188
 189
 190 3.2 Deprecated Interfaces
 191 -------------------------
 192
 193 Depending on your kernel configuration, you might find the following
 194 cpufreq-related files:
 195 /proc/cpufreq
 196 /proc/sys/cpu/*/speed
 197 /proc/sys/cpu/*/speed-min
 198 /proc/sys/cpu/*/speed-max
 199
 200 These are files for deprecated interfaces to cpufreq, which offer far
 201 less functionality. Because of this, these interfaces aren't described
 202 here.
 203