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 and ARM64
  22 1.2 x86
  23 1.3 sparc64
  24 1.4 ppc
  25 1.5 SuperH
  26 1.6 Blackfin
  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
  35
  36
  37 1. Supported Architectures and Processors
  38 =========================================
  39
  40 1.1 ARM and ARM64
  41 -----------------
  42
  43 Almost all ARM and ARM64 platforms support CPU frequency scaling.
  44
  45 1.2 x86
  46 -------
  47
  48 The following processors for the x86 architecture are supported by cpufreq:
  49
  50 AMD Elan - SC400, SC410
  51 AMD mobile K6-2+
  52 AMD mobile K6-3+
  53 AMD mobile Duron
  54 AMD mobile Athlon
  55 AMD Opteron
  56 AMD Athlon 64
  57 Cyrix Media GXm
  58 Intel mobile PIII and Intel mobile PIII-M on certain chipsets
  59 Intel Pentium 4, Intel Xeon
  60 Intel Pentium M (Centrino)
  61 National Semiconductors Geode GX
  62 Transmeta Crusoe
  63 Transmeta Efficeon
  64 VIA Cyrix 3 / C3
  65 various processors on some ACPI 2.0-compatible systems [*]
  66 And many more
  67
  68 [*] Only if "ACPI Processor Performance States" are available
  69 to the ACPI<->BIOS interface.
  70
  71
  72 1.3 sparc64
  73 -----------
  74
  75 The following processors for the sparc64 architecture are supported by
  76 cpufreq:
  77
  78 UltraSPARC-III
  79
  80
  81 1.4 ppc
  82 -------
  83
  84 Several "PowerBook" and "iBook2" notebooks are supported.
  85
  86
  87 1.5 SuperH
  88 ----------
  89
  90 All SuperH processors supporting rate rounding through the clock
  91 framework are supported by cpufreq.
  92
  93 1.6 Blackfin
  94 ------------
  95
  96 The following Blackfin processors are supported by cpufreq:
  97
  98 BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher
  99 BF531, BF532, BF533, Rev 0.3 or higher
 100 BF534, BF536, BF537, Rev 0.2 or higher
 101 BF561, Rev 0.3 or higher
 102 BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher
 103
 104
 105 2. "Policy" / "Governor" ?
 106 ==========================
 107
 108 Some CPU frequency scaling-capable processor switch between various
 109 frequencies and operating voltages "on the fly" without any kernel or
 110 user involvement. This guarantees very fast switching to a frequency
 111 which is high enough to serve the user's needs, but low enough to save
 112 power.
 113
 114
 115 2.1 Policy
 116 ----------
 117
 118 On these systems, all you can do is select the lower and upper
 119 frequency limit as well as whether you want more aggressive
 120 power-saving or more instantly available processing power.
 121
 122
 123 2.2 Governor
 124 ------------
 125
 126 On all other cpufreq implementations, these boundaries still need to
 127 be set. Then, a "governor" must be selected. Such a "governor" decides
 128 what speed the processor shall run within the boundaries. One such
 129 "governor" is the "userspace" governor. This one allows the user - or
 130 a yet-to-implement userspace program - to decide what specific speed
 131 the processor shall run at.
 132
 133
 134 3. How to change the CPU cpufreq policy and/or speed
 135 ====================================================
 136
 137 3.1 Preferred Interface: sysfs
 138 ------------------------------
 139
 140 The preferred interface is located in the sysfs filesystem. If you
 141 mounted it at /sys, the cpufreq interface is located in a subdirectory
 142 "cpufreq" within the cpu-device directory
 143 (e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).
 144
 145 affected_cpus :                 List of Online CPUs that require software
 146                                 coordination of frequency.
 147
 148 cpuinfo_cur_freq :              Current frequency of the CPU as obtained from
 149                                 the hardware, in KHz. This is the frequency
 150                                 the CPU actually runs at.
 151
 152 cpuinfo_min_freq :              this file shows the minimum operating
 153                                 frequency the processor can run at(in kHz)
 154
 155 cpuinfo_max_freq :              this file shows the maximum operating
 156                                 frequency the processor can run at(in kHz)
 157
 158 cpuinfo_transition_latency      The time it takes on this CPU to
 159                                 switch between two frequencies in nano
 160                                 seconds. If unknown or known to be
 161                                 that high that the driver does not
 162                                 work with the ondemand governor, -1
 163                                 (CPUFREQ_ETERNAL) will be returned.
 164                                 Using this information can be useful
 165                                 to choose an appropriate polling
 166                                 frequency for a kernel governor or
 167                                 userspace daemon. Make sure to not
 168                                 switch the frequency too often
 169                                 resulting in performance loss.
 170
 171 related_cpus :                  List of Online + Offline CPUs that need software
 172                                 coordination of frequency.
 173
 174 scaling_available_frequencies : List of available frequencies, in KHz.
 175
 176 scaling_available_governors :   this file shows the CPUfreq governors
 177                                 available in this kernel. You can see the
 178                                 currently activated governor in
 179
 180 scaling_cur_freq :              Current frequency of the CPU as determined by
 181                                 the governor and cpufreq core, in KHz. This is
 182                                 the frequency the kernel thinks the CPU runs
 183                                 at.
 184
 185 scaling_driver :                this file shows what cpufreq driver is
 186                                 used to set the frequency on this CPU
 187
 188 scaling_governor,               and by "echoing" the name of another
 189                                 governor you can change it. Please note
 190                                 that some governors won't load - they only
 191                                 work on some specific architectures or
 192                                 processors.
 193
 194 scaling_min_freq and
 195 scaling_max_freq                show the current "policy limits" (in
 196                                 kHz). By echoing new values into these
 197                                 files, you can change these limits.
 198                                 NOTE: when setting a policy you need to
 199                                 first set scaling_max_freq, then
 200                                 scaling_min_freq.
 201
 202 scaling_setspeed                This can be read to get the currently programmed
 203                                 value by the governor. This can be written to
 204                                 change the current frequency for a group of
 205                                 CPUs, represented by a policy. This is supported
 206                                 currently only by the userspace governor.
 207
 208 bios_limit :                    If the BIOS tells the OS to limit a CPU to
 209                                 lower frequencies, the user can read out the
 210                                 maximum available frequency from this file.
 211                                 This typically can happen through (often not
 212                                 intended) BIOS settings, restrictions
 213                                 triggered through a service processor or other
 214                                 BIOS/HW based implementations.
 215                                 This does not cover thermal ACPI limitations
 216                                 which can be detected through the generic
 217                                 thermal driver.
 218
 219 If you have selected the "userspace" governor which allows you to
 220 set the CPU operating frequency to a specific value, you can read out
 221 the current frequency in
 222
 223 scaling_setspeed.               By "echoing" a new frequency into this
 224                                 you can change the speed of the CPU,
 225                                 but only within the limits of
 226                                 scaling_min_freq and scaling_max_freq.