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 The following POWER processors are supported in powernv mode:
  86 POWER8
  87 POWER9
  88
  89 1.5 SuperH
  90 ----------
  91
  92 All SuperH processors supporting rate rounding through the clock
  93 framework are supported by cpufreq.
  94
  95 1.6 Blackfin
  96 ------------
  97
  98 The following Blackfin processors are supported by cpufreq:
  99
 100 BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher
 101 BF531, BF532, BF533, Rev 0.3 or higher
 102 BF534, BF536, BF537, Rev 0.2 or higher
 103 BF561, Rev 0.3 or higher
 104 BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher
 105
 106
 107 2. "Policy" / "Governor" ?
 108 ==========================
 109
 110 Some CPU frequency scaling-capable processor switch between various
 111 frequencies and operating voltages "on the fly" without any kernel or
 112 user involvement. This guarantees very fast switching to a frequency
 113 which is high enough to serve the user's needs, but low enough to save
 114 power.
 115
 116
 117 2.1 Policy
 118 ----------
 119
 120 On these systems, all you can do is select the lower and upper
 121 frequency limit as well as whether you want more aggressive
 122 power-saving or more instantly available processing power.
 123
 124
 125 2.2 Governor
 126 ------------
 127
 128 On all other cpufreq implementations, these boundaries still need to
 129 be set. Then, a "governor" must be selected. Such a "governor" decides
 130 what speed the processor shall run within the boundaries. One such
 131 "governor" is the "userspace" governor. This one allows the user - or
 132 a yet-to-implement userspace program - to decide what specific speed
 133 the processor shall run at.
 134
 135
 136 3. How to change the CPU cpufreq policy and/or speed
 137 ====================================================
 138
 139 3.1 Preferred Interface: sysfs
 140 ------------------------------
 141
 142 The preferred interface is located in the sysfs filesystem. If you
 143 mounted it at /sys, the cpufreq interface is located in a subdirectory
 144 "cpufreq" within the cpu-device directory
 145 (e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).
 146
 147 affected_cpus :                 List of Online CPUs that require software
 148                                 coordination of frequency.
 149
 150 cpuinfo_cur_freq :              Current frequency of the CPU as obtained from
 151                                 the hardware, in KHz. This is the frequency
 152                                 the CPU actually runs at.
 153
 154 cpuinfo_min_freq :              this file shows the minimum operating
 155                                 frequency the processor can run at(in kHz)
 156
 157 cpuinfo_max_freq :              this file shows the maximum operating
 158                                 frequency the processor can run at(in kHz)
 159
 160 cpuinfo_transition_latency      The time it takes on this CPU to
 161                                 switch between two frequencies in nano
 162                                 seconds. If unknown or known to be
 163                                 that high that the driver does not
 164                                 work with the ondemand governor, -1
 165                                 (CPUFREQ_ETERNAL) will be returned.
 166                                 Using this information can be useful
 167                                 to choose an appropriate polling
 168                                 frequency for a kernel governor or
 169                                 userspace daemon. Make sure to not
 170                                 switch the frequency too often
 171                                 resulting in performance loss.
 172
 173 related_cpus :                  List of Online + Offline CPUs that need software
 174                                 coordination of frequency.
 175
 176 scaling_available_frequencies : List of available frequencies, in KHz.
 177
 178 scaling_available_governors :   this file shows the CPUfreq governors
 179                                 available in this kernel. You can see the
 180                                 currently activated governor in
 181
 182 scaling_cur_freq :              Current frequency of the CPU as determined by
 183                                 the governor and cpufreq core, in KHz. This is
 184                                 the frequency the kernel thinks the CPU runs
 185                                 at.
 186
 187 scaling_driver :                this file shows what cpufreq driver is
 188                                 used to set the frequency on this CPU
 189
 190 scaling_governor,               and by "echoing" the name of another
 191                                 governor you can change it. Please note
 192                                 that some governors won't load - they only
 193                                 work on some specific architectures or
 194                                 processors.
 195
 196 scaling_min_freq and
 197 scaling_max_freq                show the current "policy limits" (in
 198                                 kHz). By echoing new values into these
 199                                 files, you can change these limits.
 200                                 NOTE: when setting a policy you need to
 201                                 first set scaling_max_freq, then
 202                                 scaling_min_freq.
 203
 204 scaling_setspeed                This can be read to get the currently programmed
 205                                 value by the governor. This can be written to
 206                                 change the current frequency for a group of
 207                                 CPUs, represented by a policy. This is supported
 208                                 currently only by the userspace governor.
 209
 210 bios_limit :                    If the BIOS tells the OS to limit a CPU to
 211                                 lower frequencies, the user can read out the
 212                                 maximum available frequency from this file.
 213                                 This typically can happen through (often not
 214                                 intended) BIOS settings, restrictions
 215                                 triggered through a service processor or other
 216                                 BIOS/HW based implementations.
 217                                 This does not cover thermal ACPI limitations
 218                                 which can be detected through the generic
 219                                 thermal driver.
 220
 221 If you have selected the "userspace" governor which allows you to
 222 set the CPU operating frequency to a specific value, you can read out
 223 the current frequency in
 224
 225 scaling_setspeed.               By "echoing" a new frequency into this
 226                                 you can change the speed of the CPU,
 227                                 but only within the limits of
 228                                 scaling_min_freq and scaling_max_freq.