linux/Documentation/cpu-freq/governors.txt
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   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                      C P U F r e q   G o v e r n o r s
   7
   8                   - information for users and developers -
   9
  10
  11                    Dominik Brodowski  <linux@brodo.de>
  12            some additions and corrections by Nico Golde <nico@ngolde.de>
  13
  14
  15
  16   Clock scaling allows you to change the clock speed of the CPUs on the
  17    fly. This is a nice method to save battery power, because the lower
  18            the clock speed, the less power the CPU consumes.
  19
  20
  21Contents:
  22---------
  231.   What is a CPUFreq Governor?
  24
  252.   Governors In the Linux Kernel
  262.1  Performance
  272.2  Powersave
  282.3  Userspace
  292.4  Ondemand
  302.5  Conservative
  31
  323.   The Governor Interface in the CPUfreq Core
  33
  34
  35
  361. What Is A CPUFreq Governor?
  37==============================
  38
  39Most cpufreq drivers (in fact, all except one, longrun) or even most
  40cpu frequency scaling algorithms only offer the CPU to be set to one
  41frequency. In order to offer dynamic frequency scaling, the cpufreq
  42core must be able to tell these drivers of a "target frequency". So
  43these specific drivers will be transformed to offer a "->target"
  44call instead of the existing "->setpolicy" call. For "longrun", all
  45stays the same, though.
  46
  47How to decide what frequency within the CPUfreq policy should be used?
  48That's done using "cpufreq governors". Two are already in this patch
  49-- they're the already existing "powersave" and "performance" which
  50set the frequency statically to the lowest or highest frequency,
  51respectively. At least two more such governors will be ready for
  52addition in the near future, but likely many more as there are various
  53different theories and models about dynamic frequency scaling
  54around. Using such a generic interface as cpufreq offers to scaling
  55governors, these can be tested extensively, and the best one can be
  56selected for each specific use.
  57
  58Basically, it's the following flow graph:
  59
  60CPU can be set to switch independently   |         CPU can only be set
  61      within specific "limits"           |       to specific frequencies
  62
  63                                 "CPUfreq policy"
  64                consists of frequency limits (policy->{min,max})
  65                     and CPUfreq governor to be used
  66                         /                    \
  67                        /                      \
  68                       /                       the cpufreq governor decides
  69                      /                        (dynamically or statically)
  70                     /                         what target_freq to set within
  71                    /                          the limits of policy->{min,max}
  72                   /                                \
  73                  /                                  \
  74        Using the ->setpolicy call,              Using the ->target call,
  75            the limits and the                    the frequency closest
  76             "policy" is set.                     to target_freq is set.
  77                                                  It is assured that it
  78                                                  is within policy->{min,max}
  79
  80
  812. Governors In the Linux Kernel
  82================================
  83
  842.1 Performance
  85---------------
  86
  87The CPUfreq governor "performance" sets the CPU statically to the
  88highest frequency within the borders of scaling_min_freq and
  89scaling_max_freq.
  90
  91
  922.2 Powersave
  93-------------
  94
  95The CPUfreq governor "powersave" sets the CPU statically to the
  96lowest frequency within the borders of scaling_min_freq and
  97scaling_max_freq.
  98
  99
 1002.3 Userspace
 101-------------
 102
 103The CPUfreq governor "userspace" allows the user, or any userspace
 104program running with UID "root", to set the CPU to a specific frequency
 105by making a sysfs file "scaling_setspeed" available in the CPU-device
 106directory.
 107
 108
 1092.4 Ondemand
 110------------
 111
 112The CPUfreq governor "ondemand" sets the CPU depending on the
 113current usage. To do this the CPU must have the capability to
 114switch the frequency very quickly.  There are a number of sysfs file
 115accessible parameters:
 116
 117sampling_rate: measured in uS (10^-6 seconds), this is how often you
 118want the kernel to look at the CPU usage and to make decisions on
 119what to do about the frequency.  Typically this is set to values of
 120around '10000' or more.
 121
 122show_sampling_rate_(min|max): the minimum and maximum sampling rates
 123available that you may set 'sampling_rate' to.
 124
 125up_threshold: defines what the average CPU usage between the samplings
 126of 'sampling_rate' needs to be for the kernel to make a decision on
 127whether it should increase the frequency.  For example when it is set
 128to its default value of '80' it means that between the checking
 129intervals the CPU needs to be on average more than 80% in use to then
 130decide that the CPU frequency needs to be increased.  
 131
 132ignore_nice_load: this parameter takes a value of '0' or '1'. When
 133set to '0' (its default), all processes are counted towards the
 134'cpu utilisation' value.  When set to '1', the processes that are
 135run with a 'nice' value will not count (and thus be ignored) in the
 136overall usage calculation.  This is useful if you are running a CPU
 137intensive calculation on your laptop that you do not care how long it
 138takes to complete as you can 'nice' it and prevent it from taking part
 139in the deciding process of whether to increase your CPU frequency.
 140
 141
 1422.5 Conservative
 143----------------
 144
 145The CPUfreq governor "conservative", much like the "ondemand"
 146governor, sets the CPU depending on the current usage.  It differs in
 147behaviour in that it gracefully increases and decreases the CPU speed
 148rather than jumping to max speed the moment there is any load on the
 149CPU.  This behaviour more suitable in a battery powered environment.
 150The governor is tweaked in the same manner as the "ondemand" governor
 151through sysfs with the addition of:
 152
 153freq_step: this describes what percentage steps the cpu freq should be
 154increased and decreased smoothly by.  By default the cpu frequency will
 155increase in 5% chunks of your maximum cpu frequency.  You can change this
 156value to anywhere between 0 and 100 where '0' will effectively lock your
 157CPU at a speed regardless of its load whilst '100' will, in theory, make
 158it behave identically to the "ondemand" governor.
 159
 160down_threshold: same as the 'up_threshold' found for the "ondemand"
 161governor but for the opposite direction.  For example when set to its
 162default value of '20' it means that if the CPU usage needs to be below
 16320% between samples to have the frequency decreased.
 164
 1653. The Governor Interface in the CPUfreq Core
 166=============================================
 167
 168A new governor must register itself with the CPUfreq core using
 169"cpufreq_register_governor". The struct cpufreq_governor, which has to
 170be passed to that function, must contain the following values:
 171
 172governor->name -            A unique name for this governor
 173governor->governor -        The governor callback function
 174governor->owner -           .THIS_MODULE for the governor module (if 
 175                            appropriate)
 176
 177The governor->governor callback is called with the current (or to-be-set)
 178cpufreq_policy struct for that CPU, and an unsigned int event. The
 179following events are currently defined:
 180
 181CPUFREQ_GOV_START:   This governor shall start its duty for the CPU
 182                     policy->cpu
 183CPUFREQ_GOV_STOP:    This governor shall end its duty for the CPU
 184                     policy->cpu
 185CPUFREQ_GOV_LIMITS:  The limits for CPU policy->cpu have changed to
 186                     policy->min and policy->max.
 187
 188If you need other "events" externally of your driver, _only_ use the
 189cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
 190CPUfreq core to ensure proper locking.
 191
 192
 193The CPUfreq governor may call the CPU processor driver using one of
 194these two functions:
 195
 196int cpufreq_driver_target(struct cpufreq_policy *policy,
 197                                 unsigned int target_freq,
 198                                 unsigned int relation);
 199
 200int __cpufreq_driver_target(struct cpufreq_policy *policy,
 201                                   unsigned int target_freq,
 202                                   unsigned int relation);
 203
 204target_freq must be within policy->min and policy->max, of course.
 205What's the difference between these two functions? When your governor
 206still is in a direct code path of a call to governor->governor, the
 207per-CPU cpufreq lock is still held in the cpufreq core, and there's
 208no need to lock it again (in fact, this would cause a deadlock). So
 209use __cpufreq_driver_target only in these cases. In all other cases 
 210(for example, when there's a "daemonized" function that wakes up 
 211every second), use cpufreq_driver_target to lock the cpufreq per-CPU
 212lock before the command is passed to the cpufreq processor driver.
 213
 214