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Linus Torvalds1da177e2005-04-16 15:20:36 -07001 CPU frequency and voltage scaling code in the Linux(TM) kernel
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4 L i n u x C P U F r e q
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6 C P U F r e q G o v e r n o r s
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8 - information for users and developers -
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10
11 Dominik Brodowski <linux@brodo.de>
Nico Golde594dd2c2005-06-25 14:58:33 -070012 some additions and corrections by Nico Golde <nico@ngolde.de>
Linus Torvalds1da177e2005-04-16 15:20:36 -070013
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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?
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252. Governors In the Linux Kernel
262.1 Performance
272.2 Powersave
282.3 Userspace
Nico Golde594dd2c2005-06-25 14:58:33 -0700292.4 Ondemand
Alexander Clouter537208c2005-12-01 01:09:23 -0800302.5 Conservative
Linus Torvalds1da177e2005-04-16 15:20:36 -070031
323. The Governor Interface in the CPUfreq Core
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34
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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
Matt LaPlante2fe0ae72006-10-03 22:50:39 +020060CPU can be set to switch independently | CPU can only be set
Linus Torvalds1da177e2005-04-16 15:20:36 -070061 within specific "limits" | to specific frequencies
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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}
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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
Nico Golde594dd2c2005-06-25 14:58:33 -0700922.2 Powersave
Linus Torvalds1da177e2005-04-16 15:20:36 -070093-------------
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
Nico Golde594dd2c2005-06-25 14:58:33 -07001002.3 Userspace
Linus Torvalds1da177e2005-04-16 15:20:36 -0700101-------------
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
Nico Golde594dd2c2005-06-25 14:58:33 -07001092.4 Ondemand
110------------
111
Matt LaPlantea2ffd272006-10-03 22:49:15 +0200112The CPUfreq governor "ondemand" sets the CPU depending on the
Nico Golde594dd2c2005-06-25 14:58:33 -0700113current usage. To do this the CPU must have the capability to
Alexander Clouter537208c2005-12-01 01:09:23 -0800114switch 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
Matt LaPlanted9195882008-07-25 19:45:33 -0700125up_threshold: defines what the average CPU usage between the samplings
Alexander Clouter537208c2005-12-01 01:09:23 -0800126of '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
Matt LaPlante992caac2006-10-03 22:52:05 +0200132ignore_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
Alexander Clouter537208c2005-12-01 01:09:23 -0800135run with a 'nice' value will not count (and thus be ignored) in the
Matt LaPlante992caac2006-10-03 22:52:05 +0200136overall usage calculation. This is useful if you are running a CPU
Alexander Clouter537208c2005-12-01 01:09:23 -0800137intensive 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.
Nico Golde594dd2c2005-06-25 14:58:33 -0700140
141
Alexander Clouter537208c2005-12-01 01:09:23 -08001422.5 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700164
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