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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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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 independetly | 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
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.
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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
112The CPUfreq govenor "ondemand" sets the CPU depending on the
113current 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
125up_threshold: defines what the average CPU usaged 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
132sampling_down_factor: this parameter controls the rate that the CPU
133makes a decision on when to decrease the frequency. When set to its
134default value of '5' it means that at 1/5 the sampling_rate the kernel
135makes a decision to lower the frequency. Five "lower rate" decisions
136have to be made in a row before the CPU frequency is actually lower.
137If set to '1' then the frequency decreases as quickly as it increases,
138if set to '2' it decreases at half the rate of the increase.
139
140ignore_nice_load: this parameter takes a value of '0' or '1', when set
141to '0' (its default) then all processes are counted towards towards the
142'cpu utilisation' value. When set to '1' then processes that are
143run with a 'nice' value will not count (and thus be ignored) in the
144overal usage calculation. This is useful if you are running a CPU
145intensive calculation on your laptop that you do not care how long it
146takes to complete as you can 'nice' it and prevent it from taking part
147in the deciding process of whether to increase your CPU frequency.
Nico Golde594dd2c2005-06-25 14:58:33 -0700148
149
Alexander Clouter537208c2005-12-01 01:09:23 -08001502.5 Conservative
151----------------
152
153The CPUfreq governor "conservative", much like the "ondemand"
154governor, sets the CPU depending on the current usage. It differs in
155behaviour in that it gracefully increases and decreases the CPU speed
156rather than jumping to max speed the moment there is any load on the
157CPU. This behaviour more suitable in a battery powered environment.
158The governor is tweaked in the same manner as the "ondemand" governor
159through sysfs with the addition of:
160
161freq_step: this describes what percentage steps the cpu freq should be
162increased and decreased smoothly by. By default the cpu frequency will
163increase in 5% chunks of your maximum cpu frequency. You can change this
164value to anywhere between 0 and 100 where '0' will effectively lock your
165CPU at a speed regardless of its load whilst '100' will, in theory, make
166it behave identically to the "ondemand" governor.
167
168down_threshold: same as the 'up_threshold' found for the "ondemand"
169governor but for the opposite direction. For example when set to its
170default value of '20' it means that if the CPU usage needs to be below
17120% between samples to have the frequency decreased.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700172
1733. The Governor Interface in the CPUfreq Core
174=============================================
175
176A new governor must register itself with the CPUfreq core using
177"cpufreq_register_governor". The struct cpufreq_governor, which has to
178be passed to that function, must contain the following values:
179
180governor->name - A unique name for this governor
181governor->governor - The governor callback function
182governor->owner - .THIS_MODULE for the governor module (if
183 appropriate)
184
185The governor->governor callback is called with the current (or to-be-set)
186cpufreq_policy struct for that CPU, and an unsigned int event. The
187following events are currently defined:
188
189CPUFREQ_GOV_START: This governor shall start its duty for the CPU
190 policy->cpu
191CPUFREQ_GOV_STOP: This governor shall end its duty for the CPU
192 policy->cpu
193CPUFREQ_GOV_LIMITS: The limits for CPU policy->cpu have changed to
194 policy->min and policy->max.
195
196If you need other "events" externally of your driver, _only_ use the
197cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
198CPUfreq core to ensure proper locking.
199
200
201The CPUfreq governor may call the CPU processor driver using one of
202these two functions:
203
204int cpufreq_driver_target(struct cpufreq_policy *policy,
205 unsigned int target_freq,
206 unsigned int relation);
207
208int __cpufreq_driver_target(struct cpufreq_policy *policy,
209 unsigned int target_freq,
210 unsigned int relation);
211
212target_freq must be within policy->min and policy->max, of course.
213What's the difference between these two functions? When your governor
214still is in a direct code path of a call to governor->governor, the
215per-CPU cpufreq lock is still held in the cpufreq core, and there's
216no need to lock it again (in fact, this would cause a deadlock). So
217use __cpufreq_driver_target only in these cases. In all other cases
218(for example, when there's a "daemonized" function that wakes up
219every second), use cpufreq_driver_target to lock the cpufreq per-CPU
220lock before the command is passed to the cpufreq processor driver.
221