1 CPU frequency and voltage scaling code in the Linux(TM) kernel
4 L i n u x C P U F r e q
8 - information for developers -
11 Dominik Brodowski <linux@brodo.de>
15 Clock scaling allows you to change the clock speed of the CPUs on the
16 fly. This is a nice method to save battery power, because the lower
17 the clock speed, the less power the CPU consumes.
24 1.2 Per-CPU Initialization
26 1.4 target/target_index or setpolicy?
27 1.5 target/target_index
29 1.7 get_intermediate and target_intermediate
30 2. Frequency Table Helpers
37 So, you just got a brand-new CPU / chipset with datasheets and want to
38 add cpufreq support for this CPU / chipset? Great. Here are some hints
45 First of all, in an __initcall level 7 (module_init()) or later
46 function check whether this kernel runs on the right CPU and the right
47 chipset. If so, register a struct cpufreq_driver with the CPUfreq core
48 using cpufreq_register_driver()
50 What shall this struct cpufreq_driver contain?
52 cpufreq_driver.name - The name of this driver.
54 cpufreq_driver.init - A pointer to the per-CPU initialization
57 cpufreq_driver.verify - A pointer to a "verification" function.
59 cpufreq_driver.setpolicy _or_
60 cpufreq_driver.target/
61 target_index - See below on the differences.
65 cpufreq_driver.exit - A pointer to a per-CPU cleanup
66 function called during CPU_POST_DEAD
67 phase of cpu hotplug process.
69 cpufreq_driver.stop_cpu - A pointer to a per-CPU stop function
70 called during CPU_DOWN_PREPARE phase of
73 cpufreq_driver.resume - A pointer to a per-CPU resume function
74 which is called with interrupts disabled
75 and _before_ the pre-suspend frequency
76 and/or policy is restored by a call to
77 ->target/target_index or ->setpolicy.
79 cpufreq_driver.attr - A pointer to a NULL-terminated list of
80 "struct freq_attr" which allow to
81 export values to sysfs.
83 cpufreq_driver.get_intermediate
84 and target_intermediate Used to switch to stable frequency while
85 changing CPU frequency.
88 1.2 Per-CPU Initialization
89 --------------------------
91 Whenever a new CPU is registered with the device model, or after the
92 cpufreq driver registers itself, the per-CPU initialization function
93 cpufreq_driver.init is called. It takes a struct cpufreq_policy
94 *policy as argument. What to do now?
96 If necessary, activate the CPUfreq support on your CPU.
98 Then, the driver must fill in the following values:
100 policy->cpuinfo.min_freq _and_
101 policy->cpuinfo.max_freq - the minimum and maximum frequency
102 (in kHz) which is supported by
104 policy->cpuinfo.transition_latency the time it takes on this CPU to
105 switch between two frequencies in
106 nanoseconds (if appropriate, else
107 specify CPUFREQ_ETERNAL)
109 policy->cur The current operating frequency of
110 this CPU (if appropriate)
113 policy->policy and, if necessary,
114 policy->governor must contain the "default policy" for
115 this CPU. A few moments later,
116 cpufreq_driver.verify and either
117 cpufreq_driver.setpolicy or
118 cpufreq_driver.target/target_index is called
121 For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the
122 frequency table helpers might be helpful. See the section 2 for more information
125 SMP systems normally have same clock source for a group of cpus. For these the
126 .init() would be called only once for the first online cpu. Here the .init()
127 routine must initialize policy->cpus with mask of all possible cpus (Online +
128 Offline) that share the clock. Then the core would copy this mask onto
129 policy->related_cpus and will reset policy->cpus to carry only online cpus.
135 When the user decides a new policy (consisting of
136 "policy,governor,min,max") shall be set, this policy must be validated
137 so that incompatible values can be corrected. For verifying these
138 values, a frequency table helper and/or the
139 cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned
140 int min_freq, unsigned int max_freq) function might be helpful. See
141 section 2 for details on frequency table helpers.
143 You need to make sure that at least one valid frequency (or operating
144 range) is within policy->min and policy->max. If necessary, increase
145 policy->max first, and only if this is no solution, decrease policy->min.
148 1.4 target/target_index or setpolicy?
149 ----------------------------
151 Most cpufreq drivers or even most cpu frequency scaling algorithms
152 only allow the CPU to be set to one frequency. For these, you use the
153 ->target/target_index call.
155 Some cpufreq-capable processors switch the frequency between certain
156 limits on their own. These shall use the ->setpolicy call
159 1.5. target/target_index
162 The target_index call has two arguments: struct cpufreq_policy *policy,
163 and unsigned int index (into the exposed frequency table).
165 The CPUfreq driver must set the new frequency when called here. The
166 actual frequency must be determined by freq_table[index].frequency.
168 It should always restore to earlier frequency (i.e. policy->restore_freq) in
169 case of errors, even if we switched to intermediate frequency earlier.
173 The target call has three arguments: struct cpufreq_policy *policy,
174 unsigned int target_frequency, unsigned int relation.
176 The CPUfreq driver must set the new frequency when called here. The
177 actual frequency must be determined using the following rules:
179 - keep close to "target_freq"
180 - policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)
181 - if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal
182 target_freq. ("L for lowest, but no lower than")
183 - if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal
184 target_freq. ("H for highest, but no higher than")
186 Here again the frequency table helper might assist you - see section 2
193 The setpolicy call only takes a struct cpufreq_policy *policy as
194 argument. You need to set the lower limit of the in-processor or
195 in-chipset dynamic frequency switching to policy->min, the upper limit
196 to policy->max, and -if supported- select a performance-oriented
197 setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a
198 powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check
199 the reference implementation in drivers/cpufreq/longrun.c
201 1.7 get_intermediate and target_intermediate
202 --------------------------------------------
204 Only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION unset.
206 get_intermediate should return a stable intermediate frequency platform wants to
207 switch to, and target_intermediate() should set CPU to to that frequency, before
208 jumping to the frequency corresponding to 'index'. Core will take care of
209 sending notifications and driver doesn't have to handle them in
210 target_intermediate() or target_index().
212 Drivers can return '0' from get_intermediate() in case they don't wish to switch
213 to intermediate frequency for some target frequency. In that case core will
214 directly call ->target_index().
216 NOTE: ->target_index() should restore to policy->restore_freq in case of
217 failures as core would send notifications for that.
220 2. Frequency Table Helpers
221 ==========================
223 As most cpufreq processors only allow for being set to a few specific
224 frequencies, a "frequency table" with some functions might assist in
225 some work of the processor driver. Such a "frequency table" consists
226 of an array of struct cpufreq_frequency_table entries, with any value in
227 "driver_data" you want to use, and the corresponding frequency in
228 "frequency". At the end of the table, you need to add a
229 cpufreq_frequency_table entry with frequency set to CPUFREQ_TABLE_END. And
230 if you want to skip one entry in the table, set the frequency to
231 CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending
234 By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,
235 struct cpufreq_frequency_table *table);
236 the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and
237 policy->min and policy->max are set to the same values. This is
238 helpful for the per-CPU initialization stage.
240 int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,
241 struct cpufreq_frequency_table *table);
242 assures that at least one valid frequency is within policy->min and
243 policy->max, and all other criteria are met. This is helpful for the
246 int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
247 struct cpufreq_frequency_table *table,
248 unsigned int target_freq,
249 unsigned int relation,
250 unsigned int *index);
252 is the corresponding frequency table helper for the ->target
253 stage. Just pass the values to this function, and the unsigned int
254 index returns the number of the frequency table entry which contains
255 the frequency the CPU shall be set to.
257 The following macros can be used as iterators over cpufreq_frequency_table:
259 cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency
262 cpufreq-for_each_valid_entry(pos, table) - iterates over all entries,
263 excluding CPUFREQ_ENTRY_INVALID frequencies.
264 Use arguments "pos" - a cpufreq_frequency_table * as a loop cursor and
265 "table" - the cpufreq_frequency_table * you want to iterate over.
269 struct cpufreq_frequency_table *pos, *driver_freq_table;
271 cpufreq_for_each_entry(pos, driver_freq_table) {
272 /* Do something with pos */