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 2. Frequency Table Helpers
36 So, you just got a brand-new CPU / chipset with datasheets and want to
37 add cpufreq support for this CPU / chipset? Great. Here are some hints
44 First of all, in an __initcall level 7 (module_init()) or later
45 function check whether this kernel runs on the right CPU and the right
46 chipset. If so, register a struct cpufreq_driver with the CPUfreq core
47 using cpufreq_register_driver()
49 What shall this struct cpufreq_driver contain?
51 cpufreq_driver.name - The name of this driver.
53 cpufreq_driver.init - A pointer to the per-CPU initialization
56 cpufreq_driver.verify - A pointer to a "verification" function.
58 cpufreq_driver.setpolicy _or_
59 cpufreq_driver.target/
60 target_index - See below on the differences.
64 cpufreq_driver.exit - A pointer to a per-CPU cleanup
65 function called during CPU_POST_DEAD
66 phase of cpu hotplug process.
68 cpufreq_driver.stop_cpu - A pointer to a per-CPU stop function
69 called during CPU_DOWN_PREPARE phase of
72 cpufreq_driver.resume - A pointer to a per-CPU resume function
73 which is called with interrupts disabled
74 and _before_ the pre-suspend frequency
75 and/or policy is restored by a call to
76 ->target/target_index or ->setpolicy.
78 cpufreq_driver.attr - A pointer to a NULL-terminated list of
79 "struct freq_attr" which allow to
80 export values to sysfs.
83 1.2 Per-CPU Initialization
84 --------------------------
86 Whenever a new CPU is registered with the device model, or after the
87 cpufreq driver registers itself, the per-CPU initialization function
88 cpufreq_driver.init is called. It takes a struct cpufreq_policy
89 *policy as argument. What to do now?
91 If necessary, activate the CPUfreq support on your CPU.
93 Then, the driver must fill in the following values:
95 policy->cpuinfo.min_freq _and_
96 policy->cpuinfo.max_freq - the minimum and maximum frequency
97 (in kHz) which is supported by
99 policy->cpuinfo.transition_latency the time it takes on this CPU to
100 switch between two frequencies in
101 nanoseconds (if appropriate, else
102 specify CPUFREQ_ETERNAL)
104 policy->cur The current operating frequency of
105 this CPU (if appropriate)
108 policy->policy and, if necessary,
109 policy->governor must contain the "default policy" for
110 this CPU. A few moments later,
111 cpufreq_driver.verify and either
112 cpufreq_driver.setpolicy or
113 cpufreq_driver.target/target_index is called
116 For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the
117 frequency table helpers might be helpful. See the section 2 for more information
120 SMP systems normally have same clock source for a group of cpus. For these the
121 .init() would be called only once for the first online cpu. Here the .init()
122 routine must initialize policy->cpus with mask of all possible cpus (Online +
123 Offline) that share the clock. Then the core would copy this mask onto
124 policy->related_cpus and will reset policy->cpus to carry only online cpus.
130 When the user decides a new policy (consisting of
131 "policy,governor,min,max") shall be set, this policy must be validated
132 so that incompatible values can be corrected. For verifying these
133 values, a frequency table helper and/or the
134 cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned
135 int min_freq, unsigned int max_freq) function might be helpful. See
136 section 2 for details on frequency table helpers.
138 You need to make sure that at least one valid frequency (or operating
139 range) is within policy->min and policy->max. If necessary, increase
140 policy->max first, and only if this is no solution, decrease policy->min.
143 1.4 target/target_index or setpolicy?
144 ----------------------------
146 Most cpufreq drivers or even most cpu frequency scaling algorithms
147 only allow the CPU to be set to one frequency. For these, you use the
148 ->target/target_index call.
150 Some cpufreq-capable processors switch the frequency between certain
151 limits on their own. These shall use the ->setpolicy call
154 1.4. target/target_index
157 The target_index call has two arguments: struct cpufreq_policy *policy,
158 and unsigned int index (into the exposed frequency table).
160 The CPUfreq driver must set the new frequency when called here. The
161 actual frequency must be determined by freq_table[index].frequency.
165 The target call has three arguments: struct cpufreq_policy *policy,
166 unsigned int target_frequency, unsigned int relation.
168 The CPUfreq driver must set the new frequency when called here. The
169 actual frequency must be determined using the following rules:
171 - keep close to "target_freq"
172 - policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)
173 - if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal
174 target_freq. ("L for lowest, but no lower than")
175 - if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal
176 target_freq. ("H for highest, but no higher than")
178 Here again the frequency table helper might assist you - see section 2
185 The setpolicy call only takes a struct cpufreq_policy *policy as
186 argument. You need to set the lower limit of the in-processor or
187 in-chipset dynamic frequency switching to policy->min, the upper limit
188 to policy->max, and -if supported- select a performance-oriented
189 setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a
190 powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check
191 the reference implementation in drivers/cpufreq/longrun.c
195 2. Frequency Table Helpers
196 ==========================
198 As most cpufreq processors only allow for being set to a few specific
199 frequencies, a "frequency table" with some functions might assist in
200 some work of the processor driver. Such a "frequency table" consists
201 of an array of struct cpufreq_frequency_table entries, with any value in
202 "driver_data" you want to use, and the corresponding frequency in
203 "frequency". At the end of the table, you need to add a
204 cpufreq_frequency_table entry with frequency set to CPUFREQ_TABLE_END. And
205 if you want to skip one entry in the table, set the frequency to
206 CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending
209 By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,
210 struct cpufreq_frequency_table *table);
211 the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and
212 policy->min and policy->max are set to the same values. This is
213 helpful for the per-CPU initialization stage.
215 int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,
216 struct cpufreq_frequency_table *table);
217 assures that at least one valid frequency is within policy->min and
218 policy->max, and all other criteria are met. This is helpful for the
221 int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
222 struct cpufreq_frequency_table *table,
223 unsigned int target_freq,
224 unsigned int relation,
225 unsigned int *index);
227 is the corresponding frequency table helper for the ->target
228 stage. Just pass the values to this function, and the unsigned int
229 index returns the number of the frequency table entry which contains
230 the frequency the CPU shall be set to.
232 The following macros can be used as iterators over cpufreq_frequency_table:
234 cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency
237 cpufreq-for_each_valid_entry(pos, table) - iterates over all entries,
238 excluding CPUFREQ_ENTRY_INVALID frequencies.
239 Use arguments "pos" - a cpufreq_frequency_table * as a loop cursor and
240 "table" - the cpufreq_frequency_table * you want to iterate over.
244 struct cpufreq_frequency_table *pos, *driver_freq_table;
246 cpufreq_for_each_entry(pos, driver_freq_table) {
247 /* Do something with pos */