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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 D r i v e r s |
7 | |
8 | - information for developers - |
9 | |
10 | |
11 | Dominik Brodowski <linux@brodo.de> |
12 | |
13 | |
14 | |
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. |
18 | |
19 | |
20 | Contents: |
21 | --------- |
22 | 1. What To Do? |
23 | 1.1 Initialization |
24 | 1.2 Per-CPU Initialization |
25 | 1.3 verify |
26 | 1.4 target or setpolicy? |
27 | 1.5 target |
28 | 1.6 setpolicy |
29 | 2. Frequency Table Helpers |
30 | |
31 | |
32 | |
33 | 1. What To Do? |
34 | ============== |
35 | |
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 |
38 | on what is necessary: |
39 | |
40 | |
41 | 1.1 Initialization |
42 | ------------------ |
43 | |
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() |
48 | |
49 | What shall this struct cpufreq_driver contain? |
50 | |
51 | cpufreq_driver.name - The name of this driver. |
52 | |
53 | cpufreq_driver.owner - THIS_MODULE; |
54 | |
55 | cpufreq_driver.init - A pointer to the per-CPU initialization |
56 | function. |
57 | |
58 | cpufreq_driver.verify - A pointer to a "verification" function. |
59 | |
60 | cpufreq_driver.setpolicy _or_ |
61 | cpufreq_driver.target - See below on the differences. |
62 | |
63 | And optionally |
64 | |
65 | cpufreq_driver.exit - A pointer to a per-CPU cleanup function. |
66 | |
67 | cpufreq_driver.resume - A pointer to a per-CPU resume function |
68 | which is called with interrupts disabled |
69 | and _before_ the pre-suspend frequency |
70 | and/or policy is restored by a call to |
71 | ->target or ->setpolicy. |
72 | |
73 | cpufreq_driver.attr - A pointer to a NULL-terminated list of |
74 | "struct freq_attr" which allow to |
75 | export values to sysfs. |
76 | |
77 | |
78 | 1.2 Per-CPU Initialization |
79 | -------------------------- |
80 | |
81 | Whenever a new CPU is registered with the device model, or after the |
82 | cpufreq driver registers itself, the per-CPU initialization function |
83 | cpufreq_driver.init is called. It takes a struct cpufreq_policy |
84 | *policy as argument. What to do now? |
85 | |
86 | If necessary, activate the CPUfreq support on your CPU. |
87 | |
88 | Then, the driver must fill in the following values: |
89 | |
90 | policy->cpuinfo.min_freq _and_ |
91 | policy->cpuinfo.max_freq - the minimum and maximum frequency |
92 | (in kHz) which is supported by |
93 | this CPU |
94 | policy->cpuinfo.transition_latency the time it takes on this CPU to |
95 | switch between two frequencies in |
96 | nanoseconds (if appropriate, else |
97 | specify CPUFREQ_ETERNAL) |
98 | |
99 | policy->cur The current operating frequency of |
100 | this CPU (if appropriate) |
101 | policy->min, |
102 | policy->max, |
103 | policy->policy and, if necessary, |
104 | policy->governor must contain the "default policy" for |
105 | this CPU. A few moments later, |
106 | cpufreq_driver.verify and either |
107 | cpufreq_driver.setpolicy or |
108 | cpufreq_driver.target is called with |
109 | these values. |
110 | |
111 | For setting some of these values, the frequency table helpers might be |
112 | helpful. See the section 2 for more information on them. |
113 | |
114 | |
115 | 1.3 verify |
116 | ------------ |
117 | |
118 | When the user decides a new policy (consisting of |
119 | "policy,governor,min,max") shall be set, this policy must be validated |
120 | so that incompatible values can be corrected. For verifying these |
121 | values, a frequency table helper and/or the |
122 | cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned |
123 | int min_freq, unsigned int max_freq) function might be helpful. See |
124 | section 2 for details on frequency table helpers. |
125 | |
126 | You need to make sure that at least one valid frequency (or operating |
127 | range) is within policy->min and policy->max. If necessary, increase |
128 | policy->max first, and only if this is no solution, decrease policy->min. |
129 | |
130 | |
131 | 1.4 target or setpolicy? |
132 | ---------------------------- |
133 | |
134 | Most cpufreq drivers or even most cpu frequency scaling algorithms |
135 | only allow the CPU to be set to one frequency. For these, you use the |
136 | ->target call. |
137 | |
138 | Some cpufreq-capable processors switch the frequency between certain |
139 | limits on their own. These shall use the ->setpolicy call |
140 | |
141 | |
142 | 1.4. target |
143 | ------------- |
144 | |
145 | The target call has three arguments: struct cpufreq_policy *policy, |
146 | unsigned int target_frequency, unsigned int relation. |
147 | |
148 | The CPUfreq driver must set the new frequency when called here. The |
149 | actual frequency must be determined using the following rules: |
150 | |
151 | - keep close to "target_freq" |
152 | - policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!) |
153 | - if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal |
154 | target_freq. ("L for lowest, but no lower than") |
155 | - if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal |
156 | target_freq. ("H for highest, but no higher than") |
157 | |
158 | Here again the frequency table helper might assist you - see section 2 |
159 | for details. |
160 | |
161 | |
162 | 1.5 setpolicy |
163 | --------------- |
164 | |
165 | The setpolicy call only takes a struct cpufreq_policy *policy as |
166 | argument. You need to set the lower limit of the in-processor or |
167 | in-chipset dynamic frequency switching to policy->min, the upper limit |
168 | to policy->max, and -if supported- select a performance-oriented |
169 | setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a |
170 | powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check |
171 | the reference implementation in arch/i386/kernel/cpu/cpufreq/longrun.c |
172 | |
173 | |
174 | |
175 | 2. Frequency Table Helpers |
176 | ========================== |
177 | |
178 | As most cpufreq processors only allow for being set to a few specific |
179 | frequencies, a "frequency table" with some functions might assist in |
180 | some work of the processor driver. Such a "frequency table" consists |
181 | of an array of struct cpufreq_freq_table entries, with any value in |
182 | "index" you want to use, and the corresponding frequency in |
183 | "frequency". At the end of the table, you need to add a |
184 | cpufreq_freq_table entry with frequency set to CPUFREQ_TABLE_END. And |
185 | if you want to skip one entry in the table, set the frequency to |
186 | CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending |
187 | order. |
188 | |
189 | By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy, |
190 | struct cpufreq_frequency_table *table); |
191 | the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and |
192 | policy->min and policy->max are set to the same values. This is |
193 | helpful for the per-CPU initialization stage. |
194 | |
195 | int cpufreq_frequency_table_verify(struct cpufreq_policy *policy, |
196 | struct cpufreq_frequency_table *table); |
197 | assures that at least one valid frequency is within policy->min and |
198 | policy->max, and all other criteria are met. This is helpful for the |
199 | ->verify call. |
200 | |
201 | int cpufreq_frequency_table_target(struct cpufreq_policy *policy, |
202 | struct cpufreq_frequency_table *table, |
203 | unsigned int target_freq, |
204 | unsigned int relation, |
205 | unsigned int *index); |
206 | |
207 | is the corresponding frequency table helper for the ->target |
208 | stage. Just pass the values to this function, and the unsigned int |
209 | index returns the number of the frequency table entry which contains |
210 | the frequency the CPU shall be set to. PLEASE NOTE: This is not the |
211 | "index" which is in this cpufreq_table_entry.index, but instead |
212 | cpufreq_table[index]. So, the new frequency is |
213 | cpufreq_table[index].frequency, and the value you stored into the |
214 | frequency table "index" field is |
215 | cpufreq_table[index].index. |
216 | |
217 |
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