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8f920589 | 1 | .. SPDX-License-Identifier: GPL-2.0 |
1da177e4 | 2 | |
8f920589 MCC |
3 | =============================================== |
4 | How to Implement a new CPUFreq Processor Driver | |
5 | =============================================== | |
1da177e4 | 6 | |
8f920589 | 7 | Authors: |
1da177e4 | 8 | |
1da177e4 | 9 | |
8f920589 MCC |
10 | - Dominik Brodowski <linux@brodo.de> |
11 | - Rafael J. Wysocki <rafael.j.wysocki@intel.com> | |
12 | - Viresh Kumar <viresh.kumar@linaro.org> | |
1da177e4 | 13 | |
8f920589 | 14 | .. Contents |
1da177e4 | 15 | |
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16 | 1. What To Do? |
17 | 1.1 Initialization | |
18 | 1.2 Per-CPU Initialization | |
19 | 1.3 verify | |
20 | 1.4 target/target_index or setpolicy? | |
21 | 1.5 target/target_index | |
22 | 1.6 setpolicy | |
23 | 1.7 get_intermediate and target_intermediate | |
24 | 2. Frequency Table Helpers | |
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25 | |
26 | ||
27 | ||
28 | 1. What To Do? | |
29 | ============== | |
30 | ||
31 | So, you just got a brand-new CPU / chipset with datasheets and want to | |
32 | add cpufreq support for this CPU / chipset? Great. Here are some hints | |
33 | on what is necessary: | |
34 | ||
35 | ||
36 | 1.1 Initialization | |
37 | ------------------ | |
38 | ||
39 | First of all, in an __initcall level 7 (module_init()) or later | |
40 | function check whether this kernel runs on the right CPU and the right | |
41 | chipset. If so, register a struct cpufreq_driver with the CPUfreq core | |
42 | using cpufreq_register_driver() | |
43 | ||
8f920589 | 44 | What shall this struct cpufreq_driver contain? |
1da177e4 | 45 | |
7de962c0 | 46 | .name - The name of this driver. |
1da177e4 | 47 | |
7de962c0 | 48 | .init - A pointer to the per-policy initialization function. |
1da177e4 | 49 | |
7de962c0 | 50 | .verify - A pointer to a "verification" function. |
1da177e4 | 51 | |
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52 | .setpolicy _or_ .fast_switch _or_ .target _or_ .target_index - See |
53 | below on the differences. | |
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54 | |
55 | And optionally | |
56 | ||
7de962c0 | 57 | .flags - Hints for the cpufreq core. |
367dc4aa | 58 | |
7de962c0 | 59 | .driver_data - cpufreq driver specific data. |
1da177e4 | 60 | |
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61 | .get_intermediate and target_intermediate - Used to switch to stable |
62 | frequency while changing CPU frequency. | |
1da177e4 | 63 | |
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64 | .get - Returns current frequency of the CPU. |
65 | ||
66 | .bios_limit - Returns HW/BIOS max frequency limitations for the CPU. | |
67 | ||
68 | .exit - A pointer to a per-policy cleanup function called during | |
69 | CPU_POST_DEAD phase of cpu hotplug process. | |
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70 | |
71 | .suspend - A pointer to a per-policy suspend function which is called | |
72 | with interrupts disabled and _after_ the governor is stopped for the | |
73 | policy. | |
74 | ||
75 | .resume - A pointer to a per-policy resume function which is called | |
76 | with interrupts disabled and _before_ the governor is started again. | |
77 | ||
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78 | .ready - A pointer to a per-policy ready function which is called after |
79 | the policy is fully initialized. | |
80 | ||
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81 | .attr - A pointer to a NULL-terminated list of "struct freq_attr" which |
82 | allow to export values to sysfs. | |
83 | ||
84 | .boost_enabled - If set, boost frequencies are enabled. | |
85 | ||
86 | .set_boost - A pointer to a per-policy function to enable/disable boost | |
87 | frequencies. | |
1c03a2d0 | 88 | |
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89 | |
90 | 1.2 Per-CPU Initialization | |
91 | -------------------------- | |
92 | ||
93 | Whenever a new CPU is registered with the device model, or after the | |
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94 | cpufreq driver registers itself, the per-policy initialization function |
95 | cpufreq_driver.init is called if no cpufreq policy existed for the CPU. | |
96 | Note that the .init() and .exit() routines are called only once for the | |
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97 | policy and not for each CPU managed by the policy. It takes a ``struct |
98 | cpufreq_policy *policy`` as argument. What to do now? | |
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99 | |
100 | If necessary, activate the CPUfreq support on your CPU. | |
101 | ||
102 | Then, the driver must fill in the following values: | |
103 | ||
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104 | +-----------------------------------+--------------------------------------+ |
105 | |policy->cpuinfo.min_freq _and_ | | | |
106 | |policy->cpuinfo.max_freq | the minimum and maximum frequency | | |
107 | | | (in kHz) which is supported by | | |
108 | | | this CPU | | |
109 | +-----------------------------------+--------------------------------------+ | |
110 | |policy->cpuinfo.transition_latency | the time it takes on this CPU to | | |
111 | | | switch between two frequencies in | | |
112 | | | nanoseconds (if appropriate, else | | |
113 | | | specify CPUFREQ_ETERNAL) | | |
114 | +-----------------------------------+--------------------------------------+ | |
115 | |policy->cur | The current operating frequency of | | |
116 | | | this CPU (if appropriate) | | |
117 | +-----------------------------------+--------------------------------------+ | |
118 | |policy->min, | | | |
119 | |policy->max, | | | |
120 | |policy->policy and, if necessary, | | | |
121 | |policy->governor | must contain the "default policy" for| | |
122 | | | this CPU. A few moments later, | | |
123 | | | cpufreq_driver.verify and either | | |
124 | | | cpufreq_driver.setpolicy or | | |
125 | | | cpufreq_driver.target/target_index is| | |
126 | | | called with these values. | | |
127 | +-----------------------------------+--------------------------------------+ | |
128 | |policy->cpus | Update this with the masks of the | | |
129 | | | (online + offline) CPUs that do DVFS | | |
130 | | | along with this CPU (i.e. that share| | |
131 | | | clock/voltage rails with it). | | |
132 | +-----------------------------------+--------------------------------------+ | |
1da177e4 | 133 | |
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134 | For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the |
135 | frequency table helpers might be helpful. See the section 2 for more information | |
136 | on them. | |
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137 | |
138 | ||
139 | 1.3 verify | |
7de962c0 | 140 | ---------- |
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141 | |
142 | When the user decides a new policy (consisting of | |
143 | "policy,governor,min,max") shall be set, this policy must be validated | |
144 | so that incompatible values can be corrected. For verifying these | |
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145 | values cpufreq_verify_within_limits(``struct cpufreq_policy *policy``, |
146 | ``unsigned int min_freq``, ``unsigned int max_freq``) function might be helpful. | |
7de962c0 | 147 | See section 2 for details on frequency table helpers. |
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148 | |
149 | You need to make sure that at least one valid frequency (or operating | |
150 | range) is within policy->min and policy->max. If necessary, increase | |
151 | policy->max first, and only if this is no solution, decrease policy->min. | |
152 | ||
153 | ||
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154 | 1.4 target or target_index or setpolicy or fast_switch? |
155 | ------------------------------------------------------- | |
1da177e4 | 156 | |
8f920589 | 157 | Most cpufreq drivers or even most cpu frequency scaling algorithms |
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158 | only allow the CPU frequency to be set to predefined fixed values. For |
159 | these, you use the ->target(), ->target_index() or ->fast_switch() | |
160 | callbacks. | |
1da177e4 | 161 | |
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162 | Some cpufreq capable processors switch the frequency between certain |
163 | limits on their own. These shall use the ->setpolicy() callback. | |
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164 | |
165 | ||
1c03a2d0 | 166 | 1.5. target/target_index |
7de962c0 | 167 | ------------------------ |
1da177e4 | 168 | |
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169 | The target_index call has two arguments: ``struct cpufreq_policy *policy``, |
170 | and ``unsigned int`` index (into the exposed frequency table). | |
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171 | |
172 | The CPUfreq driver must set the new frequency when called here. The | |
173 | actual frequency must be determined by freq_table[index].frequency. | |
174 | ||
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175 | It should always restore to earlier frequency (i.e. policy->restore_freq) in |
176 | case of errors, even if we switched to intermediate frequency earlier. | |
177 | ||
8f920589 | 178 | Deprecated |
9c0ebcf7 | 179 | ---------- |
8f920589 | 180 | The target call has three arguments: ``struct cpufreq_policy *policy``, |
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181 | unsigned int target_frequency, unsigned int relation. |
182 | ||
183 | The CPUfreq driver must set the new frequency when called here. The | |
184 | actual frequency must be determined using the following rules: | |
185 | ||
186 | - keep close to "target_freq" | |
187 | - policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!) | |
188 | - if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal | |
189 | target_freq. ("L for lowest, but no lower than") | |
190 | - if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal | |
191 | target_freq. ("H for highest, but no higher than") | |
192 | ||
51555c0e | 193 | Here again the frequency table helper might assist you - see section 2 |
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194 | for details. |
195 | ||
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196 | 1.6. fast_switch |
197 | ---------------- | |
1da177e4 | 198 | |
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199 | This function is used for frequency switching from scheduler's context. |
200 | Not all drivers are expected to implement it, as sleeping from within | |
201 | this callback isn't allowed. This callback must be highly optimized to | |
202 | do switching as fast as possible. | |
203 | ||
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204 | This function has two arguments: ``struct cpufreq_policy *policy`` and |
205 | ``unsigned int target_frequency``. | |
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206 | |
207 | ||
208 | 1.7 setpolicy | |
209 | ------------- | |
1da177e4 | 210 | |
8f920589 | 211 | The setpolicy call only takes a ``struct cpufreq_policy *policy`` as |
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212 | argument. You need to set the lower limit of the in-processor or |
213 | in-chipset dynamic frequency switching to policy->min, the upper limit | |
214 | to policy->max, and -if supported- select a performance-oriented | |
215 | setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a | |
216 | powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check | |
25eb650a | 217 | the reference implementation in drivers/cpufreq/longrun.c |
1da177e4 | 218 | |
7de962c0 | 219 | 1.8 get_intermediate and target_intermediate |
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220 | -------------------------------------------- |
221 | ||
222 | Only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION unset. | |
223 | ||
224 | get_intermediate should return a stable intermediate frequency platform wants to | |
54f5d13b | 225 | switch to, and target_intermediate() should set CPU to that frequency, before |
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226 | jumping to the frequency corresponding to 'index'. Core will take care of |
227 | sending notifications and driver doesn't have to handle them in | |
228 | target_intermediate() or target_index(). | |
229 | ||
230 | Drivers can return '0' from get_intermediate() in case they don't wish to switch | |
231 | to intermediate frequency for some target frequency. In that case core will | |
232 | directly call ->target_index(). | |
233 | ||
234 | NOTE: ->target_index() should restore to policy->restore_freq in case of | |
235 | failures as core would send notifications for that. | |
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236 | |
237 | ||
238 | 2. Frequency Table Helpers | |
239 | ========================== | |
240 | ||
241 | As most cpufreq processors only allow for being set to a few specific | |
242 | frequencies, a "frequency table" with some functions might assist in | |
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243 | some work of the processor driver. Such a "frequency table" consists of |
244 | an array of struct cpufreq_frequency_table entries, with driver specific | |
245 | values in "driver_data", the corresponding frequency in "frequency" and | |
246 | flags set. At the end of the table, you need to add a | |
247 | cpufreq_frequency_table entry with frequency set to CPUFREQ_TABLE_END. | |
248 | And if you want to skip one entry in the table, set the frequency to | |
249 | CPUFREQ_ENTRY_INVALID. The entries don't need to be in sorted in any | |
250 | particular order, but if they are cpufreq core will do DVFS a bit | |
251 | quickly for them as search for best match is faster. | |
252 | ||
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253 | The cpufreq table is verified automatically by the core if the policy contains a |
254 | valid pointer in its policy->freq_table field. | |
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255 | |
256 | cpufreq_frequency_table_verify() assures that at least one valid | |
257 | frequency is within policy->min and policy->max, and all other criteria | |
258 | are met. This is helpful for the ->verify call. | |
259 | ||
260 | cpufreq_frequency_table_target() is the corresponding frequency table | |
261 | helper for the ->target stage. Just pass the values to this function, | |
262 | and this function returns the of the frequency table entry which | |
263 | contains the frequency the CPU shall be set to. | |
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264 | |
265 | The following macros can be used as iterators over cpufreq_frequency_table: | |
266 | ||
267 | cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency | |
268 | table. | |
269 | ||
7de962c0 | 270 | cpufreq_for_each_valid_entry(pos, table) - iterates over all entries, |
27e289dc | 271 | excluding CPUFREQ_ENTRY_INVALID frequencies. |
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272 | Use arguments "pos" - a ``cpufreq_frequency_table *`` as a loop cursor and |
273 | "table" - the ``cpufreq_frequency_table *`` you want to iterate over. | |
27e289dc | 274 | |
8f920589 | 275 | For example:: |
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276 | |
277 | struct cpufreq_frequency_table *pos, *driver_freq_table; | |
278 | ||
279 | cpufreq_for_each_entry(pos, driver_freq_table) { | |
280 | /* Do something with pos */ | |
281 | pos->frequency = ... | |
282 | } | |
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283 | |
284 | If you need to work with the position of pos within driver_freq_table, | |
285 | do not subtract the pointers, as it is quite costly. Instead, use the | |
286 | macros cpufreq_for_each_entry_idx() and cpufreq_for_each_valid_entry_idx(). |