1 .. SPDX-License-Identifier: GPL-2.0
2
3 ==============================================
4 How to Implement a new CPUFreq Processor Drive
5 ==============================================
6
7 Authors:
8
9
10 - Dominik Brodowski <linux@brodo.de>
11 - Rafael J. Wysocki <rafael.j.wysocki@i
12 - Viresh Kumar <viresh.kumar@linaro.org
13
14 .. Contents
15
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_intermedia
24 2. Frequency Table Helpers
25
26
27
28 1. What To Do?
29 ==============
30
31 So, you just got a brand-new CPU / chipset wit
32 add cpufreq support for this CPU / chipset? Gr
33 on what is necessary:
34
35
36 1.1 Initialization
37 ------------------
38
39 First of all, in an __initcall level 7 (module
40 function check whether this kernel runs on the
41 chipset. If so, register a struct cpufreq_driv
42 using cpufreq_register_driver()
43
44 What shall this struct cpufreq_driver contain?
45
46 .name - The name of this driver.
47
48 .init - A pointer to the per-policy initializ
49
50 .verify - A pointer to a "verification" funct
51
52 .setpolicy _or_ .fast_switch _or_ .target _or
53 below on the differences.
54
55 And optionally
56
57 .flags - Hints for the cpufreq core.
58
59 .driver_data - cpufreq driver specific data.
60
61 .get_intermediate and target_intermediate - U
62 frequency while changing CPU frequency.
63
64 .get - Returns current frequency of the CPU.
65
66 .bios_limit - Returns HW/BIOS max frequency l
67
68 .exit - A pointer to a per-policy cleanup fun
69 CPU_POST_DEAD phase of cpu hotplug process.
70
71 .suspend - A pointer to a per-policy suspend
72 with interrupts disabled and _after_ the gove
73 policy.
74
75 .resume - A pointer to a per-policy resume fu
76 with interrupts disabled and _before_ the gov
77
78 .ready - A pointer to a per-policy ready func
79 the policy is fully initialized.
80
81 .attr - A pointer to a NULL-terminated list o
82 allow to export values to sysfs.
83
84 .boost_enabled - If set, boost frequencies ar
85
86 .set_boost - A pointer to a per-policy functi
87 frequencies.
88
89
90 1.2 Per-CPU Initialization
91 --------------------------
92
93 Whenever a new CPU is registered with the devi
94 cpufreq driver registers itself, the per-polic
95 cpufreq_driver.init is called if no cpufreq po
96 Note that the .init() and .exit() routines are
97 policy and not for each CPU managed by the pol
98 cpufreq_policy *policy`` as argument. What to
99
100 If necessary, activate the CPUfreq support on
101
102 Then, the driver must fill in the following va
103
104 +-----------------------------------+---------
105 |policy->cpuinfo.min_freq _and_ |
106 |policy->cpuinfo.max_freq | the mini
107 | | (in kHz)
108 | | this CPU
109 +-----------------------------------+---------
110 |policy->cpuinfo.transition_latency | the time
111 | | switch b
112 | | nanoseco
113 | | specify
114 +-----------------------------------+---------
115 |policy->cur | The curr
116 | | this CPU
117 +-----------------------------------+---------
118 |policy->min, |
119 |policy->max, |
120 |policy->policy and, if necessary, |
121 |policy->governor | must con
122 | | this CPU
123 | | cpufreq_
124 | | cpufreq_
125 | | cpufreq_
126 | | called w
127 +-----------------------------------+---------
128 |policy->cpus | Update t
129 | | (online
130 | | along wi
131 | | clock/vo
132 +-----------------------------------+---------
133
134 For setting some of these values (cpuinfo.min[
135 frequency table helpers might be helpful. See
136 on them.
137
138
139 1.3 verify
140 ----------
141
142 When the user decides a new policy (consisting
143 "policy,governor,min,max") shall be set, this
144 so that incompatible values can be corrected.
145 values cpufreq_verify_within_limits(``struct c
146 ``unsigned int min_freq``, ``unsigned int max_
147 See section 2 for details on frequency table h
148
149 You need to make sure that at least one valid
150 range) is within policy->min and policy->max.
151 policy->max first, and only if this is no solu
152
153
154 1.4 target or target_index or setpolicy or fas
155 ----------------------------------------------
156
157 Most cpufreq drivers or even most cpu frequenc
158 only allow the CPU frequency to be set to pred
159 these, you use the ->target(), ->target_index(
160 callbacks.
161
162 Some cpufreq capable processors switch the fre
163 limits on their own. These shall use the ->set
164
165
166 1.5. target/target_index
167 ------------------------
168
169 The target_index call has two arguments: ``str
170 and ``unsigned int`` index (into the exposed f
171
172 The CPUfreq driver must set the new frequency
173 actual frequency must be determined by freq_ta
174
175 It should always restore to earlier frequency
176 case of errors, even if we switched to interme
177
178 Deprecated
179 ----------
180 The target call has three arguments: ``struct
181 unsigned int target_frequency, unsigned int re
182
183 The CPUfreq driver must set the new frequency
184 actual frequency must be determined using the
185
186 - keep close to "target_freq"
187 - policy->min <= new_freq <= policy->max (THIS
188 - if relation==CPUFREQ_REL_L, try to select a
189 target_freq. ("L for lowest, but no lower th
190 - if relation==CPUFREQ_REL_H, try to select a
191 target_freq. ("H for highest, but no higher
192
193 Here again the frequency table helper might as
194 for details.
195
196 1.6. fast_switch
197 ----------------
198
199 This function is used for frequency switching
200 Not all drivers are expected to implement it,
201 this callback isn't allowed. This callback mus
202 do switching as fast as possible.
203
204 This function has two arguments: ``struct cpuf
205 ``unsigned int target_frequency``.
206
207
208 1.7 setpolicy
209 -------------
210
211 The setpolicy call only takes a ``struct cpufr
212 argument. You need to set the lower limit of t
213 in-chipset dynamic frequency switching to poli
214 to policy->max, and -if supported- select a pe
215 setting when policy->policy is CPUFREQ_POLICY_
216 powersaving-oriented setting when CPUFREQ_POLI
217 the reference implementation in drivers/cpufre
218
219 1.8 get_intermediate and target_intermediate
220 --------------------------------------------
221
222 Only for drivers with target_index() and CPUFR
223
224 get_intermediate should return a stable interm
225 switch to, and target_intermediate() should se
226 jumping to the frequency corresponding to 'ind
227 sending notifications and driver doesn't have
228 target_intermediate() or target_index().
229
230 Drivers can return '0' from get_intermediate()
231 to intermediate frequency for some target freq
232 directly call ->target_index().
233
234 NOTE: ->target_index() should restore to polic
235 failures as core would send notifications for
236
237
238 2. Frequency Table Helpers
239 ==========================
240
241 As most cpufreq processors only allow for bein
242 frequencies, a "frequency table" with some fun
243 some work of the processor driver. Such a "fre
244 an array of struct cpufreq_frequency_table ent
245 values in "driver_data", the corresponding fre
246 flags set. At the end of the table, you need t
247 cpufreq_frequency_table entry with frequency s
248 And if you want to skip one entry in the table
249 CPUFREQ_ENTRY_INVALID. The entries don't need
250 particular order, but if they are cpufreq core
251 quickly for them as search for best match is f
252
253 The cpufreq table is verified automatically by
254 valid pointer in its policy->freq_table field.
255
256 cpufreq_frequency_table_verify() assures that
257 frequency is within policy->min and policy->ma
258 are met. This is helpful for the ->verify call
259
260 cpufreq_frequency_table_target() is the corres
261 helper for the ->target stage. Just pass the v
262 and this function returns the of the frequency
263 contains the frequency the CPU shall be set to
264
265 The following macros can be used as iterators
266
267 cpufreq_for_each_entry(pos, table) - iterates
268 table.
269
270 cpufreq_for_each_valid_entry(pos, table) - ite
271 excluding CPUFREQ_ENTRY_INVALID frequencies.
272 Use arguments "pos" - a ``cpufreq_frequency_ta
273 "table" - the ``cpufreq_frequency_table *`` yo
274
275 For example::
276
277 struct cpufreq_frequency_table *pos, *
278
279 cpufreq_for_each_entry(pos, driver_fre
280 /* Do something with pos */
281 pos->frequency = ...
282 }
283
284 If you need to work with the position of pos w
285 do not subtract the pointers, as it is quite c
286 macros cpufreq_for_each_entry_idx() and cpufre
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.