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