1.. SPDX-License-Identifier: GPL-2.0 2 3=============================================== 4How to Implement a new CPUFreq Processor Driver 5=============================================== 6 7Authors: 8 9 10 - Dominik Brodowski <linux@brodo.de> 11 - Rafael J. Wysocki <rafael.j.wysocki@intel.com> 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_intermediate 24 2. Frequency Table Helpers 25 26 27 281. What To Do? 29============== 30 31So, you just got a brand-new CPU / chipset with datasheets and want to 32add cpufreq support for this CPU / chipset? Great. Here are some hints 33on what is necessary: 34 35 361.1 Initialization 37------------------ 38 39First of all, in an __initcall level 7 (module_init()) or later 40function check whether this kernel runs on the right CPU and the right 41chipset. If so, register a struct cpufreq_driver with the CPUfreq core 42using cpufreq_register_driver() 43 44What shall this struct cpufreq_driver contain? 45 46 .name - The name of this driver. 47 48 .init - A pointer to the per-policy initialization function. 49 50 .verify - A pointer to a "verification" function. 51 52 .setpolicy _or_ .fast_switch _or_ .target _or_ .target_index - See 53 below on the differences. 54 55And optionally 56 57 .flags - Hints for the cpufreq core. 58 59 .driver_data - cpufreq driver specific data. 60 61 .get_intermediate and target_intermediate - Used to switch to stable 62 frequency while changing CPU frequency. 63 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. 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 78 .attr - A pointer to a NULL-terminated list of "struct freq_attr" which 79 allow to export values to sysfs. 80 81 .boost_enabled - If set, boost frequencies are enabled. 82 83 .set_boost - A pointer to a per-policy function to enable/disable boost 84 frequencies. 85 86 871.2 Per-CPU Initialization 88-------------------------- 89 90Whenever a new CPU is registered with the device model, or after the 91cpufreq driver registers itself, the per-policy initialization function 92cpufreq_driver.init is called if no cpufreq policy existed for the CPU. 93Note that the .init() and .exit() routines are called only once for the 94policy and not for each CPU managed by the policy. It takes a ``struct 95cpufreq_policy *policy`` as argument. What to do now? 96 97If necessary, activate the CPUfreq support on your CPU. 98 99Then, the driver must fill in the following values: 100 101+-----------------------------------+--------------------------------------+ 102|policy->cpuinfo.min_freq _and_ | | 103|policy->cpuinfo.max_freq | the minimum and maximum frequency | 104| | (in kHz) which is supported by | 105| | this CPU | 106+-----------------------------------+--------------------------------------+ 107|policy->cpuinfo.transition_latency | the time it takes on this CPU to | 108| | switch between two frequencies in | 109| | nanoseconds (if appropriate, else | 110| | specify CPUFREQ_ETERNAL) | 111+-----------------------------------+--------------------------------------+ 112|policy->cur | The current operating frequency of | 113| | this CPU (if appropriate) | 114+-----------------------------------+--------------------------------------+ 115|policy->min, | | 116|policy->max, | | 117|policy->policy and, if necessary, | | 118|policy->governor | must contain the "default policy" for| 119| | this CPU. A few moments later, | 120| | cpufreq_driver.verify and either | 121| | cpufreq_driver.setpolicy or | 122| | cpufreq_driver.target/target_index is| 123| | called with these values. | 124+-----------------------------------+--------------------------------------+ 125|policy->cpus | Update this with the masks of the | 126| | (online + offline) CPUs that do DVFS | 127| | along with this CPU (i.e. that share| 128| | clock/voltage rails with it). | 129+-----------------------------------+--------------------------------------+ 130 131For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the 132frequency table helpers might be helpful. See the section 2 for more information 133on them. 134 135 1361.3 verify 137---------- 138 139When the user decides a new policy (consisting of 140"policy,governor,min,max") shall be set, this policy must be validated 141so that incompatible values can be corrected. For verifying these 142values cpufreq_verify_within_limits(``struct cpufreq_policy *policy``, 143``unsigned int min_freq``, ``unsigned int max_freq``) function might be helpful. 144See section 2 for details on frequency table helpers. 145 146You need to make sure that at least one valid frequency (or operating 147range) is within policy->min and policy->max. If necessary, increase 148policy->max first, and only if this is no solution, decrease policy->min. 149 150 1511.4 target or target_index or setpolicy or fast_switch? 152------------------------------------------------------- 153 154Most cpufreq drivers or even most cpu frequency scaling algorithms 155only allow the CPU frequency to be set to predefined fixed values. For 156these, you use the ->target(), ->target_index() or ->fast_switch() 157callbacks. 158 159Some cpufreq capable processors switch the frequency between certain 160limits on their own. These shall use the ->setpolicy() callback. 161 162 1631.5. target/target_index 164------------------------ 165 166The target_index call has two arguments: ``struct cpufreq_policy *policy``, 167and ``unsigned int`` index (into the exposed frequency table). 168 169The CPUfreq driver must set the new frequency when called here. The 170actual frequency must be determined by freq_table[index].frequency. 171 172It should always restore to earlier frequency (i.e. policy->restore_freq) in 173case of errors, even if we switched to intermediate frequency earlier. 174 175Deprecated 176---------- 177The target call has three arguments: ``struct cpufreq_policy *policy``, 178unsigned int target_frequency, unsigned int relation. 179 180The CPUfreq driver must set the new frequency when called here. The 181actual frequency must be determined using the following rules: 182 183- keep close to "target_freq" 184- policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!) 185- if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal 186 target_freq. ("L for lowest, but no lower than") 187- if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal 188 target_freq. ("H for highest, but no higher than") 189 190Here again the frequency table helper might assist you - see section 2 191for details. 192 1931.6. fast_switch 194---------------- 195 196This function is used for frequency switching from scheduler's context. 197Not all drivers are expected to implement it, as sleeping from within 198this callback isn't allowed. This callback must be highly optimized to 199do switching as fast as possible. 200 201This function has two arguments: ``struct cpufreq_policy *policy`` and 202``unsigned int target_frequency``. 203 204 2051.7 setpolicy 206------------- 207 208The setpolicy call only takes a ``struct cpufreq_policy *policy`` as 209argument. You need to set the lower limit of the in-processor or 210in-chipset dynamic frequency switching to policy->min, the upper limit 211to policy->max, and -if supported- select a performance-oriented 212setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a 213powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check 214the reference implementation in drivers/cpufreq/longrun.c 215 2161.8 get_intermediate and target_intermediate 217-------------------------------------------- 218 219Only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION unset. 220 221get_intermediate should return a stable intermediate frequency platform wants to 222switch to, and target_intermediate() should set CPU to that frequency, before 223jumping to the frequency corresponding to 'index'. Core will take care of 224sending notifications and driver doesn't have to handle them in 225target_intermediate() or target_index(). 226 227Drivers can return '0' from get_intermediate() in case they don't wish to switch 228to intermediate frequency for some target frequency. In that case core will 229directly call ->target_index(). 230 231NOTE: ->target_index() should restore to policy->restore_freq in case of 232failures as core would send notifications for that. 233 234 2352. Frequency Table Helpers 236========================== 237 238As most cpufreq processors only allow for being set to a few specific 239frequencies, a "frequency table" with some functions might assist in 240some work of the processor driver. Such a "frequency table" consists of 241an array of struct cpufreq_frequency_table entries, with driver specific 242values in "driver_data", the corresponding frequency in "frequency" and 243flags set. At the end of the table, you need to add a 244cpufreq_frequency_table entry with frequency set to CPUFREQ_TABLE_END. 245And if you want to skip one entry in the table, set the frequency to 246CPUFREQ_ENTRY_INVALID. The entries don't need to be in sorted in any 247particular order, but if they are cpufreq core will do DVFS a bit 248quickly for them as search for best match is faster. 249 250The cpufreq table is verified automatically by the core if the policy contains a 251valid pointer in its policy->freq_table field. 252 253cpufreq_frequency_table_verify() assures that at least one valid 254frequency is within policy->min and policy->max, and all other criteria 255are met. This is helpful for the ->verify call. 256 257cpufreq_frequency_table_target() is the corresponding frequency table 258helper for the ->target stage. Just pass the values to this function, 259and this function returns the of the frequency table entry which 260contains the frequency the CPU shall be set to. 261 262The following macros can be used as iterators over cpufreq_frequency_table: 263 264cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency 265table. 266 267cpufreq_for_each_valid_entry(pos, table) - iterates over all entries, 268excluding CPUFREQ_ENTRY_INVALID frequencies. 269Use arguments "pos" - a ``cpufreq_frequency_table *`` as a loop cursor and 270"table" - the ``cpufreq_frequency_table *`` you want to iterate over. 271 272For example:: 273 274 struct cpufreq_frequency_table *pos, *driver_freq_table; 275 276 cpufreq_for_each_entry(pos, driver_freq_table) { 277 /* Do something with pos */ 278 pos->frequency = ... 279 } 280 281If you need to work with the position of pos within driver_freq_table, 282do not subtract the pointers, as it is quite costly. Instead, use the 283macros cpufreq_for_each_entry_idx() and cpufreq_for_each_valid_entry_idx(). 284