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1.. SPDX-License-Identifier: GPL-2.02 3===============================================4How to Implement a new CPUFreq Processor Driver5===============================================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.. Contents15 16 1. What To Do?17 1.1 Initialization18 1.2 Per-CPU Initialization19 1.3 verify20 1.4 target/target_index or setpolicy?21 1.5 target/target_index22 1.6 setpolicy23 1.7 get_intermediate and target_intermediate24 2. Frequency Table Helpers25 26 27 281. What To Do?29==============30 31So, you just got a brand-new CPU / chipset with datasheets and want to32add cpufreq support for this CPU / chipset? Great. Here are some hints33on what is necessary:34 35 361.1 Initialization37------------------38 39First of all, in an __initcall level 7 (module_init()) or later40function check whether this kernel runs on the right CPU and the right41chipset. If so, register a struct cpufreq_driver with the CPUfreq core42using 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 - See53 below on the differences.54 55And optionally56 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 stable62 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 during69 CPU_POST_DEAD phase of cpu hotplug process.70 71 .suspend - A pointer to a per-policy suspend function which is called72 with interrupts disabled and _after_ the governor is stopped for the73 policy.74 75 .resume - A pointer to a per-policy resume function which is called76 with interrupts disabled and _before_ the governor is started again.77 78 .ready - A pointer to a per-policy ready function which is called after79 the policy is fully initialized.80 81 .attr - A pointer to a NULL-terminated list of "struct freq_attr" which82 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 boost87 frequencies.88 89 901.2 Per-CPU Initialization91--------------------------92 93Whenever a new CPU is registered with the device model, or after the94cpufreq driver registers itself, the per-policy initialization function95cpufreq_driver.init is called if no cpufreq policy existed for the CPU.96Note that the .init() and .exit() routines are called only once for the97policy and not for each CPU managed by the policy. It takes a ``struct98cpufreq_policy *policy`` as argument. What to do now?99 100If necessary, activate the CPUfreq support on your CPU.101 102Then, the driver must fill in the following values:103 104+-----------------------------------+--------------------------------------+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+-----------------------------------+--------------------------------------+133 134For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the135frequency table helpers might be helpful. See the section 2 for more information136on them.137 138 1391.3 verify140----------141 142When the user decides a new policy (consisting of143"policy,governor,min,max") shall be set, this policy must be validated144so that incompatible values can be corrected. For verifying these145values cpufreq_verify_within_limits(``struct cpufreq_policy *policy``,146``unsigned int min_freq``, ``unsigned int max_freq``) function might be helpful.147See section 2 for details on frequency table helpers.148 149You need to make sure that at least one valid frequency (or operating150range) is within policy->min and policy->max. If necessary, increase151policy->max first, and only if this is no solution, decrease policy->min.152 153 1541.4 target or target_index or setpolicy or fast_switch?155-------------------------------------------------------156 157Most cpufreq drivers or even most cpu frequency scaling algorithms158only allow the CPU frequency to be set to predefined fixed values. For159these, you use the ->target(), ->target_index() or ->fast_switch()160callbacks.161 162Some cpufreq capable processors switch the frequency between certain163limits on their own. These shall use the ->setpolicy() callback.164 165 1661.5. target/target_index167------------------------168 169The target_index call has two arguments: ``struct cpufreq_policy *policy``,170and ``unsigned int`` index (into the exposed frequency table).171 172The CPUfreq driver must set the new frequency when called here. The173actual frequency must be determined by freq_table[index].frequency.174 175It should always restore to earlier frequency (i.e. policy->restore_freq) in176case of errors, even if we switched to intermediate frequency earlier.177 178Deprecated179----------180The target call has three arguments: ``struct cpufreq_policy *policy``,181unsigned int target_frequency, unsigned int relation.182 183The CPUfreq driver must set the new frequency when called here. The184actual 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 equal189 target_freq. ("L for lowest, but no lower than")190- if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal191 target_freq. ("H for highest, but no higher than")192 193Here again the frequency table helper might assist you - see section 2194for details.195 1961.6. fast_switch197----------------198 199This function is used for frequency switching from scheduler's context.200Not all drivers are expected to implement it, as sleeping from within201this callback isn't allowed. This callback must be highly optimized to202do switching as fast as possible.203 204This function has two arguments: ``struct cpufreq_policy *policy`` and205``unsigned int target_frequency``.206 207 2081.7 setpolicy209-------------210 211The setpolicy call only takes a ``struct cpufreq_policy *policy`` as212argument. You need to set the lower limit of the in-processor or213in-chipset dynamic frequency switching to policy->min, the upper limit214to policy->max, and -if supported- select a performance-oriented215setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a216powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check217the reference implementation in drivers/cpufreq/longrun.c218 2191.8 get_intermediate and target_intermediate220--------------------------------------------221 222Only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION unset.223 224get_intermediate should return a stable intermediate frequency platform wants to225switch to, and target_intermediate() should set CPU to that frequency, before226jumping to the frequency corresponding to 'index'. Core will take care of227sending notifications and driver doesn't have to handle them in228target_intermediate() or target_index().229 230Drivers can return '0' from get_intermediate() in case they don't wish to switch231to intermediate frequency for some target frequency. In that case core will232directly call ->target_index().233 234NOTE: ->target_index() should restore to policy->restore_freq in case of235failures as core would send notifications for that.236 237 2382. Frequency Table Helpers239==========================240 241As most cpufreq processors only allow for being set to a few specific242frequencies, a "frequency table" with some functions might assist in243some work of the processor driver. Such a "frequency table" consists of244an array of struct cpufreq_frequency_table entries, with driver specific245values in "driver_data", the corresponding frequency in "frequency" and246flags set. At the end of the table, you need to add a247cpufreq_frequency_table entry with frequency set to CPUFREQ_TABLE_END.248And if you want to skip one entry in the table, set the frequency to249CPUFREQ_ENTRY_INVALID. The entries don't need to be in sorted in any250particular order, but if they are cpufreq core will do DVFS a bit251quickly for them as search for best match is faster.252 253The cpufreq table is verified automatically by the core if the policy contains a254valid pointer in its policy->freq_table field.255 256cpufreq_frequency_table_verify() assures that at least one valid257frequency is within policy->min and policy->max, and all other criteria258are met. This is helpful for the ->verify call.259 260cpufreq_frequency_table_target() is the corresponding frequency table261helper for the ->target stage. Just pass the values to this function,262and this function returns the of the frequency table entry which263contains the frequency the CPU shall be set to.264 265The following macros can be used as iterators over cpufreq_frequency_table:266 267cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency268table.269 270cpufreq_for_each_valid_entry(pos, table) - iterates over all entries,271excluding CPUFREQ_ENTRY_INVALID frequencies.272Use arguments "pos" - a ``cpufreq_frequency_table *`` as a loop cursor and273"table" - the ``cpufreq_frequency_table *`` you want to iterate over.274 275For example::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 }283 284If you need to work with the position of pos within driver_freq_table,285do not subtract the pointers, as it is quite costly. Instead, use the286macros cpufreq_for_each_entry_idx() and cpufreq_for_each_valid_entry_idx().287