1 CPU frequency and voltage scaling code in the Linux(TM) kernel 2 3 4 L i n u x C P U F r e q 5 6 C P U D r i v e r s 7 8 - information for developers - 9 10 11 Dominik Brodowski <linux@brodo.de> 12 13 14 15 Clock scaling allows you to change the clock speed of the CPUs on the 16 fly. This is a nice method to save battery power, because the lower 17 the clock speed, the less power the CPU consumes. 18 19 20Contents: 21--------- 221. What To Do? 231.1 Initialization 241.2 Per-CPU Initialization 251.3 verify 261.4 target or setpolicy? 271.5 target 281.6 setpolicy 292. Frequency Table Helpers 30 31 32 331. What To Do? 34============== 35 36So, you just got a brand-new CPU / chipset with datasheets and want to 37add cpufreq support for this CPU / chipset? Great. Here are some hints 38on what is necessary: 39 40 411.1 Initialization 42------------------ 43 44First of all, in an __initcall level 7 (module_init()) or later 45function check whether this kernel runs on the right CPU and the right 46chipset. If so, register a struct cpufreq_driver with the CPUfreq core 47using cpufreq_register_driver() 48 49What shall this struct cpufreq_driver contain? 50 51cpufreq_driver.name - The name of this driver. 52 53cpufreq_driver.owner - THIS_MODULE; 54 55cpufreq_driver.init - A pointer to the per-CPU initialization 56 function. 57 58cpufreq_driver.verify - A pointer to a "verification" function. 59 60cpufreq_driver.setpolicy _or_ 61cpufreq_driver.target - See below on the differences. 62 63And optionally 64 65cpufreq_driver.exit - A pointer to a per-CPU cleanup function. 66 67cpufreq_driver.resume - A pointer to a per-CPU resume function 68 which is called with interrupts disabled 69 and _before_ the pre-suspend frequency 70 and/or policy is restored by a call to 71 ->target or ->setpolicy. 72 73cpufreq_driver.attr - A pointer to a NULL-terminated list of 74 "struct freq_attr" which allow to 75 export values to sysfs. 76 77 781.2 Per-CPU Initialization 79-------------------------- 80 81Whenever a new CPU is registered with the device model, or after the 82cpufreq driver registers itself, the per-CPU initialization function 83cpufreq_driver.init is called. It takes a struct cpufreq_policy 84*policy as argument. What to do now? 85 86If necessary, activate the CPUfreq support on your CPU. 87 88Then, the driver must fill in the following values: 89 90policy->cpuinfo.min_freq _and_ 91policy->cpuinfo.max_freq - the minimum and maximum frequency 92 (in kHz) which is supported by 93 this CPU 94policy->cpuinfo.transition_latency the time it takes on this CPU to 95 switch between two frequencies in 96 nanoseconds (if appropriate, else 97 specify CPUFREQ_ETERNAL) 98 99policy->cur The current operating frequency of 100 this CPU (if appropriate) 101policy->min, 102policy->max, 103policy->policy and, if necessary, 104policy->governor must contain the "default policy" for 105 this CPU. A few moments later, 106 cpufreq_driver.verify and either 107 cpufreq_driver.setpolicy or 108 cpufreq_driver.target is called with 109 these values. 110 111For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the 112frequency table helpers might be helpful. See the section 2 for more information 113on them. 114 115SMP systems normally have same clock source for a group of cpus. For these the 116.init() would be called only once for the first online cpu. Here the .init() 117routine must initialize policy->cpus with mask of all possible cpus (Online + 118Offline) that share the clock. Then the core would copy this mask onto 119policy->related_cpus and will reset policy->cpus to carry only online cpus. 120 121 1221.3 verify 123------------ 124 125When the user decides a new policy (consisting of 126"policy,governor,min,max") shall be set, this policy must be validated 127so that incompatible values can be corrected. For verifying these 128values, a frequency table helper and/or the 129cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned 130int min_freq, unsigned int max_freq) function might be helpful. See 131section 2 for details on frequency table helpers. 132 133You need to make sure that at least one valid frequency (or operating 134range) is within policy->min and policy->max. If necessary, increase 135policy->max first, and only if this is no solution, decrease policy->min. 136 137 1381.4 target or setpolicy? 139---------------------------- 140 141Most cpufreq drivers or even most cpu frequency scaling algorithms 142only allow the CPU to be set to one frequency. For these, you use the 143->target call. 144 145Some cpufreq-capable processors switch the frequency between certain 146limits on their own. These shall use the ->setpolicy call 147 148 1491.4. target 150------------- 151 152The target call has three arguments: struct cpufreq_policy *policy, 153unsigned int target_frequency, unsigned int relation. 154 155The CPUfreq driver must set the new frequency when called here. The 156actual frequency must be determined using the following rules: 157 158- keep close to "target_freq" 159- policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!) 160- if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal 161 target_freq. ("L for lowest, but no lower than") 162- if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal 163 target_freq. ("H for highest, but no higher than") 164 165Here again the frequency table helper might assist you - see section 2 166for details. 167 168 1691.5 setpolicy 170--------------- 171 172The setpolicy call only takes a struct cpufreq_policy *policy as 173argument. You need to set the lower limit of the in-processor or 174in-chipset dynamic frequency switching to policy->min, the upper limit 175to policy->max, and -if supported- select a performance-oriented 176setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a 177powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check 178the reference implementation in drivers/cpufreq/longrun.c 179 180 181 1822. Frequency Table Helpers 183========================== 184 185As most cpufreq processors only allow for being set to a few specific 186frequencies, a "frequency table" with some functions might assist in 187some work of the processor driver. Such a "frequency table" consists 188of an array of struct cpufreq_frequency_table entries, with any value in 189"index" you want to use, and the corresponding frequency in 190"frequency". At the end of the table, you need to add a 191cpufreq_frequency_table entry with frequency set to CPUFREQ_TABLE_END. And 192if you want to skip one entry in the table, set the frequency to 193CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending 194order. 195 196By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy, 197 struct cpufreq_frequency_table *table); 198the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and 199policy->min and policy->max are set to the same values. This is 200helpful for the per-CPU initialization stage. 201 202int cpufreq_frequency_table_verify(struct cpufreq_policy *policy, 203 struct cpufreq_frequency_table *table); 204assures that at least one valid frequency is within policy->min and 205policy->max, and all other criteria are met. This is helpful for the 206->verify call. 207 208int cpufreq_frequency_table_target(struct cpufreq_policy *policy, 209 struct cpufreq_frequency_table *table, 210 unsigned int target_freq, 211 unsigned int relation, 212 unsigned int *index); 213 214is the corresponding frequency table helper for the ->target 215stage. Just pass the values to this function, and the unsigned int 216index returns the number of the frequency table entry which contains 217the frequency the CPU shall be set to. PLEASE NOTE: This is not the 218"index" which is in this cpufreq_table_entry.index, but instead 219cpufreq_table[index]. So, the new frequency is 220cpufreq_table[index].frequency, and the value you stored into the 221frequency table "index" field is 222cpufreq_table[index].index. 223 224