Documentation / cpu-freq / cpu-drivers.txt
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Based on kernel version 3.9. Page generated on 2013-05-02 22:56 EST.
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 20 Contents: 21 --------- 22 1. What To Do? 23 1.1 Initialization 24 1.2 Per-CPU Initialization 25 1.3 verify 26 1.4 target or setpolicy? 27 1.5 target 28 1.6 setpolicy 29 2. Frequency Table Helpers 30 31 32 33 1. What To Do? 34 ============== 35 36 So, you just got a brand-new CPU / chipset with datasheets and want to 37 add cpufreq support for this CPU / chipset? Great. Here are some hints 38 on what is necessary: 39 40 41 1.1 Initialization 42 ------------------ 43 44 First of all, in an __initcall level 7 (module_init()) or later 45 function check whether this kernel runs on the right CPU and the right 46 chipset. If so, register a struct cpufreq_driver with the CPUfreq core 47 using cpufreq_register_driver() 48 49 What shall this struct cpufreq_driver contain? 50 51 cpufreq_driver.name - The name of this driver. 52 53 cpufreq_driver.owner - THIS_MODULE; 54 55 cpufreq_driver.init - A pointer to the per-CPU initialization 56 function. 57 58 cpufreq_driver.verify - A pointer to a "verification" function. 59 60 cpufreq_driver.setpolicy _or_ 61 cpufreq_driver.target - See below on the differences. 62 63 And optionally 64 65 cpufreq_driver.exit - A pointer to a per-CPU cleanup function. 66 67 cpufreq_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 73 cpufreq_driver.attr - A pointer to a NULL-terminated list of 74 "struct freq_attr" which allow to 75 export values to sysfs. 76 77 78 1.2 Per-CPU Initialization 79 -------------------------- 80 81 Whenever a new CPU is registered with the device model, or after the 82 cpufreq driver registers itself, the per-CPU initialization function 83 cpufreq_driver.init is called. It takes a struct cpufreq_policy 84 *policy as argument. What to do now? 85 86 If necessary, activate the CPUfreq support on your CPU. 87 88 Then, the driver must fill in the following values: 89 90 policy->cpuinfo.min_freq _and_ 91 policy->cpuinfo.max_freq - the minimum and maximum frequency 92 (in kHz) which is supported by 93 this CPU 94 policy->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 99 policy->cur The current operating frequency of 100 this CPU (if appropriate) 101 policy->min, 102 policy->max, 103 policy->policy and, if necessary, 104 policy->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. 110 111 For setting some of these values, the frequency table helpers might be 112 helpful. See the section 2 for more information on them. 113 114 SMP systems normally have same clock source for a group of cpus. For these the 115 .init() would be called only once for the first online cpu. Here the .init() 116 routine must initialize policy->cpus with mask of all possible cpus (Online + 117 Offline) that share the clock. Then the core would copy this mask onto 118 policy->related_cpus and will reset policy->cpus to carry only online cpus. 119 120 121 1.3 verify 122 ------------ 123 124 When the user decides a new policy (consisting of 125 "policy,governor,min,max") shall be set, this policy must be validated 126 so that incompatible values can be corrected. For verifying these 127 values, a frequency table helper and/or the 128 cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned 129 int min_freq, unsigned int max_freq) function might be helpful. See 130 section 2 for details on frequency table helpers. 131 132 You need to make sure that at least one valid frequency (or operating 133 range) is within policy->min and policy->max. If necessary, increase 134 policy->max first, and only if this is no solution, decrease policy->min. 135 136 137 1.4 target or setpolicy? 138 ---------------------------- 139 140 Most cpufreq drivers or even most cpu frequency scaling algorithms 141 only allow the CPU to be set to one frequency. For these, you use the 142 ->target call. 143 144 Some cpufreq-capable processors switch the frequency between certain 145 limits on their own. These shall use the ->setpolicy call 146 147 148 1.4. target 149 ------------- 150 151 The target call has three arguments: struct cpufreq_policy *policy, 152 unsigned int target_frequency, unsigned int relation. 153 154 The CPUfreq driver must set the new frequency when called here. The 155 actual frequency must be determined using the following rules: 156 157 - keep close to "target_freq" 158 - policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!) 159 - if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal 160 target_freq. ("L for lowest, but no lower than") 161 - if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal 162 target_freq. ("H for highest, but no higher than") 163 164 Here again the frequency table helper might assist you - see section 2 165 for details. 166 167 168 1.5 setpolicy 169 --------------- 170 171 The setpolicy call only takes a struct cpufreq_policy *policy as 172 argument. You need to set the lower limit of the in-processor or 173 in-chipset dynamic frequency switching to policy->min, the upper limit 174 to policy->max, and -if supported- select a performance-oriented 175 setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a 176 powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check 177 the reference implementation in drivers/cpufreq/longrun.c 178 179 180 181 2. Frequency Table Helpers 182 ========================== 183 184 As most cpufreq processors only allow for being set to a few specific 185 frequencies, a "frequency table" with some functions might assist in 186 some work of the processor driver. Such a "frequency table" consists 187 of an array of struct cpufreq_freq_table entries, with any value in 188 "index" you want to use, and the corresponding frequency in 189 "frequency". At the end of the table, you need to add a 190 cpufreq_freq_table entry with frequency set to CPUFREQ_TABLE_END. And 191 if you want to skip one entry in the table, set the frequency to 192 CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending 193 order. 194 195 By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy, 196 struct cpufreq_frequency_table *table); 197 the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and 198 policy->min and policy->max are set to the same values. This is 199 helpful for the per-CPU initialization stage. 200 201 int cpufreq_frequency_table_verify(struct cpufreq_policy *policy, 202 struct cpufreq_frequency_table *table); 203 assures that at least one valid frequency is within policy->min and 204 policy->max, and all other criteria are met. This is helpful for the 205 ->verify call. 206 207 int cpufreq_frequency_table_target(struct cpufreq_policy *policy, 208 struct cpufreq_frequency_table *table, 209 unsigned int target_freq, 210 unsigned int relation, 211 unsigned int *index); 212 213 is the corresponding frequency table helper for the ->target 214 stage. Just pass the values to this function, and the unsigned int 215 index returns the number of the frequency table entry which contains 216 the frequency the CPU shall be set to. PLEASE NOTE: This is not the 217 "index" which is in this cpufreq_table_entry.index, but instead 218 cpufreq_table[index]. So, the new frequency is 219 cpufreq_table[index].frequency, and the value you stored into the 220 frequency table "index" field is 221 cpufreq_table[index].index.
- [ cpu-freq ]
- amd-powernow.txt
- boost.txt
- core.txt
- cpu-drivers.txt
- cpufreq-nforce2.txt
- cpufreq-stats.txt
- governors.txt
- index.txt
- pcc-cpufreq.txt
- user-guide.txt
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