view Documentation/cpu-freq/cpu-drivers.txt @ 524:7f8b544237bf

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Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
line source
1 CPU frequency and voltage scaling code in the Linux(TM) kernel
4 L i n u x C P U F r e q
6 C P U D r i v e r s
8 - information for developers -
11 Dominik Brodowski <linux@brodo.de>
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.
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
33 1. What To Do?
34 ==============
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:
41 1.1 Initialization
42 ------------------
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()
49 What shall this struct cpufreq_driver contain?
51 cpufreq_driver.name - The name of this driver.
53 cpufreq_driver.owner - THIS_MODULE;
55 cpufreq_driver.init - A pointer to the per-CPU initialization
56 function.
58 cpufreq_driver.verify - A pointer to a "verification" function.
60 cpufreq_driver.setpolicy _or_
61 cpufreq_driver.target - See below on the differences.
63 And optionally
65 cpufreq_driver.exit - A pointer to a per-CPU cleanup function.
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.
73 cpufreq_driver.attr - A pointer to a NULL-terminated list of
74 "struct freq_attr" which allow to
75 export values to sysfs.
78 1.2 Per-CPU Initialization
79 --------------------------
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?
86 If necessary, activate the CPUfreq support on your CPU.
88 Then, the driver must fill in the following values:
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 (if
96 appropriate, else specify
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.
111 For setting some of these values, the frequency table helpers might be
112 helpful. See the section 2 for more information on them.
115 1.3 verify
116 ------------
118 When the user decides a new policy (consisting of
119 "policy,governor,min,max") shall be set, this policy must be validated
120 so that incompatible values can be corrected. For verifying these
121 values, a frequency table helper and/or the
122 cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned
123 int min_freq, unsigned int max_freq) function might be helpful. See
124 section 2 for details on frequency table helpers.
126 You need to make sure that at least one valid frequency (or operating
127 range) is within policy->min and policy->max. If necessary, increase
128 policy->max first, and only if this is no solution, decrease policy->min.
131 1.4 target or setpolicy?
132 ----------------------------
134 Most cpufreq drivers or even most cpu frequency scaling algorithms
135 only allow the CPU to be set to one frequency. For these, you use the
136 ->target call.
138 Some cpufreq-capable processors switch the frequency between certain
139 limits on their own. These shall use the ->setpolicy call
142 1.4. target
143 -------------
145 The target call has three arguments: struct cpufreq_policy *policy,
146 unsigned int target_frequency, unsigned int relation.
148 The CPUfreq driver must set the new frequency when called here. The
149 actual frequency must be determined using the following rules:
151 - keep close to "target_freq"
152 - policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)
153 - if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal
154 target_freq. ("L for lowest, but no lower than")
155 - if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal
156 target_freq. ("H for highest, but no higher than")
158 Here again the frequency table helper might assist you - see section 3
159 for details.
162 1.5 setpolicy
163 ---------------
165 The setpolicy call only takes a struct cpufreq_policy *policy as
166 argument. You need to set the lower limit of the in-processor or
167 in-chipset dynamic frequency switching to policy->min, the upper limit
168 to policy->max, and -if supported- select a performance-oriented
169 setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a
170 powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check
171 the reference implementation in arch/i386/kernel/cpu/cpufreq/longrun.c
175 2. Frequency Table Helpers
176 ==========================
178 As most cpufreq processors only allow for being set to a few specific
179 frequencies, a "frequency table" with some functions might assist in
180 some work of the processor driver. Such a "frequency table" consists
181 of an array of struct cpufreq_freq_table entries, with any value in
182 "index" you want to use, and the corresponding frequency in
183 "frequency". At the end of the table, you need to add a
184 cpufreq_freq_table entry with frequency set to CPUFREQ_TABLE_END. And
185 if you want to skip one entry in the table, set the frequency to
186 CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending
187 order.
189 By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,
190 struct cpufreq_frequency_table *table);
191 the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and
192 policy->min and policy->max are set to the same values. This is
193 helpful for the per-CPU initialization stage.
195 int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,
196 struct cpufreq_frequency_table *table);
197 assures that at least one valid frequency is within policy->min and
198 policy->max, and all other criteria are met. This is helpful for the
199 ->verify call.
201 int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
202 struct cpufreq_frequency_table *table,
203 unsigned int target_freq,
204 unsigned int relation,
205 unsigned int *index);
207 is the corresponding frequency table helper for the ->target
208 stage. Just pass the values to this function, and the unsigned int
209 index returns the number of the frequency table entry which contains
210 the frequency the CPU shall be set to. PLEASE NOTE: This is not the
211 "index" which is in this cpufreq_table_entry.index, but instead
212 cpufreq_table[index]. So, the new frequency is
213 cpufreq_table[index].frequency, and the value you stored into the
214 frequency table "index" field is
215 cpufreq_table[index].index.