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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 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/target_index or setpolicy?
27	1.5  target/target_index
28	1.6  setpolicy
29	1.7  get_intermediate and target_intermediate
30	2.   Frequency Table Helpers
31	
32	
33	
34	1. What To Do?
35	==============
36	
37	So, you just got a brand-new CPU / chipset with datasheets and want to
38	add cpufreq support for this CPU / chipset? Great. Here are some hints
39	on what is necessary:
40	
41	
42	1.1 Initialization
43	------------------
44	
45	First of all, in an __initcall level 7 (module_init()) or later
46	function check whether this kernel runs on the right CPU and the right
47	chipset. If so, register a struct cpufreq_driver with the CPUfreq core
48	using cpufreq_register_driver()
49	
50	What shall this struct cpufreq_driver contain? 
51	
52	cpufreq_driver.name -		The name of this driver.
53	
54	cpufreq_driver.init -		A pointer to the per-CPU initialization 
55					function.
56	
57	cpufreq_driver.verify -		A pointer to a "verification" function.
58	
59	cpufreq_driver.setpolicy _or_ 
60	cpufreq_driver.target/
61	target_index		-	See below on the differences.
62	
63	And optionally
64	
65	cpufreq_driver.exit -		A pointer to a per-CPU cleanup
66					function called during CPU_POST_DEAD
67					phase of cpu hotplug process.
68	
69	cpufreq_driver.stop_cpu -	A pointer to a per-CPU stop function
70					called during CPU_DOWN_PREPARE phase of
71					cpu hotplug process.
72	
73	cpufreq_driver.resume -		A pointer to a per-CPU resume function
74					which is called with interrupts disabled
75					and _before_ the pre-suspend frequency
76					and/or policy is restored by a call to
77					->target/target_index or ->setpolicy.
78	
79	cpufreq_driver.attr -		A pointer to a NULL-terminated list of
80					"struct freq_attr" which allow to
81					export values to sysfs.
82	
83	cpufreq_driver.get_intermediate
84	and target_intermediate		Used to switch to stable frequency while
85					changing CPU frequency.
86	
87	
88	1.2 Per-CPU Initialization
89	--------------------------
90	
91	Whenever a new CPU is registered with the device model, or after the
92	cpufreq driver registers itself, the per-CPU initialization function 
93	cpufreq_driver.init is called. It takes a struct cpufreq_policy
94	*policy as argument. What to do now?
95	
96	If necessary, activate the CPUfreq support on your CPU.
97	
98	Then, the driver must fill in the following values:
99	
100	policy->cpuinfo.min_freq _and_
101	policy->cpuinfo.max_freq -	the minimum and maximum frequency 
102					(in kHz) which is supported by 
103					this CPU
104	policy->cpuinfo.transition_latency   the time it takes on this CPU to
105					switch between two frequencies in
106					nanoseconds (if appropriate, else
107					specify CPUFREQ_ETERNAL)
108	
109	policy->cur			The current operating frequency of
110					this CPU (if appropriate)
111	policy->min, 
112	policy->max, 
113	policy->policy and, if necessary,
114	policy->governor		must contain the "default policy" for
115					this CPU. A few moments later,
116					cpufreq_driver.verify and either
117					cpufreq_driver.setpolicy or
118					cpufreq_driver.target/target_index is called
119					with these values.
120	
121	For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the
122	frequency table helpers might be helpful. See the section 2 for more information
123	on them.
124	
125	SMP systems normally have same clock source for a group of cpus. For these the
126	.init() would be called only once for the first online cpu. Here the .init()
127	routine must initialize policy->cpus with mask of all possible cpus (Online +
128	Offline) that share the clock. Then the core would copy this mask onto
129	policy->related_cpus and will reset policy->cpus to carry only online cpus.
130	
131	
132	1.3 verify
133	------------
134	
135	When the user decides a new policy (consisting of
136	"policy,governor,min,max") shall be set, this policy must be validated
137	so that incompatible values can be corrected. For verifying these
138	values, a frequency table helper and/or the
139	cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned
140	int min_freq, unsigned int max_freq) function might be helpful. See
141	section 2 for details on frequency table helpers.
142	
143	You need to make sure that at least one valid frequency (or operating
144	range) is within policy->min and policy->max. If necessary, increase
145	policy->max first, and only if this is no solution, decrease policy->min.
146	
147	
148	1.4 target/target_index or setpolicy?
149	----------------------------
150	
151	Most cpufreq drivers or even most cpu frequency scaling algorithms 
152	only allow the CPU to be set to one frequency. For these, you use the
153	->target/target_index call.
154	
155	Some cpufreq-capable processors switch the frequency between certain
156	limits on their own. These shall use the ->setpolicy call
157	
158	
159	1.5. target/target_index
160	-------------
161	
162	The target_index call has two arguments: struct cpufreq_policy *policy,
163	and unsigned int index (into the exposed frequency table).
164	
165	The CPUfreq driver must set the new frequency when called here. The
166	actual frequency must be determined by freq_table[index].frequency.
167	
168	It should always restore to earlier frequency (i.e. policy->restore_freq) in
169	case of errors, even if we switched to intermediate frequency earlier.
170	
171	Deprecated:
172	----------
173	The target call has three arguments: struct cpufreq_policy *policy,
174	unsigned int target_frequency, unsigned int relation.
175	
176	The CPUfreq driver must set the new frequency when called here. The
177	actual frequency must be determined using the following rules:
178	
179	- keep close to "target_freq"
180	- policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)
181	- if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal
182	  target_freq. ("L for lowest, but no lower than")
183	- if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal
184	  target_freq. ("H for highest, but no higher than")
185	
186	Here again the frequency table helper might assist you - see section 2
187	for details.
188	
189	
190	1.6 setpolicy
191	---------------
192	
193	The setpolicy call only takes a struct cpufreq_policy *policy as
194	argument. You need to set the lower limit of the in-processor or
195	in-chipset dynamic frequency switching to policy->min, the upper limit
196	to policy->max, and -if supported- select a performance-oriented
197	setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a
198	powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check
199	the reference implementation in drivers/cpufreq/longrun.c
200	
201	1.7 get_intermediate and target_intermediate
202	--------------------------------------------
203	
204	Only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION unset.
205	
206	get_intermediate should return a stable intermediate frequency platform wants to
207	switch to, and target_intermediate() should set CPU to to that frequency, before
208	jumping to the frequency corresponding to 'index'. Core will take care of
209	sending notifications and driver doesn't have to handle them in
210	target_intermediate() or target_index().
211	
212	Drivers can return '0' from get_intermediate() in case they don't wish to switch
213	to intermediate frequency for some target frequency. In that case core will
214	directly call ->target_index().
215	
216	NOTE: ->target_index() should restore to policy->restore_freq in case of
217	failures as core would send notifications for that.
218	
219	
220	2. Frequency Table Helpers
221	==========================
222	
223	As most cpufreq processors only allow for being set to a few specific
224	frequencies, a "frequency table" with some functions might assist in
225	some work of the processor driver. Such a "frequency table" consists
226	of an array of struct cpufreq_frequency_table entries, with any value in
227	"driver_data" you want to use, and the corresponding frequency in
228	"frequency". At the end of the table, you need to add a
229	cpufreq_frequency_table entry with frequency set to CPUFREQ_TABLE_END. And
230	if you want to skip one entry in the table, set the frequency to 
231	CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending
232	order.
233	
234	By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,
235						struct cpufreq_frequency_table *table);
236	the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and
237	policy->min and policy->max are set to the same values. This is
238	helpful for the per-CPU initialization stage.
239	
240	int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,
241	                                   struct cpufreq_frequency_table *table);
242	assures that at least one valid frequency is within policy->min and
243	policy->max, and all other criteria are met. This is helpful for the
244	->verify call.
245	
246	int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
247	                                   struct cpufreq_frequency_table *table,
248	                                   unsigned int target_freq,
249	                                   unsigned int relation,
250	                                   unsigned int *index);
251	
252	is the corresponding frequency table helper for the ->target
253	stage. Just pass the values to this function, and the unsigned int
254	index returns the number of the frequency table entry which contains
255	the frequency the CPU shall be set to.
256	
257	The following macros can be used as iterators over cpufreq_frequency_table:
258	
259	cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency
260	table.
261	
262	cpufreq-for_each_valid_entry(pos, table) - iterates over all entries,
263	excluding CPUFREQ_ENTRY_INVALID frequencies.
264	Use arguments "pos" - a cpufreq_frequency_table * as a loop cursor and
265	"table" - the cpufreq_frequency_table * you want to iterate over.
266	
267	For example:
268	
269		struct cpufreq_frequency_table *pos, *driver_freq_table;
270	
271		cpufreq_for_each_entry(pos, driver_freq_table) {
272			/* Do something with pos */
273			pos->frequency = ...
274		}
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