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Documentation / cpu-freq / cpu-drivers.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 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	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.init -		A pointer to the per-CPU initialization 
54					function.
55	
56	cpufreq_driver.verify -		A pointer to a "verification" function.
57	
58	cpufreq_driver.setpolicy _or_ 
59	cpufreq_driver.target/
60	target_index		-	See below on the differences.
61	
62	And optionally
63	
64	cpufreq_driver.exit -		A pointer to a per-CPU cleanup function.
65	
66	cpufreq_driver.resume -		A pointer to a per-CPU resume function
67					which is called with interrupts disabled
68					and _before_ the pre-suspend frequency
69					and/or policy is restored by a call to
70					->target/target_index or ->setpolicy.
71	
72	cpufreq_driver.attr -		A pointer to a NULL-terminated list of
73					"struct freq_attr" which allow to
74					export values to sysfs.
75	
76	
77	1.2 Per-CPU Initialization
78	--------------------------
79	
80	Whenever a new CPU is registered with the device model, or after the
81	cpufreq driver registers itself, the per-CPU initialization function 
82	cpufreq_driver.init is called. It takes a struct cpufreq_policy
83	*policy as argument. What to do now?
84	
85	If necessary, activate the CPUfreq support on your CPU.
86	
87	Then, the driver must fill in the following values:
88	
89	policy->cpuinfo.min_freq _and_
90	policy->cpuinfo.max_freq -	the minimum and maximum frequency 
91					(in kHz) which is supported by 
92					this CPU
93	policy->cpuinfo.transition_latency   the time it takes on this CPU to
94					switch between two frequencies in
95					nanoseconds (if appropriate, else
96					specify CPUFREQ_ETERNAL)
97	
98	policy->cur			The current operating frequency of
99					this CPU (if appropriate)
100	policy->min, 
101	policy->max, 
102	policy->policy and, if necessary,
103	policy->governor		must contain the "default policy" for
104					this CPU. A few moments later,
105					cpufreq_driver.verify and either
106					cpufreq_driver.setpolicy or
107					cpufreq_driver.target/target_index is called
108					with these values.
109	
110	For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the
111	frequency table helpers might be helpful. See the section 2 for more information
112	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/target_index 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/target_index 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/target_index
149	-------------
150	
151	The target_index call has two arguments: struct cpufreq_policy *policy,
152	and unsigned int index (into the exposed frequency table).
153	
154	The CPUfreq driver must set the new frequency when called here. The
155	actual frequency must be determined by freq_table[index].frequency.
156	
157	Deprecated:
158	----------
159	The target call has three arguments: struct cpufreq_policy *policy,
160	unsigned int target_frequency, unsigned int relation.
161	
162	The CPUfreq driver must set the new frequency when called here. The
163	actual frequency must be determined using the following rules:
164	
165	- keep close to "target_freq"
166	- policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)
167	- if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal
168	  target_freq. ("L for lowest, but no lower than")
169	- if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal
170	  target_freq. ("H for highest, but no higher than")
171	
172	Here again the frequency table helper might assist you - see section 2
173	for details.
174	
175	
176	1.5 setpolicy
177	---------------
178	
179	The setpolicy call only takes a struct cpufreq_policy *policy as
180	argument. You need to set the lower limit of the in-processor or
181	in-chipset dynamic frequency switching to policy->min, the upper limit
182	to policy->max, and -if supported- select a performance-oriented
183	setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a
184	powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check
185	the reference implementation in drivers/cpufreq/longrun.c
186	
187	
188	
189	2. Frequency Table Helpers
190	==========================
191	
192	As most cpufreq processors only allow for being set to a few specific
193	frequencies, a "frequency table" with some functions might assist in
194	some work of the processor driver. Such a "frequency table" consists
195	of an array of struct cpufreq_frequency_table entries, with any value in
196	"driver_data" you want to use, and the corresponding frequency in
197	"frequency". At the end of the table, you need to add a
198	cpufreq_frequency_table entry with frequency set to CPUFREQ_TABLE_END. And
199	if you want to skip one entry in the table, set the frequency to 
200	CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending
201	order.
202	
203	By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,
204						struct cpufreq_frequency_table *table);
205	the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and
206	policy->min and policy->max are set to the same values. This is
207	helpful for the per-CPU initialization stage.
208	
209	int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,
210	                                   struct cpufreq_frequency_table *table);
211	assures that at least one valid frequency is within policy->min and
212	policy->max, and all other criteria are met. This is helpful for the
213	->verify call.
214	
215	int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
216	                                   struct cpufreq_frequency_table *table,
217	                                   unsigned int target_freq,
218	                                   unsigned int relation,
219	                                   unsigned int *index);
220	
221	is the corresponding frequency table helper for the ->target
222	stage. Just pass the values to this function, and the unsigned int
223	index returns the number of the frequency table entry which contains
224	the frequency the CPU shall be set to.
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