About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / cpu-freq / cpu-drivers.txt




Custom Search

Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 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
65					function called during CPU_POST_DEAD
66					phase of cpu hotplug process.
67	
68	cpufreq_driver.stop_cpu -	A pointer to a per-CPU stop function
69					called during CPU_DOWN_PREPARE phase of
70					cpu hotplug process.
71	
72	cpufreq_driver.resume -		A pointer to a per-CPU resume function
73					which is called with interrupts disabled
74					and _before_ the pre-suspend frequency
75					and/or policy is restored by a call to
76					->target/target_index or ->setpolicy.
77	
78	cpufreq_driver.attr -		A pointer to a NULL-terminated list of
79					"struct freq_attr" which allow to
80					export values to sysfs.
81	
82	
83	1.2 Per-CPU Initialization
84	--------------------------
85	
86	Whenever a new CPU is registered with the device model, or after the
87	cpufreq driver registers itself, the per-CPU initialization function 
88	cpufreq_driver.init is called. It takes a struct cpufreq_policy
89	*policy as argument. What to do now?
90	
91	If necessary, activate the CPUfreq support on your CPU.
92	
93	Then, the driver must fill in the following values:
94	
95	policy->cpuinfo.min_freq _and_
96	policy->cpuinfo.max_freq -	the minimum and maximum frequency 
97					(in kHz) which is supported by 
98					this CPU
99	policy->cpuinfo.transition_latency   the time it takes on this CPU to
100					switch between two frequencies in
101					nanoseconds (if appropriate, else
102					specify CPUFREQ_ETERNAL)
103	
104	policy->cur			The current operating frequency of
105					this CPU (if appropriate)
106	policy->min, 
107	policy->max, 
108	policy->policy and, if necessary,
109	policy->governor		must contain the "default policy" for
110					this CPU. A few moments later,
111					cpufreq_driver.verify and either
112					cpufreq_driver.setpolicy or
113					cpufreq_driver.target/target_index is called
114					with these values.
115	
116	For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the
117	frequency table helpers might be helpful. See the section 2 for more information
118	on them.
119	
120	SMP systems normally have same clock source for a group of cpus. For these the
121	.init() would be called only once for the first online cpu. Here the .init()
122	routine must initialize policy->cpus with mask of all possible cpus (Online +
123	Offline) that share the clock. Then the core would copy this mask onto
124	policy->related_cpus and will reset policy->cpus to carry only online cpus.
125	
126	
127	1.3 verify
128	------------
129	
130	When the user decides a new policy (consisting of
131	"policy,governor,min,max") shall be set, this policy must be validated
132	so that incompatible values can be corrected. For verifying these
133	values, a frequency table helper and/or the
134	cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned
135	int min_freq, unsigned int max_freq) function might be helpful. See
136	section 2 for details on frequency table helpers.
137	
138	You need to make sure that at least one valid frequency (or operating
139	range) is within policy->min and policy->max. If necessary, increase
140	policy->max first, and only if this is no solution, decrease policy->min.
141	
142	
143	1.4 target/target_index or setpolicy?
144	----------------------------
145	
146	Most cpufreq drivers or even most cpu frequency scaling algorithms 
147	only allow the CPU to be set to one frequency. For these, you use the
148	->target/target_index call.
149	
150	Some cpufreq-capable processors switch the frequency between certain
151	limits on their own. These shall use the ->setpolicy call
152	
153	
154	1.4. target/target_index
155	-------------
156	
157	The target_index call has two arguments: struct cpufreq_policy *policy,
158	and unsigned int index (into the exposed frequency table).
159	
160	The CPUfreq driver must set the new frequency when called here. The
161	actual frequency must be determined by freq_table[index].frequency.
162	
163	Deprecated:
164	----------
165	The target call has three arguments: struct cpufreq_policy *policy,
166	unsigned int target_frequency, unsigned int relation.
167	
168	The CPUfreq driver must set the new frequency when called here. The
169	actual frequency must be determined using the following rules:
170	
171	- keep close to "target_freq"
172	- policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)
173	- if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal
174	  target_freq. ("L for lowest, but no lower than")
175	- if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal
176	  target_freq. ("H for highest, but no higher than")
177	
178	Here again the frequency table helper might assist you - see section 2
179	for details.
180	
181	
182	1.5 setpolicy
183	---------------
184	
185	The setpolicy call only takes a struct cpufreq_policy *policy as
186	argument. You need to set the lower limit of the in-processor or
187	in-chipset dynamic frequency switching to policy->min, the upper limit
188	to policy->max, and -if supported- select a performance-oriented
189	setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a
190	powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check
191	the reference implementation in drivers/cpufreq/longrun.c
192	
193	
194	
195	2. Frequency Table Helpers
196	==========================
197	
198	As most cpufreq processors only allow for being set to a few specific
199	frequencies, a "frequency table" with some functions might assist in
200	some work of the processor driver. Such a "frequency table" consists
201	of an array of struct cpufreq_frequency_table entries, with any value in
202	"driver_data" you want to use, and the corresponding frequency in
203	"frequency". At the end of the table, you need to add a
204	cpufreq_frequency_table entry with frequency set to CPUFREQ_TABLE_END. And
205	if you want to skip one entry in the table, set the frequency to 
206	CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending
207	order.
208	
209	By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,
210						struct cpufreq_frequency_table *table);
211	the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and
212	policy->min and policy->max are set to the same values. This is
213	helpful for the per-CPU initialization stage.
214	
215	int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,
216	                                   struct cpufreq_frequency_table *table);
217	assures that at least one valid frequency is within policy->min and
218	policy->max, and all other criteria are met. This is helpful for the
219	->verify call.
220	
221	int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
222	                                   struct cpufreq_frequency_table *table,
223	                                   unsigned int target_freq,
224	                                   unsigned int relation,
225	                                   unsigned int *index);
226	
227	is the corresponding frequency table helper for the ->target
228	stage. Just pass the values to this function, and the unsigned int
229	index returns the number of the frequency table entry which contains
230	the frequency the CPU shall be set to.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.