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Based on kernel version 3.19. Page generated on 2015-02-13 21:22 EST.

1	Operating Performance Points (OPP) Library
2	==========================================
3	
4	(C) 2009-2010 Nishanth Menon <nm@ti.com>, Texas Instruments Incorporated
5	
6	Contents
7	--------
8	1. Introduction
9	2. Initial OPP List Registration
10	3. OPP Search Functions
11	4. OPP Availability Control Functions
12	5. OPP Data Retrieval Functions
13	6. Data Structures
14	
15	1. Introduction
16	===============
17	1.1 What is an Operating Performance Point (OPP)?
18	
19	Complex SoCs of today consists of a multiple sub-modules working in conjunction.
20	In an operational system executing varied use cases, not all modules in the SoC
21	need to function at their highest performing frequency all the time. To
22	facilitate this, sub-modules in a SoC are grouped into domains, allowing some
23	domains to run at lower voltage and frequency while other domains run at
24	voltage/frequency pairs that are higher.
25	
26	The set of discrete tuples consisting of frequency and voltage pairs that
27	the device will support per domain are called Operating Performance Points or
28	OPPs.
29	
30	As an example:
31	Let us consider an MPU device which supports the following:
32	{300MHz at minimum voltage of 1V}, {800MHz at minimum voltage of 1.2V},
33	{1GHz at minimum voltage of 1.3V}
34	
35	We can represent these as three OPPs as the following {Hz, uV} tuples:
36	{300000000, 1000000}
37	{800000000, 1200000}
38	{1000000000, 1300000}
39	
40	1.2 Operating Performance Points Library
41	
42	OPP library provides a set of helper functions to organize and query the OPP
43	information. The library is located in drivers/base/power/opp.c and the header
44	is located in include/linux/pm_opp.h. OPP library can be enabled by enabling
45	CONFIG_PM_OPP from power management menuconfig menu. OPP library depends on
46	CONFIG_PM as certain SoCs such as Texas Instrument's OMAP framework allows to
47	optionally boot at a certain OPP without needing cpufreq.
48	
49	Typical usage of the OPP library is as follows:
50	(users)		-> registers a set of default OPPs		-> (library)
51	SoC framework	-> modifies on required cases certain OPPs	-> OPP layer
52			-> queries to search/retrieve information	->
53	
54	OPP layer expects each domain to be represented by a unique device pointer. SoC
55	framework registers a set of initial OPPs per device with the OPP layer. This
56	list is expected to be an optimally small number typically around 5 per device.
57	This initial list contains a set of OPPs that the framework expects to be safely
58	enabled by default in the system.
59	
60	Note on OPP Availability:
61	------------------------
62	As the system proceeds to operate, SoC framework may choose to make certain
63	OPPs available or not available on each device based on various external
64	factors. Example usage: Thermal management or other exceptional situations where
65	SoC framework might choose to disable a higher frequency OPP to safely continue
66	operations until that OPP could be re-enabled if possible.
67	
68	OPP library facilitates this concept in it's implementation. The following
69	operational functions operate only on available opps:
70	opp_find_freq_{ceil, floor}, dev_pm_opp_get_voltage, dev_pm_opp_get_freq, dev_pm_opp_get_opp_count
71	
72	dev_pm_opp_find_freq_exact is meant to be used to find the opp pointer which can then
73	be used for dev_pm_opp_enable/disable functions to make an opp available as required.
74	
75	WARNING: Users of OPP library should refresh their availability count using
76	get_opp_count if dev_pm_opp_enable/disable functions are invoked for a device, the
77	exact mechanism to trigger these or the notification mechanism to other
78	dependent subsystems such as cpufreq are left to the discretion of the SoC
79	specific framework which uses the OPP library. Similar care needs to be taken
80	care to refresh the cpufreq table in cases of these operations.
81	
82	WARNING on OPP List locking mechanism:
83	-------------------------------------------------
84	OPP library uses RCU for exclusivity. RCU allows the query functions to operate
85	in multiple contexts and this synchronization mechanism is optimal for a read
86	intensive operations on data structure as the OPP library caters to.
87	
88	To ensure that the data retrieved are sane, the users such as SoC framework
89	should ensure that the section of code operating on OPP queries are locked
90	using RCU read locks. The opp_find_freq_{exact,ceil,floor},
91	opp_get_{voltage, freq, opp_count} fall into this category.
92	
93	opp_{add,enable,disable} are updaters which use mutex and implement it's own
94	RCU locking mechanisms. These functions should *NOT* be called under RCU locks
95	and other contexts that prevent blocking functions in RCU or mutex operations
96	from working.
97	
98	2. Initial OPP List Registration
99	================================
100	The SoC implementation calls dev_pm_opp_add function iteratively to add OPPs per
101	device. It is expected that the SoC framework will register the OPP entries
102	optimally- typical numbers range to be less than 5. The list generated by
103	registering the OPPs is maintained by OPP library throughout the device
104	operation. The SoC framework can subsequently control the availability of the
105	OPPs dynamically using the dev_pm_opp_enable / disable functions.
106	
107	dev_pm_opp_add - Add a new OPP for a specific domain represented by the device pointer.
108		The OPP is defined using the frequency and voltage. Once added, the OPP
109		is assumed to be available and control of it's availability can be done
110		with the dev_pm_opp_enable/disable functions. OPP library internally stores
111		and manages this information in the opp struct. This function may be
112		used by SoC framework to define a optimal list as per the demands of
113		SoC usage environment.
114	
115		WARNING: Do not use this function in interrupt context.
116	
117		Example:
118		 soc_pm_init()
119		 {
120			/* Do things */
121			r = dev_pm_opp_add(mpu_dev, 1000000, 900000);
122			if (!r) {
123				pr_err("%s: unable to register mpu opp(%d)\n", r);
124				goto no_cpufreq;
125			}
126			/* Do cpufreq things */
127		 no_cpufreq:
128			/* Do remaining things */
129		 }
130	
131	3. OPP Search Functions
132	=======================
133	High level framework such as cpufreq operates on frequencies. To map the
134	frequency back to the corresponding OPP, OPP library provides handy functions
135	to search the OPP list that OPP library internally manages. These search
136	functions return the matching pointer representing the opp if a match is
137	found, else returns error. These errors are expected to be handled by standard
138	error checks such as IS_ERR() and appropriate actions taken by the caller.
139	
140	dev_pm_opp_find_freq_exact - Search for an OPP based on an *exact* frequency and
141		availability. This function is especially useful to enable an OPP which
142		is not available by default.
143		Example: In a case when SoC framework detects a situation where a
144		higher frequency could be made available, it can use this function to
145		find the OPP prior to call the dev_pm_opp_enable to actually make it available.
146		 rcu_read_lock();
147		 opp = dev_pm_opp_find_freq_exact(dev, 1000000000, false);
148		 rcu_read_unlock();
149		 /* dont operate on the pointer.. just do a sanity check.. */
150		 if (IS_ERR(opp)) {
151			pr_err("frequency not disabled!\n");
152			/* trigger appropriate actions.. */
153		 } else {
154			dev_pm_opp_enable(dev,1000000000);
155		 }
156	
157		NOTE: This is the only search function that operates on OPPs which are
158		not available.
159	
160	dev_pm_opp_find_freq_floor - Search for an available OPP which is *at most* the
161		provided frequency. This function is useful while searching for a lesser
162		match OR operating on OPP information in the order of decreasing
163		frequency.
164		Example: To find the highest opp for a device:
165		 freq = ULONG_MAX;
166		 rcu_read_lock();
167		 dev_pm_opp_find_freq_floor(dev, &freq);
168		 rcu_read_unlock();
169	
170	dev_pm_opp_find_freq_ceil - Search for an available OPP which is *at least* the
171		provided frequency. This function is useful while searching for a
172		higher match OR operating on OPP information in the order of increasing
173		frequency.
174		Example 1: To find the lowest opp for a device:
175		 freq = 0;
176		 rcu_read_lock();
177		 dev_pm_opp_find_freq_ceil(dev, &freq);
178		 rcu_read_unlock();
179		Example 2: A simplified implementation of a SoC cpufreq_driver->target:
180		 soc_cpufreq_target(..)
181		 {
182			/* Do stuff like policy checks etc. */
183			/* Find the best frequency match for the req */
184			rcu_read_lock();
185			opp = dev_pm_opp_find_freq_ceil(dev, &freq);
186			rcu_read_unlock();
187			if (!IS_ERR(opp))
188				soc_switch_to_freq_voltage(freq);
189			else
190				/* do something when we can't satisfy the req */
191			/* do other stuff */
192		 }
193	
194	4. OPP Availability Control Functions
195	=====================================
196	A default OPP list registered with the OPP library may not cater to all possible
197	situation. The OPP library provides a set of functions to modify the
198	availability of a OPP within the OPP list. This allows SoC frameworks to have
199	fine grained dynamic control of which sets of OPPs are operationally available.
200	These functions are intended to *temporarily* remove an OPP in conditions such
201	as thermal considerations (e.g. don't use OPPx until the temperature drops).
202	
203	WARNING: Do not use these functions in interrupt context.
204	
205	dev_pm_opp_enable - Make a OPP available for operation.
206		Example: Lets say that 1GHz OPP is to be made available only if the
207		SoC temperature is lower than a certain threshold. The SoC framework
208		implementation might choose to do something as follows:
209		 if (cur_temp < temp_low_thresh) {
210			/* Enable 1GHz if it was disabled */
211			rcu_read_lock();
212			opp = dev_pm_opp_find_freq_exact(dev, 1000000000, false);
213			rcu_read_unlock();
214			/* just error check */
215			if (!IS_ERR(opp))
216				ret = dev_pm_opp_enable(dev, 1000000000);
217			else
218				goto try_something_else;
219		 }
220	
221	dev_pm_opp_disable - Make an OPP to be not available for operation
222		Example: Lets say that 1GHz OPP is to be disabled if the temperature
223		exceeds a threshold value. The SoC framework implementation might
224		choose to do something as follows:
225		 if (cur_temp > temp_high_thresh) {
226			/* Disable 1GHz if it was enabled */
227			rcu_read_lock();
228			opp = dev_pm_opp_find_freq_exact(dev, 1000000000, true);
229			rcu_read_unlock();
230			/* just error check */
231			if (!IS_ERR(opp))
232				ret = dev_pm_opp_disable(dev, 1000000000);
233			else
234				goto try_something_else;
235		 }
236	
237	5. OPP Data Retrieval Functions
238	===============================
239	Since OPP library abstracts away the OPP information, a set of functions to pull
240	information from the OPP structure is necessary. Once an OPP pointer is
241	retrieved using the search functions, the following functions can be used by SoC
242	framework to retrieve the information represented inside the OPP layer.
243	
244	dev_pm_opp_get_voltage - Retrieve the voltage represented by the opp pointer.
245		Example: At a cpufreq transition to a different frequency, SoC
246		framework requires to set the voltage represented by the OPP using
247		the regulator framework to the Power Management chip providing the
248		voltage.
249		 soc_switch_to_freq_voltage(freq)
250		 {
251			/* do things */
252			rcu_read_lock();
253			opp = dev_pm_opp_find_freq_ceil(dev, &freq);
254			v = dev_pm_opp_get_voltage(opp);
255			rcu_read_unlock();
256			if (v)
257				regulator_set_voltage(.., v);
258			/* do other things */
259		 }
260	
261	dev_pm_opp_get_freq - Retrieve the freq represented by the opp pointer.
262		Example: Lets say the SoC framework uses a couple of helper functions
263		we could pass opp pointers instead of doing additional parameters to
264		handle quiet a bit of data parameters.
265		 soc_cpufreq_target(..)
266		 {
267			/* do things.. */
268			 max_freq = ULONG_MAX;
269			 rcu_read_lock();
270			 max_opp = dev_pm_opp_find_freq_floor(dev,&max_freq);
271			 requested_opp = dev_pm_opp_find_freq_ceil(dev,&freq);
272			 if (!IS_ERR(max_opp) && !IS_ERR(requested_opp))
273				r = soc_test_validity(max_opp, requested_opp);
274			 rcu_read_unlock();
275			/* do other things */
276		 }
277		 soc_test_validity(..)
278		 {
279			 if(dev_pm_opp_get_voltage(max_opp) < dev_pm_opp_get_voltage(requested_opp))
280				 return -EINVAL;
281			 if(dev_pm_opp_get_freq(max_opp) < dev_pm_opp_get_freq(requested_opp))
282				 return -EINVAL;
283			/* do things.. */
284		 }
285	
286	dev_pm_opp_get_opp_count - Retrieve the number of available opps for a device
287		Example: Lets say a co-processor in the SoC needs to know the available
288		frequencies in a table, the main processor can notify as following:
289		 soc_notify_coproc_available_frequencies()
290		 {
291			/* Do things */
292			rcu_read_lock();
293			num_available = dev_pm_opp_get_opp_count(dev);
294			speeds = kzalloc(sizeof(u32) * num_available, GFP_KERNEL);
295			/* populate the table in increasing order */
296			freq = 0;
297			while (!IS_ERR(opp = dev_pm_opp_find_freq_ceil(dev, &freq))) {
298				speeds[i] = freq;
299				freq++;
300				i++;
301			}
302			rcu_read_unlock();
303	
304			soc_notify_coproc(AVAILABLE_FREQs, speeds, num_available);
305			/* Do other things */
306		 }
307	
308	6. Data Structures
309	==================
310	Typically an SoC contains multiple voltage domains which are variable. Each
311	domain is represented by a device pointer. The relationship to OPP can be
312	represented as follows:
313	SoC
314	 |- device 1
315	 |	|- opp 1 (availability, freq, voltage)
316	 |	|- opp 2 ..
317	 ...	...
318	 |	`- opp n ..
319	 |- device 2
320	 ...
321	 `- device m
322	
323	OPP library maintains a internal list that the SoC framework populates and
324	accessed by various functions as described above. However, the structures
325	representing the actual OPPs and domains are internal to the OPP library itself
326	to allow for suitable abstraction reusable across systems.
327	
328	struct dev_pm_opp - The internal data structure of OPP library which is used to
329		represent an OPP. In addition to the freq, voltage, availability
330		information, it also contains internal book keeping information required
331		for the OPP library to operate on.  Pointer to this structure is
332		provided back to the users such as SoC framework to be used as a
333		identifier for OPP in the interactions with OPP layer.
334	
335		WARNING: The struct dev_pm_opp pointer should not be parsed or modified by the
336		users. The defaults of for an instance is populated by dev_pm_opp_add, but the
337		availability of the OPP can be modified by dev_pm_opp_enable/disable functions.
338	
339	struct device - This is used to identify a domain to the OPP layer. The
340		nature of the device and it's implementation is left to the user of
341		OPP library such as the SoC framework.
342	
343	Overall, in a simplistic view, the data structure operations is represented as
344	following:
345	
346	Initialization / modification:
347	            +-----+        /- dev_pm_opp_enable
348	dev_pm_opp_add --> | opp | <-------
349	  |         +-----+        \- dev_pm_opp_disable
350	  \-------> domain_info(device)
351	
352	Search functions:
353	             /-- dev_pm_opp_find_freq_ceil  ---\   +-----+
354	domain_info<---- dev_pm_opp_find_freq_exact -----> | opp |
355	             \-- dev_pm_opp_find_freq_floor ---/   +-----+
356	
357	Retrieval functions:
358	+-----+     /- dev_pm_opp_get_voltage
359	| opp | <---
360	+-----+     \- dev_pm_opp_get_freq
361	
362	domain_info <- dev_pm_opp_get_opp_count
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