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Nishanth Menon4b875812013-02-26 23:53:02 +00001Operating Performance Points (OPP) Library
2==========================================
Nishanth Menone1f60b22010-10-13 00:13:10 +02003
4(C) 2009-2010 Nishanth Menon <nm@ti.com>, Texas Instruments Incorporated
5
6Contents
7--------
81. Introduction
92. Initial OPP List Registration
103. OPP Search Functions
114. OPP Availability Control Functions
125. OPP Data Retrieval Functions
136. Cpufreq Table Generation
147. Data Structures
15
161. Introduction
17===============
Nishanth Menon4b875812013-02-26 23:53:02 +0000181.1 What is an Operating Performance Point (OPP)?
19
Nishanth Menone1f60b22010-10-13 00:13:10 +020020Complex SoCs of today consists of a multiple sub-modules working in conjunction.
21In an operational system executing varied use cases, not all modules in the SoC
22need to function at their highest performing frequency all the time. To
23facilitate this, sub-modules in a SoC are grouped into domains, allowing some
Nishanth Menon4b875812013-02-26 23:53:02 +000024domains to run at lower voltage and frequency while other domains run at
25voltage/frequency pairs that are higher.
26
27The set of discrete tuples consisting of frequency and voltage pairs that
Nishanth Menone1f60b22010-10-13 00:13:10 +020028the device will support per domain are called Operating Performance Points or
29OPPs.
30
Nishanth Menon4b875812013-02-26 23:53:02 +000031As an example:
32Let us consider an MPU device which supports the following:
33{300MHz at minimum voltage of 1V}, {800MHz at minimum voltage of 1.2V},
34{1GHz at minimum voltage of 1.3V}
35
36We can represent these as three OPPs as the following {Hz, uV} tuples:
37{300000000, 1000000}
38{800000000, 1200000}
39{1000000000, 1300000}
40
411.2 Operating Performance Points Library
42
Nishanth Menone1f60b22010-10-13 00:13:10 +020043OPP library provides a set of helper functions to organize and query the OPP
44information. The library is located in drivers/base/power/opp.c and the header
45is located in include/linux/opp.h. OPP library can be enabled by enabling
46CONFIG_PM_OPP from power management menuconfig menu. OPP library depends on
47CONFIG_PM as certain SoCs such as Texas Instrument's OMAP framework allows to
48optionally boot at a certain OPP without needing cpufreq.
49
50Typical usage of the OPP library is as follows:
51(users) -> registers a set of default OPPs -> (library)
52SoC framework -> modifies on required cases certain OPPs -> OPP layer
53 -> queries to search/retrieve information ->
54
Mark Brown43e60862010-11-11 01:51:26 +010055Architectures that provide a SoC framework for OPP should select ARCH_HAS_OPP
56to make the OPP layer available.
57
Nishanth Menone1f60b22010-10-13 00:13:10 +020058OPP layer expects each domain to be represented by a unique device pointer. SoC
59framework registers a set of initial OPPs per device with the OPP layer. This
60list is expected to be an optimally small number typically around 5 per device.
61This initial list contains a set of OPPs that the framework expects to be safely
62enabled by default in the system.
63
64Note on OPP Availability:
65------------------------
66As the system proceeds to operate, SoC framework may choose to make certain
67OPPs available or not available on each device based on various external
68factors. Example usage: Thermal management or other exceptional situations where
69SoC framework might choose to disable a higher frequency OPP to safely continue
70operations until that OPP could be re-enabled if possible.
71
72OPP library facilitates this concept in it's implementation. The following
73operational functions operate only on available opps:
74opp_find_freq_{ceil, floor}, opp_get_voltage, opp_get_freq, opp_get_opp_count
75and opp_init_cpufreq_table
76
77opp_find_freq_exact is meant to be used to find the opp pointer which can then
78be used for opp_enable/disable functions to make an opp available as required.
79
80WARNING: Users of OPP library should refresh their availability count using
81get_opp_count if opp_enable/disable functions are invoked for a device, the
82exact mechanism to trigger these or the notification mechanism to other
83dependent subsystems such as cpufreq are left to the discretion of the SoC
84specific framework which uses the OPP library. Similar care needs to be taken
85care to refresh the cpufreq table in cases of these operations.
86
87WARNING on OPP List locking mechanism:
88-------------------------------------------------
89OPP library uses RCU for exclusivity. RCU allows the query functions to operate
90in multiple contexts and this synchronization mechanism is optimal for a read
91intensive operations on data structure as the OPP library caters to.
92
93To ensure that the data retrieved are sane, the users such as SoC framework
94should ensure that the section of code operating on OPP queries are locked
95using RCU read locks. The opp_find_freq_{exact,ceil,floor},
96opp_get_{voltage, freq, opp_count} fall into this category.
97
98opp_{add,enable,disable} are updaters which use mutex and implement it's own
99RCU locking mechanisms. opp_init_cpufreq_table acts as an updater and uses
100mutex to implment RCU updater strategy. These functions should *NOT* be called
101under RCU locks and other contexts that prevent blocking functions in RCU or
102mutex operations from working.
103
1042. Initial OPP List Registration
105================================
106The SoC implementation calls opp_add function iteratively to add OPPs per
107device. It is expected that the SoC framework will register the OPP entries
108optimally- typical numbers range to be less than 5. The list generated by
109registering the OPPs is maintained by OPP library throughout the device
110operation. The SoC framework can subsequently control the availability of the
111OPPs dynamically using the opp_enable / disable functions.
112
113opp_add - Add a new OPP for a specific domain represented by the device pointer.
114 The OPP is defined using the frequency and voltage. Once added, the OPP
115 is assumed to be available and control of it's availability can be done
116 with the opp_enable/disable functions. OPP library internally stores
117 and manages this information in the opp struct. This function may be
118 used by SoC framework to define a optimal list as per the demands of
119 SoC usage environment.
120
121 WARNING: Do not use this function in interrupt context.
122
123 Example:
124 soc_pm_init()
125 {
126 /* Do things */
127 r = opp_add(mpu_dev, 1000000, 900000);
128 if (!r) {
129 pr_err("%s: unable to register mpu opp(%d)\n", r);
130 goto no_cpufreq;
131 }
132 /* Do cpufreq things */
133 no_cpufreq:
134 /* Do remaining things */
135 }
136
1373. OPP Search Functions
138=======================
139High level framework such as cpufreq operates on frequencies. To map the
140frequency back to the corresponding OPP, OPP library provides handy functions
141to search the OPP list that OPP library internally manages. These search
142functions return the matching pointer representing the opp if a match is
143found, else returns error. These errors are expected to be handled by standard
144error checks such as IS_ERR() and appropriate actions taken by the caller.
145
146opp_find_freq_exact - Search for an OPP based on an *exact* frequency and
147 availability. This function is especially useful to enable an OPP which
148 is not available by default.
149 Example: In a case when SoC framework detects a situation where a
150 higher frequency could be made available, it can use this function to
151 find the OPP prior to call the opp_enable to actually make it available.
152 rcu_read_lock();
153 opp = opp_find_freq_exact(dev, 1000000000, false);
154 rcu_read_unlock();
155 /* dont operate on the pointer.. just do a sanity check.. */
156 if (IS_ERR(opp)) {
157 pr_err("frequency not disabled!\n");
158 /* trigger appropriate actions.. */
159 } else {
160 opp_enable(dev,1000000000);
161 }
162
163 NOTE: This is the only search function that operates on OPPs which are
164 not available.
165
166opp_find_freq_floor - Search for an available OPP which is *at most* the
167 provided frequency. This function is useful while searching for a lesser
168 match OR operating on OPP information in the order of decreasing
169 frequency.
170 Example: To find the highest opp for a device:
171 freq = ULONG_MAX;
172 rcu_read_lock();
173 opp_find_freq_floor(dev, &freq);
174 rcu_read_unlock();
175
176opp_find_freq_ceil - Search for an available OPP which is *at least* the
177 provided frequency. This function is useful while searching for a
178 higher match OR operating on OPP information in the order of increasing
179 frequency.
180 Example 1: To find the lowest opp for a device:
181 freq = 0;
182 rcu_read_lock();
183 opp_find_freq_ceil(dev, &freq);
184 rcu_read_unlock();
185 Example 2: A simplified implementation of a SoC cpufreq_driver->target:
186 soc_cpufreq_target(..)
187 {
188 /* Do stuff like policy checks etc. */
189 /* Find the best frequency match for the req */
190 rcu_read_lock();
191 opp = opp_find_freq_ceil(dev, &freq);
192 rcu_read_unlock();
193 if (!IS_ERR(opp))
194 soc_switch_to_freq_voltage(freq);
195 else
Lucas De Marchi25985ed2011-03-30 22:57:33 -0300196 /* do something when we can't satisfy the req */
Nishanth Menone1f60b22010-10-13 00:13:10 +0200197 /* do other stuff */
198 }
199
2004. OPP Availability Control Functions
201=====================================
202A default OPP list registered with the OPP library may not cater to all possible
203situation. The OPP library provides a set of functions to modify the
204availability of a OPP within the OPP list. This allows SoC frameworks to have
205fine grained dynamic control of which sets of OPPs are operationally available.
206These functions are intended to *temporarily* remove an OPP in conditions such
207as thermal considerations (e.g. don't use OPPx until the temperature drops).
208
209WARNING: Do not use these functions in interrupt context.
210
211opp_enable - Make a OPP available for operation.
212 Example: Lets say that 1GHz OPP is to be made available only if the
213 SoC temperature is lower than a certain threshold. The SoC framework
214 implementation might choose to do something as follows:
215 if (cur_temp < temp_low_thresh) {
216 /* Enable 1GHz if it was disabled */
217 rcu_read_lock();
218 opp = opp_find_freq_exact(dev, 1000000000, false);
219 rcu_read_unlock();
220 /* just error check */
221 if (!IS_ERR(opp))
222 ret = opp_enable(dev, 1000000000);
223 else
224 goto try_something_else;
225 }
226
227opp_disable - Make an OPP to be not available for operation
228 Example: Lets say that 1GHz OPP is to be disabled if the temperature
229 exceeds a threshold value. The SoC framework implementation might
230 choose to do something as follows:
231 if (cur_temp > temp_high_thresh) {
232 /* Disable 1GHz if it was enabled */
233 rcu_read_lock();
234 opp = opp_find_freq_exact(dev, 1000000000, true);
235 rcu_read_unlock();
236 /* just error check */
237 if (!IS_ERR(opp))
238 ret = opp_disable(dev, 1000000000);
239 else
240 goto try_something_else;
241 }
242
2435. OPP Data Retrieval Functions
244===============================
245Since OPP library abstracts away the OPP information, a set of functions to pull
246information from the OPP structure is necessary. Once an OPP pointer is
247retrieved using the search functions, the following functions can be used by SoC
248framework to retrieve the information represented inside the OPP layer.
249
250opp_get_voltage - Retrieve the voltage represented by the opp pointer.
251 Example: At a cpufreq transition to a different frequency, SoC
252 framework requires to set the voltage represented by the OPP using
253 the regulator framework to the Power Management chip providing the
254 voltage.
255 soc_switch_to_freq_voltage(freq)
256 {
257 /* do things */
258 rcu_read_lock();
259 opp = opp_find_freq_ceil(dev, &freq);
260 v = opp_get_voltage(opp);
261 rcu_read_unlock();
262 if (v)
263 regulator_set_voltage(.., v);
264 /* do other things */
265 }
266
267opp_get_freq - Retrieve the freq represented by the opp pointer.
268 Example: Lets say the SoC framework uses a couple of helper functions
269 we could pass opp pointers instead of doing additional parameters to
270 handle quiet a bit of data parameters.
271 soc_cpufreq_target(..)
272 {
273 /* do things.. */
274 max_freq = ULONG_MAX;
275 rcu_read_lock();
276 max_opp = opp_find_freq_floor(dev,&max_freq);
277 requested_opp = opp_find_freq_ceil(dev,&freq);
278 if (!IS_ERR(max_opp) && !IS_ERR(requested_opp))
279 r = soc_test_validity(max_opp, requested_opp);
280 rcu_read_unlock();
281 /* do other things */
282 }
283 soc_test_validity(..)
284 {
285 if(opp_get_voltage(max_opp) < opp_get_voltage(requested_opp))
286 return -EINVAL;
287 if(opp_get_freq(max_opp) < opp_get_freq(requested_opp))
288 return -EINVAL;
289 /* do things.. */
290 }
291
292opp_get_opp_count - Retrieve the number of available opps for a device
293 Example: Lets say a co-processor in the SoC needs to know the available
294 frequencies in a table, the main processor can notify as following:
295 soc_notify_coproc_available_frequencies()
296 {
297 /* Do things */
298 rcu_read_lock();
299 num_available = opp_get_opp_count(dev);
300 speeds = kzalloc(sizeof(u32) * num_available, GFP_KERNEL);
301 /* populate the table in increasing order */
302 freq = 0;
303 while (!IS_ERR(opp = opp_find_freq_ceil(dev, &freq))) {
304 speeds[i] = freq;
305 freq++;
306 i++;
307 }
308 rcu_read_unlock();
309
310 soc_notify_coproc(AVAILABLE_FREQs, speeds, num_available);
311 /* Do other things */
312 }
313
3146. Cpufreq Table Generation
315===========================
316opp_init_cpufreq_table - cpufreq framework typically is initialized with
317 cpufreq_frequency_table_cpuinfo which is provided with the list of
318 frequencies that are available for operation. This function provides
319 a ready to use conversion routine to translate the OPP layer's internal
320 information about the available frequencies into a format readily
321 providable to cpufreq.
322
323 WARNING: Do not use this function in interrupt context.
324
325 Example:
326 soc_pm_init()
327 {
328 /* Do things */
329 r = opp_init_cpufreq_table(dev, &freq_table);
330 if (!r)
331 cpufreq_frequency_table_cpuinfo(policy, freq_table);
332 /* Do other things */
333 }
334
335 NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
336 addition to CONFIG_PM as power management feature is required to
337 dynamically scale voltage and frequency in a system.
338
Nishanth Menon99f381d2011-06-10 20:24:57 +0200339opp_free_cpufreq_table - Free up the table allocated by opp_init_cpufreq_table
340
Nishanth Menone1f60b22010-10-13 00:13:10 +02003417. Data Structures
342==================
343Typically an SoC contains multiple voltage domains which are variable. Each
344domain is represented by a device pointer. The relationship to OPP can be
345represented as follows:
346SoC
347 |- device 1
348 | |- opp 1 (availability, freq, voltage)
349 | |- opp 2 ..
350 ... ...
351 | `- opp n ..
352 |- device 2
353 ...
354 `- device m
355
356OPP library maintains a internal list that the SoC framework populates and
357accessed by various functions as described above. However, the structures
358representing the actual OPPs and domains are internal to the OPP library itself
359to allow for suitable abstraction reusable across systems.
360
361struct opp - The internal data structure of OPP library which is used to
362 represent an OPP. In addition to the freq, voltage, availability
363 information, it also contains internal book keeping information required
364 for the OPP library to operate on. Pointer to this structure is
365 provided back to the users such as SoC framework to be used as a
366 identifier for OPP in the interactions with OPP layer.
367
368 WARNING: The struct opp pointer should not be parsed or modified by the
369 users. The defaults of for an instance is populated by opp_add, but the
370 availability of the OPP can be modified by opp_enable/disable functions.
371
372struct device - This is used to identify a domain to the OPP layer. The
373 nature of the device and it's implementation is left to the user of
374 OPP library such as the SoC framework.
375
376Overall, in a simplistic view, the data structure operations is represented as
377following:
378
379Initialization / modification:
380 +-----+ /- opp_enable
381opp_add --> | opp | <-------
382 | +-----+ \- opp_disable
383 \-------> domain_info(device)
384
385Search functions:
386 /-- opp_find_freq_ceil ---\ +-----+
387domain_info<---- opp_find_freq_exact -----> | opp |
388 \-- opp_find_freq_floor ---/ +-----+
389
390Retrieval functions:
391+-----+ /- opp_get_voltage
392| opp | <---
393+-----+ \- opp_get_freq
394
395domain_info <- opp_get_opp_count