| blob | 44d87ad3cea9fd345a774e196578a0cc8bf4d779 |
1 *=============*
2 * OPP Library *
3 *=============*
5 (C) 2009-2010 Nishanth Menon <nm@ti.com>, Texas Instruments Incorporated
7 Contents
8 --------
9 1. Introduction
10 2. Initial OPP List Registration
11 3. OPP Search Functions
12 4. OPP Availability Control Functions
13 5. OPP Data Retrieval Functions
14 6. Cpufreq Table Generation
15 7. Data Structures
17 1. Introduction
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 are loaded
24 more. The set of discrete tuples consisting of frequency and voltage pairs that
25 the device will support per domain are called Operating Performance Points or
26 OPPs.
28 OPP library provides a set of helper functions to organize and query the OPP
29 information. The library is located in drivers/base/power/opp.c and the header
30 is located in include/linux/opp.h. OPP library can be enabled by enabling
31 CONFIG_PM_OPP from power management menuconfig menu. OPP library depends on
32 CONFIG_PM as certain SoCs such as Texas Instrument's OMAP framework allows to
33 optionally boot at a certain OPP without needing cpufreq.
35 Typical usage of the OPP library is as follows:
36 (users) -> registers a set of default OPPs -> (library)
37 SoC framework -> modifies on required cases certain OPPs -> OPP layer
38 -> queries to search/retrieve information ->
40 OPP layer expects each domain to be represented by a unique device pointer. SoC
41 framework registers a set of initial OPPs per device with the OPP layer. This
42 list is expected to be an optimally small number typically around 5 per device.
43 This initial list contains a set of OPPs that the framework expects to be safely
44 enabled by default in the system.
46 Note on OPP Availability:
47 ------------------------
48 As the system proceeds to operate, SoC framework may choose to make certain
49 OPPs available or not available on each device based on various external
50 factors. Example usage: Thermal management or other exceptional situations where
51 SoC framework might choose to disable a higher frequency OPP to safely continue
52 operations until that OPP could be re-enabled if possible.
54 OPP library facilitates this concept in it's implementation. The following
55 operational functions operate only on available opps:
56 opp_find_freq_{ceil, floor}, opp_get_voltage, opp_get_freq, opp_get_opp_count
57 and opp_init_cpufreq_table
59 opp_find_freq_exact is meant to be used to find the opp pointer which can then
60 be used for opp_enable/disable functions to make an opp available as required.
62 WARNING: Users of OPP library should refresh their availability count using
63 get_opp_count if opp_enable/disable functions are invoked for a device, the
64 exact mechanism to trigger these or the notification mechanism to other
65 dependent subsystems such as cpufreq are left to the discretion of the SoC
66 specific framework which uses the OPP library. Similar care needs to be taken
67 care to refresh the cpufreq table in cases of these operations.
69 WARNING on OPP List locking mechanism:
70 -------------------------------------------------
71 OPP library uses RCU for exclusivity. RCU allows the query functions to operate
72 in multiple contexts and this synchronization mechanism is optimal for a read
73 intensive operations on data structure as the OPP library caters to.
75 To ensure that the data retrieved are sane, the users such as SoC framework
76 should ensure that the section of code operating on OPP queries are locked
77 using RCU read locks. The opp_find_freq_{exact,ceil,floor},
78 opp_get_{voltage, freq, opp_count} fall into this category.
80 opp_{add,enable,disable} are updaters which use mutex and implement it's own
81 RCU locking mechanisms. opp_init_cpufreq_table acts as an updater and uses
82 mutex to implment RCU updater strategy. These functions should *NOT* be called
83 under RCU locks and other contexts that prevent blocking functions in RCU or
84 mutex operations from working.
86 2. Initial OPP List Registration
87 ================================
88 The SoC implementation calls opp_add function iteratively to add OPPs per
89 device. It is expected that the SoC framework will register the OPP entries
90 optimally- typical numbers range to be less than 5. The list generated by
91 registering the OPPs is maintained by OPP library throughout the device
92 operation. The SoC framework can subsequently control the availability of the
93 OPPs dynamically using the opp_enable / disable functions.
95 opp_add - Add a new OPP for a specific domain represented by the device pointer.
96 The OPP is defined using the frequency and voltage. Once added, the OPP
97 is assumed to be available and control of it's availability can be done
98 with the opp_enable/disable functions. OPP library internally stores
99 and manages this information in the opp struct. This function may be
100 used by SoC framework to define a optimal list as per the demands of
101 SoC usage environment.
103 WARNING: Do not use this function in interrupt context.
105 Example:
106 soc_pm_init()
107 {
108 /* Do things */
109 r = opp_add(mpu_dev, 1000000, 900000);
110 if (!r) {
111 pr_err("%s: unable to register mpu opp(%d)\n", r);
112 goto no_cpufreq;
113 }
114 /* Do cpufreq things */
115 no_cpufreq:
116 /* Do remaining things */
117 }
119 3. OPP Search Functions
120 =======================
121 High level framework such as cpufreq operates on frequencies. To map the
122 frequency back to the corresponding OPP, OPP library provides handy functions
123 to search the OPP list that OPP library internally manages. These search
124 functions return the matching pointer representing the opp if a match is
125 found, else returns error. These errors are expected to be handled by standard
126 error checks such as IS_ERR() and appropriate actions taken by the caller.
128 opp_find_freq_exact - Search for an OPP based on an *exact* frequency and
129 availability. This function is especially useful to enable an OPP which
130 is not available by default.
131 Example: In a case when SoC framework detects a situation where a
132 higher frequency could be made available, it can use this function to
133 find the OPP prior to call the opp_enable to actually make it available.
134 rcu_read_lock();
135 opp = opp_find_freq_exact(dev, 1000000000, false);
136 rcu_read_unlock();
137 /* dont operate on the pointer.. just do a sanity check.. */
138 if (IS_ERR(opp)) {
139 pr_err("frequency not disabled!\n");
140 /* trigger appropriate actions.. */
141 } else {
142 opp_enable(dev,1000000000);
143 }
145 NOTE: This is the only search function that operates on OPPs which are
146 not available.
148 opp_find_freq_floor - Search for an available OPP which is *at most* the
149 provided frequency. This function is useful while searching for a lesser
150 match OR operating on OPP information in the order of decreasing
151 frequency.
152 Example: To find the highest opp for a device:
153 freq = ULONG_MAX;
154 rcu_read_lock();
155 opp_find_freq_floor(dev, &freq);
156 rcu_read_unlock();
158 opp_find_freq_ceil - Search for an available OPP which is *at least* the
159 provided frequency. This function is useful while searching for a
160 higher match OR operating on OPP information in the order of increasing
161 frequency.
162 Example 1: To find the lowest opp for a device:
163 freq = 0;
164 rcu_read_lock();
165 opp_find_freq_ceil(dev, &freq);
166 rcu_read_unlock();
167 Example 2: A simplified implementation of a SoC cpufreq_driver->target:
168 soc_cpufreq_target(..)
169 {
170 /* Do stuff like policy checks etc. */
171 /* Find the best frequency match for the req */
172 rcu_read_lock();
173 opp = opp_find_freq_ceil(dev, &freq);
174 rcu_read_unlock();
175 if (!IS_ERR(opp))
176 soc_switch_to_freq_voltage(freq);
177 else
178 /* do something when we cant satisfy the req */
179 /* do other stuff */
180 }
182 4. OPP Availability Control Functions
183 =====================================
184 A default OPP list registered with the OPP library may not cater to all possible
185 situation. The OPP library provides a set of functions to modify the
186 availability of a OPP within the OPP list. This allows SoC frameworks to have
187 fine grained dynamic control of which sets of OPPs are operationally available.
188 These functions are intended to *temporarily* remove an OPP in conditions such
189 as thermal considerations (e.g. don't use OPPx until the temperature drops).
191 WARNING: Do not use these functions in interrupt context.
193 opp_enable - Make a OPP available for operation.
194 Example: Lets say that 1GHz OPP is to be made available only if the
195 SoC temperature is lower than a certain threshold. The SoC framework
196 implementation might choose to do something as follows:
197 if (cur_temp < temp_low_thresh) {
198 /* Enable 1GHz if it was disabled */
199 rcu_read_lock();
200 opp = opp_find_freq_exact(dev, 1000000000, false);
201 rcu_read_unlock();
202 /* just error check */
203 if (!IS_ERR(opp))
204 ret = opp_enable(dev, 1000000000);
205 else
206 goto try_something_else;
207 }
209 opp_disable - Make an OPP to be not available for operation
210 Example: Lets say that 1GHz OPP is to be disabled if the temperature
211 exceeds a threshold value. The SoC framework implementation might
212 choose to do something as follows:
213 if (cur_temp > temp_high_thresh) {
214 /* Disable 1GHz if it was enabled */
215 rcu_read_lock();
216 opp = opp_find_freq_exact(dev, 1000000000, true);
217 rcu_read_unlock();
218 /* just error check */
219 if (!IS_ERR(opp))
220 ret = opp_disable(dev, 1000000000);
221 else
222 goto try_something_else;
223 }
225 5. OPP Data Retrieval Functions
226 ===============================
227 Since OPP library abstracts away the OPP information, a set of functions to pull
228 information from the OPP structure is necessary. Once an OPP pointer is
229 retrieved using the search functions, the following functions can be used by SoC
230 framework to retrieve the information represented inside the OPP layer.
232 opp_get_voltage - Retrieve the voltage represented by the opp pointer.
233 Example: At a cpufreq transition to a different frequency, SoC
234 framework requires to set the voltage represented by the OPP using
235 the regulator framework to the Power Management chip providing the
236 voltage.
237 soc_switch_to_freq_voltage(freq)
238 {
239 /* do things */
240 rcu_read_lock();
241 opp = opp_find_freq_ceil(dev, &freq);
242 v = opp_get_voltage(opp);
243 rcu_read_unlock();
244 if (v)
245 regulator_set_voltage(.., v);
246 /* do other things */
247 }
249 opp_get_freq - Retrieve the freq represented by the opp pointer.
250 Example: Lets say the SoC framework uses a couple of helper functions
251 we could pass opp pointers instead of doing additional parameters to
252 handle quiet a bit of data parameters.
253 soc_cpufreq_target(..)
254 {
255 /* do things.. */
256 max_freq = ULONG_MAX;
257 rcu_read_lock();
258 max_opp = opp_find_freq_floor(dev,&max_freq);
259 requested_opp = opp_find_freq_ceil(dev,&freq);
260 if (!IS_ERR(max_opp) && !IS_ERR(requested_opp))
261 r = soc_test_validity(max_opp, requested_opp);
262 rcu_read_unlock();
263 /* do other things */
264 }
265 soc_test_validity(..)
266 {
267 if(opp_get_voltage(max_opp) < opp_get_voltage(requested_opp))
268 return -EINVAL;
269 if(opp_get_freq(max_opp) < opp_get_freq(requested_opp))
270 return -EINVAL;
271 /* do things.. */
272 }
274 opp_get_opp_count - Retrieve the number of available opps for a device
275 Example: Lets say a co-processor in the SoC needs to know the available
276 frequencies in a table, the main processor can notify as following:
277 soc_notify_coproc_available_frequencies()
278 {
279 /* Do things */
280 rcu_read_lock();
281 num_available = opp_get_opp_count(dev);
282 speeds = kzalloc(sizeof(u32) * num_available, GFP_KERNEL);
283 /* populate the table in increasing order */
284 freq = 0;
285 while (!IS_ERR(opp = opp_find_freq_ceil(dev, &freq))) {
286 speeds[i] = freq;
287 freq++;
288 i++;
289 }
290 rcu_read_unlock();
292 soc_notify_coproc(AVAILABLE_FREQs, speeds, num_available);
293 /* Do other things */
294 }
296 6. Cpufreq Table Generation
297 ===========================
298 opp_init_cpufreq_table - cpufreq framework typically is initialized with
299 cpufreq_frequency_table_cpuinfo which is provided with the list of
300 frequencies that are available for operation. This function provides
301 a ready to use conversion routine to translate the OPP layer's internal
302 information about the available frequencies into a format readily
303 providable to cpufreq.
305 WARNING: Do not use this function in interrupt context.
307 Example:
308 soc_pm_init()
309 {
310 /* Do things */
311 r = opp_init_cpufreq_table(dev, &freq_table);
312 if (!r)
313 cpufreq_frequency_table_cpuinfo(policy, freq_table);
314 /* Do other things */
315 }
317 NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
318 addition to CONFIG_PM as power management feature is required to
319 dynamically scale voltage and frequency in a system.
321 7. Data Structures
322 ==================
323 Typically an SoC contains multiple voltage domains which are variable. Each
324 domain is represented by a device pointer. The relationship to OPP can be
325 represented as follows:
326 SoC
327 |- device 1
328 | |- opp 1 (availability, freq, voltage)
329 | |- opp 2 ..
330 ... ...
331 | `- opp n ..
332 |- device 2
333 ...
334 `- device m
336 OPP library maintains a internal list that the SoC framework populates and
337 accessed by various functions as described above. However, the structures
338 representing the actual OPPs and domains are internal to the OPP library itself
339 to allow for suitable abstraction reusable across systems.
341 struct opp - The internal data structure of OPP library which is used to
342 represent an OPP. In addition to the freq, voltage, availability
343 information, it also contains internal book keeping information required
344 for the OPP library to operate on. Pointer to this structure is
345 provided back to the users such as SoC framework to be used as a
346 identifier for OPP in the interactions with OPP layer.
348 WARNING: The struct opp pointer should not be parsed or modified by the
349 users. The defaults of for an instance is populated by opp_add, but the
350 availability of the OPP can be modified by opp_enable/disable functions.
352 struct device - This is used to identify a domain to the OPP layer. The
353 nature of the device and it's implementation is left to the user of
354 OPP library such as the SoC framework.
356 Overall, in a simplistic view, the data structure operations is represented as
357 following:
359 Initialization / modification:
360 +-----+ /- opp_enable
361 opp_add --> | opp | <-------
362 | +-----+ \- opp_disable
363 \-------> domain_info(device)
365 Search functions:
366 /-- opp_find_freq_ceil ---\ +-----+
367 domain_info<---- opp_find_freq_exact -----> | opp |
368 \-- opp_find_freq_floor ---/ +-----+
370 Retrieval functions:
371 +-----+ /- opp_get_voltage
372 | opp | <---
373 +-----+ \- opp_get_freq
375 domain_info <- opp_get_opp_count