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1 Operating Performance Points (OPP) Library
2 ==========================================
4 (C) 2009-2010 Nishanth Menon <nm@ti.com>, Texas Instruments Incorporated
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
15 1. Introduction
16 ===============
17 1.1 What is an Operating Performance Point (OPP)?
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.
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.
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}
35 We can represent these as three OPPs as the following {Hz, uV} tuples:
36 {300000000, 1000000}
37 {800000000, 1200000}
38 {1000000000, 1300000}
40 1.2 Operating Performance Points Library
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.
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 ->
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
115 WARNING: Do not use this function in interrupt context.
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 }
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.
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 }
157 NOTE: This is the only search function that operates on OPPs which are
158 not available.
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();
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 }
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).
203 WARNING: Do not use these functions in interrupt context.
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 }
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 }
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.
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 }
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 }
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();
304 soc_notify_coproc(AVAILABLE_FREQs, speeds, num_available);
305 /* Do other things */
306 }
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
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.
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.
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.
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.
343 Overall, in a simplistic view, the data structure operations is represented as
344 following:
346 Initialization / modification:
347 +-----+ /- dev_pm_opp_enable
348 dev_pm_opp_add --> | opp | <-------
349 | +-----+ \- dev_pm_opp_disable
350 \-------> domain_info(device)
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 ---/ +-----+
357 Retrieval functions:
358 +-----+ /- dev_pm_opp_get_voltage
359 | opp | <---
360 +-----+ \- dev_pm_opp_get_freq
362 domain_info <- dev_pm_opp_get_opp_count