Root/Documentation/power/opp.txt

1*=============*
2* OPP Library *
3*=============*
4
5(C) 2009-2010 Nishanth Menon <nm@ti.com>, Texas Instruments Incorporated
6
7Contents
8--------
91. Introduction
102. Initial OPP List Registration
113. OPP Search Functions
124. OPP Availability Control Functions
135. OPP Data Retrieval Functions
146. Cpufreq Table Generation
157. Data Structures
16
171. Introduction
18===============
19Complex SoCs of today consists of a multiple sub-modules working in conjunction.
20In an operational system executing varied use cases, not all modules in the SoC
21need to function at their highest performing frequency all the time. To
22facilitate this, sub-modules in a SoC are grouped into domains, allowing some
23domains to run at lower voltage and frequency while other domains are loaded
24more. The set of discrete tuples consisting of frequency and voltage pairs that
25the device will support per domain are called Operating Performance Points or
26OPPs.
27
28OPP library provides a set of helper functions to organize and query the OPP
29information. The library is located in drivers/base/power/opp.c and the header
30is located in include/linux/opp.h. OPP library can be enabled by enabling
31CONFIG_PM_OPP from power management menuconfig menu. OPP library depends on
32CONFIG_PM as certain SoCs such as Texas Instrument's OMAP framework allows to
33optionally boot at a certain OPP without needing cpufreq.
34
35Typical usage of the OPP library is as follows:
36(users) -> registers a set of default OPPs -> (library)
37SoC framework -> modifies on required cases certain OPPs -> OPP layer
38        -> queries to search/retrieve information ->
39
40Architectures that provide a SoC framework for OPP should select ARCH_HAS_OPP
41to make the OPP layer available.
42
43OPP layer expects each domain to be represented by a unique device pointer. SoC
44framework registers a set of initial OPPs per device with the OPP layer. This
45list is expected to be an optimally small number typically around 5 per device.
46This initial list contains a set of OPPs that the framework expects to be safely
47enabled by default in the system.
48
49Note on OPP Availability:
50------------------------
51As the system proceeds to operate, SoC framework may choose to make certain
52OPPs available or not available on each device based on various external
53factors. Example usage: Thermal management or other exceptional situations where
54SoC framework might choose to disable a higher frequency OPP to safely continue
55operations until that OPP could be re-enabled if possible.
56
57OPP library facilitates this concept in it's implementation. The following
58operational functions operate only on available opps:
59opp_find_freq_{ceil, floor}, opp_get_voltage, opp_get_freq, opp_get_opp_count
60and opp_init_cpufreq_table
61
62opp_find_freq_exact is meant to be used to find the opp pointer which can then
63be used for opp_enable/disable functions to make an opp available as required.
64
65WARNING: Users of OPP library should refresh their availability count using
66get_opp_count if opp_enable/disable functions are invoked for a device, the
67exact mechanism to trigger these or the notification mechanism to other
68dependent subsystems such as cpufreq are left to the discretion of the SoC
69specific framework which uses the OPP library. Similar care needs to be taken
70care to refresh the cpufreq table in cases of these operations.
71
72WARNING on OPP List locking mechanism:
73-------------------------------------------------
74OPP library uses RCU for exclusivity. RCU allows the query functions to operate
75in multiple contexts and this synchronization mechanism is optimal for a read
76intensive operations on data structure as the OPP library caters to.
77
78To ensure that the data retrieved are sane, the users such as SoC framework
79should ensure that the section of code operating on OPP queries are locked
80using RCU read locks. The opp_find_freq_{exact,ceil,floor},
81opp_get_{voltage, freq, opp_count} fall into this category.
82
83opp_{add,enable,disable} are updaters which use mutex and implement it's own
84RCU locking mechanisms. opp_init_cpufreq_table acts as an updater and uses
85mutex to implment RCU updater strategy. These functions should *NOT* be called
86under RCU locks and other contexts that prevent blocking functions in RCU or
87mutex operations from working.
88
892. Initial OPP List Registration
90================================
91The SoC implementation calls opp_add function iteratively to add OPPs per
92device. It is expected that the SoC framework will register the OPP entries
93optimally- typical numbers range to be less than 5. The list generated by
94registering the OPPs is maintained by OPP library throughout the device
95operation. The SoC framework can subsequently control the availability of the
96OPPs dynamically using the opp_enable / disable functions.
97
98opp_add - Add a new OPP for a specific domain represented by the device pointer.
99    The OPP is defined using the frequency and voltage. Once added, the OPP
100    is assumed to be available and control of it's availability can be done
101    with the opp_enable/disable functions. OPP library internally stores
102    and manages this information in the opp struct. This function may be
103    used by SoC framework to define a optimal list as per the demands of
104    SoC usage environment.
105
106    WARNING: Do not use this function in interrupt context.
107
108    Example:
109     soc_pm_init()
110     {
111        /* Do things */
112        r = opp_add(mpu_dev, 1000000, 900000);
113        if (!r) {
114            pr_err("%s: unable to register mpu opp(%d)\n", r);
115            goto no_cpufreq;
116        }
117        /* Do cpufreq things */
118     no_cpufreq:
119        /* Do remaining things */
120     }
121
1223. OPP Search Functions
123=======================
124High level framework such as cpufreq operates on frequencies. To map the
125frequency back to the corresponding OPP, OPP library provides handy functions
126to search the OPP list that OPP library internally manages. These search
127functions return the matching pointer representing the opp if a match is
128found, else returns error. These errors are expected to be handled by standard
129error checks such as IS_ERR() and appropriate actions taken by the caller.
130
131opp_find_freq_exact - Search for an OPP based on an *exact* frequency and
132    availability. This function is especially useful to enable an OPP which
133    is not available by default.
134    Example: In a case when SoC framework detects a situation where a
135    higher frequency could be made available, it can use this function to
136    find the OPP prior to call the opp_enable to actually make it available.
137     rcu_read_lock();
138     opp = opp_find_freq_exact(dev, 1000000000, false);
139     rcu_read_unlock();
140     /* dont operate on the pointer.. just do a sanity check.. */
141     if (IS_ERR(opp)) {
142        pr_err("frequency not disabled!\n");
143        /* trigger appropriate actions.. */
144     } else {
145        opp_enable(dev,1000000000);
146     }
147
148    NOTE: This is the only search function that operates on OPPs which are
149    not available.
150
151opp_find_freq_floor - Search for an available OPP which is *at most* the
152    provided frequency. This function is useful while searching for a lesser
153    match OR operating on OPP information in the order of decreasing
154    frequency.
155    Example: To find the highest opp for a device:
156     freq = ULONG_MAX;
157     rcu_read_lock();
158     opp_find_freq_floor(dev, &freq);
159     rcu_read_unlock();
160
161opp_find_freq_ceil - Search for an available OPP which is *at least* the
162    provided frequency. This function is useful while searching for a
163    higher match OR operating on OPP information in the order of increasing
164    frequency.
165    Example 1: To find the lowest opp for a device:
166     freq = 0;
167     rcu_read_lock();
168     opp_find_freq_ceil(dev, &freq);
169     rcu_read_unlock();
170    Example 2: A simplified implementation of a SoC cpufreq_driver->target:
171     soc_cpufreq_target(..)
172     {
173        /* Do stuff like policy checks etc. */
174        /* Find the best frequency match for the req */
175        rcu_read_lock();
176        opp = opp_find_freq_ceil(dev, &freq);
177        rcu_read_unlock();
178        if (!IS_ERR(opp))
179            soc_switch_to_freq_voltage(freq);
180        else
181            /* do something when we can't satisfy the req */
182        /* do other stuff */
183     }
184
1854. OPP Availability Control Functions
186=====================================
187A default OPP list registered with the OPP library may not cater to all possible
188situation. The OPP library provides a set of functions to modify the
189availability of a OPP within the OPP list. This allows SoC frameworks to have
190fine grained dynamic control of which sets of OPPs are operationally available.
191These functions are intended to *temporarily* remove an OPP in conditions such
192as thermal considerations (e.g. don't use OPPx until the temperature drops).
193
194WARNING: Do not use these functions in interrupt context.
195
196opp_enable - Make a OPP available for operation.
197    Example: Lets say that 1GHz OPP is to be made available only if the
198    SoC temperature is lower than a certain threshold. The SoC framework
199    implementation might choose to do something as follows:
200     if (cur_temp < temp_low_thresh) {
201        /* Enable 1GHz if it was disabled */
202        rcu_read_lock();
203        opp = opp_find_freq_exact(dev, 1000000000, false);
204        rcu_read_unlock();
205        /* just error check */
206        if (!IS_ERR(opp))
207            ret = opp_enable(dev, 1000000000);
208        else
209            goto try_something_else;
210     }
211
212opp_disable - Make an OPP to be not available for operation
213    Example: Lets say that 1GHz OPP is to be disabled if the temperature
214    exceeds a threshold value. The SoC framework implementation might
215    choose to do something as follows:
216     if (cur_temp > temp_high_thresh) {
217        /* Disable 1GHz if it was enabled */
218        rcu_read_lock();
219        opp = opp_find_freq_exact(dev, 1000000000, true);
220        rcu_read_unlock();
221        /* just error check */
222        if (!IS_ERR(opp))
223            ret = opp_disable(dev, 1000000000);
224        else
225            goto try_something_else;
226     }
227
2285. OPP Data Retrieval Functions
229===============================
230Since OPP library abstracts away the OPP information, a set of functions to pull
231information from the OPP structure is necessary. Once an OPP pointer is
232retrieved using the search functions, the following functions can be used by SoC
233framework to retrieve the information represented inside the OPP layer.
234
235opp_get_voltage - Retrieve the voltage represented by the opp pointer.
236    Example: At a cpufreq transition to a different frequency, SoC
237    framework requires to set the voltage represented by the OPP using
238    the regulator framework to the Power Management chip providing the
239    voltage.
240     soc_switch_to_freq_voltage(freq)
241     {
242        /* do things */
243        rcu_read_lock();
244        opp = opp_find_freq_ceil(dev, &freq);
245        v = opp_get_voltage(opp);
246        rcu_read_unlock();
247        if (v)
248            regulator_set_voltage(.., v);
249        /* do other things */
250     }
251
252opp_get_freq - Retrieve the freq represented by the opp pointer.
253    Example: Lets say the SoC framework uses a couple of helper functions
254    we could pass opp pointers instead of doing additional parameters to
255    handle quiet a bit of data parameters.
256     soc_cpufreq_target(..)
257     {
258        /* do things.. */
259         max_freq = ULONG_MAX;
260         rcu_read_lock();
261         max_opp = opp_find_freq_floor(dev,&max_freq);
262         requested_opp = opp_find_freq_ceil(dev,&freq);
263         if (!IS_ERR(max_opp) && !IS_ERR(requested_opp))
264            r = soc_test_validity(max_opp, requested_opp);
265         rcu_read_unlock();
266        /* do other things */
267     }
268     soc_test_validity(..)
269     {
270         if(opp_get_voltage(max_opp) < opp_get_voltage(requested_opp))
271             return -EINVAL;
272         if(opp_get_freq(max_opp) < opp_get_freq(requested_opp))
273             return -EINVAL;
274        /* do things.. */
275     }
276
277opp_get_opp_count - Retrieve the number of available opps for a device
278    Example: Lets say a co-processor in the SoC needs to know the available
279    frequencies in a table, the main processor can notify as following:
280     soc_notify_coproc_available_frequencies()
281     {
282        /* Do things */
283        rcu_read_lock();
284        num_available = opp_get_opp_count(dev);
285        speeds = kzalloc(sizeof(u32) * num_available, GFP_KERNEL);
286        /* populate the table in increasing order */
287        freq = 0;
288        while (!IS_ERR(opp = opp_find_freq_ceil(dev, &freq))) {
289            speeds[i] = freq;
290            freq++;
291            i++;
292        }
293        rcu_read_unlock();
294
295        soc_notify_coproc(AVAILABLE_FREQs, speeds, num_available);
296        /* Do other things */
297     }
298
2996. Cpufreq Table Generation
300===========================
301opp_init_cpufreq_table - cpufreq framework typically is initialized with
302    cpufreq_frequency_table_cpuinfo which is provided with the list of
303    frequencies that are available for operation. This function provides
304    a ready to use conversion routine to translate the OPP layer's internal
305    information about the available frequencies into a format readily
306    providable to cpufreq.
307
308    WARNING: Do not use this function in interrupt context.
309
310    Example:
311     soc_pm_init()
312     {
313        /* Do things */
314        r = opp_init_cpufreq_table(dev, &freq_table);
315        if (!r)
316            cpufreq_frequency_table_cpuinfo(policy, freq_table);
317        /* Do other things */
318     }
319
320    NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
321    addition to CONFIG_PM as power management feature is required to
322    dynamically scale voltage and frequency in a system.
323
324opp_free_cpufreq_table - Free up the table allocated by opp_init_cpufreq_table
325
3267. Data Structures
327==================
328Typically an SoC contains multiple voltage domains which are variable. Each
329domain is represented by a device pointer. The relationship to OPP can be
330represented as follows:
331SoC
332 |- device 1
333 | |- opp 1 (availability, freq, voltage)
334 | |- opp 2 ..
335 ... ...
336 | `- opp n ..
337 |- device 2
338 ...
339 `- device m
340
341OPP library maintains a internal list that the SoC framework populates and
342accessed by various functions as described above. However, the structures
343representing the actual OPPs and domains are internal to the OPP library itself
344to allow for suitable abstraction reusable across systems.
345
346struct opp - The internal data structure of OPP library which is used to
347    represent an OPP. In addition to the freq, voltage, availability
348    information, it also contains internal book keeping information required
349    for the OPP library to operate on. Pointer to this structure is
350    provided back to the users such as SoC framework to be used as a
351    identifier for OPP in the interactions with OPP layer.
352
353    WARNING: The struct opp pointer should not be parsed or modified by the
354    users. The defaults of for an instance is populated by opp_add, but the
355    availability of the OPP can be modified by opp_enable/disable functions.
356
357struct device - This is used to identify a domain to the OPP layer. The
358    nature of the device and it's implementation is left to the user of
359    OPP library such as the SoC framework.
360
361Overall, in a simplistic view, the data structure operations is represented as
362following:
363
364Initialization / modification:
365            +-----+ /- opp_enable
366opp_add --> | opp | <-------
367  | +-----+ \- opp_disable
368  \-------> domain_info(device)
369
370Search functions:
371             /-- opp_find_freq_ceil ---\ +-----+
372domain_info<---- opp_find_freq_exact -----> | opp |
373             \-- opp_find_freq_floor ---/ +-----+
374
375Retrieval functions:
376+-----+ /- opp_get_voltage
377| opp | <---
378+-----+ \- opp_get_freq
379
380domain_info <- opp_get_opp_count
381

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