linux/Documentation/power/opp.txt
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   1Operating Performance Points (OPP) Library
   2==========================================
   3
   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===============
  181.1 What is an Operating Performance Point (OPP)?
  19
  20Complex 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
  24domains 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
  28the device will support per domain are called Operating Performance Points or
  29OPPs.
  30
  31As 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
  43OPP 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
  55Architectures that provide a SoC framework for OPP should select ARCH_HAS_OPP
  56to make the OPP layer available.
  57
  58OPP 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
 196                        /* do something when we can't satisfy the req */
 197                /* 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
 339opp_free_cpufreq_table - Free up the table allocated by opp_init_cpufreq_table
 340
 3417. 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
 396
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