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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-11 08:27:49 +0000
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+==========================================
+Operating Performance Points (OPP) Library
+==========================================
+
+(C) 2009-2010 Nishanth Menon <nm@ti.com>, Texas Instruments Incorporated
+
+.. Contents
+
+ 1. Introduction
+ 2. Initial OPP List Registration
+ 3. OPP Search Functions
+ 4. OPP Availability Control Functions
+ 5. OPP Data Retrieval Functions
+ 6. Data Structures
+
+1. Introduction
+===============
+
+1.1 What is an Operating Performance Point (OPP)?
+-------------------------------------------------
+
+Complex SoCs of today consists of a multiple sub-modules working in conjunction.
+In an operational system executing varied use cases, not all modules in the SoC
+need to function at their highest performing frequency all the time. To
+facilitate this, sub-modules in a SoC are grouped into domains, allowing some
+domains to run at lower voltage and frequency while other domains run at
+voltage/frequency pairs that are higher.
+
+The set of discrete tuples consisting of frequency and voltage pairs that
+the device will support per domain are called Operating Performance Points or
+OPPs.
+
+As an example:
+
+Let us consider an MPU device which supports the following:
+{300MHz at minimum voltage of 1V}, {800MHz at minimum voltage of 1.2V},
+{1GHz at minimum voltage of 1.3V}
+
+We can represent these as three OPPs as the following {Hz, uV} tuples:
+
+- {300000000, 1000000}
+- {800000000, 1200000}
+- {1000000000, 1300000}
+
+1.2 Operating Performance Points Library
+----------------------------------------
+
+OPP library provides a set of helper functions to organize and query the OPP
+information. The library is located in drivers/opp/ directory and the header
+is located in include/linux/pm_opp.h. OPP library can be enabled by enabling
+CONFIG_PM_OPP from power management menuconfig menu. Certain SoCs such as Texas
+Instrument's OMAP framework allows to optionally boot at a certain OPP without
+needing cpufreq.
+
+Typical usage of the OPP library is as follows::
+
+ (users) -> registers a set of default OPPs -> (library)
+ SoC framework -> modifies on required cases certain OPPs -> OPP layer
+ -> queries to search/retrieve information ->
+
+OPP layer expects each domain to be represented by a unique device pointer. SoC
+framework registers a set of initial OPPs per device with the OPP layer. This
+list is expected to be an optimally small number typically around 5 per device.
+This initial list contains a set of OPPs that the framework expects to be safely
+enabled by default in the system.
+
+Note on OPP Availability
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+As the system proceeds to operate, SoC framework may choose to make certain
+OPPs available or not available on each device based on various external
+factors. Example usage: Thermal management or other exceptional situations where
+SoC framework might choose to disable a higher frequency OPP to safely continue
+operations until that OPP could be re-enabled if possible.
+
+OPP library facilitates this concept in its implementation. The following
+operational functions operate only on available opps:
+dev_pm_opp_find_freq_{ceil, floor}, dev_pm_opp_get_voltage, dev_pm_opp_get_freq,
+dev_pm_opp_get_opp_count.
+
+dev_pm_opp_find_freq_exact is meant to be used to find the opp pointer
+which can then be used for dev_pm_opp_enable/disable functions to make an
+opp available as required.
+
+WARNING: Users of OPP library should refresh their availability count using
+get_opp_count if dev_pm_opp_enable/disable functions are invoked for a
+device, the exact mechanism to trigger these or the notification mechanism
+to other dependent subsystems such as cpufreq are left to the discretion of
+the SoC specific framework which uses the OPP library. Similar care needs
+to be taken care to refresh the cpufreq table in cases of these operations.
+
+2. Initial OPP List Registration
+================================
+The SoC implementation calls dev_pm_opp_add function iteratively to add OPPs per
+device. It is expected that the SoC framework will register the OPP entries
+optimally- typical numbers range to be less than 5. The list generated by
+registering the OPPs is maintained by OPP library throughout the device
+operation. The SoC framework can subsequently control the availability of the
+OPPs dynamically using the dev_pm_opp_enable / disable functions.
+
+dev_pm_opp_add
+ Add a new OPP for a specific domain represented by the device pointer.
+ The OPP is defined using the frequency and voltage. Once added, the OPP
+ is assumed to be available and control of its availability can be done
+ with the dev_pm_opp_enable/disable functions. OPP library
+ internally stores and manages this information in the dev_pm_opp struct.
+ This function may be used by SoC framework to define a optimal list
+ as per the demands of SoC usage environment.
+
+ WARNING:
+ Do not use this function in interrupt context.
+
+ Example::
+
+ soc_pm_init()
+ {
+ /* Do things */
+ r = dev_pm_opp_add(mpu_dev, 1000000, 900000);
+ if (!r) {
+ pr_err("%s: unable to register mpu opp(%d)\n", r);
+ goto no_cpufreq;
+ }
+ /* Do cpufreq things */
+ no_cpufreq:
+ /* Do remaining things */
+ }
+
+3. OPP Search Functions
+=======================
+High level framework such as cpufreq operates on frequencies. To map the
+frequency back to the corresponding OPP, OPP library provides handy functions
+to search the OPP list that OPP library internally manages. These search
+functions return the matching pointer representing the opp if a match is
+found, else returns error. These errors are expected to be handled by standard
+error checks such as IS_ERR() and appropriate actions taken by the caller.
+
+Callers of these functions shall call dev_pm_opp_put() after they have used the
+OPP. Otherwise the memory for the OPP will never get freed and result in
+memleak.
+
+dev_pm_opp_find_freq_exact
+ Search for an OPP based on an *exact* frequency and
+ availability. This function is especially useful to enable an OPP which
+ is not available by default.
+ Example: In a case when SoC framework detects a situation where a
+ higher frequency could be made available, it can use this function to
+ find the OPP prior to call the dev_pm_opp_enable to actually make
+ it available::
+
+ opp = dev_pm_opp_find_freq_exact(dev, 1000000000, false);
+ dev_pm_opp_put(opp);
+ /* dont operate on the pointer.. just do a sanity check.. */
+ if (IS_ERR(opp)) {
+ pr_err("frequency not disabled!\n");
+ /* trigger appropriate actions.. */
+ } else {
+ dev_pm_opp_enable(dev,1000000000);
+ }
+
+ NOTE:
+ This is the only search function that operates on OPPs which are
+ not available.
+
+dev_pm_opp_find_freq_floor
+ Search for an available OPP which is *at most* the
+ provided frequency. This function is useful while searching for a lesser
+ match OR operating on OPP information in the order of decreasing
+ frequency.
+ Example: To find the highest opp for a device::
+
+ freq = ULONG_MAX;
+ opp = dev_pm_opp_find_freq_floor(dev, &freq);
+ dev_pm_opp_put(opp);
+
+dev_pm_opp_find_freq_ceil
+ Search for an available OPP which is *at least* the
+ provided frequency. This function is useful while searching for a
+ higher match OR operating on OPP information in the order of increasing
+ frequency.
+ Example 1: To find the lowest opp for a device::
+
+ freq = 0;
+ opp = dev_pm_opp_find_freq_ceil(dev, &freq);
+ dev_pm_opp_put(opp);
+
+ Example 2: A simplified implementation of a SoC cpufreq_driver->target::
+
+ soc_cpufreq_target(..)
+ {
+ /* Do stuff like policy checks etc. */
+ /* Find the best frequency match for the req */
+ opp = dev_pm_opp_find_freq_ceil(dev, &freq);
+ dev_pm_opp_put(opp);
+ if (!IS_ERR(opp))
+ soc_switch_to_freq_voltage(freq);
+ else
+ /* do something when we can't satisfy the req */
+ /* do other stuff */
+ }
+
+4. OPP Availability Control Functions
+=====================================
+A default OPP list registered with the OPP library may not cater to all possible
+situation. The OPP library provides a set of functions to modify the
+availability of a OPP within the OPP list. This allows SoC frameworks to have
+fine grained dynamic control of which sets of OPPs are operationally available.
+These functions are intended to *temporarily* remove an OPP in conditions such
+as thermal considerations (e.g. don't use OPPx until the temperature drops).
+
+WARNING:
+ Do not use these functions in interrupt context.
+
+dev_pm_opp_enable
+ Make a OPP available for operation.
+ Example: Lets say that 1GHz OPP is to be made available only if the
+ SoC temperature is lower than a certain threshold. The SoC framework
+ implementation might choose to do something as follows::
+
+ if (cur_temp < temp_low_thresh) {
+ /* Enable 1GHz if it was disabled */
+ opp = dev_pm_opp_find_freq_exact(dev, 1000000000, false);
+ dev_pm_opp_put(opp);
+ /* just error check */
+ if (!IS_ERR(opp))
+ ret = dev_pm_opp_enable(dev, 1000000000);
+ else
+ goto try_something_else;
+ }
+
+dev_pm_opp_disable
+ Make an OPP to be not available for operation
+ Example: Lets say that 1GHz OPP is to be disabled if the temperature
+ exceeds a threshold value. The SoC framework implementation might
+ choose to do something as follows::
+
+ if (cur_temp > temp_high_thresh) {
+ /* Disable 1GHz if it was enabled */
+ opp = dev_pm_opp_find_freq_exact(dev, 1000000000, true);
+ dev_pm_opp_put(opp);
+ /* just error check */
+ if (!IS_ERR(opp))
+ ret = dev_pm_opp_disable(dev, 1000000000);
+ else
+ goto try_something_else;
+ }
+
+5. OPP Data Retrieval Functions
+===============================
+Since OPP library abstracts away the OPP information, a set of functions to pull
+information from the dev_pm_opp structure is necessary. Once an OPP pointer is
+retrieved using the search functions, the following functions can be used by SoC
+framework to retrieve the information represented inside the OPP layer.
+
+dev_pm_opp_get_voltage
+ Retrieve the voltage represented by the opp pointer.
+ Example: At a cpufreq transition to a different frequency, SoC
+ framework requires to set the voltage represented by the OPP using
+ the regulator framework to the Power Management chip providing the
+ voltage::
+
+ soc_switch_to_freq_voltage(freq)
+ {
+ /* do things */
+ opp = dev_pm_opp_find_freq_ceil(dev, &freq);
+ v = dev_pm_opp_get_voltage(opp);
+ dev_pm_opp_put(opp);
+ if (v)
+ regulator_set_voltage(.., v);
+ /* do other things */
+ }
+
+dev_pm_opp_get_freq
+ Retrieve the freq represented by the opp pointer.
+ Example: Lets say the SoC framework uses a couple of helper functions
+ we could pass opp pointers instead of doing additional parameters to
+ handle quiet a bit of data parameters::
+
+ soc_cpufreq_target(..)
+ {
+ /* do things.. */
+ max_freq = ULONG_MAX;
+ max_opp = dev_pm_opp_find_freq_floor(dev,&max_freq);
+ requested_opp = dev_pm_opp_find_freq_ceil(dev,&freq);
+ if (!IS_ERR(max_opp) && !IS_ERR(requested_opp))
+ r = soc_test_validity(max_opp, requested_opp);
+ dev_pm_opp_put(max_opp);
+ dev_pm_opp_put(requested_opp);
+ /* do other things */
+ }
+ soc_test_validity(..)
+ {
+ if(dev_pm_opp_get_voltage(max_opp) < dev_pm_opp_get_voltage(requested_opp))
+ return -EINVAL;
+ if(dev_pm_opp_get_freq(max_opp) < dev_pm_opp_get_freq(requested_opp))
+ return -EINVAL;
+ /* do things.. */
+ }
+
+dev_pm_opp_get_opp_count
+ Retrieve the number of available opps for a device
+ Example: Lets say a co-processor in the SoC needs to know the available
+ frequencies in a table, the main processor can notify as following::
+
+ soc_notify_coproc_available_frequencies()
+ {
+ /* Do things */
+ num_available = dev_pm_opp_get_opp_count(dev);
+ speeds = kzalloc(sizeof(u32) * num_available, GFP_KERNEL);
+ /* populate the table in increasing order */
+ freq = 0;
+ while (!IS_ERR(opp = dev_pm_opp_find_freq_ceil(dev, &freq))) {
+ speeds[i] = freq;
+ freq++;
+ i++;
+ dev_pm_opp_put(opp);
+ }
+
+ soc_notify_coproc(AVAILABLE_FREQs, speeds, num_available);
+ /* Do other things */
+ }
+
+6. Data Structures
+==================
+Typically an SoC contains multiple voltage domains which are variable. Each
+domain is represented by a device pointer. The relationship to OPP can be
+represented as follows::
+
+ SoC
+ |- device 1
+ | |- opp 1 (availability, freq, voltage)
+ | |- opp 2 ..
+ ... ...
+ | `- opp n ..
+ |- device 2
+ ...
+ `- device m
+
+OPP library maintains a internal list that the SoC framework populates and
+accessed by various functions as described above. However, the structures
+representing the actual OPPs and domains are internal to the OPP library itself
+to allow for suitable abstraction reusable across systems.
+
+struct dev_pm_opp
+ The internal data structure of OPP library which is used to
+ represent an OPP. In addition to the freq, voltage, availability
+ information, it also contains internal book keeping information required
+ for the OPP library to operate on. Pointer to this structure is
+ provided back to the users such as SoC framework to be used as a
+ identifier for OPP in the interactions with OPP layer.
+
+ WARNING:
+ The struct dev_pm_opp pointer should not be parsed or modified by the
+ users. The defaults of for an instance is populated by
+ dev_pm_opp_add, but the availability of the OPP can be modified
+ by dev_pm_opp_enable/disable functions.
+
+struct device
+ This is used to identify a domain to the OPP layer. The
+ nature of the device and its implementation is left to the user of
+ OPP library such as the SoC framework.
+
+Overall, in a simplistic view, the data structure operations is represented as
+following::
+
+ Initialization / modification:
+ +-----+ /- dev_pm_opp_enable
+ dev_pm_opp_add --> | opp | <-------
+ | +-----+ \- dev_pm_opp_disable
+ \-------> domain_info(device)
+
+ Search functions:
+ /-- dev_pm_opp_find_freq_ceil ---\ +-----+
+ domain_info<---- dev_pm_opp_find_freq_exact -----> | opp |
+ \-- dev_pm_opp_find_freq_floor ---/ +-----+
+
+ Retrieval functions:
+ +-----+ /- dev_pm_opp_get_voltage
+ | opp | <---
+ +-----+ \- dev_pm_opp_get_freq
+
+ domain_info <- dev_pm_opp_get_opp_count