Documentation/power/opp.txt - kernel/msm-4.9 - Gitiles

 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. Cpufreq Table Generation
 7. 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/base/power/opp.c 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. OPP library depends on
 CONFIG_PM as 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	->

 Architectures that provide a SoC framework for OPP should select ARCH_HAS_OPP
 to make the OPP layer available.

 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 it's implementation. The following
 operational functions operate only on available opps:
 opp_find_freq_{ceil, floor}, dev_pm_opp_get_voltage, dev_pm_opp_get_freq, dev_pm_opp_get_opp_count
 and dev_pm_opp_init_cpufreq_table

 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.

 WARNING on OPP List locking mechanism:
 -------------------------------------------------
 OPP library uses RCU for exclusivity. RCU allows the query functions to operate
 in multiple contexts and this synchronization mechanism is optimal for a read
 intensive operations on data structure as the OPP library caters to.

 To ensure that the data retrieved are sane, the users such as SoC framework
 should ensure that the section of code operating on OPP queries are locked
 using RCU read locks. The opp_find_freq_{exact,ceil,floor},
 opp_get_{voltage, freq, opp_count} fall into this category.

 opp_{add,enable,disable} are updaters which use mutex and implement it's own
 RCU locking mechanisms. dev_pm_opp_init_cpufreq_table acts as an updater and uses
 mutex to implment RCU updater strategy. These functions should *NOT* be called
 under RCU locks and other contexts that prevent blocking functions in RCU or
 mutex operations from working.

 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 it's availability can be done
 	with the dev_pm_opp_enable/disable functions. OPP library internally stores
 	and manages this information in the 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.

 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.
 	 rcu_read_lock();
 	 opp = dev_pm_opp_find_freq_exact(dev, 1000000000, false);
 	 rcu_read_unlock();
 	 /* 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;
 	 rcu_read_lock();
 	 dev_pm_opp_find_freq_floor(dev, &freq);
 	 rcu_read_unlock();

 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;
 	 rcu_read_lock();
 	 dev_pm_opp_find_freq_ceil(dev, &freq);
 	 rcu_read_unlock();
 	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 */
 		rcu_read_lock();
 		opp = dev_pm_opp_find_freq_ceil(dev, &freq);
 		rcu_read_unlock();
 		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 */
 		rcu_read_lock();
 		opp = dev_pm_opp_find_freq_exact(dev, 1000000000, false);
 		rcu_read_unlock();
 		/* 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 */
 		rcu_read_lock();
 		opp = dev_pm_opp_find_freq_exact(dev, 1000000000, true);
 		rcu_read_unlock();
 		/* 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 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 */
 		rcu_read_lock();
 		opp = dev_pm_opp_find_freq_ceil(dev, &freq);
 		v = dev_pm_opp_get_voltage(opp);
 		rcu_read_unlock();
 		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;
 		 rcu_read_lock();
 		 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);
 		 rcu_read_unlock();
 		/* 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 */
 		rcu_read_lock();
 		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++;
 		}
 		rcu_read_unlock();

 		soc_notify_coproc(AVAILABLE_FREQs, speeds, num_available);
 		/* Do other things */
 	 }

 6. Cpufreq Table Generation
 ===========================
 dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with
 	cpufreq_frequency_table_cpuinfo which is provided with the list of
 	frequencies that are available for operation. This function provides
 	a ready to use conversion routine to translate the OPP layer's internal
 	information about the available frequencies into a format readily
 	providable to cpufreq.

 	WARNING: Do not use this function in interrupt context.

 	Example:
 	 soc_pm_init()
 	 {
 		/* Do things */
 		r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
 		if (!r)
 			cpufreq_frequency_table_cpuinfo(policy, freq_table);
 		/* Do other things */
 	 }

 	NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
 	addition to CONFIG_PM as power management feature is required to
 	dynamically scale voltage and frequency in a system.

 dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table

 7. 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 it's 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
	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. Cpufreq Table Generation
	7. 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/base/power/opp.c 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. OPP library depends on
	CONFIG_PM as 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 ->

	Architectures that provide a SoC framework for OPP should select ARCH_HAS_OPP
	to make the OPP layer available.

	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 it's implementation. The following
	operational functions operate only on available opps:
	opp_find_freq_{ceil, floor}, dev_pm_opp_get_voltage, dev_pm_opp_get_freq, dev_pm_opp_get_opp_count
	and dev_pm_opp_init_cpufreq_table

	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.

	WARNING on OPP List locking mechanism:
	-------------------------------------------------
	OPP library uses RCU for exclusivity. RCU allows the query functions to operate
	in multiple contexts and this synchronization mechanism is optimal for a read
	intensive operations on data structure as the OPP library caters to.

	To ensure that the data retrieved are sane, the users such as SoC framework
	should ensure that the section of code operating on OPP queries are locked
	using RCU read locks. The opp_find_freq_{exact,ceil,floor},
	opp_get_{voltage, freq, opp_count} fall into this category.

	opp_{add,enable,disable} are updaters which use mutex and implement it's own
	RCU locking mechanisms. dev_pm_opp_init_cpufreq_table acts as an updater and uses
	mutex to implment RCU updater strategy. These functions should NOT be called
	under RCU locks and other contexts that prevent blocking functions in RCU or
	mutex operations from working.

	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 it's availability can be done
	with the dev_pm_opp_enable/disable functions. OPP library internally stores
	and manages this information in the 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.

	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.
	rcu_read_lock();
	opp = dev_pm_opp_find_freq_exact(dev, 1000000000, false);
	rcu_read_unlock();
	/* 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;
	rcu_read_lock();
	dev_pm_opp_find_freq_floor(dev, &freq);
	rcu_read_unlock();

	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;
	rcu_read_lock();
	dev_pm_opp_find_freq_ceil(dev, &freq);
	rcu_read_unlock();
	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 */
	rcu_read_lock();
	opp = dev_pm_opp_find_freq_ceil(dev, &freq);
	rcu_read_unlock();
	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 */
	rcu_read_lock();
	opp = dev_pm_opp_find_freq_exact(dev, 1000000000, false);
	rcu_read_unlock();
	/* 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 */
	rcu_read_lock();
	opp = dev_pm_opp_find_freq_exact(dev, 1000000000, true);
	rcu_read_unlock();
	/* 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 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 */
	rcu_read_lock();
	opp = dev_pm_opp_find_freq_ceil(dev, &freq);
	v = dev_pm_opp_get_voltage(opp);
	rcu_read_unlock();
	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;
	rcu_read_lock();
	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);
	rcu_read_unlock();
	/* 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 */
	rcu_read_lock();
	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++;
	}
	rcu_read_unlock();

	soc_notify_coproc(AVAILABLE_FREQs, speeds, num_available);
	/* Do other things */
	}

	6. Cpufreq Table Generation
	===========================
	dev_pm_opp_init_cpufreq_table - cpufreq framework typically is initialized with
	cpufreq_frequency_table_cpuinfo which is provided with the list of
	frequencies that are available for operation. This function provides
	a ready to use conversion routine to translate the OPP layer's internal
	information about the available frequencies into a format readily
	providable to cpufreq.

	WARNING: Do not use this function in interrupt context.

	Example:
	soc_pm_init()
	{
	/* Do things */
	r = dev_pm_opp_init_cpufreq_table(dev, &freq_table);
	if (!r)
	cpufreq_frequency_table_cpuinfo(policy, freq_table);
	/* Do other things */
	}

	NOTE: This function is available only if CONFIG_CPU_FREQ is enabled in
	addition to CONFIG_PM as power management feature is required to
	dynamically scale voltage and frequency in a system.

	dev_pm_opp_free_cpufreq_table - Free up the table allocated by dev_pm_opp_init_cpufreq_table

	7. 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 it's 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