cachepc-linux

opp.rst (13991B)

==========================================
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