PM / OPP: improve introductory documentation

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* OPP Library *

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Operating Performance Points (OPP) Library

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(C) 2009-2010 Nishanth Menon <nm@ti.com>, Texas Instruments Incorporated

1. Introduction

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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 are loaded

more. The set of discrete tuples consisting of frequency and voltage pairs that

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/opp.h. OPP library can be enabled by enabling