regulator: documentation - overview

This adds overview documentation describing the regulator framework and
nomenclature used in the interface specific documentation and code.

Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>

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+Linux voltage and current regulator framework
+=============================================
+
+About
+=====
+
+This framework is designed to provide a standard kernel interface to control
+voltage and current regulators.
+
+The intention is to allow systems to dynamically control regulator power output
+in order to save power and prolong battery life. This applies to both voltage
+regulators (where voltage output is controllable) and current sinks (where
+current limit is controllable).
+
+(C) 2008  Wolfson Microelectronics PLC.
+Author: Liam Girdwood <lg@opensource.wolfsonmicro.com>
+
+
+Nomenclature
+============
+
+Some terms used in this document:-
+
+  o Regulator    - Electronic device that supplies power to other devices.
+                   Most regulators can enable and disable their output whilst
+                   some can control their output voltage and or current.
+
+                   Input Voltage -> Regulator -> Output Voltage
+
+
+  o PMIC         - Power Management IC. An IC that contains numerous regulators
+                   and often contains other susbsystems.
+
+
+  o Consumer     - Electronic device that is supplied power by a regulator.
+                   Consumers can be classified into two types:-
+
+                   Static: consumer does not change it's supply voltage or
+                   current limit. It only needs to enable or disable it's
+                   power supply. It's supply voltage is set by the hardware,
+                   bootloader, firmware or kernel board initialisation code.
+
+                   Dynamic: consumer needs to change it's supply voltage or
+                   current limit to meet operation demands.
+
+
+  o Power Domain - Electronic circuit that is supplied it's input power by the
+                   output power of a regulator, switch or by another power
+                   domain.
+
+                   The supply regulator may be behind a switch(s). i.e.
+
+                   Regulator -+-> Switch-1 -+-> Switch-2 --> [Consumer A]
+                              |             |
+                              |             +-> [Consumer B], [Consumer C]
+                              |
+                              +-> [Consumer D], [Consumer E]
+
+                   That is one regulator and three power domains:
+
+                   Domain 1: Switch-1, Consumers D & E.
+                   Domain 2: Switch-2, Consumers B & C.
+                   Domain 3: Consumer A.
+
+                   and this represents a "supplies" relationship:
+
+                   Domain-1 --> Domain-2 --> Domain-3.
+
+                   A power domain may have regulators that are supplied power
+                   by other regulators. i.e.
+
+                   Regulator-1 -+-> Regulator-2 -+-> [Consumer A]
+                                |
+                                +-> [Consumer B]
+
+                   This gives us two regulators and two power domains:
+
+                   Domain 1: Regulator-2, Consumer B.
+                   Domain 2: Consumer A.
+
+                   and a "supplies" relationship:
+
+                   Domain-1 --> Domain-2
+
+
+  o Constraints  - Constraints are used to define power levels for performance
+                   and hardware protection. Constraints exist at three levels:
+
+                   Regulator Level: This is defined by the regulator hardware
+                   operating parameters and is specified in the regulator
+                   datasheet. i.e.
+
+                     - voltage output is in the range 800mV -> 3500mV.
+                     - regulator current output limit is 20mA @ 5V but is
+                       10mA @ 10V.
+
+                   Power Domain Level: This is defined in software by kernel
+                   level board initialisation code. It is used to constrain a
+                   power domain to a particular power range. i.e.
+
+                     - Domain-1 voltage is 3300mV
+                     - Domain-2 voltage is 1400mV -> 1600mV
+                     - Domain-3 current limit is 0mA -> 20mA.
+
+                   Consumer Level: This is defined by consumer drivers
+                   dynamically setting voltage or current limit levels.
+
+                   e.g. a consumer backlight driver asks for a current increase
+                   from 5mA to 10mA to increase LCD illumination. This passes
+                   to through the levels as follows :-
+
+                   Consumer: need to increase LCD brightness. Lookup and
+                   request next current mA value in brightness table (the
+                   consumer driver could be used on several different
+                   personalities based upon the same reference device).
+
+                   Power Domain: is the new current limit within the domain
+                   operating limits for this domain and system state (e.g.
+                   battery power, USB power)
+
+                   Regulator Domains: is the new current limit within the
+                   regulator operating parameters for input/ouput voltage.
+
+                   If the regulator request passes all the constraint tests
+                   then the new regulator value is applied.
+
+
+Design
+======
+
+The framework is designed and targeted at SoC based devices but may also be
+relevant to non SoC devices and is split into the following four interfaces:-
+
+
+   1. Consumer driver interface.
+
+      This uses a similar API to the kernel clock interface in that consumer
+      drivers can get and put a regulator (like they can with clocks atm) and
+      get/set voltage, current limit, mode, enable and disable. This should
+      allow consumers complete control over their supply voltage and current
+      limit. This also compiles out if not in use so drivers can be reused in
+      systems with no regulator based power control.
+
+        See Documentation/power/regulator/consumer.txt
+
+   2. Regulator driver interface.
+
+      This allows regulator drivers to register their regulators and provide
+      operations to the core. It also has a notifier call chain for propagating
+      regulator events to clients.
+
+        See Documentation/power/regulator/regulator.txt
+
+   3. Machine interface.
+
+      This interface is for machine specific code and allows the creation of
+      voltage/current domains (with constraints) for each regulator. It can
+      provide regulator constraints that will prevent device damage through
+      overvoltage or over current caused by buggy client drivers. It also
+      allows the creation of a regulator tree whereby some regulators are
+      supplied by others (similar to a clock tree).
+
+        See Documentation/power/regulator/machine.txt
+
+   4. Userspace ABI.
+
+      The framework also exports a lot of useful voltage/current/opmode data to
+      userspace via sysfs. This could be used to help monitor device power
+      consumption and status.
+
+        See Documentation/ABI/testing/regulator-sysfs.txt