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Introduction

------------

The configuration database is collection of configuration options

organized in a tree structure:

        +- Code maturity level options

        |  +- Prompt for development and/or incomplete code/drivers

        +- General setup

        |  +- Networking support

        |  +- System V IPC

        |  +- BSD Process Accounting

        |  +- Sysctl support

        +- Loadable module support

        |  +- Enable loadable module support

        |     +- Set version information on all module symbols

        |     +- Kernel module loader

        +- ...

Every entry has its own dependencies. These dependencies are used

to determine the visibility of an entry. Any child entry is only

visible if its parent entry is also visible.

Menu entries

------------

Most entries define a config option, all other entries help to organize

them. A single configuration option is defined like this:

config MODVERSIONS

        bool "Set version information on all module symbols"

        depends MODULES

        help

          Usually, modules have to be recompiled whenever you switch to a new

          kernel.  ...

Every line starts with a key word and can be followed by multiple

arguments.  "config" starts a new config entry. The following lines

define attributes for this config option. Attributes can be the type of

the config option, input prompt, dependencies, help text and default

values. A config option can be defined multiple times with the same

name, but every definition can have only a single input prompt and the

type must not conflict.

Menu attributes

---------------

A menu entry can have a number of attributes. Not all of them are

applicable everywhere (see syntax).

- type definition: "bool"/"tristate"/"string"/"hex"/"integer"

  Every config option must have a type. There are only two basic types:

  tristate and string, the other types are based on these two. The type

  definition optionally accepts an input prompt, so these two examples

  are equivalent:

        bool "Networking support"

  and

        bool

        prompt "Networking support"

- input prompt: "prompt"  ["if" ]

  Every menu entry can have at most one prompt, which is used to display

  to the user. Optionally dependencies only for this prompt can be added

  with "if".

- default value: "default"  ["if" ]

  A config option can have any number of default values. If multiple

  default values are visible, only the first defined one is active.

  Default values are not limited to the menu entry, where they are

  defined, this means the default can be defined somewhere else or be

  overridden by an earlier definition.

  The default value is only assigned to the config symbol if no other

  value was set by the user (via the input prompt above). If an input

  prompt is visible the default value is presented to the user and can

  be overridden by him.

  Optionally dependencies only for this default value can be added with

  "if".

- dependencies: "depends on"/"requires" 

  This defines a dependency for this menu entry. If multiple

  dependencies are defined they are connected with '&&'. Dependencies

  are applied to all other options within this menu entry (which also

  accept an "if" expression), so these two examples are equivalent:

        bool "foo" if BAR

        default y if BAR

  and

        depends on BAR

        bool "foo"

        default y

- reverse dependencies: "select"  ["if" ]

  While normal dependencies reduce the upper limit of a symbol (see

  below), reverse dependencies can be used to force a lower limit of

  another symbol. The value of the current menu symbol is used as the

  minimal value  can be set to. If  is selected multiple

  times, the limit is set to the largest selection.

  Reverse dependencies can only be used with boolean or tristate

  symbols.

- numerical ranges: "range"   ["if" ]

  This allows to limit the range of possible input values for integer

  and hex symbols. The user can only input a value which is larger than

  or equal to the first symbol and smaller than or equal to the second

  symbol.

- help text: "help" or "---help---"

  This defines a help text. The end of the help text is determined by

  the indentation level, this means it ends at the first line which has

  a smaller indentation than the first line of the help text.

  "---help---" and "help" do not differ in behaviour, "---help---" is

  used to help visually seperate configuration logic from help within

  the file as an aid to developers.

Menu dependencies

-----------------

Dependencies define the visibility of a menu entry and can also reduce

the input range of tristate symbols. The tristate logic used in the

expressions uses one more state than normal boolean logic to express the

module state. Dependency expressions have the following syntax:

 ::=                              (1)

            '='                 (2)

            '!='                (3)

           '('  ')'                       (4)

           '!'                            (5)

            '&&'                    (6)

            '||'                    (7)

Expressions are listed in decreasing order of precedence.

(1) Convert the symbol into an expression. Boolean and tristate symbols

    are simply converted into the respective expression values. All

    other symbol types result in 'n'.

(2) If the values of both symbols are equal, it returns 'y',

    otherwise 'n'.

(3) If the values of both symbols are equal, it returns 'n',

    otherwise 'y'.

(4) Returns the value of the expression. Used to override precedence.

(5) Returns the result of (2-/expr/).

(6) Returns the result of max(/expr/, /expr/).

(7) Returns the result of min(/expr/, /expr/).

An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2

respectively for calculations). A menu entry becomes visible when it's

expression evaluates to 'm' or 'y'.

There are two types of symbols: constant and nonconstant symbols.

Nonconstant symbols are the most common ones and are defined with the

'config' statement. Nonconstant symbols consist entirely of alphanumeric

characters or underscores.

Constant symbols are only part of expressions. Constant symbols are

always surrounded by single or double quotes. Within the quote any

other character is allowed and the quotes can be escaped using '\'.

Menu structure

--------------

The position of a menu entry in the tree is determined in two ways. First

it can be specified explicitly:

menu "Network device support"

        depends NET

config NETDEVICES

        ...

endmenu

All entries within the "menu" ... "endmenu" block become a submenu of

"Network device support". All subentries inherit the dependencies from

the menu entry, e.g. this means the dependency "NET" is added to the

dependency list of the config option NETDEVICES.

The other way to generate the menu structure is done by analyzing the

dependencies. If a menu entry somehow depends on the previous entry, it

can be made a submenu of it. First, the previous (parent) symbol must

be part of the dependency list and then one of these two conditions

must be true:

- the child entry must become invisible, if the parent is set to 'n'

- the child entry must only be visible, if the parent is visible

config MODULES

        bool "Enable loadable module support"

config MODVERSIONS

        bool "Set version information on all module symbols"

        depends MODULES

comment "module support disabled"

        depends !MODULES

MODVERSIONS directly depends on MODULES, this means it's only visible if

MODULES is different from 'n'. The comment on the other hand is always

visible when MODULES is visible (the (empty) dependency of MODULES is

also part of the comment dependencies).

Kconfig syntax

--------------

The configuration file describes a series of menu entries, where every

line starts with a keyword (except help texts). The following keywords

end a menu entry:

- config

- menuconfig

- choice/endchoice

- comment

- menu/endmenu

- if/endif

- source

The first five also start the definition of a menu entry.

config:

        "config" 

This defines a config symbol  and accepts any of above

attributes as options.

menuconfig:

        "menuconfig" 

This is similiar to the simple config entry above, but it also gives a

hint to front ends, that all suboptions should be displayed as a

separate list of options.

choices:

        "choice"

        "endchoice"

This defines a choice group and accepts any of above attributes as

options. A choice can only be of type bool or tristate, while a boolean

choice only allows a single config entry to be selected, a tristate

choice also allows any number of config entries to be set to 'm'. This

can be used if multiple drivers for a single hardware exists and only a

single driver can be compiled/loaded into the kernel, but all drivers

can be compiled as modules.

A choice accepts another option "optional", which allows to set the

choice to 'n' and no entry needs to be selected.

comment:

        "comment" 

This defines a comment which is displayed to the user during the

configuration process and is also echoed to the output files. The only

possible options are dependencies.

menu:

        "menu" 

        "endmenu"

This defines a menu block, see "Menu structure" above for more

information. The only possible options are dependencies.

if:

        "if" 

        "endif"

This defines an if block. The dependency expression  is appended

to all enclosed menu entries.

source:

        "source" 

This reads the specified configuration file. This file is always parsed.