Introduction
------------

The configuration database is a 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 on 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"/"int"
  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" <prompt> ["if" <expr>]
  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" <expr> ["if" <expr>]
  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".

- type definition + default value:
	"def_bool"/"def_tristate" <expr> ["if" <expr>]
  This is a shorthand notation for a type definition plus a value.
  Optionally dependencies for this default value can be added with "if".

- dependencies: "depends on" <expr>
  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" <symbol> ["if" <expr>]
  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 <symbol> can be set to. If <symbol> is selected multiple
  times, the limit is set to the largest selection.
  Reverse dependencies can only be used with boolean or tristate
  symbols.
  Note:
	select is evil.... select will by brute force set a symbol
	equal to 'y' without visiting the dependencies. So abusing
	select you are able to select a symbol FOO even if FOO depends
	on BAR that is not set. In general use select only for
	non-visible symbols (no prompts anywhere) and for symbols with
	no dependencies. That will limit the usefulness but on the
	other hand avoid the illegal configurations all over. kconfig
	should one day warn about such things.

- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
  This allows to limit the range of possible input values for int
  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 separate 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:

<expr> ::= <symbol>                             (1)
           <symbol> '=' <symbol>                (2)
           <symbol> '!=' <symbol>               (3)
           '(' <expr> ')'                       (4)
           '!' <expr>                           (5)
           <expr> '&&' <expr>                   (6)
           <expr> '||' <expr>                   (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 min(/expr/, /expr/).
(7) Returns the result of max(/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 non-constant symbols.
Non-constant symbols are the most common ones and are defined with the
'config' statement. Non-constant 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 on 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 on MODULES

comment "module support disabled"
	depends on !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" <symbol>
	<config options>

This defines a config symbol <symbol> and accepts any of above
attributes as options.

menuconfig:
	"menuconfig" <symbol>
	<config options>

This is similar 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"
	<choice options>
	<choice block>
	"endchoice"

This defines a choice group and accepts any of the 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" <prompt>
	<comment options>

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" <prompt>
	<menu options>
	<menu block>
	"endmenu"

This defines a menu block, see "Menu structure" above for more
information. The only possible options are dependencies.

if:

	"if" <expr>
	<if block>
	"endif"

This defines an if block. The dependency expression <expr> is appended
to all enclosed menu entries.

source:

	"source" <prompt>

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

mainmenu:

	"mainmenu" <prompt>

This sets the config program's title bar if the config program chooses
to use it.


Kconfig hints
-------------
This is a collection of Kconfig tips, most of which aren't obvious at
first glance and most of which have become idioms in several Kconfig
files.

Build as module only
~~~~~~~~~~~~~~~~~~~~
To restrict a component build to module-only, qualify its config symbol
with "depends on m".  E.g.:

config FOO
	depends on BAR && m

limits FOO to module (=m) or disabled (=n).


Build limited by a third config symbol which may be =y or =m
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A common idiom that we see (and sometimes have problems with) is this:

When option C in B (module or subsystem) uses interfaces from A (module
or subsystem), and both A and B are tristate (could be =y or =m if they
were independent of each other, but they aren't), then we need to limit
C such that it cannot be built statically if A is built as a loadable
module.  (C already depends on B, so there is no dependency issue to
take care of here.)

If A is linked statically into the kernel image, C can be built
statically or as loadable module(s).  However, if A is built as loadable
module(s), then C must be restricted to loadable module(s) also.  This
can be expressed in kconfig language as:

config C
	depends on A = y || A = B

or for real examples, use this command in a kernel tree:

$ find . -name Kconfig\* | xargs grep -ns "depends on.*=.*||.*=" | grep -v orig