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Based on kernel version 3.9. Page generated on 2013-05-02 23:09 EST.

	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 should be used with care. select will force
		a symbol to a value without visiting the dependencies.
		By 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.
	
	- limiting menu display: "visible if" <expr>
	  This attribute is only applicable to menu blocks, if the condition is
	  false, the menu block is not displayed to the user (the symbols
	  contained there can still be selected by other symbols, though). It is
	  similar to a conditional "prompt" attribute for individual menu
	  entries. Default value of "visible" is true.
	
	- 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.
	
	- misc options: "option" <symbol>[=<value>]
	  Various less common options can be defined via this option syntax,
	  which can modify the behaviour of the menu entry and its config
	  symbol. These options are currently possible:
	
	  - "defconfig_list"
	    This declares a list of default entries which can be used when
	    looking for the default configuration (which is used when the main
	    .config doesn't exists yet.)
	
	  - "modules"
	    This declares the symbol to be used as the MODULES symbol, which
	    enables the third modular state for all config symbols.
	
	  - "env"=<value>
	    This imports the environment variable into Kconfig. It behaves like
	    a default, except that the value comes from the environment, this
	    also means that the behaviour when mixing it with normal defaults is
	    undefined at this point. The symbol is currently not exported back
	    to the build environment (if this is desired, it can be done via
	    another symbol).
	
	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 its
	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" [symbol]
		<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.
	If no [symbol] is associated with a choice, then you can not have multiple
	definitions of that choice. If a [symbol] is associated to the choice,
	then you may define the same choice (ie. with the same entries) in another
	place.
	
	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 and "visible"
	attributes.
	
	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. It should be placed at the top of the configuration, before any
	other statement.
	
	
	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.
	
	Adding common features and make the usage configurable
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	It is a common idiom to implement a feature/functionality that are
	relevant for some architectures but not all.
	The recommended way to do so is to use a config variable named HAVE_*
	that is defined in a common Kconfig file and selected by the relevant
	architectures.
	An example is the generic IOMAP functionality.
	
	We would in lib/Kconfig see:
	
	# Generic IOMAP is used to ...
	config HAVE_GENERIC_IOMAP
	
	config GENERIC_IOMAP
		depends on HAVE_GENERIC_IOMAP && FOO
	
	And in lib/Makefile we would see:
	obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
	
	For each architecture using the generic IOMAP functionality we would see:
	
	config X86
		select ...
		select HAVE_GENERIC_IOMAP
		select ...
	
	Note: we use the existing config option and avoid creating a new
	config variable to select HAVE_GENERIC_IOMAP.
	
	Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
	introduced to overcome the limitation of select which will force a
	config option to 'y' no matter the dependencies.
	The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
	situation where select forces a symbol equals to 'y'.
	
	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).

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• 00-INDEX
• kbuild.txt
• kconfig-language.txt
• kconfig.txt
• makefiles.txt
• modules.txt
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