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-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).
-
-Kconfig symbol existence
-~~~~~~~~~~~~~~~~~~~~~~~~
-The following two methods produce the same kconfig symbol dependencies
-but differ greatly in kconfig symbol existence (production) in the
-generated config file.
-
-case 1:
-
-config FOO
- tristate "about foo"
- depends on BAR
-
-vs. case 2:
-
-if BAR
-config FOO
- tristate "about foo"
-endif
-
-In case 1, the symbol FOO will always exist in the config file (given
-no other dependencies). In case 2, the symbol FOO will only exist in
-the config file if BAR is enabled.