--- /dev/null
+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.
+ At most one symbol may have the "modules" option set.
+
+ - "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).
+
+ - "allnoconfig_y"
+ This declares the symbol as one that should have the value y when
+ using "allnoconfig". Used for symbols that hide other symbols.
+
+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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