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).
  
  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).