linux/Documentation/kbuild/kconfig-language.txt
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   1Introduction
   2------------
   3
   4The configuration database is a collection of configuration options
   5organized in a tree structure:
   6
   7        +- Code maturity level options
   8        |  +- Prompt for development and/or incomplete code/drivers
   9        +- General setup
  10        |  +- Networking support
  11        |  +- System V IPC
  12        |  +- BSD Process Accounting
  13        |  +- Sysctl support
  14        +- Loadable module support
  15        |  +- Enable loadable module support
  16        |     +- Set version information on all module symbols
  17        |     +- Kernel module loader
  18        +- ...
  19
  20Every entry has its own dependencies. These dependencies are used
  21to determine the visibility of an entry. Any child entry is only
  22visible if its parent entry is also visible.
  23
  24Menu entries
  25------------
  26
  27Most entries define a config option; all other entries help to organize
  28them. A single configuration option is defined like this:
  29
  30config MODVERSIONS
  31        bool "Set version information on all module symbols"
  32        depends on MODULES
  33        help
  34          Usually, modules have to be recompiled whenever you switch to a new
  35          kernel.  ...
  36
  37Every line starts with a key word and can be followed by multiple
  38arguments.  "config" starts a new config entry. The following lines
  39define attributes for this config option. Attributes can be the type of
  40the config option, input prompt, dependencies, help text and default
  41values. A config option can be defined multiple times with the same
  42name, but every definition can have only a single input prompt and the
  43type must not conflict.
  44
  45Menu attributes
  46---------------
  47
  48A menu entry can have a number of attributes. Not all of them are
  49applicable everywhere (see syntax).
  50
  51- type definition: "bool"/"tristate"/"string"/"hex"/"int"
  52  Every config option must have a type. There are only two basic types:
  53  tristate and string; the other types are based on these two. The type
  54  definition optionally accepts an input prompt, so these two examples
  55  are equivalent:
  56
  57        bool "Networking support"
  58  and
  59        bool
  60        prompt "Networking support"
  61
  62- input prompt: "prompt" <prompt> ["if" <expr>]
  63  Every menu entry can have at most one prompt, which is used to display
  64  to the user. Optionally dependencies only for this prompt can be added
  65  with "if".
  66
  67- default value: "default" <expr> ["if" <expr>]
  68  A config option can have any number of default values. If multiple
  69  default values are visible, only the first defined one is active.
  70  Default values are not limited to the menu entry where they are
  71  defined. This means the default can be defined somewhere else or be
  72  overridden by an earlier definition.
  73  The default value is only assigned to the config symbol if no other
  74  value was set by the user (via the input prompt above). If an input
  75  prompt is visible the default value is presented to the user and can
  76  be overridden by him.
  77  Optionally, dependencies only for this default value can be added with
  78  "if".
  79
  80- type definition + default value:
  81        "def_bool"/"def_tristate" <expr> ["if" <expr>]
  82  This is a shorthand notation for a type definition plus a value.
  83  Optionally dependencies for this default value can be added with "if".
  84
  85- dependencies: "depends on" <expr>
  86  This defines a dependency for this menu entry. If multiple
  87  dependencies are defined, they are connected with '&&'. Dependencies
  88  are applied to all other options within this menu entry (which also
  89  accept an "if" expression), so these two examples are equivalent:
  90
  91        bool "foo" if BAR
  92        default y if BAR
  93  and
  94        depends on BAR
  95        bool "foo"
  96        default y
  97
  98- reverse dependencies: "select" <symbol> ["if" <expr>]
  99  While normal dependencies reduce the upper limit of a symbol (see
 100  below), reverse dependencies can be used to force a lower limit of
 101  another symbol. The value of the current menu symbol is used as the
 102  minimal value <symbol> can be set to. If <symbol> is selected multiple
 103  times, the limit is set to the largest selection.
 104  Reverse dependencies can only be used with boolean or tristate
 105  symbols.
 106  Note:
 107        select should be used with care. select will force
 108        a symbol to a value without visiting the dependencies.
 109        By abusing select you are able to select a symbol FOO even
 110        if FOO depends on BAR that is not set.
 111        In general use select only for non-visible symbols
 112        (no prompts anywhere) and for symbols with no dependencies.
 113        That will limit the usefulness but on the other hand avoid
 114        the illegal configurations all over.
 115        kconfig should one day warn about such things.
 116
 117- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
 118  This allows to limit the range of possible input values for int
 119  and hex symbols. The user can only input a value which is larger than
 120  or equal to the first symbol and smaller than or equal to the second
 121  symbol.
 122
 123- help text: "help" or "---help---"
 124  This defines a help text. The end of the help text is determined by
 125  the indentation level, this means it ends at the first line which has
 126  a smaller indentation than the first line of the help text.
 127  "---help---" and "help" do not differ in behaviour, "---help---" is
 128  used to help visually separate configuration logic from help within
 129  the file as an aid to developers.
 130
 131- misc options: "option" <symbol>[=<value>]
 132  Various less common options can be defined via this option syntax,
 133  which can modify the behaviour of the menu entry and its config
 134  symbol. These options are currently possible:
 135
 136  - "defconfig_list"
 137    This declares a list of default entries which can be used when
 138    looking for the default configuration (which is used when the main
 139    .config doesn't exists yet.)
 140
 141  - "modules"
 142    This declares the symbol to be used as the MODULES symbol, which
 143    enables the third modular state for all config symbols.
 144
 145  - "env"=<value>
 146    This imports the environment variable into Kconfig. It behaves like
 147    a default, except that the value comes from the environment, this
 148    also means that the behaviour when mixing it with normal defaults is
 149    undefined at this point. The symbol is currently not exported back
 150    to the build environment (if this is desired, it can be done via
 151    another symbol).
 152
 153Menu dependencies
 154-----------------
 155
 156Dependencies define the visibility of a menu entry and can also reduce
 157the input range of tristate symbols. The tristate logic used in the
 158expressions uses one more state than normal boolean logic to express the
 159module state. Dependency expressions have the following syntax:
 160
 161<expr> ::= <symbol>                             (1)
 162           <symbol> '=' <symbol>                (2)
 163           <symbol> '!=' <symbol>               (3)
 164           '(' <expr> ')'                       (4)
 165           '!' <expr>                           (5)
 166           <expr> '&&' <expr>                   (6)
 167           <expr> '||' <expr>                   (7)
 168
 169Expressions are listed in decreasing order of precedence. 
 170
 171(1) Convert the symbol into an expression. Boolean and tristate symbols
 172    are simply converted into the respective expression values. All
 173    other symbol types result in 'n'.
 174(2) If the values of both symbols are equal, it returns 'y',
 175    otherwise 'n'.
 176(3) If the values of both symbols are equal, it returns 'n',
 177    otherwise 'y'.
 178(4) Returns the value of the expression. Used to override precedence.
 179(5) Returns the result of (2-/expr/).
 180(6) Returns the result of min(/expr/, /expr/).
 181(7) Returns the result of max(/expr/, /expr/).
 182
 183An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
 184respectively for calculations). A menu entry becomes visible when it's
 185expression evaluates to 'm' or 'y'.
 186
 187There are two types of symbols: constant and non-constant symbols.
 188Non-constant symbols are the most common ones and are defined with the
 189'config' statement. Non-constant symbols consist entirely of alphanumeric
 190characters or underscores.
 191Constant symbols are only part of expressions. Constant symbols are
 192always surrounded by single or double quotes. Within the quote, any
 193other character is allowed and the quotes can be escaped using '\'.
 194
 195Menu structure
 196--------------
 197
 198The position of a menu entry in the tree is determined in two ways. First
 199it can be specified explicitly:
 200
 201menu "Network device support"
 202        depends on NET
 203
 204config NETDEVICES
 205        ...
 206
 207endmenu
 208
 209All entries within the "menu" ... "endmenu" block become a submenu of
 210"Network device support". All subentries inherit the dependencies from
 211the menu entry, e.g. this means the dependency "NET" is added to the
 212dependency list of the config option NETDEVICES.
 213
 214The other way to generate the menu structure is done by analyzing the
 215dependencies. If a menu entry somehow depends on the previous entry, it
 216can be made a submenu of it. First, the previous (parent) symbol must
 217be part of the dependency list and then one of these two conditions
 218must be true:
 219- the child entry must become invisible, if the parent is set to 'n'
 220- the child entry must only be visible, if the parent is visible
 221
 222config MODULES
 223        bool "Enable loadable module support"
 224
 225config MODVERSIONS
 226        bool "Set version information on all module symbols"
 227        depends on MODULES
 228
 229comment "module support disabled"
 230        depends on !MODULES
 231
 232MODVERSIONS directly depends on MODULES, this means it's only visible if
 233MODULES is different from 'n'. The comment on the other hand is always
 234visible when MODULES is visible (the (empty) dependency of MODULES is
 235also part of the comment dependencies).
 236
 237
 238Kconfig syntax
 239--------------
 240
 241The configuration file describes a series of menu entries, where every
 242line starts with a keyword (except help texts). The following keywords
 243end a menu entry:
 244- config
 245- menuconfig
 246- choice/endchoice
 247- comment
 248- menu/endmenu
 249- if/endif
 250- source
 251The first five also start the definition of a menu entry.
 252
 253config:
 254
 255        "config" <symbol>
 256        <config options>
 257
 258This defines a config symbol <symbol> and accepts any of above
 259attributes as options.
 260
 261menuconfig:
 262        "menuconfig" <symbol>
 263        <config options>
 264
 265This is similar to the simple config entry above, but it also gives a
 266hint to front ends, that all suboptions should be displayed as a
 267separate list of options.
 268
 269choices:
 270
 271        "choice"
 272        <choice options>
 273        <choice block>
 274        "endchoice"
 275
 276This defines a choice group and accepts any of the above attributes as
 277options. A choice can only be of type bool or tristate, while a boolean
 278choice only allows a single config entry to be selected, a tristate
 279choice also allows any number of config entries to be set to 'm'. This
 280can be used if multiple drivers for a single hardware exists and only a
 281single driver can be compiled/loaded into the kernel, but all drivers
 282can be compiled as modules.
 283A choice accepts another option "optional", which allows to set the
 284choice to 'n' and no entry needs to be selected.
 285
 286comment:
 287
 288        "comment" <prompt>
 289        <comment options>
 290
 291This defines a comment which is displayed to the user during the
 292configuration process and is also echoed to the output files. The only
 293possible options are dependencies.
 294
 295menu:
 296
 297        "menu" <prompt>
 298        <menu options>
 299        <menu block>
 300        "endmenu"
 301
 302This defines a menu block, see "Menu structure" above for more
 303information. The only possible options are dependencies.
 304
 305if:
 306
 307        "if" <expr>
 308        <if block>
 309        "endif"
 310
 311This defines an if block. The dependency expression <expr> is appended
 312to all enclosed menu entries.
 313
 314source:
 315
 316        "source" <prompt>
 317
 318This reads the specified configuration file. This file is always parsed.
 319
 320mainmenu:
 321
 322        "mainmenu" <prompt>
 323
 324This sets the config program's title bar if the config program chooses
 325to use it.
 326
 327
 328Kconfig hints
 329-------------
 330This is a collection of Kconfig tips, most of which aren't obvious at
 331first glance and most of which have become idioms in several Kconfig
 332files.
 333
 334Adding common features and make the usage configurable
 335~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 336It is a common idiom to implement a feature/functionality that are
 337relevant for some architectures but not all.
 338The recommended way to do so is to use a config variable named HAVE_*
 339that is defined in a common Kconfig file and selected by the relevant
 340architectures.
 341An example is the generic IOMAP functionality.
 342
 343We would in lib/Kconfig see:
 344
 345# Generic IOMAP is used to ...
 346config HAVE_GENERIC_IOMAP
 347
 348config GENERIC_IOMAP
 349        depends on HAVE_GENERIC_IOMAP && FOO
 350
 351And in lib/Makefile we would see:
 352obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
 353
 354For each architecture using the generic IOMAP functionality we would see:
 355
 356config X86
 357        select ...
 358        select HAVE_GENERIC_IOMAP
 359        select ...
 360
 361Note: we use the existing config option and avoid creating a new
 362config variable to select HAVE_GENERIC_IOMAP.
 363
 364Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
 365introduced to overcome the limitation of select which will force a
 366config option to 'y' no matter the dependencies.
 367The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
 368situation where select forces a symbol equals to 'y'.
 369
 370Build as module only
 371~~~~~~~~~~~~~~~~~~~~
 372To restrict a component build to module-only, qualify its config symbol
 373with "depends on m".  E.g.:
 374
 375config FOO
 376        depends on BAR && m
 377
 378limits FOO to module (=m) or disabled (=n).
 379
 380