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
 116- limiting menu display: "visible if" <expr>
 117  This attribute is only applicable to menu blocks, if the condition is
 118  false, the menu block is not displayed to the user (the symbols
 119  contained there can still be selected by other symbols, though). It is
 120  similar to a conditional "prompt" attribute for individual menu
 121  entries. Default value of "visible" is true.
 122
 123- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
 124  This allows to limit the range of possible input values for int
 125  and hex symbols. The user can only input a value which is larger than
 126  or equal to the first symbol and smaller than or equal to the second
 127  symbol.
 128
 129- help text: "help" or "---help---"
 130  This defines a help text. The end of the help text is determined by
 131  the indentation level, this means it ends at the first line which has
 132  a smaller indentation than the first line of the help text.
 133  "---help---" and "help" do not differ in behaviour, "---help---" is
 134  used to help visually separate configuration logic from help within
 135  the file as an aid to developers.
 136
 137- misc options: "option" <symbol>[=<value>]
 138  Various less common options can be defined via this option syntax,
 139  which can modify the behaviour of the menu entry and its config
 140  symbol. These options are currently possible:
 141
 142  - "defconfig_list"
 143    This declares a list of default entries which can be used when
 144    looking for the default configuration (which is used when the main
 145    .config doesn't exists yet.)
 146
 147  - "modules"
 148    This declares the symbol to be used as the MODULES symbol, which
 149    enables the third modular state for all config symbols.
 150
 151  - "env"=<value>
 152    This imports the environment variable into Kconfig. It behaves like
 153    a default, except that the value comes from the environment, this
 154    also means that the behaviour when mixing it with normal defaults is
 155    undefined at this point. The symbol is currently not exported back
 156    to the build environment (if this is desired, it can be done via
 157    another symbol).
 158
 159Menu dependencies
 160-----------------
 161
 162Dependencies define the visibility of a menu entry and can also reduce
 163the input range of tristate symbols. The tristate logic used in the
 164expressions uses one more state than normal boolean logic to express the
 165module state. Dependency expressions have the following syntax:
 166
 167<expr> ::= <symbol>                             (1)
 168           <symbol> '=' <symbol>                (2)
 169           <symbol> '!=' <symbol>               (3)
 170           '(' <expr> ')'                       (4)
 171           '!' <expr>                           (5)
 172           <expr> '&&' <expr>                   (6)
 173           <expr> '||' <expr>                   (7)
 174
 175Expressions are listed in decreasing order of precedence. 
 176
 177(1) Convert the symbol into an expression. Boolean and tristate symbols
 178    are simply converted into the respective expression values. All
 179    other symbol types result in 'n'.
 180(2) If the values of both symbols are equal, it returns 'y',
 181    otherwise 'n'.
 182(3) If the values of both symbols are equal, it returns 'n',
 183    otherwise 'y'.
 184(4) Returns the value of the expression. Used to override precedence.
 185(5) Returns the result of (2-/expr/).
 186(6) Returns the result of min(/expr/, /expr/).
 187(7) Returns the result of max(/expr/, /expr/).
 188
 189An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
 190respectively for calculations). A menu entry becomes visible when its
 191expression evaluates to 'm' or 'y'.
 192
 193There are two types of symbols: constant and non-constant symbols.
 194Non-constant symbols are the most common ones and are defined with the
 195'config' statement. Non-constant symbols consist entirely of alphanumeric
 196characters or underscores.
 197Constant symbols are only part of expressions. Constant symbols are
 198always surrounded by single or double quotes. Within the quote, any
 199other character is allowed and the quotes can be escaped using '\'.
 200
 201Menu structure
 202--------------
 203
 204The position of a menu entry in the tree is determined in two ways. First
 205it can be specified explicitly:
 206
 207menu "Network device support"
 208        depends on NET
 209
 210config NETDEVICES
 211        ...
 212
 213endmenu
 214
 215All entries within the "menu" ... "endmenu" block become a submenu of
 216"Network device support". All subentries inherit the dependencies from
 217the menu entry, e.g. this means the dependency "NET" is added to the
 218dependency list of the config option NETDEVICES.
 219
 220The other way to generate the menu structure is done by analyzing the
 221dependencies. If a menu entry somehow depends on the previous entry, it
 222can be made a submenu of it. First, the previous (parent) symbol must
 223be part of the dependency list and then one of these two conditions
 224must be true:
 225- the child entry must become invisible, if the parent is set to 'n'
 226- the child entry must only be visible, if the parent is visible
 227
 228config MODULES
 229        bool "Enable loadable module support"
 230
 231config MODVERSIONS
 232        bool "Set version information on all module symbols"
 233        depends on MODULES
 234
 235comment "module support disabled"
 236        depends on !MODULES
 237
 238MODVERSIONS directly depends on MODULES, this means it's only visible if
 239MODULES is different from 'n'. The comment on the other hand is always
 240visible when MODULES is visible (the (empty) dependency of MODULES is
 241also part of the comment dependencies).
 242
 243
 244Kconfig syntax
 245--------------
 246
 247The configuration file describes a series of menu entries, where every
 248line starts with a keyword (except help texts). The following keywords
 249end a menu entry:
 250- config
 251- menuconfig
 252- choice/endchoice
 253- comment
 254- menu/endmenu
 255- if/endif
 256- source
 257The first five also start the definition of a menu entry.
 258
 259config:
 260
 261        "config" <symbol>
 262        <config options>
 263
 264This defines a config symbol <symbol> and accepts any of above
 265attributes as options.
 266
 267menuconfig:
 268        "menuconfig" <symbol>
 269        <config options>
 270
 271This is similar to the simple config entry above, but it also gives a
 272hint to front ends, that all suboptions should be displayed as a
 273separate list of options.
 274
 275choices:
 276
 277        "choice" [symbol]
 278        <choice options>
 279        <choice block>
 280        "endchoice"
 281
 282This defines a choice group and accepts any of the above attributes as
 283options. A choice can only be of type bool or tristate, while a boolean
 284choice only allows a single config entry to be selected, a tristate
 285choice also allows any number of config entries to be set to 'm'. This
 286can be used if multiple drivers for a single hardware exists and only a
 287single driver can be compiled/loaded into the kernel, but all drivers
 288can be compiled as modules.
 289A choice accepts another option "optional", which allows to set the
 290choice to 'n' and no entry needs to be selected.
 291If no [symbol] is associated with a choice, then you can not have multiple
 292definitions of that choice. If a [symbol] is associated to the choice,
 293then you may define the same choice (ie. with the same entries) in another
 294place.
 295
 296comment:
 297
 298        "comment" <prompt>
 299        <comment options>
 300
 301This defines a comment which is displayed to the user during the
 302configuration process and is also echoed to the output files. The only
 303possible options are dependencies.
 304
 305menu:
 306
 307        "menu" <prompt>
 308        <menu options>
 309        <menu block>
 310        "endmenu"
 311
 312This defines a menu block, see "Menu structure" above for more
 313information. The only possible options are dependencies and "visible"
 314attributes.
 315
 316if:
 317
 318        "if" <expr>
 319        <if block>
 320        "endif"
 321
 322This defines an if block. The dependency expression <expr> is appended
 323to all enclosed menu entries.
 324
 325source:
 326
 327        "source" <prompt>
 328
 329This reads the specified configuration file. This file is always parsed.
 330
 331mainmenu:
 332
 333        "mainmenu" <prompt>
 334
 335This sets the config program's title bar if the config program chooses
 336to use it. It should be placed at the top of the configuration, before any
 337other statement.
 338
 339
 340Kconfig hints
 341-------------
 342This is a collection of Kconfig tips, most of which aren't obvious at
 343first glance and most of which have become idioms in several Kconfig
 344files.
 345
 346Adding common features and make the usage configurable
 347~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 348It is a common idiom to implement a feature/functionality that are
 349relevant for some architectures but not all.
 350The recommended way to do so is to use a config variable named HAVE_*
 351that is defined in a common Kconfig file and selected by the relevant
 352architectures.
 353An example is the generic IOMAP functionality.
 354
 355We would in lib/Kconfig see:
 356
 357# Generic IOMAP is used to ...
 358config HAVE_GENERIC_IOMAP
 359
 360config GENERIC_IOMAP
 361        depends on HAVE_GENERIC_IOMAP && FOO
 362
 363And in lib/Makefile we would see:
 364obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
 365
 366For each architecture using the generic IOMAP functionality we would see:
 367
 368config X86
 369        select ...
 370        select HAVE_GENERIC_IOMAP
 371        select ...
 372
 373Note: we use the existing config option and avoid creating a new
 374config variable to select HAVE_GENERIC_IOMAP.
 375
 376Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
 377introduced to overcome the limitation of select which will force a
 378config option to 'y' no matter the dependencies.
 379The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
 380situation where select forces a symbol equals to 'y'.
 381
 382Build as module only
 383~~~~~~~~~~~~~~~~~~~~
 384To restrict a component build to module-only, qualify its config symbol
 385with "depends on m".  E.g.:
 386
 387config FOO
 388        depends on BAR && m
 389
 390limits FOO to module (=m) or disabled (=n).
 391
 392Kconfig symbol existence
 393~~~~~~~~~~~~~~~~~~~~~~~~
 394The following two methods produce the same kconfig symbol dependencies
 395but differ greatly in kconfig symbol existence (production) in the
 396generated config file.
 397
 398case 1:
 399
 400config FOO
 401        tristate "about foo"
 402        depends on BAR
 403
 404vs. case 2:
 405
 406if BAR
 407config FOO
 408        tristate "about foo"
 409endif
 410
 411In case 1, the symbol FOO will always exist in the config file (given
 412no other dependencies).  In case 2, the symbol FOO will only exist in
 413the config file if BAR is enabled.
 414
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