Documentation/kbuild/kconfig-language.txt

   1 Introduction
   2 ------------
   3
   4 The configuration database is a collection of configuration options
   5 organized 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
  20 Every entry has its own dependencies. These dependencies are used
  21 to determine the visibility of an entry. Any child entry is only
  22 visible if its parent entry is also visible.
  23
  24 Menu entries
  25 ------------
  26
  27 Most entries define a config option; all other entries help to organize
  28 them. A single configuration option is defined like this:
  29
  30 config 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
  37 Every line starts with a key word and can be followed by multiple
  38 arguments.  "config" starts a new config entry. The following lines
  39 define attributes for this config option. Attributes can be the type of
  40 the config option, input prompt, dependencies, help text and default
  41 values. A config option can be defined multiple times with the same
  42 name, but every definition can have only a single input prompt and the
  43 type must not conflict.
  44
  45 Menu attributes
  46 ---------------
  47
  48 A menu entry can have a number of attributes. Not all of them are
  49 applicable 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
 159 Menu dependencies
 160 -----------------
 161
 162 Dependencies define the visibility of a menu entry and can also reduce
 163 the input range of tristate symbols. The tristate logic used in the
 164 expressions uses one more state than normal boolean logic to express the
 165 module 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
 175 Expressions 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
 189 An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
 190 respectively for calculations). A menu entry becomes visible when its
 191 expression evaluates to 'm' or 'y'.
 192
 193 There are two types of symbols: constant and non-constant symbols.
 194 Non-constant symbols are the most common ones and are defined with the
 195 'config' statement. Non-constant symbols consist entirely of alphanumeric
 196 characters or underscores.
 197 Constant symbols are only part of expressions. Constant symbols are
 198 always surrounded by single or double quotes. Within the quote, any
 199 other character is allowed and the quotes can be escaped using '\'.
 200
 201 Menu structure
 202 --------------
 203
 204 The position of a menu entry in the tree is determined in two ways. First
 205 it can be specified explicitly:
 206
 207 menu "Network device support"
 208         depends on NET
 209
 210 config NETDEVICES
 211         ...
 212
 213 endmenu
 214
 215 All entries within the "menu" ... "endmenu" block become a submenu of
 216 "Network device support". All subentries inherit the dependencies from
 217 the menu entry, e.g. this means the dependency "NET" is added to the
 218 dependency list of the config option NETDEVICES.
 219
 220 The other way to generate the menu structure is done by analyzing the
 221 dependencies. If a menu entry somehow depends on the previous entry, it
 222 can be made a submenu of it. First, the previous (parent) symbol must
 223 be part of the dependency list and then one of these two conditions
 224 must 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
 228 config MODULES
 229         bool "Enable loadable module support"
 230
 231 config MODVERSIONS
 232         bool "Set version information on all module symbols"
 233         depends on MODULES
 234
 235 comment "module support disabled"
 236         depends on !MODULES
 237
 238 MODVERSIONS directly depends on MODULES, this means it's only visible if
 239 MODULES is different from 'n'. The comment on the other hand is always
 240 visible when MODULES is visible (the (empty) dependency of MODULES is
 241 also part of the comment dependencies).
 242
 243
 244 Kconfig syntax
 245 --------------
 246
 247 The configuration file describes a series of menu entries, where every
 248 line starts with a keyword (except help texts). The following keywords
 249 end a menu entry:
 250 - config
 251 - menuconfig
 252 - choice/endchoice
 253 - comment
 254 - menu/endmenu
 255 - if/endif
 256 - source
 257 The first five also start the definition of a menu entry.
 258
 259 config:
 260
 261         "config" <symbol>
 262         <config options>
 263
 264 This defines a config symbol <symbol> and accepts any of above
 265 attributes as options.
 266
 267 menuconfig:
 268         "menuconfig" <symbol>
 269         <config options>
 270
 271 This is similar to the simple config entry above, but it also gives a
 272 hint to front ends, that all suboptions should be displayed as a
 273 separate list of options.
 274
 275 choices:
 276
 277         "choice" [symbol]
 278         <choice options>
 279         <choice block>
 280         "endchoice"
 281
 282 This defines a choice group and accepts any of the above attributes as
 283 options. A choice can only be of type bool or tristate, while a boolean
 284 choice only allows a single config entry to be selected, a tristate
 285 choice also allows any number of config entries to be set to 'm'. This
 286 can be used if multiple drivers for a single hardware exists and only a
 287 single driver can be compiled/loaded into the kernel, but all drivers
 288 can be compiled as modules.
 289 A choice accepts another option "optional", which allows to set the
 290 choice to 'n' and no entry needs to be selected.
 291 If no [symbol] is associated with a choice, then you can not have multiple
 292 definitions of that choice. If a [symbol] is associated to the choice,
 293 then you may define the same choice (ie. with the same entries) in another
 294 place.
 295
 296 comment:
 297
 298         "comment" <prompt>
 299         <comment options>
 300
 301 This defines a comment which is displayed to the user during the
 302 configuration process and is also echoed to the output files. The only
 303 possible options are dependencies.
 304
 305 menu:
 306
 307         "menu" <prompt>
 308         <menu options>
 309         <menu block>
 310         "endmenu"
 311
 312 This defines a menu block, see "Menu structure" above for more
 313 information. The only possible options are dependencies and "visible"
 314 attributes.
 315
 316 if:
 317
 318         "if" <expr>
 319         <if block>
 320         "endif"
 321
 322 This defines an if block. The dependency expression <expr> is appended
 323 to all enclosed menu entries.
 324
 325 source:
 326
 327         "source" <prompt>
 328
 329 This reads the specified configuration file. This file is always parsed.
 330
 331 mainmenu:
 332
 333         "mainmenu" <prompt>
 334
 335 This sets the config program's title bar if the config program chooses
 336 to use it. It should be placed at the top of the configuration, before any
 337 other statement.
 338
 339
 340 Kconfig hints
 341 -------------
 342 This is a collection of Kconfig tips, most of which aren't obvious at
 343 first glance and most of which have become idioms in several Kconfig
 344 files.
 345
 346 Adding common features and make the usage configurable
 347 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 348 It is a common idiom to implement a feature/functionality that are
 349 relevant for some architectures but not all.
 350 The recommended way to do so is to use a config variable named HAVE_*
 351 that is defined in a common Kconfig file and selected by the relevant
 352 architectures.
 353 An example is the generic IOMAP functionality.
 354
 355 We would in lib/Kconfig see:
 356
 357 # Generic IOMAP is used to ...
 358 config HAVE_GENERIC_IOMAP
 359
 360 config GENERIC_IOMAP
 361         depends on HAVE_GENERIC_IOMAP && FOO
 362
 363 And in lib/Makefile we would see:
 364 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
 365
 366 For each architecture using the generic IOMAP functionality we would see:
 367
 368 config X86
 369         select ...
 370         select HAVE_GENERIC_IOMAP
 371         select ...
 372
 373 Note: we use the existing config option and avoid creating a new
 374 config variable to select HAVE_GENERIC_IOMAP.
 375
 376 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
 377 introduced to overcome the limitation of select which will force a
 378 config option to 'y' no matter the dependencies.
 379 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
 380 situation where select forces a symbol equals to 'y'.
 381
 382 Build as module only
 383 ~~~~~~~~~~~~~~~~~~~~
 384 To restrict a component build to module-only, qualify its config symbol
 385 with "depends on m".  E.g.:
 386
 387 config FOO
 388         depends on BAR && m
 389
 390 limits FOO to module (=m) or disabled (=n).
 391
 392 Kconfig symbol existence
 393 ~~~~~~~~~~~~~~~~~~~~~~~~
 394 The following two methods produce the same kconfig symbol dependencies
 395 but differ greatly in kconfig symbol existence (production) in the
 396 generated config file.
 397
 398 case 1:
 399
 400 config FOO
 401         tristate "about foo"
 402         depends on BAR
 403
 404 vs. case 2:
 405
 406 if BAR
 407 config FOO
 408         tristate "about foo"
 409 endif
 410
 411 In case 1, the symbol FOO will always exist in the config file (given
 412 no other dependencies).  In case 2, the symbol FOO will only exist in
 413 the config file if BAR is enabled.