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