Linux/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"  ["if" ] 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"  ["if" ] 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"  ["if" ] 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"  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"  ["if" ] 99   While normal dependencies reduce the upper limit of a symbol (see100   below), reverse dependencies can be used to force a lower limit of101   another symbol. The value of the current menu symbol is used as the102   minimal value  can be set to. If  is selected multiple103   times, the limit is set to the largest selection.104   Reverse dependencies can only be used with boolean or tristate105   symbols.106   Note:107         select should be used with care. select will force108         a symbol to a value without visiting the dependencies.109         By abusing select you are able to select a symbol FOO even110         if FOO depends on BAR that is not set.111         In general use select only for non-visible symbols112         (no prompts anywhere) and for symbols with no dependencies.113         That will limit the usefulness but on the other hand avoid114         the illegal configurations all over.115 116 - limiting menu display: "visible if" 117   This attribute is only applicable to menu blocks, if the condition is118   false, the menu block is not displayed to the user (the symbols119   contained there can still be selected by other symbols, though). It is120   similar to a conditional "prompt" attribude for individual menu121   entries. Default value of "visible" is true.122 123 - numerical ranges: "range"   ["if" ]124   This allows to limit the range of possible input values for int125   and hex symbols. The user can only input a value which is larger than126   or equal to the first symbol and smaller than or equal to the second127   symbol.128 129 - help text: "help" or "---help---"130   This defines a help text. The end of the help text is determined by131   the indentation level, this means it ends at the first line which has132   a smaller indentation than the first line of the help text.133   "---help---" and "help" do not differ in behaviour, "---help---" is134   used to help visually separate configuration logic from help within135   the file as an aid to developers.136 137 - misc options: "option" [=]138   Various less common options can be defined via this option syntax,139   which can modify the behaviour of the menu entry and its config140   symbol. These options are currently possible:141 142   - "defconfig_list"143     This declares a list of default entries which can be used when144     looking for the default configuration (which is used when the main145     .config doesn't exists yet.)146 147   - "modules"148     This declares the symbol to be used as the MODULES symbol, which149     enables the third modular state for all config symbols.150 151   - "env"=152     This imports the environment variable into Kconfig. It behaves like153     a default, except that the value comes from the environment, this154     also means that the behaviour when mixing it with normal defaults is155     undefined at this point. The symbol is currently not exported back156     to the build environment (if this is desired, it can be done via157     another symbol).158 159 Menu dependencies160 -----------------161 162 Dependencies define the visibility of a menu entry and can also reduce163 the input range of tristate symbols. The tristate logic used in the164 expressions uses one more state than normal boolean logic to express the165 module state. Dependency expressions have the following syntax:166 167  ::=                              (1)168             '='                 (2)169             '!='                (3)170            '('  ')'                       (4)171            '!'                            (5)172             '&&'                    (6)173             '||'                    (7)174 175 Expressions are listed in decreasing order of precedence. 176 177 (1) Convert the symbol into an expression. Boolean and tristate symbols178     are simply converted into the respective expression values. All179     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, 2190 respectively for calculations). A menu entry becomes visible when its191 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 the195 'config' statement. Non-constant symbols consist entirely of alphanumeric196 characters or underscores.197 Constant symbols are only part of expressions. Constant symbols are198 always surrounded by single or double quotes. Within the quote, any199 other character is allowed and the quotes can be escaped using '\'.200 201 Menu structure202 --------------203 204 The position of a menu entry in the tree is determined in two ways. First205 it can be specified explicitly:206 207 menu "Network device support"208         depends on NET209 210 config NETDEVICES211         ...212 213 endmenu214 215 All entries within the "menu" ... "endmenu" block become a submenu of216 "Network device support". All subentries inherit the dependencies from217 the menu entry, e.g. this means the dependency "NET" is added to the218 dependency list of the config option NETDEVICES.219 220 The other way to generate the menu structure is done by analyzing the221 dependencies. If a menu entry somehow depends on the previous entry, it222 can be made a submenu of it. First, the previous (parent) symbol must223 be part of the dependency list and then one of these two conditions224 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 visible227 228 config MODULES229         bool "Enable loadable module support"230 231 config MODVERSIONS232         bool "Set version information on all module symbols"233         depends on MODULES234 235 comment "module support disabled"236         depends on !MODULES237 238 MODVERSIONS directly depends on MODULES, this means it's only visible if239 MODULES is different from 'n'. The comment on the other hand is always240 visible when MODULES is visible (the (empty) dependency of MODULES is241 also part of the comment dependencies).242 243 244 Kconfig syntax245 --------------246 247 The configuration file describes a series of menu entries, where every248 line starts with a keyword (except help texts). The following keywords249 end a menu entry:250 - config251 - menuconfig252 - choice/endchoice253 - comment254 - menu/endmenu255 - if/endif256 - source257 The first five also start the definition of a menu entry.258 259 config:260 261         "config" 262         263 264 This defines a config symbol  and accepts any of above265 attributes as options.266 267 menuconfig:268         "menuconfig" 269         270 271 This is similar to the simple config entry above, but it also gives a272 hint to front ends, that all suboptions should be displayed as a273 separate list of options.274 275 choices:276 277         "choice" [symbol]278         279         280         "endchoice"281 282 This defines a choice group and accepts any of the above attributes as283 options. A choice can only be of type bool or tristate, while a boolean284 choice only allows a single config entry to be selected, a tristate285 choice also allows any number of config entries to be set to 'm'. This286 can be used if multiple drivers for a single hardware exists and only a287 single driver can be compiled/loaded into the kernel, but all drivers288 can be compiled as modules.289 A choice accepts another option "optional", which allows to set the290 choice to 'n' and no entry needs to be selected.291 If no [symbol] is associated with a choice, then you can not have multiple292 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 another294 place.295 296 comment:297 298         "comment" 299         300 301 This defines a comment which is displayed to the user during the302 configuration process and is also echoed to the output files. The only303 possible options are dependencies.304 305 menu:306 307         "menu" 308         309         310         "endmenu"311 312 This defines a menu block, see "Menu structure" above for more313 information. The only possible options are dependencies and "visible"314 attributes.315 316 if:317 318         "if" 319         320         "endif"321 322 This defines an if block. The dependency expression  is appended323 to all enclosed menu entries.324 325 source:326 327         "source" 328 329 This reads the specified configuration file. This file is always parsed.330 331 mainmenu:332 333         "mainmenu" 334 335 This sets the config program's title bar if the config program chooses336 to use it. It should be placed at the top of the configuration, before any337 other statement.338 339 340 Kconfig hints341 -------------342 This is a collection of Kconfig tips, most of which aren't obvious at343 first glance and most of which have become idioms in several Kconfig344 files.345 346 Adding common features and make the usage configurable347 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~348 It is a common idiom to implement a feature/functionality that are349 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 relevant352 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_IOMAP359 360 config GENERIC_IOMAP361         depends on HAVE_GENERIC_IOMAP && FOO362 363 And in lib/Makefile we would see:364 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o365 366 For each architecture using the generic IOMAP functionality we would see:367 368 config X86369         select ...370         select HAVE_GENERIC_IOMAP371         select ...372 373 Note: we use the existing config option and avoid creating a new374 config variable to select HAVE_GENERIC_IOMAP.375 376 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is377 introduced to overcome the limitation of select which will force a378 config option to 'y' no matter the dependencies.379 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the380 situation where select forces a symbol equals to 'y'.381 382 Build as module only383 ~~~~~~~~~~~~~~~~~~~~384 To restrict a component build to module-only, qualify its config symbol385 with "depends on m".  E.g.:386 387 config FOO388         depends on BAR && m389 390 limits FOO to module (=m) or disabled (=n).391 392 Kconfig symbol existence393 ~~~~~~~~~~~~~~~~~~~~~~~~394 The following two methods produce the same kconfig symbol dependencies395 but differ greatly in kconfig symbol existence (production) in the396 generated config file.397 398 case 1:399 400 config FOO401         tristate "about foo"402         depends on BAR403 404 vs. case 2:405 406 if BAR407 config FOO408         tristate "about foo"409 endif410 411 In case 1, the symbol FOO will always exist in the config file (given412 no other dependencies).  In case 2, the symbol FOO will only exist in413 the config file if BAR is enabled.