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