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