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 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 164Menu dependencies 165----------------- 166 167Dependencies define the visibility of a menu entry and can also reduce 168the input range of tristate symbols. The tristate logic used in the 169expressions uses one more state than normal boolean logic to express the 170module 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 180Expressions 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 194An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 195respectively for calculations). A menu entry becomes visible when its 196expression evaluates to 'm' or 'y'. 197 198There are two types of symbols: constant and non-constant symbols. 199Non-constant symbols are the most common ones and are defined with the 200'config' statement. Non-constant symbols consist entirely of alphanumeric 201characters or underscores. 202Constant symbols are only part of expressions. Constant symbols are 203always surrounded by single or double quotes. Within the quote, any 204other character is allowed and the quotes can be escaped using '\'. 205 206Menu structure 207-------------- 208 209The position of a menu entry in the tree is determined in two ways. First 210it can be specified explicitly: 211 212menu "Network device support" 213 depends on NET 214 215config NETDEVICES 216 ... 217 218endmenu 219 220All entries within the "menu" ... "endmenu" block become a submenu of 221"Network device support". All subentries inherit the dependencies from 222the menu entry, e.g. this means the dependency "NET" is added to the 223dependency list of the config option NETDEVICES. 224 225The other way to generate the menu structure is done by analyzing the 226dependencies. If a menu entry somehow depends on the previous entry, it 227can be made a submenu of it. First, the previous (parent) symbol must 228be part of the dependency list and then one of these two conditions 229must 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 233config MODULES 234 bool "Enable loadable module support" 235 236config MODVERSIONS 237 bool "Set version information on all module symbols" 238 depends on MODULES 239 240comment "module support disabled" 241 depends on !MODULES 242 243MODVERSIONS directly depends on MODULES, this means it's only visible if 244MODULES is different from 'n'. The comment on the other hand is always 245visible when MODULES is visible (the (empty) dependency of MODULES is 246also part of the comment dependencies). 247 248 249Kconfig syntax 250-------------- 251 252The configuration file describes a series of menu entries, where every 253line starts with a keyword (except help texts). The following keywords 254end a menu entry: 255- config 256- menuconfig 257- choice/endchoice 258- comment 259- menu/endmenu 260- if/endif 261- source 262The first five also start the definition of a menu entry. 263 264config: 265 266 "config" <symbol> 267 <config options> 268 269This defines a config symbol <symbol> and accepts any of above 270attributes as options. 271 272menuconfig: 273 "menuconfig" <symbol> 274 <config options> 275 276This is similar to the simple config entry above, but it also gives a 277hint to front ends, that all suboptions should be displayed as a 278separate list of options. 279 280choices: 281 282 "choice" [symbol] 283 <choice options> 284 <choice block> 285 "endchoice" 286 287This defines a choice group and accepts any of the above attributes as 288options. A choice can only be of type bool or tristate, while a boolean 289choice only allows a single config entry to be selected, a tristate 290choice also allows any number of config entries to be set to 'm'. This 291can be used if multiple drivers for a single hardware exists and only a 292single driver can be compiled/loaded into the kernel, but all drivers 293can be compiled as modules. 294A choice accepts another option "optional", which allows to set the 295choice to 'n' and no entry needs to be selected. 296If no [symbol] is associated with a choice, then you can not have multiple 297definitions of that choice. If a [symbol] is associated to the choice, 298then you may define the same choice (ie. with the same entries) in another 299place. 300 301comment: 302 303 "comment" <prompt> 304 <comment options> 305 306This defines a comment which is displayed to the user during the 307configuration process and is also echoed to the output files. The only 308possible options are dependencies. 309 310menu: 311 312 "menu" <prompt> 313 <menu options> 314 <menu block> 315 "endmenu" 316 317This defines a menu block, see "Menu structure" above for more 318information. The only possible options are dependencies and "visible" 319attributes. 320 321if: 322 323 "if" <expr> 324 <if block> 325 "endif" 326 327This defines an if block. The dependency expression <expr> is appended 328to all enclosed menu entries. 329 330source: 331 332 "source" <prompt> 333 334This reads the specified configuration file. This file is always parsed. 335 336mainmenu: 337 338 "mainmenu" <prompt> 339 340This sets the config program's title bar if the config program chooses 341to use it. It should be placed at the top of the configuration, before any 342other statement. 343 344 345Kconfig hints 346------------- 347This is a collection of Kconfig tips, most of which aren't obvious at 348first glance and most of which have become idioms in several Kconfig 349files. 350 351Adding common features and make the usage configurable 352~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 353It is a common idiom to implement a feature/functionality that are 354relevant for some architectures but not all. 355The recommended way to do so is to use a config variable named HAVE_* 356that is defined in a common Kconfig file and selected by the relevant 357architectures. 358An example is the generic IOMAP functionality. 359 360We would in lib/Kconfig see: 361 362# Generic IOMAP is used to ... 363config HAVE_GENERIC_IOMAP 364 365config GENERIC_IOMAP 366 depends on HAVE_GENERIC_IOMAP && FOO 367 368And in lib/Makefile we would see: 369obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 370 371For each architecture using the generic IOMAP functionality we would see: 372 373config X86 374 select ... 375 select HAVE_GENERIC_IOMAP 376 select ... 377 378Note: we use the existing config option and avoid creating a new 379config variable to select HAVE_GENERIC_IOMAP. 380 381Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 382introduced to overcome the limitation of select which will force a 383config option to 'y' no matter the dependencies. 384The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 385situation where select forces a symbol equals to 'y'. 386 387Build as module only 388~~~~~~~~~~~~~~~~~~~~ 389To restrict a component build to module-only, qualify its config symbol 390with "depends on m". E.g.: 391 392config FOO 393 depends on BAR && m 394 395limits FOO to module (=m) or disabled (=n). 396