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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 kconfig should one day warn about such things. 116 117 - numerical ranges: "range" <symbol> <symbol> ["if" <expr>] 118 This allows to limit the range of possible input values for int 119 and hex symbols. The user can only input a value which is larger than 120 or equal to the first symbol and smaller than or equal to the second 121 symbol. 122 123 - help text: "help" or "---help---" 124 This defines a help text. The end of the help text is determined by 125 the indentation level, this means it ends at the first line which has 126 a smaller indentation than the first line of the help text. 127 "---help---" and "help" do not differ in behaviour, "---help---" is 128 used to help visually separate configuration logic from help within 129 the file as an aid to developers. 130 131 - misc options: "option" <symbol>[=<value>] 132 Various less common options can be defined via this option syntax, 133 which can modify the behaviour of the menu entry and its config 134 symbol. These options are currently possible: 135 136 - "defconfig_list" 137 This declares a list of default entries which can be used when 138 looking for the default configuration (which is used when the main 139 .config doesn't exists yet.) 140 141 - "modules" 142 This declares the symbol to be used as the MODULES symbol, which 143 enables the third modular state for all config symbols. 144 145 - "env"=<value> 146 This imports the environment variable into Kconfig. It behaves like 147 a default, except that the value comes from the environment, this 148 also means that the behaviour when mixing it with normal defaults is 149 undefined at this point. The symbol is currently not exported back 150 to the build environment (if this is desired, it can be done via 151 another symbol). 152 153 Menu dependencies 154 ----------------- 155 156 Dependencies define the visibility of a menu entry and can also reduce 157 the input range of tristate symbols. The tristate logic used in the 158 expressions uses one more state than normal boolean logic to express the 159 module state. Dependency expressions have the following syntax: 160 161 <expr> ::= <symbol> (1) 162 <symbol> '=' <symbol> (2) 163 <symbol> '!=' <symbol> (3) 164 '(' <expr> ')' (4) 165 '!' <expr> (5) 166 <expr> '&&' <expr> (6) 167 <expr> '||' <expr> (7) 168 169 Expressions are listed in decreasing order of precedence. 170 171 (1) Convert the symbol into an expression. Boolean and tristate symbols 172 are simply converted into the respective expression values. All 173 other symbol types result in 'n'. 174 (2) If the values of both symbols are equal, it returns 'y', 175 otherwise 'n'. 176 (3) If the values of both symbols are equal, it returns 'n', 177 otherwise 'y'. 178 (4) Returns the value of the expression. Used to override precedence. 179 (5) Returns the result of (2-/expr/). 180 (6) Returns the result of min(/expr/, /expr/). 181 (7) Returns the result of max(/expr/, /expr/). 182 183 An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 184 respectively for calculations). A menu entry becomes visible when it's 185 expression evaluates to 'm' or 'y'. 186 187 There are two types of symbols: constant and non-constant symbols. 188 Non-constant symbols are the most common ones and are defined with the 189 'config' statement. Non-constant symbols consist entirely of alphanumeric 190 characters or underscores. 191 Constant symbols are only part of expressions. Constant symbols are 192 always surrounded by single or double quotes. Within the quote, any 193 other character is allowed and the quotes can be escaped using '\'. 194 195 Menu structure 196 -------------- 197 198 The position of a menu entry in the tree is determined in two ways. First 199 it can be specified explicitly: 200 201 menu "Network device support" 202 depends on NET 203 204 config NETDEVICES 205 ... 206 207 endmenu 208 209 All entries within the "menu" ... "endmenu" block become a submenu of 210 "Network device support". All subentries inherit the dependencies from 211 the menu entry, e.g. this means the dependency "NET" is added to the 212 dependency list of the config option NETDEVICES. 213 214 The other way to generate the menu structure is done by analyzing the 215 dependencies. If a menu entry somehow depends on the previous entry, it 216 can be made a submenu of it. First, the previous (parent) symbol must 217 be part of the dependency list and then one of these two conditions 218 must be true: 219 - the child entry must become invisible, if the parent is set to 'n' 220 - the child entry must only be visible, if the parent is visible 221 222 config MODULES 223 bool "Enable loadable module support" 224 225 config MODVERSIONS 226 bool "Set version information on all module symbols" 227 depends on MODULES 228 229 comment "module support disabled" 230 depends on !MODULES 231 232 MODVERSIONS directly depends on MODULES, this means it's only visible if 233 MODULES is different from 'n'. The comment on the other hand is always 234 visible when MODULES is visible (the (empty) dependency of MODULES is 235 also part of the comment dependencies). 236 237 238 Kconfig syntax 239 -------------- 240 241 The configuration file describes a series of menu entries, where every 242 line starts with a keyword (except help texts). The following keywords 243 end a menu entry: 244 - config 245 - menuconfig 246 - choice/endchoice 247 - comment 248 - menu/endmenu 249 - if/endif 250 - source 251 The first five also start the definition of a menu entry. 252 253 config: 254 255 "config" <symbol> 256 <config options> 257 258 This defines a config symbol <symbol> and accepts any of above 259 attributes as options. 260 261 menuconfig: 262 "menuconfig" <symbol> 263 <config options> 264 265 This is similar to the simple config entry above, but it also gives a 266 hint to front ends, that all suboptions should be displayed as a 267 separate list of options. 268 269 choices: 270 271 "choice" 272 <choice options> 273 <choice block> 274 "endchoice" 275 276 This defines a choice group and accepts any of the above attributes as 277 options. A choice can only be of type bool or tristate, while a boolean 278 choice only allows a single config entry to be selected, a tristate 279 choice also allows any number of config entries to be set to 'm'. This 280 can be used if multiple drivers for a single hardware exists and only a 281 single driver can be compiled/loaded into the kernel, but all drivers 282 can be compiled as modules. 283 A choice accepts another option "optional", which allows to set the 284 choice to 'n' and no entry needs to be selected. 285 286 comment: 287 288 "comment" <prompt> 289 <comment options> 290 291 This defines a comment which is displayed to the user during the 292 configuration process and is also echoed to the output files. The only 293 possible options are dependencies. 294 295 menu: 296 297 "menu" <prompt> 298 <menu options> 299 <menu block> 300 "endmenu" 301 302 This defines a menu block, see "Menu structure" above for more 303 information. The only possible options are dependencies. 304 305 if: 306 307 "if" <expr> 308 <if block> 309 "endif" 310 311 This defines an if block. The dependency expression <expr> is appended 312 to all enclosed menu entries. 313 314 source: 315 316 "source" <prompt> 317 318 This reads the specified configuration file. This file is always parsed. 319 320 mainmenu: 321 322 "mainmenu" <prompt> 323 324 This sets the config program's title bar if the config program chooses 325 to use it. 326 327 328 Kconfig hints 329 ------------- 330 This is a collection of Kconfig tips, most of which aren't obvious at 331 first glance and most of which have become idioms in several Kconfig 332 files. 333 334 Adding common features and make the usage configurable 335 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 336 It is a common idiom to implement a feature/functionality that are 337 relevant for some architectures but not all. 338 The recommended way to do so is to use a config variable named HAVE_* 339 that is defined in a common Kconfig file and selected by the relevant 340 architectures. 341 An example is the generic IOMAP functionality. 342 343 We would in lib/Kconfig see: 344 345 # Generic IOMAP is used to ... 346 config HAVE_GENERIC_IOMAP 347 348 config GENERIC_IOMAP 349 depends on HAVE_GENERIC_IOMAP && FOO 350 351 And in lib/Makefile we would see: 352 obj-$(CONFIG_GENERIC_IOMAP) += iomap.o 353 354 For each architecture using the generic IOMAP functionality we would see: 355 356 config X86 357 select ... 358 select HAVE_GENERIC_IOMAP 359 select ... 360 361 Note: we use the existing config option and avoid creating a new 362 config variable to select HAVE_GENERIC_IOMAP. 363 364 Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is 365 introduced to overcome the limitation of select which will force a 366 config option to 'y' no matter the dependencies. 367 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the 368 situation where select forces a symbol equals to 'y'. 369 370 Build as module only 371 ~~~~~~~~~~~~~~~~~~~~ 372 To restrict a component build to module-only, qualify its config symbol 373 with "depends on m". E.g.: 374 375 config FOO 376 depends on BAR && m 377 378 limits FOO to module (=m) or disabled (=n). 379
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