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kconfig-language.txt 13 KB

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  1. Introduction
  2. ------------
  3. The configuration database is a collection of configuration options
  4. organized in a tree structure:
  5. +- Code maturity level options
  6. | +- Prompt for development and/or incomplete code/drivers
  7. +- General setup
  8. | +- Networking support
  9. | +- System V IPC
  10. | +- BSD Process Accounting
  11. | +- Sysctl support
  12. +- Loadable module support
  13. | +- Enable loadable module support
  14. | +- Set version information on all module symbols
  15. | +- Kernel module loader
  16. +- ...
  17. Every entry has its own dependencies. These dependencies are used
  18. to determine the visibility of an entry. Any child entry is only
  19. visible if its parent entry is also visible.
  20. Menu entries
  21. ------------
  22. Most entries define a config option; all other entries help to organize
  23. them. A single configuration option is defined like this:
  24. config MODVERSIONS
  25. bool "Set version information on all module symbols"
  26. depends on MODULES
  27. help
  28. Usually, modules have to be recompiled whenever you switch to a new
  29. kernel. ...
  30. Every line starts with a key word and can be followed by multiple
  31. arguments. "config" starts a new config entry. The following lines
  32. define attributes for this config option. Attributes can be the type of
  33. the config option, input prompt, dependencies, help text and default
  34. values. A config option can be defined multiple times with the same
  35. name, but every definition can have only a single input prompt and the
  36. type must not conflict.
  37. Menu attributes
  38. ---------------
  39. A menu entry can have a number of attributes. Not all of them are
  40. applicable everywhere (see syntax).
  41. - type definition: "bool"/"tristate"/"string"/"hex"/"int"
  42. Every config option must have a type. There are only two basic types:
  43. tristate and string; the other types are based on these two. The type
  44. definition optionally accepts an input prompt, so these two examples
  45. are equivalent:
  46. bool "Networking support"
  47. and
  48. bool
  49. prompt "Networking support"
  50. - input prompt: "prompt" <prompt> ["if" <expr>]
  51. Every menu entry can have at most one prompt, which is used to display
  52. to the user. Optionally dependencies only for this prompt can be added
  53. with "if".
  54. - default value: "default" <expr> ["if" <expr>]
  55. A config option can have any number of default values. If multiple
  56. default values are visible, only the first defined one is active.
  57. Default values are not limited to the menu entry where they are
  58. defined. This means the default can be defined somewhere else or be
  59. overridden by an earlier definition.
  60. The default value is only assigned to the config symbol if no other
  61. value was set by the user (via the input prompt above). If an input
  62. prompt is visible the default value is presented to the user and can
  63. be overridden by him.
  64. Optionally, dependencies only for this default value can be added with
  65. "if".
  66. - type definition + default value:
  67. "def_bool"/"def_tristate" <expr> ["if" <expr>]
  68. This is a shorthand notation for a type definition plus a value.
  69. Optionally dependencies for this default value can be added with "if".
  70. - dependencies: "depends on" <expr>
  71. This defines a dependency for this menu entry. If multiple
  72. dependencies are defined, they are connected with '&&'. Dependencies
  73. are applied to all other options within this menu entry (which also
  74. accept an "if" expression), so these two examples are equivalent:
  75. bool "foo" if BAR
  76. default y if BAR
  77. and
  78. depends on BAR
  79. bool "foo"
  80. default y
  81. - reverse dependencies: "select" <symbol> [<expr>] ["if" <expr>]
  82. While normal dependencies reduce the upper limit of a symbol (see
  83. below), reverse dependencies can be used to force a lower limit of
  84. another symbol. The value of the current menu symbol is used as the
  85. minimal value <symbol> can be set to. If <symbol> is selected multiple
  86. times, the limit is set to the largest selection.
  87. Reverse dependencies without the optional <expr> can only be used with
  88. boolean or tristate symbols. If the optional <expr> is supplied,
  89. the <symbol> will be set to that value if possible.
  90. Note:
  91. select should be used with care. select will force
  92. a symbol to a value without visiting the dependencies.
  93. By abusing select you are able to select a symbol FOO even
  94. if FOO depends on BAR that is not set.
  95. In general use select only for non-visible symbols
  96. (no prompts anywhere) and for symbols with no dependencies.
  97. That will limit the usefulness but on the other hand avoid
  98. the illegal configurations all over.
  99. kconfig should one day warn about such things.
  100. - numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
  101. This allows to limit the range of possible input values for int
  102. and hex symbols. The user can only input a value which is larger than
  103. or equal to the first symbol and smaller than or equal to the second
  104. symbol.
  105. - help text: "help" or "---help---"
  106. This defines a help text. The end of the help text is determined by
  107. the indentation level, this means it ends at the first line which has
  108. a smaller indentation than the first line of the help text.
  109. "---help---" and "help" do not differ in behaviour, "---help---" is
  110. used to help visually separate configuration logic from help within
  111. the file as an aid to developers.
  112. - misc options: "option" <symbol>[=<value>]
  113. Various less common options can be defined via this option syntax,
  114. which can modify the behaviour of the menu entry and its config
  115. symbol. These options are currently possible:
  116. - "defconfig_list"
  117. This declares a list of default entries which can be used when
  118. looking for the default configuration (which is used when the main
  119. .config doesn't exists yet.)
  120. - "modules"
  121. This declares the symbol to be used as the MODULES symbol, which
  122. enables the third modular state for all config symbols.
  123. - "env"=<value>
  124. This imports the environment variable into Kconfig. It behaves like
  125. a default, except that the value comes from the environment, this
  126. also means that the behaviour when mixing it with normal defaults is
  127. undefined at this point. The symbol is currently not exported back
  128. to the build environment (if this is desired, it can be done via
  129. another symbol).
  130. Menu dependencies
  131. -----------------
  132. Dependencies define the visibility of a menu entry and can also reduce
  133. the input range of tristate symbols. The tristate logic used in the
  134. expressions uses one more state than normal boolean logic to express the
  135. module state. Dependency expressions have the following syntax:
  136. <expr> ::= <symbol> (1)
  137. <symbol> '=' <symbol> (2)
  138. <symbol> '!=' <symbol> (3)
  139. '(' <expr> ')' (4)
  140. '!' <expr> (5)
  141. <expr> '&&' <expr> (6)
  142. <expr> '||' <expr> (7)
  143. Expressions are listed in decreasing order of precedence.
  144. (1) Convert the symbol into an expression. Boolean and tristate symbols
  145. are simply converted into the respective expression values. All
  146. other symbol types result in 'n'.
  147. (2) If the values of both symbols are equal, it returns 'y',
  148. otherwise 'n'.
  149. (3) If the values of both symbols are equal, it returns 'n',
  150. otherwise 'y'.
  151. (4) Returns the value of the expression. Used to override precedence.
  152. (5) Returns the result of (2-/expr/).
  153. (6) Returns the result of min(/expr/, /expr/).
  154. (7) Returns the result of max(/expr/, /expr/).
  155. An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
  156. respectively for calculations). A menu entry becomes visible when it's
  157. expression evaluates to 'm' or 'y'.
  158. There are two types of symbols: constant and non-constant symbols.
  159. Non-constant symbols are the most common ones and are defined with the
  160. 'config' statement. Non-constant symbols consist entirely of alphanumeric
  161. characters or underscores.
  162. Constant symbols are only part of expressions. Constant symbols are
  163. always surrounded by single or double quotes. Within the quote, any
  164. other character is allowed and the quotes can be escaped using '\'.
  165. Menu structure
  166. --------------
  167. The position of a menu entry in the tree is determined in two ways. First
  168. it can be specified explicitly:
  169. menu "Network device support"
  170. depends on NET
  171. config NETDEVICES
  172. ...
  173. endmenu
  174. All entries within the "menu" ... "endmenu" block become a submenu of
  175. "Network device support". All subentries inherit the dependencies from
  176. the menu entry, e.g. this means the dependency "NET" is added to the
  177. dependency list of the config option NETDEVICES.
  178. The other way to generate the menu structure is done by analyzing the
  179. dependencies. If a menu entry somehow depends on the previous entry, it
  180. can be made a submenu of it. First, the previous (parent) symbol must
  181. be part of the dependency list and then one of these two conditions
  182. must be true:
  183. - the child entry must become invisible, if the parent is set to 'n'
  184. - the child entry must only be visible, if the parent is visible
  185. config MODULES
  186. bool "Enable loadable module support"
  187. config MODVERSIONS
  188. bool "Set version information on all module symbols"
  189. depends on MODULES
  190. comment "module support disabled"
  191. depends on !MODULES
  192. MODVERSIONS directly depends on MODULES, this means it's only visible if
  193. MODULES is different from 'n'. The comment on the other hand is always
  194. visible when MODULES is visible (the (empty) dependency of MODULES is
  195. also part of the comment dependencies).
  196. Kconfig syntax
  197. --------------
  198. The configuration file describes a series of menu entries, where every
  199. line starts with a keyword (except help texts). The following keywords
  200. end a menu entry:
  201. - config
  202. - menuconfig
  203. - choice/endchoice
  204. - comment
  205. - menu/endmenu
  206. - if/endif
  207. - source
  208. The first five also start the definition of a menu entry.
  209. config:
  210. "config" <symbol>
  211. <config options>
  212. This defines a config symbol <symbol> and accepts any of above
  213. attributes as options.
  214. menuconfig:
  215. "menuconfig" <symbol>
  216. <config options>
  217. This is similar to the simple config entry above, but it also gives a
  218. hint to front ends, that all suboptions should be displayed as a
  219. separate list of options.
  220. choices:
  221. "choice"
  222. <choice options>
  223. <choice block>
  224. "endchoice"
  225. This defines a choice group and accepts any of the above attributes as
  226. options. A choice can only be of type bool or tristate, while a boolean
  227. choice only allows a single config entry to be selected, a tristate
  228. choice also allows any number of config entries to be set to 'm'. This
  229. can be used if multiple drivers for a single hardware exists and only a
  230. single driver can be compiled/loaded into the kernel, but all drivers
  231. can be compiled as modules.
  232. A choice accepts another option "optional", which allows to set the
  233. choice to 'n' and no entry needs to be selected.
  234. comment:
  235. "comment" <prompt>
  236. <comment options>
  237. This defines a comment which is displayed to the user during the
  238. configuration process and is also echoed to the output files. The only
  239. possible options are dependencies.
  240. menu:
  241. "menu" <prompt>
  242. <menu options>
  243. <menu block>
  244. "endmenu"
  245. This defines a menu block, see "Menu structure" above for more
  246. information. The only possible options are dependencies.
  247. if:
  248. "if" <expr>
  249. <if block>
  250. "endif"
  251. This defines an if block. The dependency expression <expr> is appended
  252. to all enclosed menu entries.
  253. source:
  254. "source" <prompt>
  255. This reads the specified configuration file. This file is always parsed.
  256. mainmenu:
  257. "mainmenu" <prompt>
  258. This sets the config program's title bar if the config program chooses
  259. to use it.
  260. Kconfig hints
  261. -------------
  262. This is a collection of Kconfig tips, most of which aren't obvious at
  263. first glance and most of which have become idioms in several Kconfig
  264. files.
  265. Adding common features and make the usage configurable
  266. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  267. It is a common idiom to implement a feature/functionality that are
  268. relevant for some architectures but not all.
  269. The recommended way to do so is to use a config variable named HAVE_*
  270. that is defined in a common Kconfig file and selected by the relevant
  271. architectures.
  272. An example is the generic IOMAP functionality.
  273. We would in lib/Kconfig see:
  274. # Generic IOMAP is used to ...
  275. config HAVE_GENERIC_IOMAP
  276. config GENERIC_IOMAP
  277. depends on HAVE_GENERIC_IOMAP && FOO
  278. And in lib/Makefile we would see:
  279. obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
  280. For each architecture using the generic IOMAP functionality we would see:
  281. config X86
  282. select ...
  283. select HAVE_GENERIC_IOMAP
  284. select ...
  285. Note: we use the existing config option and avoid creating a new
  286. config variable to select HAVE_GENERIC_IOMAP.
  287. Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
  288. introduced to overcome the limitation of select which will force a
  289. config option to 'y' no matter the dependencies.
  290. The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
  291. situation where select forces a symbol equals to 'y'.
  292. Build as module only
  293. ~~~~~~~~~~~~~~~~~~~~
  294. To restrict a component build to module-only, qualify its config symbol
  295. with "depends on m". E.g.:
  296. config FOO
  297. depends on BAR && m
  298. limits FOO to module (=m) or disabled (=n).