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