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diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt new file mode 100644 index 000000000..350f733bf --- /dev/null +++ b/Documentation/kbuild/kconfig-language.txt @@ -0,0 +1,395 @@ +Introduction +------------ + +The configuration database is a collection of configuration options +organized in a tree structure: + + +- Code maturity level options + | +- Prompt for development and/or incomplete code/drivers + +- General setup + | +- Networking support + | +- System V IPC + | +- BSD Process Accounting + | +- Sysctl support + +- Loadable module support + | +- Enable loadable module support + | +- Set version information on all module symbols + | +- Kernel module loader + +- ... + +Every entry has its own dependencies. These dependencies are used +to determine the visibility of an entry. Any child entry is only +visible if its parent entry is also visible. + +Menu entries +------------ + +Most entries define a config option; all other entries help to organize +them. A single configuration option is defined like this: + +config MODVERSIONS + bool "Set version information on all module symbols" + depends on MODULES + help + Usually, modules have to be recompiled whenever you switch to a new + kernel. ... + +Every line starts with a key word and can be followed by multiple +arguments. "config" starts a new config entry. The following lines +define attributes for this config option. Attributes can be the type of +the config option, input prompt, dependencies, help text and default +values. A config option can be defined multiple times with the same +name, but every definition can have only a single input prompt and the +type must not conflict. + +Menu attributes +--------------- + +A menu entry can have a number of attributes. Not all of them are +applicable everywhere (see syntax). + +- type definition: "bool"/"tristate"/"string"/"hex"/"int" + Every config option must have a type. There are only two basic types: + tristate and string; the other types are based on these two. The type + definition optionally accepts an input prompt, so these two examples + are equivalent: + + bool "Networking support" + and + bool + prompt "Networking support" + +- input prompt: "prompt" <prompt> ["if" <expr>] + Every menu entry can have at most one prompt, which is used to display + to the user. Optionally dependencies only for this prompt can be added + with "if". + +- default value: "default" <expr> ["if" <expr>] + A config option can have any number of default values. If multiple + default values are visible, only the first defined one is active. + Default values are not limited to the menu entry where they are + defined. This means the default can be defined somewhere else or be + overridden by an earlier definition. + The default value is only assigned to the config symbol if no other + value was set by the user (via the input prompt above). If an input + prompt is visible the default value is presented to the user and can + be overridden by him. + Optionally, dependencies only for this default value can be added with + "if". + +- type definition + default value: + "def_bool"/"def_tristate" <expr> ["if" <expr>] + This is a shorthand notation for a type definition plus a value. + Optionally dependencies for this default value can be added with "if". + +- dependencies: "depends on" <expr> + This defines a dependency for this menu entry. If multiple + dependencies are defined, they are connected with '&&'. Dependencies + are applied to all other options within this menu entry (which also + accept an "if" expression), so these two examples are equivalent: + + bool "foo" if BAR + default y if BAR + and + depends on BAR + bool "foo" + default y + +- reverse dependencies: "select" <symbol> ["if" <expr>] + While normal dependencies reduce the upper limit of a symbol (see + below), reverse dependencies can be used to force a lower limit of + another symbol. The value of the current menu symbol is used as the + minimal value <symbol> can be set to. If <symbol> is selected multiple + times, the limit is set to the largest selection. + Reverse dependencies can only be used with boolean or tristate + symbols. + Note: + select should be used with care. select will force + a symbol to a value without visiting the dependencies. + By abusing select you are able to select a symbol FOO even + if FOO depends on BAR that is not set. + In general use select only for non-visible symbols + (no prompts anywhere) and for symbols with no dependencies. + That will limit the usefulness but on the other hand avoid + the illegal configurations all over. + +- limiting menu display: "visible if" <expr> + This attribute is only applicable to menu blocks, if the condition is + false, the menu block is not displayed to the user (the symbols + contained there can still be selected by other symbols, though). It is + similar to a conditional "prompt" attribute for individual menu + entries. Default value of "visible" is true. + +- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] + This allows to limit the range of possible input values for int + and hex symbols. The user can only input a value which is larger than + or equal to the first symbol and smaller than or equal to the second + symbol. + +- help text: "help" or "---help---" + This defines a help text. The end of the help text is determined by + the indentation level, this means it ends at the first line which has + a smaller indentation than the first line of the help text. + "---help---" and "help" do not differ in behaviour, "---help---" is + used to help visually separate configuration logic from help within + the file as an aid to developers. + +- misc options: "option" <symbol>[=<value>] + Various less common options can be defined via this option syntax, + which can modify the behaviour of the menu entry and its config + symbol. These options are currently possible: + + - "defconfig_list" + This declares a list of default entries which can be used when + looking for the default configuration (which is used when the main + .config doesn't exists yet.) + + - "modules" + This declares the symbol to be used as the MODULES symbol, which + enables the third modular state for all config symbols. + At most one symbol may have the "modules" option set. + + - "env"=<value> + This imports the environment variable into Kconfig. It behaves like + a default, except that the value comes from the environment, this + also means that the behaviour when mixing it with normal defaults is + undefined at this point. The symbol is currently not exported back + to the build environment (if this is desired, it can be done via + another symbol). + + - "allnoconfig_y" + This declares the symbol as one that should have the value y when + using "allnoconfig". Used for symbols that hide other symbols. + +Menu dependencies +----------------- + +Dependencies define the visibility of a menu entry and can also reduce +the input range of tristate symbols. The tristate logic used in the +expressions uses one more state than normal boolean logic to express the +module state. Dependency expressions have the following syntax: + +<expr> ::= <symbol> (1) + <symbol> '=' <symbol> (2) + <symbol> '!=' <symbol> (3) + '(' <expr> ')' (4) + '!' <expr> (5) + <expr> '&&' <expr> (6) + <expr> '||' <expr> (7) + +Expressions are listed in decreasing order of precedence. + +(1) Convert the symbol into an expression. Boolean and tristate symbols + are simply converted into the respective expression values. All + other symbol types result in 'n'. +(2) If the values of both symbols are equal, it returns 'y', + otherwise 'n'. +(3) If the values of both symbols are equal, it returns 'n', + otherwise 'y'. +(4) Returns the value of the expression. Used to override precedence. +(5) Returns the result of (2-/expr/). +(6) Returns the result of min(/expr/, /expr/). +(7) Returns the result of max(/expr/, /expr/). + +An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 +respectively for calculations). A menu entry becomes visible when its +expression evaluates to 'm' or 'y'. + +There are two types of symbols: constant and non-constant symbols. +Non-constant symbols are the most common ones and are defined with the +'config' statement. Non-constant symbols consist entirely of alphanumeric +characters or underscores. +Constant symbols are only part of expressions. Constant symbols are +always surrounded by single or double quotes. Within the quote, any +other character is allowed and the quotes can be escaped using '\'. + +Menu structure +-------------- + +The position of a menu entry in the tree is determined in two ways. First +it can be specified explicitly: + +menu "Network device support" + depends on NET + +config NETDEVICES + ... + +endmenu + +All entries within the "menu" ... "endmenu" block become a submenu of +"Network device support". All subentries inherit the dependencies from +the menu entry, e.g. this means the dependency "NET" is added to the +dependency list of the config option NETDEVICES. + +The other way to generate the menu structure is done by analyzing the +dependencies. If a menu entry somehow depends on the previous entry, it +can be made a submenu of it. First, the previous (parent) symbol must +be part of the dependency list and then one of these two conditions +must be true: +- the child entry must become invisible, if the parent is set to 'n' +- the child entry must only be visible, if the parent is visible + +config MODULES + bool "Enable loadable module support" + +config MODVERSIONS + bool "Set version information on all module symbols" + depends on MODULES + +comment "module support disabled" + depends on !MODULES + +MODVERSIONS directly depends on MODULES, this means it's only visible if +MODULES is different from 'n'. The comment on the other hand is always +visible when MODULES is visible (the (empty) dependency of MODULES is +also part of the comment dependencies). + + +Kconfig syntax +-------------- + +The configuration file describes a series of menu entries, where every +line starts with a keyword (except help texts). The following keywords +end a menu entry: +- config +- menuconfig +- choice/endchoice +- comment +- menu/endmenu +- if/endif +- source +The first five also start the definition of a menu entry. + +config: + + "config" <symbol> + <config options> + +This defines a config symbol <symbol> and accepts any of above +attributes as options. + +menuconfig: + "menuconfig" <symbol> + <config options> + +This is similar to the simple config entry above, but it also gives a +hint to front ends, that all suboptions should be displayed as a +separate list of options. + +choices: + + "choice" [symbol] + <choice options> + <choice block> + "endchoice" + +This defines a choice group and accepts any of the above attributes as +options. A choice can only be of type bool or tristate, while a boolean +choice only allows a single config entry to be selected, a tristate +choice also allows any number of config entries to be set to 'm'. This +can be used if multiple drivers for a single hardware exists and only a +single driver can be compiled/loaded into the kernel, but all drivers +can be compiled as modules. +A choice accepts another option "optional", which allows to set the +choice to 'n' and no entry needs to be selected. +If no [symbol] is associated with a choice, then you can not have multiple +definitions of that choice. If a [symbol] is associated to the choice, +then you may define the same choice (ie. with the same entries) in another +place. + +comment: + + "comment" <prompt> + <comment options> + +This defines a comment which is displayed to the user during the +configuration process and is also echoed to the output files. The only +possible options are dependencies. + +menu: + + "menu" <prompt> + <menu options> + <menu block> + "endmenu" + +This defines a menu block, see "Menu structure" above for more +information. The only possible options are dependencies and "visible" +attributes. + +if: + + "if" <expr> + <if block> + "endif" + +This defines an if block. The dependency expression <expr> is appended +to all enclosed menu entries. + +source: + + "source" <prompt> + +This reads the specified configuration file. This file is always parsed. + +mainmenu: + + "mainmenu" <prompt> + +This sets the config program's title bar if the config program chooses +to use it. It should be placed at the top of the configuration, before any +other statement. + + +Kconfig hints +------------- +This is a collection of Kconfig tips, most of which aren't obvious at +first glance and most of which have become idioms in several Kconfig +files. + +Adding common features and make the usage configurable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It is a common idiom to implement a feature/functionality that are +relevant for some architectures but not all. +The recommended way to do so is to use a config variable named HAVE_* +that is defined in a common Kconfig file and selected by the relevant +architectures. +An example is the generic IOMAP functionality. + +We would in lib/Kconfig see: + +# Generic IOMAP is used to ... +config HAVE_GENERIC_IOMAP + +config GENERIC_IOMAP + depends on HAVE_GENERIC_IOMAP && FOO + +And in lib/Makefile we would see: +obj-$(CONFIG_GENERIC_IOMAP) += iomap.o + +For each architecture using the generic IOMAP functionality we would see: + +config X86 + select ... + select HAVE_GENERIC_IOMAP + select ... + +Note: we use the existing config option and avoid creating a new +config variable to select HAVE_GENERIC_IOMAP. + +Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is +introduced to overcome the limitation of select which will force a +config option to 'y' no matter the dependencies. +The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the +situation where select forces a symbol equals to 'y'. + +Build as module only +~~~~~~~~~~~~~~~~~~~~ +To restrict a component build to module-only, qualify its config symbol +with "depends on m". E.g.: + +config FOO + depends on BAR && m + +limits FOO to module (=m) or disabled (=n). |