forked from Minki/linux
a33f32244d
Fix obvious cases of "it's" being used when "its" was meant. Signed-off-by: Francis Galiegue <fgaliegue@gmail.com> Acked-by: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
380 lines
13 KiB
Plaintext
380 lines
13 KiB
Plaintext
Introduction
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------------
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The configuration database is a collection of configuration options
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organized in a tree structure:
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+- Code maturity level options
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| +- Prompt for development and/or incomplete code/drivers
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+- General setup
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| +- Networking support
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| +- System V IPC
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| +- BSD Process Accounting
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| +- Sysctl support
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+- Loadable module support
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| +- Enable loadable module support
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| +- Set version information on all module symbols
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| +- Kernel module loader
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+- ...
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Every entry has its own dependencies. These dependencies are used
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to determine the visibility of an entry. Any child entry is only
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visible if its parent entry is also visible.
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Menu entries
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------------
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Most entries define a config option; all other entries help to organize
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them. A single configuration option is defined like this:
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config MODVERSIONS
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bool "Set version information on all module symbols"
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depends on MODULES
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help
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Usually, modules have to be recompiled whenever you switch to a new
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kernel. ...
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Every line starts with a key word and can be followed by multiple
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arguments. "config" starts a new config entry. The following lines
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define attributes for this config option. Attributes can be the type of
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the config option, input prompt, dependencies, help text and default
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values. A config option can be defined multiple times with the same
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name, but every definition can have only a single input prompt and the
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type must not conflict.
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Menu attributes
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---------------
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A menu entry can have a number of attributes. Not all of them are
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applicable everywhere (see syntax).
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- type definition: "bool"/"tristate"/"string"/"hex"/"int"
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Every config option must have a type. There are only two basic types:
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tristate and string; the other types are based on these two. The type
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definition optionally accepts an input prompt, so these two examples
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are equivalent:
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bool "Networking support"
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and
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bool
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prompt "Networking support"
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- input prompt: "prompt" <prompt> ["if" <expr>]
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Every menu entry can have at most one prompt, which is used to display
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to the user. Optionally dependencies only for this prompt can be added
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with "if".
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- default value: "default" <expr> ["if" <expr>]
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A config option can have any number of default values. If multiple
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default values are visible, only the first defined one is active.
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Default values are not limited to the menu entry where they are
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defined. This means the default can be defined somewhere else or be
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overridden by an earlier definition.
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The default value is only assigned to the config symbol if no other
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value was set by the user (via the input prompt above). If an input
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prompt is visible the default value is presented to the user and can
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be overridden by him.
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Optionally, dependencies only for this default value can be added with
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"if".
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- type definition + default value:
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"def_bool"/"def_tristate" <expr> ["if" <expr>]
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This is a shorthand notation for a type definition plus a value.
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Optionally dependencies for this default value can be added with "if".
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- dependencies: "depends on" <expr>
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This defines a dependency for this menu entry. If multiple
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dependencies are defined, they are connected with '&&'. Dependencies
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are applied to all other options within this menu entry (which also
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accept an "if" expression), so these two examples are equivalent:
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bool "foo" if BAR
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default y if BAR
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and
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depends on BAR
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bool "foo"
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default y
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- reverse dependencies: "select" <symbol> ["if" <expr>]
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While normal dependencies reduce the upper limit of a symbol (see
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below), reverse dependencies can be used to force a lower limit of
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another symbol. The value of the current menu symbol is used as the
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minimal value <symbol> can be set to. If <symbol> is selected multiple
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times, the limit is set to the largest selection.
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Reverse dependencies can only be used with boolean or tristate
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symbols.
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Note:
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select should be used with care. select will force
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a symbol to a value without visiting the dependencies.
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By abusing select you are able to select a symbol FOO even
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if FOO depends on BAR that is not set.
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In general use select only for non-visible symbols
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(no prompts anywhere) and for symbols with no dependencies.
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That will limit the usefulness but on the other hand avoid
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the illegal configurations all over.
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kconfig should one day warn about such things.
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- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
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This allows to limit the range of possible input values for int
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and hex symbols. The user can only input a value which is larger than
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or equal to the first symbol and smaller than or equal to the second
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symbol.
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- help text: "help" or "---help---"
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This defines a help text. The end of the help text is determined by
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the indentation level, this means it ends at the first line which has
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a smaller indentation than the first line of the help text.
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"---help---" and "help" do not differ in behaviour, "---help---" is
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used to help visually separate configuration logic from help within
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the file as an aid to developers.
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- misc options: "option" <symbol>[=<value>]
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Various less common options can be defined via this option syntax,
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which can modify the behaviour of the menu entry and its config
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symbol. These options are currently possible:
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- "defconfig_list"
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This declares a list of default entries which can be used when
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looking for the default configuration (which is used when the main
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.config doesn't exists yet.)
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- "modules"
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This declares the symbol to be used as the MODULES symbol, which
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enables the third modular state for all config symbols.
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- "env"=<value>
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This imports the environment variable into Kconfig. It behaves like
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a default, except that the value comes from the environment, this
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also means that the behaviour when mixing it with normal defaults is
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undefined at this point. The symbol is currently not exported back
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to the build environment (if this is desired, it can be done via
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another symbol).
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Menu dependencies
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-----------------
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Dependencies define the visibility of a menu entry and can also reduce
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the input range of tristate symbols. The tristate logic used in the
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expressions uses one more state than normal boolean logic to express the
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module state. Dependency expressions have the following syntax:
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<expr> ::= <symbol> (1)
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<symbol> '=' <symbol> (2)
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<symbol> '!=' <symbol> (3)
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'(' <expr> ')' (4)
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'!' <expr> (5)
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<expr> '&&' <expr> (6)
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<expr> '||' <expr> (7)
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Expressions are listed in decreasing order of precedence.
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(1) Convert the symbol into an expression. Boolean and tristate symbols
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are simply converted into the respective expression values. All
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other symbol types result in 'n'.
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(2) If the values of both symbols are equal, it returns 'y',
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otherwise 'n'.
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(3) If the values of both symbols are equal, it returns 'n',
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otherwise 'y'.
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(4) Returns the value of the expression. Used to override precedence.
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(5) Returns the result of (2-/expr/).
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(6) Returns the result of min(/expr/, /expr/).
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(7) Returns the result of max(/expr/, /expr/).
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An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
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respectively for calculations). A menu entry becomes visible when its
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expression evaluates to 'm' or 'y'.
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There are two types of symbols: constant and non-constant symbols.
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Non-constant symbols are the most common ones and are defined with the
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'config' statement. Non-constant symbols consist entirely of alphanumeric
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characters or underscores.
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Constant symbols are only part of expressions. Constant symbols are
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always surrounded by single or double quotes. Within the quote, any
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other character is allowed and the quotes can be escaped using '\'.
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Menu structure
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--------------
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The position of a menu entry in the tree is determined in two ways. First
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it can be specified explicitly:
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menu "Network device support"
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depends on NET
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config NETDEVICES
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...
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endmenu
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All entries within the "menu" ... "endmenu" block become a submenu of
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"Network device support". All subentries inherit the dependencies from
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the menu entry, e.g. this means the dependency "NET" is added to the
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dependency list of the config option NETDEVICES.
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The other way to generate the menu structure is done by analyzing the
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dependencies. If a menu entry somehow depends on the previous entry, it
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can be made a submenu of it. First, the previous (parent) symbol must
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be part of the dependency list and then one of these two conditions
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must be true:
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- the child entry must become invisible, if the parent is set to 'n'
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- the child entry must only be visible, if the parent is visible
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config MODULES
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bool "Enable loadable module support"
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config MODVERSIONS
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bool "Set version information on all module symbols"
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depends on MODULES
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comment "module support disabled"
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depends on !MODULES
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MODVERSIONS directly depends on MODULES, this means it's only visible if
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MODULES is different from 'n'. The comment on the other hand is always
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visible when MODULES is visible (the (empty) dependency of MODULES is
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also part of the comment dependencies).
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Kconfig syntax
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--------------
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The configuration file describes a series of menu entries, where every
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line starts with a keyword (except help texts). The following keywords
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end a menu entry:
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- config
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- menuconfig
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- choice/endchoice
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- comment
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- menu/endmenu
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- if/endif
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- source
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The first five also start the definition of a menu entry.
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config:
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"config" <symbol>
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<config options>
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This defines a config symbol <symbol> and accepts any of above
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attributes as options.
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menuconfig:
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"menuconfig" <symbol>
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<config options>
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This is similar to the simple config entry above, but it also gives a
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hint to front ends, that all suboptions should be displayed as a
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separate list of options.
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choices:
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"choice"
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<choice options>
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<choice block>
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"endchoice"
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This defines a choice group and accepts any of the above attributes as
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options. A choice can only be of type bool or tristate, while a boolean
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choice only allows a single config entry to be selected, a tristate
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choice also allows any number of config entries to be set to 'm'. This
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can be used if multiple drivers for a single hardware exists and only a
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single driver can be compiled/loaded into the kernel, but all drivers
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can be compiled as modules.
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A choice accepts another option "optional", which allows to set the
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choice to 'n' and no entry needs to be selected.
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comment:
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"comment" <prompt>
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<comment options>
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This defines a comment which is displayed to the user during the
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configuration process and is also echoed to the output files. The only
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possible options are dependencies.
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menu:
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"menu" <prompt>
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<menu options>
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<menu block>
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"endmenu"
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This defines a menu block, see "Menu structure" above for more
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information. The only possible options are dependencies.
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if:
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"if" <expr>
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<if block>
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"endif"
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This defines an if block. The dependency expression <expr> is appended
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to all enclosed menu entries.
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source:
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"source" <prompt>
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This reads the specified configuration file. This file is always parsed.
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mainmenu:
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"mainmenu" <prompt>
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This sets the config program's title bar if the config program chooses
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to use it.
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Kconfig hints
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-------------
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This is a collection of Kconfig tips, most of which aren't obvious at
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first glance and most of which have become idioms in several Kconfig
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files.
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Adding common features and make the usage configurable
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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It is a common idiom to implement a feature/functionality that are
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relevant for some architectures but not all.
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The recommended way to do so is to use a config variable named HAVE_*
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that is defined in a common Kconfig file and selected by the relevant
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architectures.
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An example is the generic IOMAP functionality.
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We would in lib/Kconfig see:
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# Generic IOMAP is used to ...
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config HAVE_GENERIC_IOMAP
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config GENERIC_IOMAP
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depends on HAVE_GENERIC_IOMAP && FOO
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And in lib/Makefile we would see:
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obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
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For each architecture using the generic IOMAP functionality we would see:
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config X86
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select ...
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select HAVE_GENERIC_IOMAP
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select ...
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Note: we use the existing config option and avoid creating a new
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config variable to select HAVE_GENERIC_IOMAP.
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Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
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introduced to overcome the limitation of select which will force a
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config option to 'y' no matter the dependencies.
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The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
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situation where select forces a symbol equals to 'y'.
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Build as module only
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~~~~~~~~~~~~~~~~~~~~
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To restrict a component build to module-only, qualify its config symbol
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with "depends on m". E.g.:
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config FOO
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depends on BAR && m
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limits FOO to module (=m) or disabled (=n).
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