Introduction
------------
-The configuration database is collection of configuration options
+The configuration database is a collection of configuration options
organized in a tree structure:
+- Code maturity level options
Menu entries
------------
-Most entries define a config option, all other entries help to organize
+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 MODULES
+ depends on MODULES
help
Usually, modules have to be recompiled whenever you switch to a new
kernel. ...
- 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
+ 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:
Optionally, dependencies only for this default value can be added with
"if".
-- dependencies: "depends on"/"requires" <expr>
+- 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
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.
+ kconfig should one day warn about such things.
- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
This allows to limit the range of possible input values for int
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.
+
+ - "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).
Menu dependencies
-----------------
respectively for calculations). A menu entry becomes visible when it's
expression evaluates to 'm' or 'y'.
-There are two types of symbols: constant and nonconstant symbols.
-Nonconstant symbols are the most common ones and are defined with the
-'config' statement. Nonconstant symbols consist entirely of alphanumeric
+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
it can be specified explicitly:
menu "Network device support"
- depends NET
+ depends on NET
config NETDEVICES
...
config MODVERSIONS
bool "Set version information on all module symbols"
- depends MODULES
+ depends on MODULES
comment "module support disabled"
- depends !MODULES
+ 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
"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.
+
+
+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).
+
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