This article is not strictly a word-by-word translation, and only the part of concern is translated, which comes from linux-4.1.15/Documentation/kbuild/kconfig-language.txt
Introduction
The configuration database is a collection of configuration options
organized in a tree structure:
The organized data is a tree structure, like the following
+- 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.
Each entry (compiled item) has its own dependencies. These dependencies determine the visibility of the entrance. If the child item is to be visible, the upper-level option must also be visible.
Menu entries
Most of the compilation items are configurable, the following is an example.
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.
Each line has a keyword, followed by several parameters. config defines a new configuration item. The next few lines determine the attributes of this configuration item. The attributes include: configuration, prompt, dependency, help information and default value , A configuration attribute can be defined multiple times, but the prompt cannot be the same type or 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:
Types are: "bool", "tristate", "string", "hex", "int"
Each configuration item must have a type, there are two basic types: tristate and string, other types are based on these two types, the type line can allow prompt information
bool "Networking support"
and
bool
prompt "Networking support"
The effect of these two writing methods is the same
-
input prompt: “prompt” [“if” ]
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”.
Each menu option can only have at most one input prompt displayed to the user. You can use "if" to indicate the dependency of the prompt -
default value: “default” [“if” ]
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”.
A configuration option can have any number of default values. If there are multiple default values, only the first defined value is valid. The default value is not necessarily defined in the definition of the configuration item, which means that the default value can be defined anywhere or earlier Define coverage
If the user does not set it (through the input prompt above), the value of the configuration option is the default value. If the input prompt can be displayed, the default value will be displayed to the user and the user can modify it.
The dependency of the default value can be added with "if"
-
type definition + default value:
“def_bool”/“def_tristate” [“if” ]
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”
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:
Define a dependency for the configuration item. If there are multiple dependencies, you can use the "&&" symbol, and other items of the configuration item can also use dependencies, and also accept "if". The following writing is equivalent
Only depends on A, this A is configured, this configuration item can be configured
bool "foo" if BAR
default y if BAR
and
depends on BAR
bool "foo"
default y
-
reverse dependencies: “select” [“if” ]
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 can be set to. If is selected multiple
times, the limit is set to the largest selection.
Reverse dependencies can only be used with boolean or tristate
symbols.
Reverse dependencies use "select",
I configured it myself, all other options have to be configured
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”
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” [“if” ]
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.
help: defines a help text, the end of the help text is determined by the indentation level -
misc options: “option” [=]
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”=
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
The dependency determines the visibility of the compiled item, and also limits the input range of the tristate attribute. Tristate has one more state than boolean. The dependency expression has the following symbols
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.
The expressions are listed in 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’.
An expression may have'n','m' or'y' (0, 1, 2 for calculation), and a configuration item is visible if it is configured as'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
There are two ways to define menu items, the first is explicit
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 configuration items of the "Network device support" menu are packaged in the structure of "menu"… "endmenu", and all the dependencies of the lower-level configuration items inherit the upper-level menu. In other words, the NETDEVICES configuration item also depends on "NET"
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
The second way to generate a menu is to analyze dependencies. If the configuration item depends on the previously defined configuration item, then this configuration item is a submenu of the previous configuration item
First of all, if the parent project (former) is a dependency of this project, and the following two conditions must be met
1. If the parent menu is set to "n", the submenu must be visible,
1. If the parent menu is visible, the submenu must be 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).
MODVERSIONS directly depends on MODULES, which means that as long as MODULES is not "n", MODVERSIONS is visible
Kconfig syntax
The configuration file has many menus, the keywords are as follows:
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.
The first five are used to define a menu entry
config:
"config" <symbol>
<config options>
This defines a config 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"
There are only two types of configuration items defined in choice: bool or tristate, only one of the bool type can be selected in this choice block (similar to single selection), and the number of tristate types can be unlimited The configuration item is configured as "m", (module). This feature can be used for a hardware device corresponding to multiple drivers, so that all drivers can be compiled in the form of modules, but only one can be Compile into the kernel
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"
Define a menu block
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 is appended
to all enclosed menu entries.
source:
"source" <prompt>
Similar to include
This reads the specified configuration file. This file is always parsed.
mainmenu:
"mainmenu" <prompt>
Top menu
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).