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 only
visible when MODULES is set to 'n'.


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.  If no type is
specified for a choice, it's type will be determined by the type of
the first choice element in the group or remain unknown if none of the
choice elements have a type specified, as well.

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).

Kconfig recursive dependency limitations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you've hit the Kconfig error: "recursive dependency detected" you've run
into a recursive dependency issue with Kconfig, a recursive dependency can be
summarized as a circular dependency. The kconfig tools need to ensure that
Kconfig files comply with specified configuration requirements. In order to do
that kconfig must determine the values that are possible for all Kconfig
symbols, this is currently not possible if there is a circular relation
between two or more Kconfig symbols. For more details refer to the "Simple
Kconfig recursive issue" subsection below. Kconfig does not do recursive
dependency resolution; this has a few implications for Kconfig file writers.
We'll first explain why this issues exists and then provide an example
technical limitation which this brings upon Kconfig developers. Eager
developers wishing to try to address this limitation should read the next
subsections.

Simple Kconfig recursive issue
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Read: Documentation/kbuild/Kconfig.recursion-issue-01

Test with:

make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig

Cumulative Kconfig recursive issue
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Read: Documentation/kbuild/Kconfig.recursion-issue-02

Test with:

make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig

Practical solutions to kconfig recursive issue
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Developers who run into the recursive Kconfig issue have three options
at their disposal. We document them below and also provide a list of
historical issues resolved through these different solutions.

  a) Remove any superfluous "select FOO" or "depends on FOO"
  b) Match dependency semantics:
	b1) Swap all "select FOO" to "depends on FOO" or,
	b2) Swap all "depends on FOO" to "select FOO"

The resolution to a) can be tested with the sample Kconfig file
Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
since CORE_BELL_A depends on CORE. At times it may not be possible to remove
some dependency criteria, for such cases you can work with solution b).

The two different resolutions for b) can be tested in the sample Kconfig file
Documentation/kbuild/Kconfig.recursion-issue-02.

Below is a list of examples of prior fixes for these types of recursive issues;
all errors appear to involve one or more select's and one or more "depends on".

commit          fix
======          ===
06b718c01208    select A -> depends on A
c22eacfe82f9    depends on A -> depends on B
6a91e854442c    select A -> depends on A
118c565a8f2e    select A -> select B
f004e5594705    select A -> depends on A
c7861f37b4c6    depends on A -> (null)
80c69915e5fb    select A -> (null)              (1)
c2218e26c0d0    select A -> depends on A        (1)
d6ae99d04e1c    select A -> depends on A
95ca19cf8cbf    select A -> depends on A
8f057d7bca54    depends on A -> (null)
8f057d7bca54    depends on A -> select A
a0701f04846e    select A -> depends on A
0c8b92f7f259    depends on A -> (null)
e4e9e0540928    select A -> depends on A        (2)
7453ea886e87    depends on A > (null)           (1)
7b1fff7e4fdf    select A -> depends on A
86c747d2a4f0    select A -> depends on A
d9f9ab51e55e    select A -> depends on A
0c51a4d8abd6    depends on A -> select A        (3)
e98062ed6dc4    select A -> depends on A        (3)
91e5d284a7f1    select A -> (null)

(1) Partial (or no) quote of error.
(2) That seems to be the gist of that fix.
(3) Same error.

Future kconfig work
~~~~~~~~~~~~~~~~~~~

Work on kconfig is welcomed on both areas of clarifying semantics and on
evaluating the use of a full SAT solver for it. A full SAT solver can be
desirable to enable more complex dependency mappings and / or queries,
for instance on possible use case for a SAT solver could be that of handling
the current known recursive dependency issues. It is not known if this would
address such issues but such evaluation is desirable. If support for a full SAT
solver proves too complex or that it cannot address recursive dependency issues
Kconfig should have at least clear and well defined semantics which also
addresses and documents limitations or requirements such as the ones dealing
with recursive dependencies.

Further work on both of these areas is welcomed on Kconfig. We elaborate
on both of these in the next two subsections.

Semantics of Kconfig
~~~~~~~~~~~~~~~~~~~~

The use of Kconfig is broad, Linux is now only one of Kconfig's users:
one study has completed a broad analysis of Kconfig use in 12 projects [0].
Despite its widespread use, and although this document does a reasonable job
in documenting basic Kconfig syntax a more precise definition of Kconfig
semantics is welcomed. One project deduced Kconfig semantics through
the use of the xconfig configurator [1]. Work should be done to confirm if
the deduced semantics matches our intended Kconfig design goals.

Having well defined semantics can be useful for tools for practical
evaluation of depenencies, for instance one such use known case was work to
express in boolean abstraction of the inferred semantics of Kconfig to
translate Kconfig logic into boolean formulas and run a SAT solver on this to
find dead code / features (always inactive), 114 dead features were found in
Linux using this methodology [1] (Section 8: Threats to validity).

Confirming this could prove useful as Kconfig stands as one of the the leading
industrial variability modeling languages [1] [2]. Its study would help
evaluate practical uses of such languages, their use was only theoretical
and real world requirements were not well understood. As it stands though
only reverse engineering techniques have been used to deduce semantics from
variability modeling languages such as Kconfig [3].

[0] http://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf
[1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
[2] http://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf
[3] http://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf

Full SAT solver for Kconfig
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Although SAT solvers [0] haven't yet been used by Kconfig directly, as noted in
the previous subsection, work has been done however to express in boolean
abstraction the inferred semantics of Kconfig to translate Kconfig logic into
boolean formulas and run a SAT solver on it [1]. Another known related project
is CADOS [2] (former VAMOS [3]) and the tools, mainly undertaker [4], which has
been introduced first with [5].  The basic concept of undertaker is to exract
variability models from Kconfig, and put them together with a propositional
formula extracted from CPP #ifdefs and build-rules into a SAT solver in order
to find dead code, dead files, and dead symbols. If using a SAT solver is
desirable on Kconfig one approach would be to evaluate repurposing such efforts
somehow on Kconfig. There is enough interest from mentors of existing projects
to not only help advise how to integrate this work upstream but also help
maintain it long term. Interested developers should visit:

http://kernelnewbies.org/KernelProjects/kconfig-sat

[0] http://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf
[1] http://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
[2] https://cados.cs.fau.de
[3] https://vamos.cs.fau.de
[4] https://undertaker.cs.fau.de
[5] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf