Kconfig配置文件

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内核源码树的目录下都有两个文件Kconfig和Makefile。分布到各目录的Kconfig文件构成了一个分布式的内核配置数据库,每个Kconfig文件分别描述了所属目录源文档相关的内核配置菜单。在内核配置make menuconfig(或xconfig等)时,从Kconfig中读出来菜单,用户选择后保存到.config这个内核配置文档中。在内核编译时,主目录中的Makefile调用这个.config文件,就知道了用户的选择。
Kconfig对应着内核的配置菜单。如果想添加新的驱动到内核的源码中,就需要修改Kconfig文件。
简例:
config I2C_CHARDEV
    tristate "I2C device interface"
    help
      Say Y here to use i2c-* device files, usually found in the /dev
      directory on your system.  They make it possible to have user-space
      programs use the I2C bus.  Information on how to do this is
      contained in the file <file:Documentation/i2c/dev-interface>.

      This support is also available as a module.  If so, the module 
      will be called i2c-dev.
上述为Kconfig文件的I2C_CHARDEV配置选项。这个选项tristate是一个三态配置选项,它意味着模块要么编译为内核,要么编译为内核模块,要么不编译。当选项为Y时,表示编译入内核;当选项为M时,表示编译为模块;当选项为N时,表示不编译。
语法:
Kconfig语法较为简单,其语法在Documentation/kbuild/kconfig-language.txt文件中做了介绍。归纳起来Kconfig的语法主要包括以下几个方面。
1、主要语法总览
Kconfig配置文件描述了一系列的菜单入口。除了帮助信息之外,每一行都以一个关键字开始,这些关键字如下:config、menuconfig、choice/endchoice、comment、menu/endmenu、if/endif。前五个关键字都定义了一个菜单选项,if/endif是一个条件选项。
2、菜单入口(config)
大多数内核配置选项都对应Kconfig中的一个菜单。每行都是以关键字开始,并可以有多个参数。config关键字定义一个新的配置选项,之后几行定义该配置选项的属性。属性可以有类型、输入提示(input prompt)、依赖关系、帮助信息和默认值等。
可以出现两个相同的配置选项,但每个选项只能有一个输入提示并且类型还不能冲突。每个配置选项都必须指定一种类型,包括bool、tristate、string、hex和int,其中tristate和string是两种基本的类型,其它类型都是基于这两种类型的。类型定义后面紧跟输入提示,这些提示将显示在配置菜单中。
输入提示的一般语法如下:
prompt <prompt> ["if" <expr>]
其中prompt是关键字,表示一个输入提示。<prompt>是一个提示信息。可选项if用来表示该提示的依赖关系。
默认值的语法如下:
default <expr> [if <expr>]
一个配置选项可以有多个默认值,但是只有第一个默认值是有效的。只有config选项才能配置默认值。
依赖关系如下:
depends on <expr>
如果定义了多个依赖关系,那么可以用"&&"来连接,表示与的关系。依赖关系可以应用到菜单的所有其它选择中。depends能够限定一个config选项的能力,即如果A依赖于B,则在B被配置为Y的情况下,A可以为Y、M、N;在B被配置为M的情况下,A可以为M、N;在B被配置为N的情况下,A只能为N,表示禁用该功能。
帮助信息的语法如下:
help (或者--help--)
    begin
    ...
    end
可以用"help"或者"--help--"定义帮助信息。帮助信息可以在开发人员配置内核时给出提示。
3、菜单结构(menu)
菜单结构一般作为菜单入口的父菜单。菜单入口在菜单结构中的位置可由两种方式决定。
第一种方式如下:
menu "Network device support"
    depends on NET
config NETDEVICES
...
endmenu 
menu和endmenu为菜单结构关键字,处在其中的config选项是菜单入口。菜单入口NETDEVICES是菜单结构Network device support的子菜单。depends on NET是主菜单menu的依赖项,只有在配置NET的情况下,才可以配置Network device support菜单项。并且,所有子菜单选项都会继承父菜单的依赖关系,例如,Network device support对NET的依赖将被加到配置选项NETDEVICES的依赖关系中。
第二种方式是通过分析依赖关系生成菜单结构。如果一个菜单选项在一定程度上依赖另一个菜单选项,那么它就成为该选项的子菜单。如果父菜单选项为Y或M,那么子菜单可见,如果父菜单为N,那么子菜单就不可见,例如:
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
由语句"depends on MODULES"可知,MODVERSIONS直接依赖于MODULES,所以MODVERSIONS是MODULES的子菜单。如果MODULES不为N,那么MODVERSIONS是可见的。
4、选择菜单(choice)
选择菜单定义一组选项。此选项的类型只能是boolean或tristate型。该选项的语法如下:
"choice"
<choice options>
<choice block>
"endchoice"
在一个硬件有多个驱动的情况下可以使用choice菜单,使用choice菜单可以实现最终只有一个驱动被编译进内核中。choice菜单可以接受的另一个选项是optional,这样选项就被设置为N,表示没有被选中。
5、注释菜单(comment)
注释菜单定义了配置过程中显示给用户的注释。此注释也可以被输出到文件中,以被查看。注释语法如下:
comment <prompt>
<comment options>
在注释中唯一可以定义的属性是依赖关系,其它的属性不可以定义。

文档:
kconfig.txt:

This file contains some assistance for using "make *config".

Use "make help" to list all of the possible configuration targets.

The xconfig ('qconf'), menuconfig ('mconf'), and nconfig ('nconf')
programs also have embedded help text.  Be sure to check that for
navigation, search, and other general help text.

======================================================================
General
--------------------------------------------------

New kernel releases often introduce new config symbols.  Often more
important, new kernel releases may rename config symbols.  When
this happens, using a previously working .config file and running
"make oldconfig" won't necessarily produce a working new kernel
for you, so you may find that you need to see what NEW kernel
symbols have been introduced.

To see a list of new config symbols, use

    cp user/some/old.config .config
    make listnewconfig

and the config program will list any new symbols, one per line.

Alternatively, you can use the brute force method:

    make oldconfig
    scripts/diffconfig .config.old .config | less

______________________________________________________________________
Environment variables for '*config'

KCONFIG_CONFIG
--------------------------------------------------
This environment variable can be used to specify a default kernel config
file name to override the default name of ".config".

KCONFIG_OVERWRITECONFIG
--------------------------------------------------
If you set KCONFIG_OVERWRITECONFIG in the environment, Kconfig will not
break symlinks when .config is a symlink to somewhere else.

CONFIG_
--------------------------------------------------
If you set CONFIG_ in the environment, Kconfig will prefix all symbols
with its value when saving the configuration, instead of using the default,
"CONFIG_".

______________________________________________________________________
Environment variables for '{allyes/allmod/allno/rand}config'

KCONFIG_ALLCONFIG
--------------------------------------------------
(partially based on lkml email from/by Rob Landley, re: miniconfig)
--------------------------------------------------
The allyesconfig/allmodconfig/allnoconfig/randconfig variants can also
use the environment variable KCONFIG_ALLCONFIG as a flag or a filename
that contains config symbols that the user requires to be set to a
specific value.  If KCONFIG_ALLCONFIG is used without a filename where
KCONFIG_ALLCONFIG == "" or KCONFIG_ALLCONFIG == "1", "make *config"
checks for a file named "all{yes/mod/no/def/random}.config"
(corresponding to the *config command that was used) for symbol values
that are to be forced.  If this file is not found, it checks for a
file named "all.config" to contain forced values.

This enables you to create "miniature" config (miniconfig) or custom
config files containing just the config symbols that you are interested
in.  Then the kernel config system generates the full .config file,
including symbols of your miniconfig file.

This 'KCONFIG_ALLCONFIG' file is a config file which contains
(usually a subset of all) preset config symbols.  These variable
settings are still subject to normal dependency checks.

Examples:
    KCONFIG_ALLCONFIG=custom-notebook.config make allnoconfig
or
    KCONFIG_ALLCONFIG=mini.config make allnoconfig
or
    make KCONFIG_ALLCONFIG=mini.config allnoconfig

These examples will disable most options (allnoconfig) but enable or
disable the options that are explicitly listed in the specified
mini-config files.

______________________________________________________________________
Environment variables for 'randconfig'

KCONFIG_SEED
--------------------------------------------------
You can set this to the integer value used to seed the RNG, if you want
to somehow debug the behaviour of the kconfig parser/frontends.
If not set, the current time will be used.

KCONFIG_PROBABILITY
--------------------------------------------------
This variable can be used to skew the probabilities. This variable can
be unset or empty, or set to three different formats:
    KCONFIG_PROBABILITY     y:n split           y:m:n split
    -----------------------------------------------------------------
    unset or empty          50  : 50            33  : 33  : 34
    N                        N  : 100-N         N/2 : N/2 : 100-N
    [1] N:M                     N+M : 100-(N+M)      N  :  M  : 100-(N+M)
    [2] N:M:L                    N  : 100-N          M  :  L  : 100-(M+L)

where N, M and L are integers (in base 10) in the range [0,100], and so
that:
    [1] N+M is in the range [0,100]
    [2] M+L is in the range [0,100]

Examples:
    KCONFIG_PROBABILITY=10
        10% of booleans will be set to 'y', 90% to 'n'
        5% of tristates will be set to 'y', 5% to 'm', 90% to 'n'
    KCONFIG_PROBABILITY=15:25
        40% of booleans will be set to 'y', 60% to 'n'
        15% of tristates will be set to 'y', 25% to 'm', 60% to 'n'
    KCONFIG_PROBABILITY=10:15:15
        10% of booleans will be set to 'y', 90% to 'n'
        15% of tristates will be set to 'y', 15% to 'm', 70% to 'n'

______________________________________________________________________
Environment variables for 'syncconfig'

KCONFIG_NOSILENTUPDATE
--------------------------------------------------
If this variable has a non-blank value, it prevents silent kernel
config updates (requires explicit updates).

KCONFIG_AUTOCONFIG
--------------------------------------------------
This environment variable can be set to specify the path & name of the
"auto.conf" file.  Its default value is "include/config/auto.conf".

KCONFIG_TRISTATE
--------------------------------------------------
This environment variable can be set to specify the path & name of the
"tristate.conf" file.  Its default value is "include/config/tristate.conf".

KCONFIG_AUTOHEADER
--------------------------------------------------
This environment variable can be set to specify the path & name of the
"autoconf.h" (header) file.
Its default value is "include/generated/autoconf.h".


======================================================================
menuconfig
--------------------------------------------------

SEARCHING for CONFIG symbols

Searching in menuconfig:

    The Search function searches for kernel configuration symbol
    names, so you have to know something close to what you are
    looking for.

    Example:
        /hotplug
        This lists all config symbols that contain "hotplug",
        e.g., HOTPLUG_CPU, MEMORY_HOTPLUG.

    For search help, enter / followed by TAB-TAB (to highlight
    <Help>) and Enter.  This will tell you that you can also use
    regular expressions (regexes) in the search string, so if you
    are not interested in MEMORY_HOTPLUG, you could try

        /^hotplug

    When searching, symbols are sorted thus:
      - first, exact matches, sorted alphabetically (an exact match
        is when the search matches the complete symbol name);
      - then, other matches, sorted alphabetically.
    For example: ^ATH.K matches:
        ATH5K ATH9K ATH5K_AHB ATH5K_DEBUG [...] ATH6KL ATH6KL_DEBUG
        [...] ATH9K_AHB ATH9K_BTCOEX_SUPPORT ATH9K_COMMON [...]
    of which only ATH5K and ATH9K match exactly and so are sorted
    first (and in alphabetical order), then come all other symbols,
    sorted in alphabetical order.

______________________________________________________________________
User interface options for 'menuconfig'

MENUCONFIG_COLOR
--------------------------------------------------
It is possible to select different color themes using the variable
MENUCONFIG_COLOR.  To select a theme use:

    make MENUCONFIG_COLOR=<theme> menuconfig

Available themes are:
  mono       => selects colors suitable for monochrome displays
  blackbg    => selects a color scheme with black background
  classic    => theme with blue background. The classic look
  bluetitle  => a LCD friendly version of classic. (default)

MENUCONFIG_MODE
--------------------------------------------------
This mode shows all sub-menus in one large tree.

Example:
    make MENUCONFIG_MODE=single_menu menuconfig


======================================================================
nconfig
--------------------------------------------------

nconfig is an alternate text-based configurator.  It lists function
keys across the bottom of the terminal (window) that execute commands.
You can also just use the corresponding numeric key to execute the
commands unless you are in a data entry window.  E.g., instead of F6
for Save, you can just press 6.

Use F1 for Global help or F3 for the Short help menu.

Searching in nconfig:

    You can search either in the menu entry "prompt" strings
    or in the configuration symbols.

    Use / to begin a search through the menu entries.  This does
    not support regular expressions.  Use <Down> or <Up> for
    Next hit and Previous hit, respectively.  Use <Esc> to
    terminate the search mode.

    F8 (SymSearch) searches the configuration symbols for the
    given string or regular expression (regex).

NCONFIG_MODE
--------------------------------------------------
This mode shows all sub-menus in one large tree.

Example:
    make NCONFIG_MODE=single_menu nconfig


======================================================================
xconfig
--------------------------------------------------

Searching in xconfig:

    The Search function searches for kernel configuration symbol
    names, so you have to know something close to what you are
    looking for.

    Example:
        Ctrl-F hotplug
    or
        Menu: File, Search, hotplug

    lists all config symbol entries that contain "hotplug" in
    the symbol name.  In this Search dialog, you may change the
    config setting for any of the entries that are not grayed out.
    You can also enter a different search string without having
    to return to the main menu.


======================================================================
gconfig
--------------------------------------------------

Searching in gconfig:

    There is no search command in gconfig.  However, gconfig does
    have several different viewing choices, modes, and options.

###
 

kconfig-language.txt:

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".

 The default value deliberately defaults to 'n' in order to avoid bloating the
 build. With few exceptions, new config options should not change this. The
 intent is for "make oldconfig" to add as little as possible to the config from
 release to release.

 Note:
    Things that merit "default y/m" include:

    a) A new Kconfig option for something that used to always be built
       should be "default y".

    b) A new gatekeeping Kconfig option that hides/shows other Kconfig
       options (but does not generate any code of its own), should be
       "default y" so people will see those other options.

    c) Sub-driver behavior or similar options for a driver that is
       "default n". This allows you to provide sane defaults.

    d) Hardware or infrastructure that everybody expects, such as CONFIG_NET
       or CONFIG_BLOCK. These are rare exceptions.

- 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.

- weak reverse dependencies: "imply" <symbol> ["if" <expr>]
  This is similar to "select" as it enforces a lower limit on another
  symbol except that the "implied" symbol's value may still be set to n
  from a direct dependency or with a visible prompt.

  Given the following example:

  config FOO
    tristate
    imply BAZ

  config BAZ
    tristate
    depends on BAR

  The following values are possible:

    FOO        BAR        BAZ's default    choice for BAZ
    ---        ---        -------------    --------------
    n        y        n        N/m/y
    m        y        m        M/y/n
    y        y        y        Y/n
    y        n        *        N

  This is useful e.g. with multiple drivers that want to indicate their
  ability to hook into a secondary subsystem while allowing the user to
  configure that subsystem out without also having to unset these drivers.

- 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.

  - "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)
           <symbol1> '<' <symbol2>              (4)
           <symbol1> '>' <symbol2>              (4)
           <symbol1> '<=' <symbol2>             (4)
           <symbol1> '>=' <symbol2>             (4)
           '(' <expr> ')'                       (5)
           '!' <expr>                           (6)
           <expr> '&&' <expr>                   (7)
           <expr> '||' <expr>                   (8)

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) If value of <symbol1> is respectively lower, greater, lower-or-equal,
    or greater-or-equal than value of <symbol2>, it returns 'y',
    otherwise 'n'.
(5) Returns the value of the expression. Used to override precedence.
(6) Returns the result of (2-/expr/).
(7) Returns the result of min(/expr/, /expr/).
(8) 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. To make sure all the suboptions will really
show up under the menuconfig entry and not outside of it, every item
from the <config options> list must depend on the menuconfig symbol.
In practice, this is achieved by using one of the next two constructs:

(1):
menuconfig M
if M
    config C1
    config C2
endif

(2):
menuconfig M
config C1
    depends on M
config C2
    depends on M

In the following examples (3) and (4), C1 and C2 still have the M
dependency, but will not appear under menuconfig M anymore, because
of C0, which doesn't depend on M:

(3):
menuconfig M
    config C0
if M
    config C1
    config C2
endif

(4):
menuconfig M
config C0
config C1
    depends on M
config C2
    depends on M

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, its 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 (i.e. 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 source file comment:

An unquoted '#' character anywhere in a source file line indicates
the beginning of a source file comment.  The remainder of that line
is a comment.


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'.

Adding features that need compiler support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are several features that need compiler support. The recommended way
to describe the dependency on the compiler feature is to use "depends on"
followed by a test macro.

config STACKPROTECTOR
    bool "Stack Protector buffer overflow detection"
    depends on $(cc-option,-fstack-protector)
    ...

If you need to expose a compiler capability to makefiles and/or C source files,
CC_HAS_ is the recommended prefix for the config option.

config CC_HAS_STACKPROTECTOR_NONE
    def_bool $(cc-option,-fno-stack-protector)

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 two 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
 

Kconfig.select-break:

# Select broken dependency issue
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# Test with:
#
# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.select-break menuconfig
#
# kconfig will not complain and enable this layout for configuration. This is
# currently a feature of kconfig, given select was designed to be heavy handed.
# Kconfig currently does not check the list of symbols listed on a symbol's
# "select" list, this is done on purpose to help load a set of known required
# symbols. Because of this use of select should be used with caution. An
# example of this issue is below.
#
# The option B and C are clearly contradicting with respect to A.
# However, when A is set, C can be set as well because Kconfig does not
# visit the dependencies of the select target (in this case B).  And since
# Kconfig does not visit the dependencies, it breaks the dependencies of B
# (!A).

mainmenu "Simple example to demo kconfig select broken dependency issue"

config A
    bool "CONFIG A"

config B
    bool "CONFIG B"
    depends on !A

config C
    bool "CONFIG C"
    depends on A
    select B
 

Kconfig.recursion-issue-01:

# Simple Kconfig recursive issue
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# Test with:
#
# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
#
# This Kconfig file has a simple recursive dependency issue. In order to
# understand why this recursive dependency issue occurs lets consider what
# Kconfig needs to address. We iterate over what Kconfig needs to address
# by stepping through the questions it needs to address sequentially.
#
#  * What values are possible for CORE?
#
# CORE_BELL_A_ADVANCED selects CORE, which means that it influences the values
# that are possible for CORE. So for example if CORE_BELL_A_ADVANCED is 'y',
# CORE must be 'y' too.
#
#  * What influences CORE_BELL_A_ADVANCED ?
#
# As the name implies CORE_BELL_A_ADVANCED is an advanced feature of
# CORE_BELL_A so naturally it depends on CORE_BELL_A. So if CORE_BELL_A is 'y'
# we know CORE_BELL_A_ADVANCED can be 'y' too.
#
#   * What influences CORE_BELL_A ?
#
# CORE_BELL_A depends on CORE, so CORE influences CORE_BELL_A.
#
# But that is a problem, because this means that in order to determine
# what values are possible for CORE we ended up needing to address questions
# regarding possible values of CORE itself again. Answering the original
# question of what are the possible values of CORE would make the kconfig
# tools run in a loop. When this happens Kconfig exits and complains about
# the "recursive dependency detected" error.
#
# Reading the Documentation/kbuild/Kconfig.recursion-issue-01 file it may be
# obvious that an easy to solution to this problem should just be the removal
# of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
# since CORE_BELL_A depends on CORE. Recursive dependency issues are not always
# so trivial to resolve, we provide another example below of practical
# implications of this recursive issue where the solution is perhaps not so
# easy to understand. Note that matching semantics on the dependency on
# CORE also consist of a solution to this recursive problem.

mainmenu "Simple example to demo kconfig recursive dependency issue"

config CORE
    tristate

config CORE_BELL_A
    tristate
    depends on CORE

config CORE_BELL_A_ADVANCED
    tristate
    depends on CORE_BELL_A
    select CORE
 

Kconfig.recursion-issue-02:

# Cumulative Kconfig recursive issue
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# Test with:
#
# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
#
# The recursive limitations with Kconfig has some non intuitive implications on
# kconfig sematics which are documented here. One known practical implication
# of the recursive limitation is that drivers cannot negate features from other
# drivers if they share a common core requirement and use disjoint semantics to
# annotate those requirements, ie, some drivers use "depends on" while others
# use "select". For instance it means if a driver A and driver B share the same
# core requirement, and one uses "select" while the other uses "depends on" to
# annotate this, all features that driver A selects cannot now be negated by
# driver B.
#
# A perhaps not so obvious implication of this is that, if semantics on these
# core requirements are not carefully synced, as drivers evolve features
# they select or depend on end up becoming shared requirements which cannot be
# negated by other drivers.
#
# The example provided in Documentation/kbuild/Kconfig.recursion-issue-02
# describes a simple driver core layout of example features a kernel might
# have. Let's assume we have some CORE functionality, then the kernel has a
# series of bells and whistles it desires to implement, its not so advanced so
# it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If
# CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects
# CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which
# other bells in the system cannot negate. The reason for this issue is
# due to the disjoint use of semantics on expressing each bell's relationship
# with CORE, one uses "depends on" while the other uses "select". Another
# more important reason is that kconfig does not check for dependencies listed
# under 'select' for a symbol, when such symbols are selected kconfig them
# as mandatory required symbols. For more details on the heavy handed nature
# of select refer to Documentation/kbuild/Kconfig.select-break
#
# To fix this the "depends on CORE" must be changed to "select CORE", or the
# "select CORE" must be changed to "depends on CORE".
#
# For an example real world scenario issue refer to the attempt to remove
# "select FW_LOADER" [0], in the end the simple alternative solution to this
# problem consisted on matching semantics with newly introduced features.
#
# [0] http://lkml.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com

mainmenu "Simple example to demo cumulative kconfig recursive dependency implication"

config CORE
    tristate

config CORE_BELL_A
    tristate
    depends on CORE

config CORE_BELL_A_ADVANCED
    tristate
    select CORE_BELL_A

config CORE_BELL_B
    tristate
    depends on !CORE_BELL_A
    select CORE
 

kconfig-macro-language.txt:

Concept
-------

The basic idea was inspired by Make. When we look at Make, we notice sort of
two languages in one. One language describes dependency graphs consisting of
targets and prerequisites. The other is a macro language for performing textual
substitution.

There is clear distinction between the two language stages. For example, you
can write a makefile like follows:

    APP := foo
    SRC := foo.c
    CC := gcc

    $(APP): $(SRC)
            $(CC) -o $(APP) $(SRC)

The macro language replaces the variable references with their expanded form,
and handles as if the source file were input like follows:

    foo: foo.c
            gcc -o foo foo.c

Then, Make analyzes the dependency graph and determines the targets to be
updated.

The idea is quite similar in Kconfig - it is possible to describe a Kconfig
file like this:

    CC := gcc

    config CC_HAS_FOO
            def_bool $(shell, $(srctree)/scripts/gcc-check-foo.sh $(CC))

The macro language in Kconfig processes the source file into the following
intermediate:

    config CC_HAS_FOO
            def_bool y

Then, Kconfig moves onto the evaluation stage to resolve inter-symbol
dependency as explained in kconfig-language.txt.


Variables
---------

Like in Make, a variable in Kconfig works as a macro variable.  A macro
variable is expanded "in place" to yield a text string that may then be
expanded further. To get the value of a variable, enclose the variable name in
$( ). The parentheses are required even for single-letter variable names; $X is
a syntax error. The curly brace form as in ${CC} is not supported either.

There are two types of variables: simply expanded variables and recursively
expanded variables.

A simply expanded variable is defined using the := assignment operator. Its
righthand side is expanded immediately upon reading the line from the Kconfig
file.

A recursively expanded variable is defined using the = assignment operator.
Its righthand side is simply stored as the value of the variable without
expanding it in any way. Instead, the expansion is performed when the variable
is used.

There is another type of assignment operator; += is used to append text to a
variable. The righthand side of += is expanded immediately if the lefthand
side was originally defined as a simple variable. Otherwise, its evaluation is
deferred.

The variable reference can take parameters, in the following form:

  $(name,arg1,arg2,arg3)

You can consider the parameterized reference as a function. (more precisely,
"user-defined function" in contrast to "built-in function" listed below).

Useful functions must be expanded when they are used since the same function is
expanded differently if different parameters are passed. Hence, a user-defined
function is defined using the = assignment operator. The parameters are
referenced within the body definition with $(1), $(2), etc.

In fact, recursively expanded variables and user-defined functions are the same
internally. (In other words, "variable" is "function with zero argument".)
When we say "variable" in a broad sense, it includes "user-defined function".


Built-in functions
------------------

Like Make, Kconfig provides several built-in functions. Every function takes a
particular number of arguments.

In Make, every built-in function takes at least one argument. Kconfig allows
zero argument for built-in functions, such as $(fileno), $(lineno). You could
consider those as "built-in variable", but it is just a matter of how we call
it after all. Let's say "built-in function" here to refer to natively supported
functionality.

Kconfig currently supports the following built-in functions.

 - $(shell,command)

  The "shell" function accepts a single argument that is expanded and passed
  to a subshell for execution. The standard output of the command is then read
  and returned as the value of the function. Every newline in the output is
  replaced with a space. Any trailing newlines are deleted. The standard error
  is not returned, nor is any program exit status.

 - $(info,text)

  The "info" function takes a single argument and prints it to stdout.
  It evaluates to an empty string.

 - $(warning-if,condition,text)

  The "warning-if" function takes two arguments. If the condition part is "y",
  the text part is sent to stderr. The text is prefixed with the name of the
  current Kconfig file and the current line number.

 - $(error-if,condition,text)

  The "error-if" function is similar to "warning-if", but it terminates the
  parsing immediately if the condition part is "y".

 - $(filename)

  The 'filename' takes no argument, and $(filename) is expanded to the file
  name being parsed.

 - $(lineno)

  The 'lineno' takes no argument, and $(lineno) is expanded to the line number
  being parsed.


Make vs Kconfig
---------------

Kconfig adopts Make-like macro language, but the function call syntax is
slightly different.

A function call in Make looks like this:

  $(func-name arg1,arg2,arg3)

The function name and the first argument are separated by at least one
whitespace. Then, leading whitespaces are trimmed from the first argument,
while whitespaces in the other arguments are kept. You need to use a kind of
trick to start the first parameter with spaces. For example, if you want
to make "info" function print "  hello", you can write like follows:

  empty :=
  space := $(empty) $(empty)
  $(info $(space)$(space)hello)

Kconfig uses only commas for delimiters, and keeps all whitespaces in the
function call. Some people prefer putting a space after each comma delimiter:

  $(func-name, arg1, arg2, arg3)

In this case, "func-name" will receive " arg1", " arg2", " arg3". The presence
of leading spaces may matter depending on the function. The same applies to
Make - for example, $(subst .c, .o, $(sources)) is a typical mistake; it
replaces ".c" with " .o".

In Make, a user-defined function is referenced by using a built-in function,
'call', like this:

    $(call my-func,arg1,arg2,arg3)

Kconfig invokes user-defined functions and built-in functions in the same way.
The omission of 'call' makes the syntax shorter.

In Make, some functions treat commas verbatim instead of argument separators.
For example, $(shell echo hello, world) runs the command "echo hello, world".
Likewise, $(info hello, world) prints "hello, world" to stdout. You could say
this is _useful_ inconsistency.

In Kconfig, for simpler implementation and grammatical consistency, commas that
appear in the $( ) context are always delimiters. It means

  $(shell, echo hello, world)

is an error because it is passing two parameters where the 'shell' function
accepts only one. To pass commas in arguments, you can use the following trick:

  comma := ,
  $(shell, echo hello$(comma) world)


Caveats
-------

A variable (or function) cannot be expanded across tokens. So, you cannot use
a variable as a shorthand for an expression that consists of multiple tokens.
The following works:

    RANGE_MIN := 1
    RANGE_MAX := 3

    config FOO
            int "foo"
            range $(RANGE_MIN) $(RANGE_MAX)

But, the following does not work:

    RANGES := 1 3

    config FOO
            int "foo"
            range $(RANGES)

A variable cannot be expanded to any keyword in Kconfig.  The following does
not work:

    MY_TYPE := tristate

    config FOO
            $(MY_TYPE) "foo"
            default y

Obviously from the design, $(shell command) is expanded in the textual
substitution phase. You cannot pass symbols to the 'shell' function.
The following does not work as expected.

    config ENDIAN_FLAG
            string
            default "-mbig-endian" if CPU_BIG_ENDIAN
            default "-mlittle-endian" if CPU_LITTLE_ENDIAN

    config CC_HAS_ENDIAN_FLAG
            def_bool $(shell $(srctree)/scripts/gcc-check-flag ENDIAN_FLAG)

Instead, you can do like follows so that any function call is statically
expanded.

    config CC_HAS_ENDIAN_FLAG
            bool
            default $(shell $(srctree)/scripts/gcc-check-flag -mbig-endian) if CPU_BIG_ENDIAN
            default $(shell $(srctree)/scripts/gcc-check-flag -mlittle-endian) if CPU_LITTLE_ENDIAN
 

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