Setting Kconfig configuration values

The menuconfig and guiconfig interfaces can be used to test out configurations during application development. This page explains how to make settings permanent.

All Kconfig options can be searched in the Kconfig search page.

Note

Before making changes to Kconfig files, it’s a good idea to also go through the Kconfig - Tips and Best Practices page.

Visible and invisible Kconfig symbols

When making Kconfig changes, it’s important to understand the difference between visible and invisible symbols.

  • A visible symbol is a symbol defined with a prompt. Visible symbols show up in the interactive configuration interfaces (hence visible), and can be set in configuration files.

    Here’s an example of a visible symbol:

    config FPU
   bool "Support floating point operations"
   depends on HAS_FPU

    The symbol is shown like this in menuconfig, where it can be toggled:

    [ ] Support floating point operations

  • An invisible symbol is a symbol without a prompt. Invisible symbols are not shown in the interactive configuration interfaces, and users have no direct control over their value. They instead get their value from defaults or from other symbols.

    Here’s an example of an invisible symbol:

    config CPU_HAS_FPU
   bool
   help
     This symbol is y if the CPU has a hardware floating point unit.

    In this case, CPU_HAS_FPU is enabled through other symbols having select CPU_HAS_FPU.

Setting symbols in configuration files

Visible symbols can be configured by setting them in configuration files. The initial configuration is produced by merging a *_defconfig file for the board with application settings, usually from prj.conf. See The Initial Configuration below for more details.

Assignments in configuration files use this syntax:

CONFIG_<symbol name>=<value>

There should be no spaces around the equals sign.

bool symbols can be enabled or disabled by setting them to y or n, respectively. The FPU symbol from the example above could be enabled like this:

CONFIG_FPU=y

Note

A boolean symbol can also be set to n with a comment formatted like this:

# CONFIG_SOME_OTHER_BOOL is not set

This is the format you will see in the merged configuration saved to zephyr/.config in the build directory.

This style is accepted for historical reasons: Kconfig configuration files can be parsed as makefiles (though Zephyr doesn’t use this). Having n-valued symbols correspond to unset variables simplifies tests in Make.

Other symbol types are assigned like this:

CONFIG_SOME_STRING="cool value"
CONFIG_SOME_INT=123

Comments use a #:

# This is a comment

Assignments in configuration files are only respected if the dependencies for the symbol are satisfied. A warning is printed otherwise. To figure out what the dependencies of a symbol are, use one of the interactive configuration interfaces (you can jump directly to a symbol with /), or look up the symbol in the Kconfig search page.

The Initial Configuration

The initial configuration for an application comes from merging configuration settings from three sources:

  1. A BOARD-specific configuration file stored in boards/<VENDOR>/<BOARD>/<BOARD>_defconfig

  2. Any CMake cache entries prefix with CONFIG_

  3. The application configuration

The application configuration can come from the sources below (each file is known as a Kconfig fragment, which are then merged to get the final configuration used for a particular build). By default, prj.conf is used.

  1. If CONF_FILE is set, the configuration file(s) specified in it are merged and used as the application configuration. CONF_FILE can be set in various ways:

    1. In CMakeLists.txt, before calling find_package(Zephyr)

    2. By passing -DCONF_FILE=<conf file(s)>, either directly or via west

    3. From the CMake variable cache

    Furthermore if CONF_FILE is set as single configuration file of the form prj_<build>.conf and if file boards/<BOARD>_<build>.conf exists in same folder as file prj_<build>.conf, the result of merging prj_<build>.conf and boards/<BOARD>_<build>.conf is used - note that this feature is deprecated, File Suffixes should be used instead.

  2. Otherwise, if boards/<BOARD>.conf exists in the application configuration directory, the result of merging it with prj.conf is used.

  3. Otherwise, if board revisions are used and boards/<BOARD>_<revision>.conf exists in the application configuration directory, the result of merging it with prj.conf and boards/<BOARD>.conf is used.

  4. Otherwise, prj.conf is used from the application configuration directory. If it does not exist then a fatal error will be emitted.

Furthermore, applications can have SoC overlay configuration that is applied to it, the file socs/<SOC>_<BOARD_QUALIFIERS>.conf will be applied if it exists, after the main project configuration has been applied and before any board overlay configuration files have been applied.

All configuration files will be taken from the application’s configuration directory except for files with an absolute path that are given with the CONF_FILE, EXTRA_CONF_FILE, DTC_OVERLAY_FILE, and EXTRA_DTC_OVERLAY_FILE arguments. For these, a file in a Zephyr module can be referred by escaping the Zephyr module dir variable like this \${ZEPHYR_<module>_MODULE_DIR}/<path-to>/<file> when setting any of said variables in the application’s CMakeLists.txt.

See Application Configuration Directory on how the application configuration directory is defined.

If a symbol is assigned both in <BOARD>_defconfig and in the application configuration, the value set in the application configuration takes precedence.

The merged configuration is saved to zephyr/.config in the build directory.

As long as zephyr/.config exists and is up-to-date (is newer than any BOARD and application configuration files), it will be used in preference to producing a new merged configuration. zephyr/.config is also the configuration that gets modified when making changes in the interactive configuration interfaces.

Tracking Kconfig symbols

It is possible to create Kconfig symbols which takes the default value of another Kconfig symbol.

This is valuable when you want a symbol specific to an application or subsystem but do not want to rely directly on the common symbol. For example, you might want to decouple the settings so they can be independently configured, or to ensure you always have a locally named setting, even if the external setting name changes. is later changed.

For example, consider the common FOO_STRING setting where a subsystem wants to have a SUB_FOO_STRING but still allow for customization.

This can be done like this:

config FOO_STRING
        string "Foo"
        default "foo"

config SUB_FOO_STRING
        string "Sub-foo"
        default FOO_STRING

This ensures that the default value of SUB_FOO_STRING is identical to FOO_STRING while still allows users to configure both settings independently.

It is also possible to make SUB_FOO_STRING invisible and thereby keep the two symbols in sync, unless the value of the tracking symbol is changed in a defconfig file.

config FOO_STRING
        string "Foo"
        default "foo"

config SUB_FOO_STRING
        string
        default FOO_STRING
        help
          Hidden symbol which follows FOO_STRING
          Can be changed through *.defconfig files.

Configuring invisible Kconfig symbols

When making changes to the default configuration for a board, you might have to configure invisible symbols. This is done in boards/<VENDOR>/<BOARD>/Kconfig.defconfig, which is a regular Kconfig file.

Note

Assignments in .config files have no effect on invisible symbols, so this scheme is not just an organizational issue.

Assigning values in Kconfig.defconfig relies on defining a Kconfig symbol in multiple locations. As an example, say we want to set FOO_WIDTH below to 32:

config FOO_WIDTH
    int

To do this, we extend the definition of FOO_WIDTH as follows, in Kconfig.defconfig:

if BOARD_MY_BOARD

config FOO_WIDTH
    default 32

endif

Note

Since the type of the symbol (int) has already been given at the first definition location, it does not need to be repeated here. Only giving the type once at the “base” definition of the symbol is a good idea for reasons explained in Common Kconfig shorthands.

default values in Kconfig.defconfig files have priority over default values given on the “base” definition of a symbol. Internally, this is implemented by including the Kconfig.defconfig files first. Kconfig uses the first default with a satisfied condition, where an empty condition corresponds to if y (is always satisfied).

Note that conditions from surrounding top-level ifs are propagated to symbol properties, so the above default is equivalent to default 32 if BOARD_MY_BOARD.

Multiple symbol definitions

When a symbol is defined in multiple locations, each definition acts as an independent symbol that happens to share the same name. This means that properties are not appended to previous definitions. If the conditions for ANY definition result in the symbol resolving to y, the symbol will be y. It is therefore not possible to make the dependencies of a symbol more restrictive by defining it in multiple locations.

For example, the dependencies of the symbol FOO below are satisfied if either DEP1 OR DEP2 are true, it does not require both:

config FOO
   ...
   depends on DEP1

config FOO
   ...
   depends on DEP2

Warning

Symbols without explicit dependencies still follow the above rule. A symbol without any dependencies will result in the symbol always being assignable. The definition below will result in FOO always being enabled by default, regardless of the value of DEP1.

config FOO
   bool "FOO"
   depends on DEP1

config FOO
   default y

This dependency weakening can be avoided with the configdefault extension if the desire is only to add a new default without modifying any other behaviour of the symbol.

Note

When making changes to Kconfig.defconfig files, always check the symbol’s direct dependencies in one of the interactive configuration interfaces afterwards. It is often necessary to repeat dependencies from the base definition of the symbol to avoid weakening a symbol’s dependencies.

Motivation for Kconfig.defconfig files

One motivation for this configuration scheme is to avoid making fixed BOARD-specific settings configurable in the interactive configuration interfaces. If all board configuration were done via <BOARD>_defconfig, all symbols would have to be visible, as values given in <BOARD>_defconfig have no effect on invisible symbols.

Having fixed settings be user-configurable would clutter up the configuration interfaces and make them harder to understand, and would make it easier to accidentally create broken configurations.

When dealing with fixed board-specific settings, also consider whether they should be handled via devicetree instead.

Configuring choices

There are two ways to configure a Kconfig choice:

  1. By setting one of the choice symbols to y in a configuration file.

    Setting one choice symbol to y automatically gives all other choice symbols the value n.

    If multiple choice symbols are set to y, only the last one set to y will be honored (the rest will get the value n). This allows a choice selection from a board defconfig file to be overridden from an application prj.conf file.

  2. By changing the default of the choice in Kconfig.defconfig.

    As with symbols, changing the default for a choice is done by defining the choice in multiple locations. For this to work, the choice must have a name.

    As an example, assume that a choice has the following base definition (here, the name of the choice is FOO):

    choice FOO
    bool "Foo choice"
    default B

config A
    bool "A"

config B
    bool "B"

endchoice

    To change the default symbol of FOO to A, you would add the following definition to Kconfig.defconfig:

    choice FOO
    default A
endchoice

The Kconfig.defconfig method should be used when the dependencies of the choice might not be satisfied. In that case, you’re setting the default selection whenever the user makes the choice visible.

More Kconfig resources

The Kconfig - Tips and Best Practices page has some tips for writing Kconfig files.

The kconfiglib.py docstring (at the top of the file) goes over how symbol values are calculated in detail.