Component Configuration Guide

This guide is intended to describe how to define configuration options for components in ESP-IDF. Following topics will be covered:

  • How to define new configuration options for components.

  • Basic syntax of Kconfig language.

  • How to ensure backward compatibility.

How Configuration Works in ESP-IDF

备注

More detailed information about the configuration system in ESP-IDF can be found in the Project Configuration.

ESP-IDF uses unified way to configure the project, build system, ESP-IDF framework itself and external components. This configuration tool is called Kconfig.

Configuration options are defined in Kconfig files. ESP-IDF contains the top-level Kconfig file in the root of the framework. Each component can have its own Kconfig file defining configuration options specific to that component, as well as relations between the options. Relations between config options can spread across multiple Kconfig files from different sources. In other words, configuration option from Component_A can depend on a configuration option from Component_B, even if Component_B is e.g. maintained by another developer.

When the configuration is saved (for more information about configuration editing, see e.g. Project Configuration Guide), the values in the sdkconfig file are updated, as well as those in sdkconfig.h, sdkconfig.cmake and sdkconfig.json.

How to Define New Configuration Options for Your Component

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If you plan to write Kconfig configuration files for your component, but you are not familiar with Kconfig language it is recommended to refer to the esp-idf-kconfig Documentation, where an in-depth guide is provided.

To define new configuration options for components, you need to:

  1. Create the Kconfig and/or Kconfig.projbuild in the root folder of the component.

  2. Define the configuration options in the Kconfig and/or Kconfig.projbuild file. It is generally a good practice to wrap them in the menu-endmenu block. You can see a minimal example below. Complete documentation of the Kconfig language can be found in the Kconfig Documentation.

When your component is used in a project, the Kconfig and/or Kconfig.projbuild will be automatically discovered and shown in the menuconfig tool.

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Difference between Kconfig and Kconfig.projbuild files:

  • Kconfig: configuration options in this file will be shown under Component configuration in the menuconfig tool.

  • Kconfig.projbuild: configuration options in this file will be shown in the top menu of the menuconfig tool.

Example:

menu "Motors configuration"

    config SUBLIGHT_DRIVE_ENABLED
        bool "Enable sublight drive"
        default n
        depends on SPACE_SHIP
        help
            This option enables sublight on our spaceship.

endmenu

备注

Visibility and dependencies:

In the example above, the SUBLIGHT_DRIVE_ENABLED configuration option has a dependency on the SPACE_SHIP configuration option. This option can origin from a different component. If the SPACE_SHIP option is not set or is not defined in the current configuration (e.g. the component containing this option was not included in the project), the dependency will not be satisfied and the SUBLIGHT_DRIVE_ENABLED option will not be shown in the menuconfig tool.

For more information about the visibility and dependencies, please refer to the Kconfig Documentation.

How Values are Assigned to Configuration Options

Configuration process automatically decides which value a given configuration option should have and saves it to the sdkconfig file (if the sdkconfig file does not exist, it is created, otherwise it is updated). This process is run during the idf.py build, idf.py reconfigure, idf.py menuconfig or idf.py set-target commands. The idf.py set-target command deletes previous sdkconfig (if any) and will always create a new one.

After sdkconfig file is created/updated, it contains all currently visible configuration options together with values currently assigned to them. It is important to mention that once the sdkconfig file is created, all the configuration options that are contained in it are locked, meaning their values are not changed automatically, and can only be changed manually via idf.py menuconfig. Read further for in-depth explanation or skip to the Workarounds and Best Practices section.

There are three sources a configuration option can get its value from, where later sources may override the previous ones:

  1. Kconfig file. This is the source of default values for configuration options. Every configuration option should have its default value(s) defined as a part of its definition. Default values can be conditioned. This means that given default value will be used only if its condition holds. For more information how to define default values, please refer to the default option description in Kconfig Documentation.

  2. sdkconfig.defaults file. If it exists, configuration options that have a value assigned in this file will be assigned this value, ignoring the default value (the one obtained from the Kconfig file). Configuration options not set in this file are still free to change based on the conditions of their default values!

  3. sdkconfig file. If it exists, configuration options that have a value assigned in this file will be assigned this value, ignoring the one obtained from the Kconfig or sdkconfig.defaults file (or caused by default value change during sdkconfig.defaults file processing).

As it was mentioned, sdkconfig file is created as a result of the configuration process and it contains all currently visible configuration options together with values currently assigned to them. This has a very important consequence: once sdkconfig file is created, conditional default values from Kconfig files won't be reevaluated, even if given configuration option was not set by the user. See an example:

config FEATURE_X
    bool "Enable feature X"
    default n

config MODE
    string "Mode"
    default "fast" if FEATURE_X
    default "safe"

Assume sdkconfig does not exist yet (or was deleted) and you run idf.py menuconfig:

  1. According to the Kconfig file, FEATURE_X is n by default, so MODE gets the conditional default "safe".

  2. During the configuration process, sdkconfig file is created and now contains CONFIG_MODE="safe" explicitly.

  3. Menuconfig is entered.

Because sdkconfig file was created before entering menuconfig, if you enable FEATURE_X, the MODE's conditional default "fast" if FEATURE_X statement won't be applied because CONFIG_MODE="safe" is already set in sdkconfig and overrides any other value set in previous steps.

This is a known behavior of the Kconfig system and is not a bug, although it may seem somewhat unintuitive at first glance.

Workarounds and Best Practices

How to work around this behavior depends on the exact situation:

  1. Values need to be set only in specific situations: Instruct the user to set all the necessary values in menuconfig and do not rely on conditional defaults from Kconfig files. This solution requires users to do some manual work and if users delete their sdkconfig file, they would need to set the values again.

  2. Values need to be set often: You can instruct users to create a sdkconfig.defaults in their project root folder and set these values there. This file is described in the sdkconfig.defaults section of Configuration Structure. It is important to mention that components themselves cannot provide these files in a way that would be automatically picked up by the build system. Instead, users need to be instructed to create this file inside their project and set the values there. This solution requires users to do some manual work, but it is more convenient than the previous one. After the sdkconfig.defaults file is created, it is necessary to delete the ``sdkconfig`` file and run idf.py menuconfig again to pick up the new values.

Thanks to the way how configuration system works, having necessary values set in sdkconfig.defaults file will also ensure that conditional default values from Kconfig files are reevaluated for configuration options that were not set there.

See the following example:

config FEATURE_X
    bool "Enable feature X"
    default n

config MODE
    string "Mode"
    default "fast" if FEATURE_X
    default "safe"

sdkconfig.defaults (placed in the project root folder):

CONFIG_FEATURE_X=y

Assume sdkconfig does not exist yet (or was deleted) and you run idf.py menuconfig:

  1. Default values from Kconfig file are used. FEATURE_X is n by default, so MODE gets the conditional default "safe".

  2. sdkconfig.defaults file is processed. FEATURE_X is explicitly set to y there. However, MODE's default value will be reevaluated (MODE is not explicitly set in sdkconfig.defaults file and sdkconfig file does not exist yet). Because FEATURE_X is now set to y and the conditional default default "fast" if FEATURE_X applies, MODE will get the conditional default "fast".

  3. sdkconfig file is created and contains CONFIG_MODE="fast" explicitly. This will once again lock the value of MODE configuration option, this time to "fast".

How to Ensure Backward Compatibility

In general, renaming a Kconfig option of a component is a breaking API change, just like renaming a function is. ESP-IDF contains a mechanism which makes it possible to maintain backward compatibility when renaming configuration options. This mechanism is based on sdkconfig.rename files which include pairs of configuration option names. File structure is described below.

When renaming configuration options of a component, create the sdkconfig.rename file in the root folder of the component. Every line in this file should contain one of the following pairs:

  • CONFIG_OLD_NAME CONFIG_NEW_NAME if the new option is a direct replacement of the old option.

  • CONFIG_OLD_NAME !CONFIG_NEW_NAME if the new option is a Boolean inversion of the old option.

The project configuration tool (invoked by idf.py menuconfig) will automatically find it and generate the compatibility statements in sdkconfig for the user.

For more information about the sdkconfig.rename file, please refer to the sdkconfig.rename section of Configuration Structure.

Detailed explanation of the backward compatibility mechanism:

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This part of the guide is intended to explain the behavior of the backward compatibility mechanism in ESP-IDF in greater detail. It is not necessary to understand this mechanism as it it done automatically, but it is described here for the sake of completeness.

If the user has set any value for the old config option (e.g. old config name is used in sdkconfig or sdkconfig.defaults) without sdkconfig.rename file provided, this value would be silently ignored. This behavior is the default of the Kconfig system and is not a bug. In the original project (configuration of the linux kernel) this behavior was desired and is still desired in many projects.

This behavior is suppressed in ESP-IDF by the the configuration tool (invoked by idf.py menuconfig). This tool generates compatibility statements for all the renamed options in the sdkconfig file. In more detail, the following approach is used to prevent the above mentioned situation:

  1. Configuration tool searches the whole ESP-IDF folder for sdkconfig.rename files. If the project target (<chip>) matches the last suffix of any sdkconfig.rename.<chip> file, the file will be used in the next step as well.

  2. After collecting all the relevant files, the sdkconfig file (and sdkconfig.h/json/cmake files if any) is post-processed. A block of compatibility statements for all the renamed options is added during the post-process to the end of the file(s). The block starts with # Deprecated options for backward compatibility and ends with # End of deprecated options.

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