Default Values and Their Inference

In original kconfiglib, when the configuration was written out to e.g. sdkconfig file, all the values were considered user-set, no matter if the user actually set them (even indirectly via e.g. sdkconfig.defaults). This behavior resulted in rather confusing situations; when e.g. conditional default values were not updated if the condition was changed. Consider following example:

# Kconfig file

config A
    bool "Option A"
    default y

config B
    int "Option B"
    depends on A
    default 42 if A
    default 0 if !A

# sdkconfig file
# Let's suppose both values were not set by the user, but rather automatically inferred
CONFIG_A=y
CONFIG_B=42

If user would run menuconfig tool and change the value of CONFIG_A to “n”, the CONFIG_B would still be set to 42, even though it should be 0. This is because the value of CONFIG_B was considered user-set, even though it was not. In order to fix this behavior, the inference of default values was reworked in esp-idf-kconfig. During writeout of the sdkconfig file, configuration system now distinguishes between user-set and inferred (default) values. Default values have a # default: comment/pragma preceding given line:

# sdkconfig file. Explicitly stated that both values are default.
# default:
CONFIG_A=y
# default:
CONFIG_B=42

The configuration system now determines the value for given config option as follows. Difference from original approach used in esp-idf-kconfig<3 are marked with (new), list is ordered by priority (from highest to lowest):

  1. Value set by the user in current run of menuconfig tool.

  2. User-set value from sdkconfig file.

  3. Value from sdkconfig.defaults file. These values are also considered user-set.

  4. (new) Default value from sdkconfig file.

  5. Default value from the Kconfig file.

Note

The sdkconfig.defaults, despite its name, contains user-set values. The word “defaults” in the file name does not refer to default values as configuration system understands them, but rather default values (overrides) for given project.

Differing Default Values Between sdkconfig and Kconfig Files

When e.g. updating components or ESP-IDF itself, it may happen that the default value for given configuration option will differ between sdkconfig and Kconfig:

# Kconfig file

config C
    int "Option A"
    default 100 # in the previous version of the Kconfig file, it was 42

# sdkconfig file
# CONFIG_C still have default value from previous version of Kconfig file
# default:
CONFIG_C=42

In this case, configuration system notifies the user that sdkconfig and Kconfig default values are different. Default behavior is to use the value from sdkconfig file, in order to maintain backward compatibility. Configuration system also supports to choose a default value source via the KCONFIG_DEFAULTS_POLICY environment variable. The following values are supported:

  • sdkconfig - use the value from sdkconfig file (default).

  • kconfig - use the value from Kconfig file.

  • interactive - ask the user to choose the source of the default value.