Kconfig tips and best practices

This page covers some Kconfig best practices and explains some Kconfig behaviors and features that might be cryptic or that are easily overlooked.

Note

The official Kconfig documentation is kconfig-language.txt and kconfig-macro-language.txt.

What to turn into Kconfig options

When deciding whether something belongs in Kconfig, it helps to distinguish between symbols that have prompts and symbols that don’t.

If a symbol has a prompt (e.g. bool "Enable foo"), then the user can change the symbol’s value in the menuconfig interface (or by manually editing configuration files). Therefore, only put a prompt on a symbol if it makes sense for the user to change its value.

In Zephyr, Kconfig configuration is done after selecting a machine, so in general, it does not make sense to put a prompt on a symbol that corresponds to a fixed machine-specific setting. Usually, such settings should be handled via device tree (.dts) files instead.

Symbols without prompts can’t be configured directly by the user (they derive their value from other symbols), so less restrictions apply to them. If some derived setting is easier to calculate in Kconfig than e.g. during the build, then do it in Kconfig, but keep the distinction between symbols with and without prompts in mind.

See the optional prompts section for a way to deal with settings that are fixed on some machines and configurable on other machines.

Best practices for select statements

The select statement is used to force one symbol to y whenever another symbol is y. For example, the following code forces CONSOLE to y whenever USB_CONSOLE is y:

config CONSOLE
     bool "Console support"

...

config USB_CONSOLE
     bool "USB console support"
     select CONSOLE

This section covers some pitfalls and good uses for select.

select pitfalls

select might seem like a generally useful feature at first, but can cause configuration issues if overused.

For example, say that a new dependency is added to the CONSOLE symbol above, by a developer who is unaware of the USB_CONSOLE symbol (or simply forgot about it):

config CONSOLE
     bool "Console support"
     depends on STRING_ROUTINES

Enabling USB_CONSOLE now forces CONSOLE to y, even if STRING_ROUTINES is n.

To fix the problem, the STRING_ROUTINES dependency needs to be added to USB_CONSOLE as well:

config USB_CONSOLE
     bool "USB console support"
     select CONSOLE
     depends on STRING_ROUTINES

...

config STRING_ROUTINES
     bool "Include string routines"

More insidious cases with dependencies inherited from if and menu statements are common.

An alternative attempt to solve the issue might be to turn the depends on into another select:

config CONSOLE
     bool "Console support"
     select STRING_ROUTINES

...

config USB_CONSOLE
     bool "USB console support"
     select CONSOLE

In practice, this often amplifies the problem, because any dependencies added to STRING_ROUTINES now need to be copied to both CONSOLE and USB_CONSOLE.

In general, whenever the dependencies of a symbol are updated, the dependencies of all symbols that (directly or indirectly) select it have to be updated as well. This is very often overlooked in practice, even for the simplest case above.

Chains of symbols selecting each other should be avoided in particular, except for simple helper symbols, as covered below in Using select for helper symbols.

Liberal use of select also tends to make Kconfig files harder to read, both due to the extra dependencies and due to the non-local nature of select, which hides ways in which a symbol might get enabled.

Alternatives to select

For the example in the previous section, a better solution is usually to turn the select into a depends on:

config CONSOLE
     bool "Console support"

...

config USB_CONSOLE
     bool "USB console support"
     depends on CONSOLE

This makes it impossible to generate an invalid configuration, and means that dependencies only ever have to be updated in a single spot.

An objection to using depends on here might be that configuration files that enable USB_CONSOLE now also need to enable CONSOLE:

CONFIG_CONSOLE=y
CONFIG_USB_CONSOLE=y

This comes down to a trade-off, but if enabling CONSOLE is the norm, then a mitigation is to make CONSOLE default to y:

config CONSOLE
     bool "Console support"
     default y

This gives just a single assignment in configuration files:

CONFIG_USB_CONSOLE=y

Note that configuration files that do not want CONSOLE enabled now have to explicitly disable it:

CONFIG_CONSOLE=n

Using select for helper symbols

A good and safe use of select is for setting “helper” symbols that capture some condition. Such helper symbols should preferably have no prompt or dependencies.

For example, a helper symbol for indicating that a particular CPU/SoC has an FPU could be defined as follows:

config CPU_HAS_FPU
     bool
     help
       If y, the CPU has an FPU

...

config SOC_FOO
     bool "FOO SoC"
     select CPU_HAS_FPU

...

config SOC_BAR
     bool "BAR SoC"
     select CPU_HAS_FPU

This makes it possible for other symbols to check for FPU support in a generic way, without having to look for particular architectures:

config FLOAT
     bool "Support floating point operations"
     depends on CPU_HAS_FPU

The alternative would be to have dependencies like the following, possibly duplicated in several spots:

config FLOAT
     bool "Support floating point operations"
     depends on SOC_FOO || SOC_BAR || ...

Invisible helper symbols can also be useful without select. For example, the following code defines a helper symbol that has the value y if the machine has some arbitrarily-defined “large” amount of memory:

config LARGE_MEM
     def_bool MEM_SIZE >= 64

Note

This is short for the following:

config LARGE_MEM
  bool
  default MEM_SIZE >= 64

select recommendations

In summary, here are some recommended practices for select:

  • Avoid selecting symbols with prompts or dependencies. Prefer depends on. If depends on causes annoying bloat in configuration files, consider adding a Kconfig default for the most common value.

    Rare exceptions might include cases where you’re sure that the dependencies of the selecting and selected symbol will never drift out of sync, e.g. when dealing with two simple symbols defined close to one another within the same if.

    Common sense applies, but be aware that select often causes issues in practice. depends on is usually a cleaner and safer solution.

  • Select simple helper symbols without prompts and dependencies however much you like. They’re a great tool for simplifying Kconfig files.

Style recommendations and shorthands

This section gives some style recommendations and explains some common Kconfig shorthands.

Factoring out common dependencies

If a sequence of symbols/choices share a common dependency, the dependency can be factored out with an if.

As an example, consider the following code:

config FOO
     bool "Foo"
     depends on DEP

config BAR
     bool "Bar"
     depends on DEP

choice
     prompt "Choice"
     depends on DEP

config BAZ
     bool "Baz"

config QAZ
     bool "Qaz"

endchoice

Here, the DEP dependency can be factored out like this:

if DEP

config FOO
     bool "Foo"

config BAR
     bool "Bar"

choice
     prompt "Choice"

config BAZ
     bool "Baz"

config QAZ
     bool "Qaz"

endchoice

endif # DEP

Note

Internally, the second version of the code is transformed into the first.

If a sequence of symbols/choices with shared dependencies are all in the same menu, the dependency can be put on the menu itself:

menu "Foo features"
     depends on FOO_SUPPORT

config FOO_FEATURE_1
     bool "Foo feature 1"

config FOO_FEATURE_2
     bool "Foo feature 2"

endmenu

If FOO_SUPPORT is n, the entire menu disappears.

Redundant defaults

bool symbols implicitly default to n, and string symbols implicitly default to the empty string. Therefore, default n and default "" are (almost) always redundant.

The recommended style in Zephyr is to skip redundant defaults for bool and string symbols. That also generates clearer documentation: (Implicitly defaults to n instead of n if <dependencies, possibly inherited>).

Note

The one case where default n/default "" is not redundant is when defining a symbol in multiple locations and wanting to override e.g. a default y on a later definition.

Defaults should always be given for int and hex symbols, however, as they implicitly default to the empty string. This is partly for compatibility with the C Kconfig tools, though an implicit 0 default might be less likely to be what was intended compared to other symbol types as well.

Common shorthands

Kconfig has two shorthands that deal with prompts and defaults.

  • <type> "prompt" is a shorthand for giving a symbol/choice a type and a prompt at the same time. These two definitions are equal:

    config FOO
       bool "foo"
    
    config FOO
       bool
       prompt "foo"
    

    The first style, with the shorthand, is preferred in Zephyr.

  • def_<type> <value> is a shorthand for giving a type and a value at the same time. These two definitions are equal:

    config FOO
       def_bool BAR && BAZ
    
    config FOO
       bool
       default BAR && BAZ
    

Using both the <type> "prompt" and the def_<type> <value> shorthand in the same definition is redundant, since it gives the type twice.

The def_<type> <value> shorthand is generally only useful for symbols without prompts, and somewhat obscure.

Note

For a symbol defined in multiple locations (e.g., in a Kconfig.defconfig file in Zephyr), it is best to only give the symbol type for the “base” definition of the symbol, and to use default (instead of def_<type> value) for the remaining definitions. That way, if the base definition of the symbol is removed, the symbol ends up without a type, which generates a warning that points to the other definitions. That makes the extra definitions easier to discover and remove.

Lesser-known/used Kconfig features

This section lists some more obscure Kconfig behaviors and features that might still come in handy.

The imply statement

The imply statement is similar to select, but respects dependencies and doesn’t force a value. For example, the following code could be used to enable USB keyboard support by default on the FOO SoC, while still allowing the user to turn it off:

config SOC_FOO
     bool "FOO SoC"
     imply USB_KEYBOARD

...

config USB_KEYBOARD
     bool "USB keyboard support"

imply acts like a suggestion, whereas select forces a value.

Optional prompts

A condition can be put on a symbol’s prompt to make it optionally configurable by the user. For example, a value MASK that’s hardcoded to 0xFF on some boards and configurable on others could be expressed as follows:

config MASK
     hex "Bitmask" if HAS_CONFIGURABLE_MASK
     default 0xFF

Note

This is short for the following:

config MASK
  hex
  prompt "Bitmask" if HAS_CONFIGURABLE_MASK
  default 0xFF

The HAS_CONFIGURABLE_MASK helper symbol would get selected by boards to indicate that MASK is configurable. When MASK is configurable, it will also default to 0xFF.

Optional choices

Defining a choice with the optional keyword allows the whole choice to be toggled off to select none of the symbols:

choice
     prompt "Use legacy protocol"
     optional

config LEGACY_PROTOCOL_1
     bool "Legacy protocol 1"

config LEGACY_PROTOCOL_2
     bool "Legacy protocol 2"

endchoice

In the menuconfig interface, this will be displayed e.g. as [*] Use legacy protocol (LEGACY_PROTOCOL_1) --->, where the choice can be toggled off to enable neither of the symbols.

visible if conditions

Putting a visible if condition on a menu hides the menu and all the symbols within it, while still allowing symbol default values to kick in.

As a motivating example, consider the following code:

menu "Foo subsystem"
     depends on HAS_CONFIGURABLE_FOO

config FOO_SETTING_1
     int "Foo setting 1"
     default 1

config FOO_SETTING_2
     int "Foo setting 2"
     default 2

endmenu

When HAS_CONFIGURABLE_FOO is n, no configuration output is generated for FOO_SETTING_1 and FOO_SETTING_2, as the code above is logically equivalent to the following code:

config FOO_SETTING_1
     int "Foo setting 1"
     default 1
     depends on HAS_CONFIGURABLE_FOO

config FOO_SETTING_2
     int "Foo setting 2"
     default 2
     depends on HAS_CONFIGURABLE_FOO

If we want the symbols to still get their default values even when HAS_CONFIGURABLE_FOO is n, but not be configurable by the user, then we can use visible if instead:

menu "Foo subsystem"
     visible if HAS_CONFIGURABLE_FOO

config FOO_SETTING_1
     int "Foo setting 1"
     default 1

config FOO_SETTING_2
     int "Foo setting 2"
     default 2

endmenu

This is logically equivalent to the following:

config FOO_SETTING_1
     int "Foo setting 1" if HAS_CONFIGURABLE_FOO
     default 1

config FOO_SETTING_2
     int "Foo setting 2" if HAS_CONFIGURABLE_FOO
     default 2

Note

See the optional prompts section for the meaning of the conditions on the prompts.

When HAS_CONFIGURABLE is n, we now get the following configuration output for the symbols, instead of no output:

...
CONFIG_FOO_SETTING_1=1
CONFIG_FOO_SETTING_2=2
...

Other resources

The Intro to symbol values section in the Kconfiglib docstring goes over how symbols values are calculated in more detail.