Kconfig - Tips and Best Practices¶
- What to turn into Kconfig options
- Style recommendations and shorthands
- Lesser-known/used Kconfig features
- Kconfig Functions
- Other resources
This page covers some Kconfig best practices and explains some Kconfig behaviors and features that might be cryptic or that are easily overlooked.
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.
select statement is used to force one symbol to
y whenever another
y. For example, the following code forces
config CONSOLE bool "Console support" ... config USB_CONSOLE bool "USB console support" select CONSOLE
This section covers some pitfalls and good uses for
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
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
USB_CONSOLE now forces
y, even if
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
statements are common.
An alternative attempt to solve the issue might be to turn the
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
STRING_ROUTINES now need to be copied to both
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
which hides ways in which a symbol might get enabled.
For the example in the previous section, a better solution is usually to turn
select into a
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
USB_CONSOLE now also need to enable
This comes down to a trade-off, but if enabling
CONSOLE is the norm, then a
mitigation is to make
CONSOLE default to
config CONSOLE bool "Console support" default y
This gives just a single assignment in configuration files:
Note that configuration files that do not want
CONSOLE enabled now have to
explicitly disable it:
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
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
This is short for the following:
config LARGE_MEM bool default MEM_SIZE >= 64
In summary, here are some recommended practices for
Avoid selecting symbols with prompts or dependencies. Prefer
depends on. If
depends oncauses 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
Common sense applies, but be aware that
selectoften causes issues in practice.
depends onis 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.
This section gives some style recommendations and explains some common Kconfig shorthands.
If a sequence of symbols/choices share a common dependency, the dependency can
be factored out with an
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
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
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
n, the entire menu disappears.
bool symbols implicitly default to
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
string symbols. That also generates clearer documentation: (Implicitly
defaults to n instead of n if <dependencies, possibly inherited>).
The one case where
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
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.
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.
def_<type> <value> shorthand is generally only useful for symbols
without prompts, and somewhat obscure.
For a symbol defined in multiple locations (e.g., in a
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
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.
This section lists some more obscure Kconfig behaviors and features that might still come in handy.
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.
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
This is short for the following:
config MASK hex prompt "Bitmask" if HAS_CONFIGURABLE_MASK default 0xFF
HAS_CONFIGURABLE_MASK helper symbol would get selected by boards to
MASK is configurable. When
MASK is configurable, it will
also default to 0xFF.
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 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
n, no configuration output is generated
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
n, but not be configurable by the user, then we
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
See the optional prompts section for the meaning of the conditions on the prompts.
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 ...
Kconfiglib provides user-defined preprocessor functions that we use in Zephyr to expose Device Tree information to Kconfig. For example, we can get the default value for a Kconfig symbol from the device tree.
The following example shows the usage of the
boards/arm/mimxrt1020_evk/Kconfig.defconfig config FLASH_SIZE default $(dt_int_val,DT_NXP_IMX_FLEXSPI_402A8000_SIZE_1,K)
In this example if we examine the generated generated_dts_board.conf file as part of the Zephyr build we’d find the following entry:
dt_int_val will search the generated_dts_board.conf that is derived from
the dts for the board and match the
The function than will than scale the value by
1024. This effective causes
the above to look like:
config FLASH_SIZE default 8192