.. _kconfig_tips_and_tricks: 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.rst `__ and `kconfig-macro-language.rst `__. .. contents:: :local: :depth: 2 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`` or ``guiconfig`` interface (see :ref:`menuconfig`), or by manually editing configuration files. Conversely, a symbol without a prompt can never be changed directly by the user, not even by manually editing configuration files. Only put a prompt on a symbol if it makes sense for the user to change its value. Symbols without prompts are called *hidden* or *invisible* symbols, because they don't show up in ``menuconfig`` and ``guiconfig``. Symbols that have prompts can also be invisible, when their dependencies are not satisfied. 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. What not to turn into Kconfig options ************************************* In Zephyr, Kconfig configuration is done after selecting a target board. In general, it does not make sense to use Kconfig for a value that corresponds to a fixed machine-specific setting. Usually, such settings should be handled via :ref:`devicetree ` instead. In particular, avoid adding new Kconfig options of the following types: Options that specify a device in the system by name =================================================== For example, if you are writing an I2C device driver, avoid creating an option named ``MY_DEVICE_I2C_BUS_NAME`` for specifying the bus node your device is controlled by. See :ref:`dt-drivers-that-depend` for alternatives. Similarly, if your application depends on a hardware-specific PWM device to control an RGB LED, avoid creating an option like ``MY_PWM_DEVICE_NAME``. See :ref:`dt-apps-that-depend` for alternatives. Options that specify fixed hardware configuration ================================================= For example, avoid Kconfig options specifying a GPIO pin. An alternative applicable to device drivers is to define a GPIO specifier with type phandle-array in the device binding, and using the :ref:`devicetree-gpio-api` devicetree API from C. Similar advice applies to other cases where devicetree.h provides :ref:`devicetree-hw-api` for referring to other nodes in the system. Search the source code for drivers using these APIs for examples. An application-specific devicetree :ref:`binding ` to identify board specific properties may be appropriate. See :zephyr_file:`tests/drivers/gpio/gpio_basic_api` for an example. For applications, see :zephyr:code-sample:`blinky` for a devicetree-based alternative. ``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``: .. code-block:: kconfig 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): .. code-block:: kconfig 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: .. code-block:: kconfig 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``: .. code-block:: kconfig 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 :ref:`good_select_use`. 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``: .. code-block:: kconfig 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``: .. code-block:: cfg 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``: .. code-block:: kconfig config CONSOLE bool "Console support" default y This gives just a single assignment in configuration files: .. code-block:: cfg CONFIG_USB_CONSOLE=y Note that configuration files that do not want ``CONSOLE`` enabled now have to explicitly disable it: .. code-block:: cfg CONFIG_CONSOLE=n .. _good_select_use: 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: .. code-block:: kconfig 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: .. code-block:: kconfig config FPU 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: .. code-block:: kconfig config FPU 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: .. code-block:: kconfig config LARGE_MEM def_bool MEM_SIZE >= 64 .. note:: This is short for the following: .. code-block:: kconfig 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. - An exemption are busses like I2C and SPI, and following the same thought process things like MFD as well. Drivers on these busses should use ``select`` to allow the automatic activation of the necessary bus drivers when devices on the bus are enabled in the devicetree. .. code-block:: kconfig config ADC_FOO bool "external SPI ADC foo driver" select SPI (Lack of) conditional includes ****************************** ``if`` blocks add dependencies to each item within the ``if``, as if ``depends on`` was used. A common misunderstanding related to ``if`` is to think that the following code conditionally includes the file :file:`Kconfig.other`: .. code-block:: kconfig if DEP source "Kconfig.other" endif In reality, there are no conditional includes in Kconfig. ``if`` has no special meaning around a ``source``. .. note:: Conditional includes would be impossible to implement, because ``if`` conditions may contain (either directly or indirectly) forward references to symbols that haven't been defined yet. Say that :file:`Kconfig.other` above contains this definition: .. code-block:: kconfig config FOO bool "Support foo" In this case, ``FOO`` will end up with this definition: .. code-block:: kconfig config FOO bool "Support foo" depends on DEP Note that it is redundant to add ``depends on DEP`` to the definition of ``FOO`` in :file:`Kconfig.other`, because the ``DEP`` dependency has already been added by ``if DEP``. In general, try to avoid adding redundant dependencies. They can make the structure of the Kconfig files harder to understand, and also make changes more error-prone, since it can be hard to spot that the same dependency is added twice. "Stuck" symbols in menuconfig and guiconfig ******************************************* There is a common subtle gotcha related to interdependent configuration symbols with prompts. Consider these symbols: .. code-block:: kconfig config FOO bool "Foo" config STACK_SIZE hex "Stack size" default 0x200 if FOO default 0x100 Assume that the intention here is to use a larger stack whenever ``FOO`` is enabled, and that the configuration initially has ``FOO`` disabled. Also, remember that Zephyr creates an initial configuration in :file:`zephyr/.config` in the build directory by merging configuration files (including e.g. :file:`prj.conf`). This configuration file exists before ``menuconfig`` or ``guiconfig`` is run. When first entering the configuration interface, the value of ``STACK_SIZE`` is 0x100, as expected. After enabling ``FOO``, you might reasonably expect the value of ``STACK_SIZE`` to change to 0x200, but it stays as 0x100. To understand what's going on, remember that ``STACK_SIZE`` has a prompt, meaning it is user-configurable, and consider that all Kconfig has to go on from the initial configuration is this: .. code-block:: cfg CONFIG_STACK_SIZE=0x100 Since Kconfig can't know if the 0x100 value came from a ``default`` or was typed in by the user, it has to assume that it came from the user. Since ``STACK_SIZE`` is user-configurable, the value from the configuration file is respected, and any symbol defaults are ignored. This is why the value of ``STACK_SIZE`` appears to be "frozen" at 0x100 when toggling ``FOO``. The right fix depends on what the intention is. Here's some different scenarios with suggestions: - If ``STACK_SIZE`` can always be derived automatically and does not need to be user-configurable, then just remove the prompt: .. code-block:: kconfig config STACK_SIZE hex default 0x200 if FOO default 0x100 Symbols without prompts ignore any value from the saved configuration. - If ``STACK_SIZE`` should usually be user-configurable, but needs to be set to 0x200 when ``FOO`` is enabled, then disable its prompt when ``FOO`` is enabled, as described in `optional prompts`_: .. code-block:: kconfig config STACK_SIZE hex "Stack size" if !FOO default 0x200 if FOO default 0x100 - If ``STACK_SIZE`` should usually be derived automatically, but needs to be set to a custom value in rare circumstances, then add another option for making ``STACK_SIZE`` user-configurable: .. code-block:: kconfig config CUSTOM_STACK_SIZE bool "Use a custom stack size" help Enable this if you need to use a custom stack size. When disabled, a suitable stack size is calculated automatically. config STACK_SIZE hex "Stack size" if CUSTOM_STACK_SIZE default 0x200 if FOO default 0x100 As long as ``CUSTOM_STACK_SIZE`` is disabled, ``STACK_SIZE`` will ignore the value from the saved configuration. It is a good idea to try out changes in the ``menuconfig`` or ``guiconfig`` interface, to make sure that things behave the way you expect. This is especially true when making moderately complex changes like these. Assignments to promptless symbols in configuration files ******************************************************** Assignments to hidden (promptless, also called *invisible*) symbols in configuration files are always ignored. Hidden symbols get their value indirectly from other symbols, via e.g. ``default`` and ``select``. A common source of confusion is opening the output configuration file (:file:`zephyr/.config`), seeing a bunch of assignments to hidden symbols, and assuming that those assignments must be respected when the configuration is read back in by Kconfig. In reality, all assignments to hidden symbols in :file:`zephyr/.config` are ignored by Kconfig, like for other configuration files. To understand why :file:`zephyr/.config` still includes assignments to hidden symbols, it helps to realize that :file:`zephyr/.config` serves two separate purposes: 1. It holds the saved configuration, and 2. it holds configuration output. :file:`zephyr/.config` is parsed by the CMake files to let them query configuration settings, for example. The assignments to hidden symbols in :file:`zephyr/.config` are just configuration output. Kconfig itself ignores assignments to hidden symbols when calculating symbol values. .. note:: A *minimal configuration*, which can be generated from within the :ref:`menuconfig and guiconfig interfaces `, could be considered closer to just a saved configuration, without the full configuration output. ``depends on`` and ``string``/``int``/``hex`` symbols ***************************************************** ``depends on`` works not just for ``bool`` symbols, but also for ``string``, ``int``, and ``hex`` symbols (and for choices). The Kconfig definitions below will hide the ``FOO_DEVICE_FREQUENCY`` symbol and disable any configuration output for it when ``FOO_DEVICE`` is disabled. .. code-block:: kconfig config FOO_DEVICE bool "Foo device" config FOO_DEVICE_FREQUENCY int "Foo device frequency" depends on FOO_DEVICE In general, it's a good idea to check that only relevant symbols are ever shown in the ``menuconfig``/``guiconfig`` interface. Having ``FOO_DEVICE_FREQUENCY`` show up when ``FOO_DEVICE`` is disabled (and possibly hidden) makes the relationship between the symbols harder to understand, even if code never looks at ``FOO_DEVICE_FREQUENCY`` when ``FOO_DEVICE`` is disabled. ``menuconfig`` symbols ********************** If the definition of a symbol ``FOO`` is immediately followed by other symbols that depend on ``FOO``, then those symbols become children of ``FOO``. If ``FOO`` is defined with ``config FOO``, then the children are shown indented relative to ``FOO``. Defining ``FOO`` with ``menuconfig FOO`` instead puts the children in a separate menu rooted at ``FOO``. ``menuconfig`` has no effect on evaluation. It's just a display option. ``menuconfig`` can cut down on the number of menus and make the menu structure easier to navigate. For example, say you have the following definitions: .. code-block:: kconfig menu "Foo subsystem" config FOO_SUBSYSTEM bool "Foo subsystem" if FOO_SUBSYSTEM config FOO_FEATURE_1 bool "Foo feature 1" config FOO_FEATURE_2 bool "Foo feature 2" config FOO_FREQUENCY int "Foo frequency" ... lots of other FOO-related symbols endif # FOO_SUBSYSTEM endmenu In this case, it's probably better to get rid of the ``menu`` and turn ``FOO_SUBSYSTEM`` into a ``menuconfig`` symbol: .. code-block:: kconfig menuconfig FOO_SUBSYSTEM bool "Foo subsystem" if FOO_SUBSYSTEM config FOO_FEATURE_1 bool "Foo feature 1" config FOO_FEATURE_2 bool "Foo feature 2" config FOO_FREQUENCY int "Foo frequency" ... lots of other FOO-related symbols endif # FOO_SUBSYSTEM In the ``menuconfig`` interface, this will be displayed as follows: .. code-block:: none [*] Foo subsystem ---> Note that making a symbol without children a ``menuconfig`` is meaningless. It should be avoided, because it looks identical to a symbol with all children invisible: .. code-block:: none [*] I have no children ---- [*] All my children are invisible ---- Commas in macro arguments ************************* Kconfig uses commas to separate macro arguments. This means a construct like this will fail: .. code-block:: kconfig config FOO bool default y if $(dt_chosen_enabled,"zephyr,bar") To solve this problem, create a variable with the text and use this variable as argument, as follows: .. code-block:: kconfig DT_CHOSEN_ZEPHYR_BAR := zephyr,bar config FOO bool default y if $(dt_chosen_enabled,$(DT_CHOSEN_ZEPHYR_BAR)) Checking changes in menuconfig/guiconfig **************************************** When adding new symbols or making other changes to Kconfig files, it is a good idea to look up the symbols in :ref:`menuconfig or guiconfig ` afterwards. To get to a symbol quickly, use the jump-to feature (press :kbd:`/`). Here are some things to check: * Are the symbols placed in a good spot? Check that they appear in a menu where they make sense, close to related symbols. If one symbol depends on another, then it's often a good idea to place it right after the symbol it depends on. It will then be shown indented relative to the symbol it depends on in the ``menuconfig`` interface, and in a separate menu rooted at the symbol in ``guiconfig``. This also works if several symbols are placed after the symbol they depend on. * Is it easy to guess what the symbols do from their prompts? * If many symbols are added, do all combinations of values they can be set to make sense? For example, if two symbols ``FOO_SUPPORT`` and ``NO_FOO_SUPPORT`` are added, and both can be enabled at the same time, then that makes a nonsensical configuration. In this case, it's probably better to have a single ``FOO_SUPPORT`` symbol. * Are there any duplicated dependencies? This can be checked by selecting a symbol and pressing :kbd:`?` to view the symbol information. If there are duplicated dependencies, then use the ``Included via ...`` path shown in the symbol information to figure out where they come from. Checking changes with :file:`scripts/kconfig/lint.py` ***************************************************** After you make Kconfig changes, you can use the :zephyr_file:`scripts/kconfig/lint.py` script to check for some potential issues, like unused symbols and symbols that are impossible to enable. Use ``--help`` to see available options. Some checks are necessarily a bit heuristic, so a symbol being flagged by a check does not necessarily mean there's a problem. If a check returns a false positive e.g. due to token pasting in C (``CONFIG_FOO_##index##_BAR``), just ignore it. When investigating an unknown symbol ``FOO_BAR``, it is a good idea to run ``git grep FOO_BAR`` to look for references. It is also a good idea to search for some components of the symbol name with e.g. ``git grep FOO`` and ``git grep BAR``, as it can help uncover token pasting. 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: .. code-block:: kconfig 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: .. code-block:: kconfig 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: .. code-block:: kconfig 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 *). 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. 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. Note that a ``default n`` does not override a previously defined ``default y``. That is, FOO will be set to ``n`` in the example below. If the ``default n`` was omitted in the first definition, FOO would have been set to ``y``. .. code-block:: kconfig config FOO bool "foo" default n config FOO bool "foo" default y In the following example FOO will get the value ``y``. .. code-block:: kconfig config FOO bool "foo" default y config FOO bool "foo" default n .. _kconfig_shorthands: Common Kconfig shorthands ========================= Kconfig has two shorthands that deal with prompts and defaults. - `` "prompt"`` is a shorthand for giving a symbol/choice a type and a prompt at the same time. These two definitions are equal: .. code-block:: kconfig config FOO bool "foo" .. code-block:: kconfig config FOO bool prompt "foo" The first style, with the shorthand, is preferred in Zephyr. - ``def_ `` is a shorthand for giving a type and a value at the same time. These two definitions are equal: .. code-block:: kconfig config FOO def_bool BAR && BAZ .. code-block:: kconfig config FOO bool default BAR && BAZ Using both the `` "prompt"`` and the ``def_ `` shorthand in the same definition is redundant, since it gives the type twice. The ``def_ `` 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_ 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. Prompt strings ============== For a Kconfig symbol that enables a driver/subsystem FOO, consider having just "Foo" as the prompt, instead of "Enable Foo support" or the like. It will usually be clear in the context of an option that can be toggled on/off, and makes things consistent. Header comments and other nits ============================== A few formatting nits, to help keep things consistent: - Use this format for any header comments at the top of ``Kconfig`` files: .. code-block:: none # (Blank line) # Copyright (c) 2019 ... # SPDX-License-Identifier: (Blank line) (Kconfig definitions) - Format comments as ``# Comment`` rather than ``#Comment`` - Put a blank line before/after each top-level ``if`` and ``endif`` - Use a single tab for each indentation - Indent help text with two extra spaces 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: .. code-block:: kconfig 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: .. code-block:: kconfig config MASK hex "Bitmask" if HAS_CONFIGURABLE_MASK default 0xFF .. note:: This is short for the following: .. code-block:: kconfig 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: .. code-block:: kconfig 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: .. code-block:: kconfig 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: .. code-block:: kconfig 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: .. code-block:: kconfig 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: .. code-block:: kconfig 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: .. code-block:: cfg ... 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.