.. SPDX-License-Identifier: Apache-2.0 .. _kconfig_tips_and_tricks: Kconfig - Tips and Best Practices ################################# .. contents:: :local: :depth: 2 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 `_. 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 devicetree (``.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`` 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:: none 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:: none 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:: none 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:: none 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:: none 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:: none 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:: none config CONSOLE bool "Console support" default y This gives just a single assignment in configuration files: .. code-block:: none CONFIG_USB_CONSOLE=y Note that configuration files that do not want ``CONSOLE`` enabled now have to explicitly disable it: .. code-block:: none 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:: none 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:: none 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: .. code-block:: none 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: .. code-block:: none config LARGE_MEM def_bool MEM_SIZE >= 64 .. note:: This is short for the following: .. code-block:: none 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. (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:: none 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:: none config FOO bool "Support foo" In this case, ``FOO`` will end up with this definition: .. code-block:: none 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. ``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:: none 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`` 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:: none 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:: none 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 ---- Checking changes in ``menuconfig`` ********************************** When adding new symbols or making other changes to Kconfig files, it is a good idea to look up the symbols in the :ref:`menuconfig ` interface afterwards. To get to a symbol quickly, use the menuconfig's 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. 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. 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:: none 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:: none 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:: none 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 *). .. 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. - `` "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:: none config FOO bool "foo" .. code-block:: none 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:: none config FOO def_bool BAR && BAZ .. code-block:: none 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. 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:: none 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:: none config MASK hex "Bitmask" if HAS_CONFIGURABLE_MASK default 0xFF .. note:: This is short for the following: .. code-block:: none 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:: none 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:: none 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:: none 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:: none 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:: none 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:: none ... CONFIG_FOO_SETTING_1=1 CONFIG_FOO_SETTING_2=2 ... .. _kconfig-functions: Kconfig Functions ***************** Kconfiglib provides user-defined preprocessor functions that we use in Zephyr to expose devicetree information to Kconfig. For example, we can get the default value for a Kconfig symbol from the devicetree. Devicetree Related Functions ============================ See the Python docstrings in ``scripts/kconfig/kconfigfunctions.py`` for more details on the functions. The ``*_int`` version of each function returns the value as a decimal integer, while the ``*_hex`` version returns a hexidecimal value starting with ``0x``. .. code-block:: none dt_chosen_reg_addr_int(kconf, _, chosen, index=0, unit=None): dt_chosen_reg_addr_hex(kconf, _, chosen, index=0, unit=None): dt_chosen_reg_size_int(kconf, _, chosen, index=0, unit=None): dt_chosen_reg_size_hex(kconf, _, chosen, index=0, unit=None): dt_node_reg_addr_int(kconf, _, path, index=0, unit=None): dt_node_reg_addr_hex(kconf, _, path, index=0, unit=None): dt_node_reg_size_int(kconf, _, path, index=0, unit=None): dt_node_reg_size_hex(kconf, _, path, index=0, unit=None): dt_compat_enabled(kconf, _, compat): dt_node_has_bool_prop(kconf, _, path, prop): Example Usage ------------- The following example shows the usage of the ``dt_node_reg_addr_hex`` function. This function will take a path to a devicetree node and register the register address of that node: .. code-block:: none boards/riscv/hifive1_revb/Kconfig.defconfig config FLASH_BASE_ADDRESS default $(dt_node_reg_addr_hex,/soc/spi@10014000,1) In this example if we examine the dts file for the board: .. code-block:: none spi0: spi@10014000 { compatible = "sifive,spi0"; reg = <0x10014000 0x1000 0x20010000 0x3c0900>; reg-names = "control", "mem"; ... }; The ``dt_node_reg_addr_hex`` will search the dts file for a node at the path ``/soc/spi@10014000``. The function than will extract the register address at the index 1. This effective gets the value of ``0x20010000`` and causes the above to look like: .. code-block:: none config FLASH_BASE_ADDRESS default 0x20010000 Other resources *************** The *Intro to symbol values* section in the `Kconfiglib docstring `_ goes over how symbols values are calculated in more detail.