build/kconfig/tips.rst

.. _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
   <https://www.kernel.org/doc/html/latest/kbuild/kconfig-language.html>`__
   and `kconfig-macro-language.rst
   <https://www.kernel.org/doc/html/latest/kbuild/kconfig-macro-language.html>`__.

.. 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 <dt-guide>` 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 <dt-bindings>` 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.


(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 <menuconfig>`, 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 <menuconfig>`
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 <dependencies, possibly inherited>*).

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.

- ``<type> "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_<type> <value>`` 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 ``<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.


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

     # <Overview of symbols defined in the file, preferably in plain English>
     (Blank line)
     # Copyright (c) 2019 ...
     # SPDX-License-Identifier: <License>
     (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
<https://github.com/ulfalizer/Kconfiglib/blob/master/kconfiglib.py>`__ goes
over how symbols values are calculated in more detail.