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doc: kconfig: Touch up preprocessor docs and put it on a separate page

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Touch up the Kconfig preprocessor function documentation a bit and add
some more details to the example. Use the Kconfig preprocessor call
syntax instead of Python syntax in the function synopses.

Also move the preprocessor docs to a separate page, and add a top-level
Kconfig section to User and Developer Guides that links to the Kconfig
tips page and the preprocessor page.

Trying to avoid the Kconfig tips page becoming a dumping ground for
random Kconfig documentation.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
This commit is contained in:
Ulf Magnusson 2019-12-03 03:48:55 +01:00 • committed by Carles Cufí
commit 4255bf948c
3 changed files with 932 additions and 916 deletions
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doc/guides/kconfig/index.rst
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@ -1,918 +1,8 @@
.. SPDX-License-Identifier: Apache-2.0
Kconfig
#######
.. _kconfig_tips_and_tricks:
.. toctree::
:maxdepth: 1
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
<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>`_.
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 <override_kernel_conf>`
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.
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:: 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 <dependencies, possibly inherited>*).
.. note::
The one case where ``default n``/``default ""`` is not redundant is when
defining a symbol in multiple locations and wanting to override e.g. a
``default y`` on a later definition.
Defaults *should* always be given for ``int`` and ``hex`` symbols, however, as
they implicitly default to the empty string. This is partly for compatibility
with the C Kconfig tools, though an implicit 0 default might be less likely to
be what was intended compared to other symbol types as well.
Common shorthands
=================
Kconfig has two shorthands that deal with prompts and defaults.
- ``<type> "prompt"`` is a shorthand for giving a symbol/choice a type and a
prompt at the same time. These two definitions are equal:
.. 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_<type> <value>`` 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 ``<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``
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 hexadecimal
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_chosen_enabled(kconf, _, chosen):
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
<https://github.com/ulfalizer/Kconfiglib/blob/master/kconfiglib.py>`_ goes over
how symbols values are calculated in more detail.
tips.rst
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.. _kconfig-functions:
Custom Kconfig Preprocessor Functions
#####################################
Kconfiglib supports custom Kconfig preprocessor functions written in Python.
These functions are defined in
:zephyr_file:`scripts/kconfig/kconfigfunctions.py`.
.. note::
The official Kconfig preprocessor documentation can be found `here
<https://www.kernel.org/doc/html/latest/kbuild/kconfig-macro-language.html>`__.
Most of the custom preprocessor functions are used to get devicetree
information into Kconfig. For example, the default value of a Kconfig symbol
can be fetched from a devicetree ``reg`` property.
Devicetree-related Functions
****************************
The functions listed below are used to get devicetree information into Kconfig.
See the Python docstrings in :zephyr_file:`scripts/kconfig/kconfigfunctions.py`
for detailed documentation.
The ``*_int`` version of each function returns the value as a decimal integer,
while the ``*_hex`` version returns a hexadecimal value starting with ``0x``.
.. code-block:: none
$(dt_chosen_reg_addr_int,<property in /chosen>[,<index>,<unit>])
$(dt_chosen_reg_addr_hex,<property in /chosen>[,<index>,<unit>])
$(dt_chosen_reg_size_int,<property in /chosen>[,<index>,<unit>])
$(dt_chosen_reg_size_hex,<property in /chosen>[,<index>,<unit>])
$(dt_node_reg_addr_int,<node path>[,<index>,<unit>])
$(dt_node_reg_addr_hex,<node path>[,<index>,<unit>])
$(dt_node_reg_size_int,<node path>[,<index>,<unit>])
$(dt_node_reg_size_hex,<node path>[,<index>,<unit>])
$(dt_compat_enabled,<compatible string>)
$(dt_chosen_enabled,<property in /chosen>)
$(dt_node_has_bool_prop,<node path>,<prop>)
Example Usage
=============
Assume that the devicetree for some board looks like this:
.. code-block:: none
{
soc {
#address-cells = <1>;
#size-cells = <1>;
spi0: spi@10014000 {
compatible = "sifive,spi0";
reg = <0x10014000 0x1000 0x20010000 0x3c0900>;
reg-names = "control", "mem";
...
};
};
The second entry in ``reg`` in ``spi@1001400`` (``<0x20010000 0x3c0900>``)
corresponds to ``mem``, and has the address ``0x20010000``. This address can be
inserted into Kconfig as follows:
.. code-block:: none
config FLASH_BASE_ADDRESS
default $(dt_node_reg_addr_hex,/soc/spi@1001400,1)
After preprocessor expansion, this turns into the definition below:
.. code-block:: none
config FLASH_BASE_ADDRESS
default 0x20010000

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.. _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`` 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 <override_kernel_conf>`
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.
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:: 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 <dependencies, possibly inherited>*).
.. note::
The one case where ``default n``/``default ""`` is not redundant is when
defining a symbol in multiple locations and wanting to override e.g. a
``default y`` on a later definition.
Defaults *should* always be given for ``int`` and ``hex`` symbols, however, as
they implicitly default to the empty string. This is partly for compatibility
with the C Kconfig tools, though an implicit 0 default might be less likely to
be what was intended compared to other symbol types as well.
Common shorthands
=================
Kconfig has two shorthands that deal with prompts and defaults.
- ``<type> "prompt"`` is a shorthand for giving a symbol/choice a type and a
prompt at the same time. These two definitions are equal:
.. 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_<type> <value>`` 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 ``<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``
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
...
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.