u-boot/doc/develop/system_configuration.rst

.. SPDX-License-Identifier: GPL-2.0+

System configuration
====================

There are a number of different aspects to configuring U-Boot to build and then
run on a given platform or set of platforms. Broadly speaking, some aspects of
the world can be configured at run time and others must be done at build time.
In general run time configuration is preferred over build time configuration.
But when making these decisions, we also need to consider if we're talking about
a feature that could be useful to virtually every platform or something specific
to a single hardware platform. The resulting image size is also another
important consideration. Finally, run time configuration has additional overhead
both in terms of resource requirements and wall clock time. All of this means
that care must be taken when writing new code to select the most appropriate
configuration mechanism.

When adding new features to U-Boot, be they a new subsystem or SoC support or
new platform for an existing supported SoC, the preferred configuration order
is:

#. Hardware based run time configuration. Examples of this include reading
   processor specific registers, or a set of board specific GPIOs or an EEPROM
   with a known format to it. These are the cases where we either cannot or
   should not be relying on device tree checks. We use this for cases such as
   optimized boot time or starting with a generic device tree and then enabling
   or disabling features as we boot.

#. Making use of our Kconfig infrastructure and C preprocessor macros that have
   the prefix ``CONFIG``. This is the primary method of build time
   configuration. This is generally the best fit for when we want to enable or
   disable some sort of feature, such as the SoC or network support. The
   ``CONFIG`` prefix for C preprocessor macros is strictly reserved for Kconfig
   usage only.

#. Making use of the :doc:`device tree <devicetree/control>` to determine at
   run time how to configure a feature that we have enabled via Kconfig. For
   example, we would use Kconfig to enable an I2C chip driver, but use the device
   tree to know where the I2C chip resides in memory and other details we need
   in order to configure the bus.

#. Making use of C header files directly and defining C preprocessor macros that
   have the ``CFG`` prefix. While the ``CFG`` prefix is reserved for this build
   time configuration mechanism, the usage is ad hoc. This is to be used when the
   previously mentioned mechanisms are not possible, or for legacy code that has
   not been converted.

Dynamic run time configuration methods.
---------------------------------------

Details of hardware specific run time configuration methods are found within the
documentation for a given processor family or board.

Details of how to use run time configuration based on :doc:`driver model
<driver-model/index>` are covered in that documentation section.

Static build time configuration methods
---------------------------------------

There are two mechanisms used to control the build time configuration of U-Boot.
One is utilizing Kconfig and ``CONFIG`` prefixed macros and the other is ad hoc
usage of ``CFG`` prefixed macros. Both of these are used when it is either not
possible or not practical to make a run time determination about some
functionality of the hardware or a required software feature or similar. Each of
these has their own places where they are better suited than the other for use.

The `Kconfig language
<https://www.kernel.org/doc/html/latest/kbuild/kconfig-language.html>`_ is well
documented and used in a number of projects, including the Linux kernel. We
implement this with the Kconfig files found throughout our sources. This
mechanism is the preferred way of exposing new configuration options as there
are a number of ways for both users and system integrators to manage and change
these options. Some common examples here are to enable a specific command within
U-Boot or even a whole subsystem such as NAND flash or network connectivity.

The ``CFG`` mechanism is implemented directly as C preprocessor values or
macros, depending on what they are in turn describing. While we have some
functionality that is very reasonable to expose to the end user to enable or
disable we have other places where we need to describe things such as register
locations or values, memory map ranges and so on. When practical, we should be
getting these values from the device tree. However, there are cases where this
is either not practical due to when we need the information and may not have a
device tree yet or due to legacy reasons code has not been rewritten.

When to use each mechanism
^^^^^^^^^^^^^^^^^^^^^^^^^^

While there are some cases where it should be fairly obvious where to use each
mechanism, as for example a command would done via Kconfig, a new I2C driver
should use Kconfig and be configured via driver model and a header of values
generated by an external tool should be ``CFG``, there will be cases where it's
less clear and one needs to take care when implementing it. In general,
configuration *options* should be done in Kconfig and configuration *settings*
should done in driver model or ``CFG``. Let us discuss things to keep in mind
when picking the appropriate mechanism.

A thing to keep in mind is that we have a strong preference for using Kconfig as
the primary build time configuration mechanism. Options expressed this way let
us easily express dependencies and abstractions. In addition, given that many
projects use this mechanism means it has a broad set of tooling and existing
knowledge base.

Consider the example of a SHA256 hardware acceleration engine. This would be a
feature of the SoC and so something to not ask the user if it exists, but we
would want to have our generic framework for such engines be optionally
available and depend on knowing we have this engine on a given hardware
platform. Expressing this should be done as a hidden Kconfig symbol that is
``select``'ed by the SoC symbol which would in turn be ``select``'ed by the
board option, which is user visible. Hardware features that are either present
or not present should be expressed in Kconfig and in a similar manner, features
which will always have a constant value such as "this SoC always has 4 cores and
4 threads per core" should be as well.

This brings us to differentiating between a configuration *setting* versus a
hardware feature. To build on the previous example, while we may know the number
of cores and threads, it's possible that within a given family of SoCs the base
addresses of peripherals has changed, but the register offsets within have not.
The preference in this case is to get our information from the device tree and
perform run time configuration. However, this is not always practical and in
those cases we instead rely on the ``CFG`` mechanism. While it would be possible
to use Kconfig in this case, it would result in using calculated rather than
constructed values, resulting in less clear code. Consider the example of a set
of register values for a memory controller. Defining this as a series of logical
ORs and shifts based on other defines is more clear than the Kconfig entry that
set the calculated value alone.

When it has been determined that the practical solution is to utilize the
``CFG`` mechanism, the next decision is where to place these settings. It is
strongly encouraged to place these in the architecture header files, if they are
generic to a given SoC, or under the board directory if board specific. Placing
them under the board.h file in the *include/configs/* directory should be seen
as a last resort.