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Tom Rini53d2ab92022-07-29 12:07:38 -04001.. SPDX-License-Identifier: GPL-2.0+
2
3System configuration
4====================
5
6There are a number of different aspects to configuring U-Boot to build and then
7run on a given platform or set of platforms. Broadly speaking, some aspects of
8the world can be configured at run time and others must be done at build time.
9In general run time configuration is preferred over build time configuration.
10But when making these decisions, we also need to consider if we're talking about
11a feature that could be useful to virtually every platform or something specific
12to a single hardware platform. The resulting image size is also another
13important consideration. Finally, run time configuration has additional overhead
14both in terms of resource requirements and wall clock time. All of this means
15that care must be taken when writing new code to select the most appropriate
16configuration mechanism.
17
18When adding new features to U-Boot, be they a new subsystem or SoC support or
19new platform for an existing supported SoC, the preferred configuration order
20is:
21
22#. Hardware based run time configuration. Examples of this include reading
23 processor specific registers, or a set of board specific GPIOs or an EEPROM
24 with a known format to it. These are the cases where we either cannot or
25 should not be relying on device tree checks. We use this for cases such as
26 optimized boot time or starting with a generic device tree and then enabling
27 or disabling features as we boot.
28
29#. Making use of our Kconfig infrastructure and C preprocessor macros that have
30 the prefix ``CONFIG``. This is the primary method of build time
31 configuration. This is generally the best fit for when we want to enable or
32 disable some sort of feature, such as the SoC or network support. The
33 ``CONFIG`` prefix for C preprocessor macros is strictly reserved for Kconfig
34 usage only.
35
36#. Making use of the :doc:`device tree <devicetree/control>` to determine at
37 run time how to configure a feature that we have enabled via Kconfig. For
38 example, we would use Kconfig to enable an I2C chip driver, but use the device
39 tree to know where the I2C chip resides in memory and other details we need
40 in order to configure the bus.
41
42#. Making use of C header files directly and defining C preprocessor macros that
43 have the ``CFG`` prefix. While the ``CFG`` prefix is reserved for this build
44 time configuration mechanism, the usage is ad hoc. This is to be used when the
45 previously mentioned mechanisms are not possible, or for legacy code that has
46 not been converted.
47
48Dynamic run time configuration methods.
49---------------------------------------
50
51Details of hardware specific run time configuration methods are found within the
52documentation for a given processor family or board.
53
54Details of how to use run time configuration based on :doc:`driver model
55<driver-model/index>` are covered in that documentation section.
56
57Static build time configuration methods
58---------------------------------------
59
60There are two mechanisms used to control the build time configuration of U-Boot.
61One is utilizing Kconfig and ``CONFIG`` prefixed macros and the other is ad hoc
62usage of ``CFG`` prefixed macros. Both of these are used when it is either not
63possible or not practical to make a run time determination about some
64functionality of the hardware or a required software feature or similar. Each of
65these has their own places where they are better suited than the other for use.
66
67The `Kconfig language
68<https://www.kernel.org/doc/html/latest/kbuild/kconfig-language.html>`_ is well
69documented and used in a number of projects, including the Linux kernel. We
70implement this with the Kconfig files found throughout our sources. This
71mechanism is the preferred way of exposing new configuration options as there
72are a number of ways for both users and system integrators to manage and change
73these options. Some common examples here are to enable a specific command within
74U-Boot or even a whole subsystem such as NAND flash or network connectivity.
75
76The ``CFG`` mechanism is implemented directly as C preprocessor values or
77macros, depending on what they are in turn describing. While we have some
78functionality that is very reasonable to expose to the end user to enable or
79disable we have other places where we need to describe things such as register
80locations or values, memory map ranges and so on. When practical, we should be
81getting these values from the device tree. However, there are cases where this
82is either not practical due to when we need the information and may not have a
83device tree yet or due to legacy reasons code has not been rewritten.
84
85When to use each mechanism
86^^^^^^^^^^^^^^^^^^^^^^^^^^
87
88While there are some cases where it should be fairly obvious where to use each
Maxim Cournoyere7d962b2022-12-16 21:09:40 -050089mechanism, as for example a command would be done via Kconfig, a new I2C driver
Tom Rini53d2ab92022-07-29 12:07:38 -040090should use Kconfig and be configured via driver model and a header of values
91generated by an external tool should be ``CFG``, there will be cases where it's
92less clear and one needs to take care when implementing it. In general,
93configuration *options* should be done in Kconfig and configuration *settings*
Maxim Cournoyere7d962b2022-12-16 21:09:40 -050094should be done in driver model or ``CFG``. Let us discuss things to keep in mind
Tom Rini53d2ab92022-07-29 12:07:38 -040095when picking the appropriate mechanism.
96
97A thing to keep in mind is that we have a strong preference for using Kconfig as
98the primary build time configuration mechanism. Options expressed this way let
99us easily express dependencies and abstractions. In addition, given that many
100projects use this mechanism means it has a broad set of tooling and existing
101knowledge base.
102
103Consider the example of a SHA256 hardware acceleration engine. This would be a
104feature of the SoC and so something to not ask the user if it exists, but we
105would want to have our generic framework for such engines be optionally
106available and depend on knowing we have this engine on a given hardware
107platform. Expressing this should be done as a hidden Kconfig symbol that is
108``select``'ed by the SoC symbol which would in turn be ``select``'ed by the
109board option, which is user visible. Hardware features that are either present
110or not present should be expressed in Kconfig and in a similar manner, features
111which will always have a constant value such as "this SoC always has 4 cores and
1124 threads per core" should be as well.
113
114This brings us to differentiating between a configuration *setting* versus a
115hardware feature. To build on the previous example, while we may know the number
116of cores and threads, it's possible that within a given family of SoCs the base
117addresses of peripherals has changed, but the register offsets within have not.
118The preference in this case is to get our information from the device tree and
119perform run time configuration. However, this is not always practical and in
120those cases we instead rely on the ``CFG`` mechanism. While it would be possible
121to use Kconfig in this case, it would result in using calculated rather than
122constructed values, resulting in less clear code. Consider the example of a set
123of register values for a memory controller. Defining this as a series of logical
124ORs and shifts based on other defines is more clear than the Kconfig entry that
Maxim Cournoyere7d962b2022-12-16 21:09:40 -0500125sets the calculated value alone.
Tom Rini53d2ab92022-07-29 12:07:38 -0400126
127When it has been determined that the practical solution is to utilize the
128``CFG`` mechanism, the next decision is where to place these settings. It is
129strongly encouraged to place these in the architecture header files, if they are
130generic to a given SoC, or under the board directory if board specific. Placing
131them under the board.h file in the *include/configs/* directory should be seen
132as a last resort.