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"Das U-Boot" Source Tree
1.. SPDX-License-Identifier: GPL-2.0-or-later
2
3Kconfig in U-Boot
4=================
5
6This document describes the configuration infrastructure of U-Boot.
7
8The conventional configuration was replaced by Kconfig at v2014.10-rc1 release.
9
10Language Specification
11----------------------
12
13The Kconfig configuration language originates in Linux kernel.
14See the Linux document
15`Kconfig Language <https://www.kernel.org/doc/html/latest/kbuild/kconfig-language.html>`_
16for a description of Kconfig.
17
18Difference from Linux's Kconfig
19-------------------------------
20
21Here are some worth-mentioning configuration targets.
22
23silentoldconfig
24 This target updates .config, include/generated/autoconf.h and
25 include/configs/* as in Linux. In U-Boot, it also does the following
26 for the compatibility with the old configuration system:
27
28 * create a symbolic link "arch/${ARCH}/include/asm/arch" pointing to
29 the SoC/CPU specific header directory
30 * create include/config.h
31 * create include/autoconf.mk
32 * create spl/include/autoconf.mk (SPL and TPL only)
33 * create tpl/include/autoconf.mk (TPL only)
34
35 If we could completely switch to Kconfig in a long run
36 (i.e. remove all the include/configs/\*.h), those additional processings
37 above would be removed.
38
39defconfig
40 In U-Boot, "make defconfig" is a shorthand of "make sandbox_defconfig"
41
42<board>_defconfig
43 Now it works as in Linux.
44 The prefixes such as "+S:" in \*_defconfig are deprecated.
45 You can simply remove the prefixes. Do not add them for new boards.
46
47<board>_config
48 This does not exist in Linux's Kconfig.
49 "make <board>_config" works the same as "make <board>_defconfig".
50 Prior to Kconfig, in U-Boot, "make <board>_config" was used for the
51 configuration. It is still supported for backward compatibility, so
52 we do not need to update the distro recipes.
53
54The other configuration targets work as in Linux Kernel.
55
56Migration steps to Kconfig
57--------------------------
58
59Prior to Kconfig, the C preprocessor based board configuration had been used
60in U-Boot.
61
62Although Kconfig was introduced and some configs have been moved to Kconfig,
63many of configs are still defined in C header files. It will take a very
64long term to move all of them to Kconfig. In the interim, the two different
65configuration infrastructures should coexist.
66The configuration files are generated by both Kconfig and the old preprocessor
67based configuration as follows:
68
69Configuration files for use in C sources
70 - include/generated/autoconf.h (generated by Kconfig for Normal)
71 - include/configs/<board>.h (exists for all boards)
72
73Configuration file for use in makefiles
74 - include/config/auto.conf (generated by Kconfig)
75 - include/autoconf.mk (generated by the old config for Normal)
76 - spl/include/autoconfig.mk (generated by the old config for SPL)
77 - tpl/include/autoconfig.mk (generated by the old config for TPL)
78
79When adding a new CONFIG macro, it is highly recommended to add it to Kconfig
80rather than to a header file.
81
82Conversion from boards.cfg to Kconfig
83-------------------------------------
84
85Prior to Kconfig, boards.cfg was a primary database that contained Arch, CPU,
86SoC, etc. of all the supported boards. It was deleted when switching to
87Kconfig. Each field of boards.cfg was converted as follows:
88
89=========== ====================================================
90From To
91=========== ====================================================
92Arch CONFIG_SYS_ARCH defined by Kconfig
93Board CONFIG_SYS_BOARD defined by Kconfig
94CPU CONFIG_SYS_CPU defined by Kconfig
95Maintainers "M:" entry of MAINTAINERS
96SoC CONFIG_SYS_SOC defined by Kconfig
97Status "S:" entry of MAINTAINERS
98Target File name of defconfig (configs/<target>\_defconfig)
99Vendor CONFIG_SYS_VENDOR defined by Kconfig
100=========== ====================================================
101
102Tips to add/remove boards
103-------------------------
104
105When adding a new board, the following steps are generally needed:
106
1071. Add a header file include/configs/<target>.h
108
1092. Make sure to define necessary CONFIG_SYS_* in Kconfig:
110
111 * Define CONFIG_SYS_CPU="cpu" to compile arch/<arch>/cpu/<cpu>
112 * Define CONFIG_SYS_SOC="soc" to compile arch/<arch>/cpu/<cpu>/<soc>
113 * Define CONFIG_SYS_VENDOR="vendor" to compile board/<vendor>/common/\*
114 and board/<vendor>/<board>/\*
115 * Define CONFIG_SYS_BOARD="board" to compile board/<board>/\*
116 (or board/<vendor>/<board>/* if CONFIG_SYS_VENDOR is defined)
117 Define CONFIG_SYS_CONFIG_NAME="target" to include
118 include/configs/<target>.h
119
1203. Add a new entry to the board select menu in Kconfig.
121 The board select menu is located in arch/<arch>/Kconfig or
122 arch/<arch>/\*/Kconfig.
123
1244. Add a MAINTAINERS file
125 It is generally placed at board/<board>/MAINTAINERS or
126 board/<vendor>/<board>/MAINTAINERS
127
1285. Add configs/<target>_defconfig
129
130When removing an obsolete board, the following steps are generally needed:
131
1321. Remove configs/<target>_defconfig
133
1342. Remove include/configs/<target>.h if it is not used by any other boards
135
1363. Remove board/<vendor>/<board>/\* or board/<board>/\* if it is not used
137 by any other boards
138
1394. Update MAINTAINERS if necessary
140
1415. Remove the unused entry from the board select menu in Kconfig
142
1436. Add an entry to doc/README.scrapyard
144
145TODO
146----
147
148* In the pre-Kconfig, a single board had multiple entries in the boards.cfg
149 file with differences in the option fields. The corresponding defconfig
150 files were auto-generated when switching to Kconfig. Now we have too many
151 defconfig files compared with the number of the supported boards. It is
152 recommended to have only one defconfig per board and allow users to select
153 the config options.
154
155* Move the config macros in header files to Kconfig. When we move at least
156 macros used in makefiles, we can drop include/autoconfig.mk, which makes
157 the build scripts much simpler.