1Kconfig in U-Boot 2================= 3 4This document describes the configuration infrastructure of U-Boot. 5 6The conventional configuration was replaced by Kconfig at v2014.10-rc1 release. 7 8 9Language Specification 10---------------------- 11 12Kconfig originates in Linux Kernel. 13See the file "Documentation/kbuild/kconfig*.txt" in your Linux Kernel 14source directory for a basic specification of Kconfig. 15 16 17Difference from Linux's Kconfig 18------------------------------- 19 20Here are some worth-mentioning configuration targets. 21 22- silentoldconfig 23 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 39- defconfig 40 41 In U-Boot, "make defconfig" is a shorthand of "make sandbox_defconfig" 42 43- <board>_defconfig 44 45 Now it works as in Linux. 46 The prefixes such as "+S:" in *_defconfig are deprecated. 47 You can simply remove the prefixes. Do not add them for new boards. 48 49- <board>_config 50 51 This does not exist in Linux's Kconfig. 52 "make <board>_config" works the same as "make <board>_defconfig". 53 Prior to Kconfig, in U-Boot, "make <board>_config" was used for the 54 configuration. It is still supported for backward compatibility, so 55 we do not need to update the distro recipes. 56 57 58The other configuration targets work as in Linux Kernel. 59 60 61Migration steps to Kconfig 62-------------------------- 63 64Prior to Kconfig, the C preprocessor based board configuration had been used 65in U-Boot. 66 67Although Kconfig was introduced and some configs have been moved to Kconfig, 68many of configs are still defined in C header files. It will take a very 69long term to move all of them to Kconfig. In the interim, the two different 70configuration infrastructures should coexist. 71The configuration files are generated by both Kconfig and the old preprocessor 72based configuration as follows: 73 74Configuration files for use in C sources 75 - include/generated/autoconf.h (generated by Kconfig for Normal) 76 - include/configs/<board>.h (exists for all boards) 77 78Configuration file for use in makefiles 79 - include/config/auto.conf (generated by Kconfig) 80 - include/autoconf.mk (generated by the old config for Normal) 81 - spl/include/autoconfig.mk (generated by the old config for SPL) 82 - tpl/include/autoconfig.mk (generated by the old config for TPL) 83 84When adding a new CONFIG macro, it is highly recommended to add it to Kconfig 85rather than to a header file. 86 87 88Conversion from boards.cfg to Kconfig 89------------------------------------- 90 91Prior to Kconfig, boards.cfg was a primary database that contained Arch, CPU, 92SoC, etc. of all the supported boards. It was deleted when switching to 93Kconfig. Each field of boards.cfg was converted as follows: 94 95 Status -> "S:" entry of MAINTAINERS 96 Arch -> CONFIG_SYS_ARCH defined by Kconfig 97 CPU -> CONFIG_SYS_CPU defined by Kconfig 98 SoC -> CONFIG_SYS_SOC defined by Kconfig 99 Vendor -> CONFIG_SYS_VENDOR defined by Kconfig 100 Board -> CONFIG_SYS_BOARD defined by Kconfig 101 Target -> File name of defconfig (configs/<target>_defconfig) 102 Options -> CONFIG_SYS_EXTRA_OPTIONS defined by Kconfig 103 Maintainers -> "M:" entry of MAINTAINERS 104 105 106Tips to add/remove boards 107------------------------- 108 109When adding a new board, the following steps are generally needed: 110 111 [1] Add a header file include/configs/<target>.h 112 [2] Make sure to define necessary CONFIG_SYS_* in Kconfig: 113 Define CONFIG_SYS_CPU="cpu" to compile arch/<arch>/cpu/<cpu> 114 Define CONFIG_SYS_SOC="soc" to compile arch/<arch>/cpu/<cpu>/<soc> 115 Define CONFIG_SYS_VENDOR="vendor" to compile board/<vendor>/common/* 116 and board/<vendor>/<board>/* 117 Define CONFIG_SYS_BOARD="board" to compile board/<board>/* 118 (or board/<vendor>/<board>/* if CONFIG_SYS_VENDOR is defined) 119 Define CONFIG_SYS_CONFIG_NAME="target" to include 120 include/configs/<target>.h 121 [3] Add a new entry to the board select menu in Kconfig. 122 The board select menu is located in arch/<arch>/Kconfig or 123 arch/<arch>/*/Kconfig. 124 [4] Add a MAINTAINERS file 125 It is generally placed at board/<board>/MAINTAINERS or 126 board/<vendor>/<board>/MAINTAINERS 127 [5] Add configs/<target>_defconfig 128 129When removing an obsolete board, the following steps are generally needed: 130 131 [1] Remove configs/<target>_defconfig 132 [2] Remove include/configs/<target>.h if it is not used by any other boards 133 [3] Remove board/<vendor>/<board>/* or board/<board>/* if it is not used 134 by any other boards 135 [4] Update MAINTAINERS if necessary 136 [5] Remove the unused entry from the board select menu in Kconfig 137 [6] Add an entry to doc/README.scrapyard 138 139 140TODO 141---- 142 143- The option field of boards.cfg, which was used for the pre-Kconfig 144 configuration, moved to CONFIG_SYS_EXTRA_OPTIONS verbatim now. 145 Board maintainers are expected to implement proper Kconfig options 146 and switch over to them. Eventually CONFIG_SYS_EXTRA_OPTIONS will go away. 147 CONFIG_SYS_EXTRA_OPTIONS should not be used for new boards. 148 149- In the pre-Kconfig, a single board had multiple entries in the boards.cfg 150 file with differences in the option fields. The corresponding defconfig 151 files were auto-generated when switching to Kconfig. Now we have too many 152 defconfig files compared with the number of the supported boards. It is 153 recommended to have only one defconfig per board and allow users to select 154 the config options. 155 156- Move the config macros in header files to Kconfig. When we move at least 157 macros used in makefiles, we can drop include/autoconfig.mk, which makes 158 the build scripts much simpler. 159