diff options
authorMasahiro Yamada <>2014-08-16 00:50:30 +0900
committerTom Rini <>2014-08-21 12:01:11 -0400
commit6933b5c9f3b6844b5fc3b81b3c8157cb17eaea3e (patch)
parent67ee22b0681305ba1099c508eecade996abc1c65 (diff)
README.kconfig: add initial version of Kconfig document
Signed-off-by: Masahiro Yamada <>
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+Kconfig in U-Boot
+This document describes the configuration infrastructure of U-Boot.
+The conventional configuration was replaced by Kconfig at v2014.10-rc1 release.
+Language Specification
+Kconfig originates in Linux Kernel.
+See the file "Documentation/kbuild/kconfig*.txt" in your Linux Kernel
+source directory for a basic specification of Kconfig.
+Difference from Linux's Kconfig
+The biggest difference between Linux Kernel and U-Boot in terms of the
+configuration is that U-Boot has to configure multiple boot images per board:
+Normal, SPL, TPL.
+Kconfig functions need to be expanded for U-Boot to handle multiple images.
+The files scripts/kconfig/* were imported from Linux Kernel and adjusted
+for that purpose.
+See below for how each configuration target works in U-Boot:
+- config, nconfig, menuconfig, xconfig, gconfig
+ These targets are used to configure Normal and create (or modify) the
+ .config file. For SPL configuration, the configutation targets are prefixed
+ with "spl/", for example "make spl/config", "make spl/menuconfig", etc.
+ Those targets create or modify the spl/.config file. Likewise, run
+ "make tpl/config", "make tpl/menuconfig", etc. for TPL.
+- silentoldconfig
+ This target updates .config, include/generated/autoconf.h and
+ include/configs/*. In U-Boot, the same thing is done for SPL, TPL,
+ if supported by the target board. Depending on whether CONFIG_SPL and
+ CONFIG_TPL are defined, "make silentoldconfig" iterates three times at most
+ changing the work directory.
+ To sum up, "make silentoldconfig" possibly updates:
+ - .config, include/generated/autoconf.h, include/config/*
+ - spl/.config, spl/include/generated/autoconf.h, spl/include/config/*
+ (in case CONFIG_SPL=y)
+ - tpl/.config, tpl/include/generated/autoconf.h, tpl/include/config/*
+ (in case CONFIG_TPL=y)
+- defconfig, <board>_defconfig
+ The target "<board>_defconfig" is used to create the .config based on the
+ file configs/<board>_defconfig. The "defconfig" target is the same
+ except it checks for a file specified with KBUILD_DEFCONFIG environment.
+ Note:
+ The defconfig files are placed under the "configs" directory,
+ not "arch/$(ARCH)/configs". This is because "ARCH" is not necessarily
+ given from the command line for the U-Boot configuration and build.
+ The defconfig file format in U-Boot has the special syntax; each line has
+ "<condition>:" prefix to show which image(s) the line is valid for.
+ For example,
+ Here, the "<condition>:" prefix is one of:
+ None - the line is valid only for Normal image
+ S: - the line is valid only for SPL image
+ T: - the line is valid only for TPL image
+ ST: - the line is valid for SPL and TPL images
+ +S: - the line is valid for Normal and SPL images
+ +T: - the line is valid for Normal and TPL images
+ +ST: - the line is valid for Normal, SPL and SPL images
+ So, if neither CONFIG_SPL nor CONFIG_TPL is defined, the defconfig file
+ has no "<condition>:" part and therefore has the same form as in Linux.
+ From the example defconfig shown above, three separete configuration sets
+ are generated and used for creating .config, spl/.config and tpl/.config.
+ - Input for the default configuration of Normal
+ - Input for the default configuration of SPL
+ - Input for the default configuration of TPL
+- savedefconfig
+ This is the reverse operation of "make defconfig". If neither CONFIG_SPL
+ nor CONFIG_TPL is defined in the .config file, it works like "savedefconfig"
+ in Linux Kernel: creates the minimal set of config based on the .config
+ and saves it into the "defconfig" file. If CONFIG_SPL (and CONFIG_TPL) is
+ defined, the common lines among .config, spl/.config (and tpl/.config) are
+ coalesced together with "<condition:>" prefix for each line as shown above.
+ This file can be used as an input of "defconfig" target.
+Migration steps to Kconfig
+Prior to Kconfig, the C preprocessor based board configuration had been used
+in U-Boot.
+Although Kconfig was introduced and some configs have been moved to Kconfig,
+many of configs are still defined in C header files. It will take a very
+long term to move all of them to Kconfig. In the interim, the two different
+configuration infrastructures should coexist.
+The configuration files are generated by both Kconfig and the old preprocessor
+based configuration as follows:
+Configuration files for use in C sources
+ - include/generated/autoconf.h (generated by Kconfig for Normal)
+ - spl/include/generated/autoconf.h (generated by Kconfig for SPL)
+ - tpl/include/generated/autoconf.h (generated by Kconfig for TPL)
+ - include/configs/<board>.h (exists for all boards)
+Configuration file for use in makefiles
+ - include/config/auto.conf (generated by Kconfig for Normal)
+ - spl/include/config/auto.conf (generated by Kconfig for SPL)
+ - tpl/include/config/auto.conf (generated by Kconfig for TPL)
+ - include/ (generated by the old config for Normal)
+ - spl/include/ (generated by the old config for SPL)
+ - tpl/include/ (generated by the old config for TPL)
+When adding a new CONFIG macro, it is highly recommended to add it to Kconfig
+rather than to a header file.
+Conversion from boards.cfg to Kconfig
+Prior to Kconfig, boards.cfg was a primary database that contained Arch, CPU,
+SoC, etc. of all the supported boards. It was deleted when switching to
+Kconfig. Each field of boards.cfg was converted as follows:
+ Status -> "S:" entry of MAINTAINERS
+ Arch -> CONFIG_SYS_ARCH defined by Kconfig
+ CPU -> CONFIG_SYS_CPU defined by Kconfig
+ SoC -> CONFIG_SYS_SOC defined by Kconfig
+ Vendor -> CONFIG_SYS_VENDOR defined by Kconfig
+ Board -> CONFIG_SYS_BOARD defined by Kconfig
+ Target -> File name of defconfig (configs/<target>_defconfig)
+ Options -> CONFIG_SYS_EXTRA_OPTIONS defined by Kconfig
+ Maintainers -> "M:" entry of MAINTAINERS
+Tips to add/remove boards
+When adding a new board, the following steps are generally needed:
+ [1] Add a header file include/configs/<target>.h
+ [2] Make sure to define necessary CONFIG_SYS_* in Kconfig:
+ Define CONFIG_SYS_CPU="cpu" to compile arch/<arch>/cpu/<cpu>
+ Define CONFIG_SYS_SOC="soc" to compile arch/<arch>/cpu/<cpu>/<soc>
+ Define CONFIG_SYS_VENDOR="vendor" to compile board/<vendor>/common/*
+ and board/<vendor>/<board>/*
+ Define CONFIG_SYS_BOARD="board" to compile board/<board>/*
+ (or board/<vendor>/<board>/* if CONFIG_SYS_VENDOR is defined)
+ Define CONFIG_SYS_CONFIG_NAME="target" to include
+ include/configs/<target>.h
+ [3] Add a new entry to the board select menu in Kconfig.
+ The board select menu is located in arch/<arch>/Kconfig or
+ arch/<arch>/*/Kconfig.
+ [4] Add a MAINTAINERS file
+ It is generally placed at board/<board>/MAINTAINERS or
+ board/<vendor>/<board>/MAINTAINERS
+ [5] Add configs/<target>_defconfig
+When removing an obsolete board, the following steps are generally needed:
+ [1] Remove configs/<target>_defconfig
+ [2] Remove include/configs/<target>.h if it is not used by any other boards
+ [3] Remove board/<vendor>/<board>/* or board/<board>/* if it is not used
+ by any other boards
+ [4] Update MAINTAINERS if necessary
+ [5] Remove the unused entry from the board select menu in Kconfig
+ [6] Add an entry to doc/README.scrapyard
+- The option field of boards.cfg, which was used for the pre-Kconfig
+ configuration, moved to CONFIG_SYS_EXTRA_OPTIONS verbatim now.
+ Board maintainers are expected to implement proper Kconfig options
+ and switch over to them. Eventually CONFIG_SYS_EXTRA_OPTIONS will go away.
+ CONFIG_SYS_EXTRA_OPTIONS should not be used for new boards.
+- In the pre-Kconfig, a single board had multiple entries in the boards.cfg
+ file with differences in the option fields. The correspoing defconfig files
+ were auto-generated when switching to Kconfig. Now we have too many
+ defconfig files compared with the number of the supported boards. It is
+ recommended to have only one defconfig per board and allow users to select
+ the config options.
+- Move the config macros in header files to Kconfig. When we move at least
+ macros used in makefiles, we can drop include/, which makes
+ the build scripts much simpler.
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