Kconfig in U-Boot
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=================
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This document describes the configuration infrastructure of U-Boot.
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The conventional configuration was replaced by Kconfig at v2014.10-rc1 release.
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Language Specification
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----------------------
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Kconfig originates in Linux Kernel.
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See the file "Documentation/kbuild/kconfig*.txt" in your Linux Kernel
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source directory for a basic specification of Kconfig.
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Difference from Linux's Kconfig
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-------------------------------
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Here are some worth-mentioning configuration targets.
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- silentoldconfig
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This target updates .config, include/generated/autoconf.h and
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include/configs/* as in Linux. In U-Boot, it also does the following
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for the compatibility with the old configuration system:
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* create a symbolic link "arch/${ARCH}/include/asm/arch" pointing to
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the SoC/CPU specific header directory
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* create include/config.h
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* create include/autoconf.mk
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* create spl/include/autoconf.mk (SPL and TPL only)
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* create tpl/include/autoconf.mk (TPL only)
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If we could completely switch to Kconfig in a long run
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(i.e. remove all the include/configs/*.h), those additional processings
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above would be removed.
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- defconfig
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In U-Boot, "make defconfig" is a shorthand of "make sandbox_defconfig"
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- <board>_defconfig
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Now it works as in Linux.
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The prefixes such as "+S:" in *_defconfig are deprecated.
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You can simply remove the prefixes. Do not add them for new boards.
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- <board>_config
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This does not exist in Linux's Kconfig.
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"make <board>_config" works the same as "make <board>_defconfig".
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Prior to Kconfig, in U-Boot, "make <board>_config" was used for the
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configuration. It is still supported for backward compatibility, so
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we do not need to update the distro recipes.
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The other configuration targets work as in Linux Kernel.
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Migration steps to Kconfig
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--------------------------
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Prior to Kconfig, the C preprocessor based board configuration had been used
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in U-Boot.
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Although Kconfig was introduced and some configs have been moved to Kconfig,
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many of configs are still defined in C header files. It will take a very
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long term to move all of them to Kconfig. In the interim, the two different
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configuration infrastructures should coexist.
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The configuration files are generated by both Kconfig and the old preprocessor
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based configuration as follows:
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Configuration files for use in C sources
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- include/generated/autoconf.h (generated by Kconfig for Normal)
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- include/configs/<board>.h (exists for all boards)
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Configuration file for use in makefiles
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- include/config/auto.conf (generated by Kconfig)
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- include/autoconf.mk (generated by the old config for Normal)
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- spl/include/autoconfig.mk (generated by the old config for SPL)
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- tpl/include/autoconfig.mk (generated by the old config for TPL)
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When adding a new CONFIG macro, it is highly recommended to add it to Kconfig
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rather than to a header file.
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Conversion from boards.cfg to Kconfig
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-------------------------------------
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Prior to Kconfig, boards.cfg was a primary database that contained Arch, CPU,
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SoC, etc. of all the supported boards. It was deleted when switching to
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Kconfig. Each field of boards.cfg was converted as follows:
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Status -> "S:" entry of MAINTAINERS
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Arch -> CONFIG_SYS_ARCH defined by Kconfig
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CPU -> CONFIG_SYS_CPU defined by Kconfig
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SoC -> CONFIG_SYS_SOC defined by Kconfig
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Vendor -> CONFIG_SYS_VENDOR defined by Kconfig
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Board -> CONFIG_SYS_BOARD defined by Kconfig
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Target -> File name of defconfig (configs/<target>_defconfig)
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Options -> CONFIG_SYS_EXTRA_OPTIONS defined by Kconfig
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Maintainers -> "M:" entry of MAINTAINERS
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Tips to add/remove boards
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-------------------------
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When adding a new board, the following steps are generally needed:
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[1] Add a header file include/configs/<target>.h
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[2] Make sure to define necessary CONFIG_SYS_* in Kconfig:
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Define CONFIG_SYS_CPU="cpu" to compile arch/<arch>/cpu/<cpu>
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Define CONFIG_SYS_SOC="soc" to compile arch/<arch>/cpu/<cpu>/<soc>
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Define CONFIG_SYS_VENDOR="vendor" to compile board/<vendor>/common/*
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and board/<vendor>/<board>/*
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Define CONFIG_SYS_BOARD="board" to compile board/<board>/*
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(or board/<vendor>/<board>/* if CONFIG_SYS_VENDOR is defined)
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Define CONFIG_SYS_CONFIG_NAME="target" to include
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include/configs/<target>.h
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[3] Add a new entry to the board select menu in Kconfig.
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The board select menu is located in arch/<arch>/Kconfig or
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arch/<arch>/*/Kconfig.
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[4] Add a MAINTAINERS file
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It is generally placed at board/<board>/MAINTAINERS or
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board/<vendor>/<board>/MAINTAINERS
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[5] Add configs/<target>_defconfig
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When removing an obsolete board, the following steps are generally needed:
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[1] Remove configs/<target>_defconfig
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[2] Remove include/configs/<target>.h if it is not used by any other boards
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[3] Remove board/<vendor>/<board>/* or board/<board>/* if it is not used
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by any other boards
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[4] Update MAINTAINERS if necessary
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[5] Remove the unused entry from the board select menu in Kconfig
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[6] Add an entry to doc/README.scrapyard
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TODO
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----
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- The option field of boards.cfg, which was used for the pre-Kconfig
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configuration, moved to CONFIG_SYS_EXTRA_OPTIONS verbatim now.
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Board maintainers are expected to implement proper Kconfig options
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and switch over to them. Eventually CONFIG_SYS_EXTRA_OPTIONS will go away.
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CONFIG_SYS_EXTRA_OPTIONS should not be used for new boards.
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- In the pre-Kconfig, a single board had multiple entries in the boards.cfg
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file with differences in the option fields. The corresponding defconfig
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files were auto-generated when switching to Kconfig. Now we have too many
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defconfig files compared with the number of the supported boards. It is
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recommended to have only one defconfig per board and allow users to select
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the config options.
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- Move the config macros in header files to Kconfig. When we move at least
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macros used in makefiles, we can drop include/autoconfig.mk, which makes
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the build scripts much simpler.
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