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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.
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 TPL 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.
- <board>_config
This does not exist in Linux's Kconfig.
Prior to Kconfig, in U-Boot, "make <board>_config" was used for the
configuration. It is still supported for backward compatibility and
its behavior is the same as "make <board>_defconfig".
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
[3] Add a new entry to the board select menu in Kconfig.
The board select menu is located in arch/<arch>/Kconfig or
[4] Add a MAINTAINERS file
It is generally placed at board/<board>/MAINTAINERS or
[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.