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

The Kconfig configuration language originates in Linux kernel. See the Linux document Kconfig Language for a description of Kconfig.

Difference from Linux’s Kconfig

Here are some worth-mentioning configuration targets.

silentoldconfig

This target updates .config, include/generated/autoconf.h and include/configs/* as in Linux. In U-Boot, it also does the following for the compatibility with the old configuration system:

  • create a symbolic link “arch/${ARCH}/include/asm/arch” pointing to the SoC/CPU specific header directory

  • create include/config.h

  • create include/autoconf.mk

  • create spl/include/autoconf.mk (SPL and TPL only)

  • create tpl/include/autoconf.mk (TPL only)

If we could completely switch to Kconfig in a long run (i.e. remove all the include/configs/*.h), those additional processings above would be removed.

defconfig

In U-Boot, “make defconfig” is a shorthand of “make sandbox_defconfig”

<board>_defconfig

Now it works as in Linux. The prefixes such as “+S:” in *_defconfig are deprecated. You can simply remove the prefixes. Do not add them for new boards.

<board>_config

This does not exist in Linux’s Kconfig. “make <board>_config” works the same as “make <board>_defconfig”. Prior to Kconfig, in U-Boot, “make <board>_config” was used for the configuration. It is still supported for backward compatibility, so we do not need to update the distro recipes.

The other configuration targets work as in Linux Kernel.

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)

  • include/configs/<board>.h (exists for all boards)

Configuration file for use in makefiles

  • include/config/auto.conf (generated by Kconfig)

  • include/autoconf.mk (generated by the old config for Normal)

  • spl/include/autoconfig.mk (generated by the old config for SPL)

  • tpl/include/autoconfig.mk (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:

From

To

Arch

CONFIG_SYS_ARCH defined by Kconfig

Board

CONFIG_SYS_BOARD defined by Kconfig

CPU

CONFIG_SYS_CPU defined by Kconfig

Maintainers

“M:” entry of MAINTAINERS

SoC

CONFIG_SYS_SOC defined by Kconfig

Status

“S:” entry of MAINTAINERS

Target

File name of defconfig (configs/<target>_defconfig)

Vendor

CONFIG_SYS_VENDOR defined by Kconfig

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

TODO

  • In the pre-Kconfig, a single board had multiple entries in the boards.cfg file with differences in the option fields. The corresponding 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/autoconfig.mk, which makes the build scripts much simpler.