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,

  CONFIG_FOO=100
  S:CONFIG_FOO=200
  T:CONFIG_FOO=300
  ST:CONFIG_BAR=y
  +S:CONFIG_BAZ=y
  +T:CONFIG_QUX=y
  +ST:CONFIG_QUUX=y

  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
     CONFIG_FOO=100
     CONFIG_BAZ=y
     CONFIG_QUX=y
     CONFIG_QUUX=y

  - Input for the default configuration of SPL
     CONFIG_FOO=200
     CONFIG_BAR=y
     CONFIG_BAZ=y
     CONFIG_QUUX=y

  - Input for the default configuration of TPL
     CONFIG_FOO=300
     CONFIG_BAR=y
     CONFIG_QUX=y
     CONFIG_QUUX=y

- 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/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:

 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


TODO
----

- 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/autoconfig.mk, which makes
  the build scripts much simpler.