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When Kconfig for U-boot was examined, one of the biggest issues was how to support multiple images (Normal, SPL, TPL). There were actually two options, "single .config" and "multiple .config". After some discussions and thought experiments, I chose the latter, i.e. to create ".config", "spl/.config", "tpl/.config" for Normal, SPL, TPL, respectively. It is true that the "multiple .config" strategy provided us the maximum flexibility and helped to avoid duplicating CONFIGs among Normal, SPL, TPL, but I have noticed some fatal problems: [1] It is impossible to share CONFIG options across the images. If you change the configuration of Main image, you often have to adjust some SPL configurations correspondingly. Currently, we cannot handle the dependencies between them. It means one of the biggest advantages of Kconfig is lost. [2] It is too painful to change both ".config" and "spl/.config". Sunxi guys started to work around this problem by creating a new configuration target. Commitcbdd9a9737(sunxi: kconfig: Add %_felconfig rule to enable FEL build of sunxi platforms.) added "make *_felconfig" to enable CONFIG_SPL_FEL on both images. Changing the configuration of multiple images in one command is a generic demand. The current implementation cannot propose any good solution about this. [3] Kconfig files are getting ugly and difficult to understand. Commitb724bd7d63(dm: Kconfig: Move CONFIG_SYS_MALLOC_F_LEN to Kconfig) has sprinkled "if !SPL_BUILD" over the Kconfig files. [4] The build system got more complicated than it should be. To adjust Linux-originated Kconfig to U-Boot, the helper script "scripts/multiconfig.sh" was introduced. Writing a complicated text processor is a shell script sometimes caused problems. Now I believe the "single .config" will serve us better. With it, all the problems above would go away. Instead, we will have to add some CONFIG_SPL_* (and CONFIG_TPL_*) options such as CONFIG_SPL_DM, but we will not have much. Anyway, this is what we do now in scripts/Makefile.spl. I admit my mistake with my apology and this commit switches to the single .config configuration. It is not so difficult to do that: - Remove unnecessary processings from scripts/multiconfig.sh This file will remain for a while to support the current defconfig format. It will be removed after more cleanups are done. - Adjust some makefiles and Kconfigs - Add some entries to include/config_uncmd_spl.h and the new file scripts/Makefile.uncmd_spl. Some CONFIG options that are not supported on SPL must be disabled because one .config is shared between SPL and U-Boot proper going forward. I know this is not a beautiful solution and I think we can do better, but let's see how much we will have to describe them. - update doc/README.kconfig More cleaning up patches will follow this. Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com> Reviewed-by: Simon Glass <sjg@chromium.org>
158 lines
6 KiB
Text
158 lines
6 KiB
Text
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 followings
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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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