path: root/doc
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authorMasahiro Yamada <>2015-02-24 22:26:20 +0900
committerTom Rini <>2015-02-24 17:06:23 -0500
commite02ee2548afe22f7d197b89df2ae7bd72f1df630 (patch)
tree1c3c0dfbdd5bca4a88689aa4aafb308e8c872035 /doc
parent66afaef22889e2685aa500c047b66ac003eea264 (diff)
kconfig: switch to single .config configuration
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. Commit cbdd9a9737cc (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. Commit b724bd7d6349 (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/" 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/ 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 <> Reviewed-by: Simon Glass <>
Diffstat (limited to 'doc')
1 files changed, 32 insertions, 100 deletions
diff --git a/doc/README.kconfig b/doc/README.kconfig
index 69dc45970b..288d17d232 100644
--- a/doc/README.kconfig
+++ b/doc/README.kconfig
@@ -17,109 +17,45 @@ 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.
+Here are some worth-mentioning configuration targets.
-See below for how each configuration target works in U-Boot:
+- silentoldconfig
-- config, nconfig, menuconfig, xconfig, gconfig
+ This target updates .config, include/generated/autoconf.h and
+ include/configs/* as in Linux. In U-Boot, it also does the followings
+ for the compatibility with the old configuration system:
- 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.
+ * create a symbolic link "arch/${ARCH}/include/asm/arch" pointing to
+ the SoC/CPU specific header directory
+ * create include/config.h
+ * create include/
+ * create spl/include/ (SPL and TPL only)
+ * create tpl/include/ (TPL only)
-- silentoldconfig
+ If we could completely switch to Kconfig in a long run
+ (i.e. remove all the include/configs/*.h), those additional processings
+ above would be removed.
- This target updates .config, include/generated/autoconf.h and
- include/configs/*. In U-Boot, the same thing is done for SPL, TPL,
- if supported by the target board. Depending on whether CONFIG_SPL and
- CONFIG_TPL are defined, "make silentoldconfig" iterates three times at most
- changing the work directory.
- To sum up, "make silentoldconfig" possibly updates:
- - .config, include/generated/autoconf.h, include/config/*
- - spl/.config, spl/include/generated/autoconf.h, spl/include/config/*
- (in case CONFIG_SPL=y)
- - tpl/.config, tpl/include/generated/autoconf.h, tpl/include/config/*
- (in case CONFIG_TPL=y)
-- defconfig, <board>_defconfig
- The target "<board>_defconfig" is used to create the .config based on the
- file configs/<board>_defconfig. The "defconfig" target is the same
- except it checks for a file specified with KBUILD_DEFCONFIG environment.
- Note:
- The defconfig files are placed under the "configs" directory,
- not "arch/$(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,
- 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.
+- 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 and
- its behavior is the same as "make <board>_defconfig".
+ 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
@@ -137,14 +73,10 @@ 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/config/auto.conf (generated by Kconfig)
- 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)
@@ -215,8 +147,8 @@ TODO
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
+ 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.
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