220 lines
8.7 KiB
Plaintext
220 lines
8.7 KiB
Plaintext
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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The biggest difference between Linux Kernel and U-Boot in terms of the
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configuration is that U-Boot has to configure multiple boot images per board:
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Normal, SPL, TPL.
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Kconfig functions need to be expanded for U-Boot to handle multiple images.
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The files scripts/kconfig/* were imported from Linux Kernel and adjusted
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for that purpose.
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See below for how each configuration target works in U-Boot:
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- config, nconfig, menuconfig, xconfig, gconfig
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These targets are used to configure Normal and create (or modify) the
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.config file. For SPL configuration, the configutation targets are prefixed
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with "spl/", for example "make spl/config", "make spl/menuconfig", etc.
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Those targets create or modify the spl/.config file. Likewise, run
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"make tpl/config", "make tpl/menuconfig", etc. for TPL.
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- silentoldconfig
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This target updates .config, include/generated/autoconf.h and
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include/configs/*. In U-Boot, the same thing is done for SPL, TPL,
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if supported by the target board. Depending on whether CONFIG_SPL and
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CONFIG_TPL are defined, "make silentoldconfig" iterates three times at most
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changing the work directory.
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To sum up, "make silentoldconfig" possibly updates:
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- .config, include/generated/autoconf.h, include/config/*
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- spl/.config, spl/include/generated/autoconf.h, spl/include/config/*
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(in case CONFIG_SPL=y)
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- tpl/.config, tpl/include/generated/autoconf.h, tpl/include/config/*
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(in case CONFIG_TPL=y)
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- defconfig, <board>_defconfig
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The target "<board>_defconfig" is used to create the .config based on the
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file configs/<board>_defconfig. The "defconfig" target is the same
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except it checks for a file specified with KBUILD_DEFCONFIG environment.
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Note:
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The defconfig files are placed under the "configs" directory,
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not "arch/$(ARCH)/configs". This is because "ARCH" is not necessarily
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given from the command line for the U-Boot configuration and build.
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The defconfig file format in U-Boot has the special syntax; each line has
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"<condition>:" prefix to show which image(s) the line is valid for.
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For example,
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CONFIG_FOO=100
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S:CONFIG_FOO=200
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T:CONFIG_FOO=300
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ST:CONFIG_BAR=y
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+S:CONFIG_BAZ=y
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+T:CONFIG_QUX=y
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+ST:CONFIG_QUUX=y
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Here, the "<condition>:" prefix is one of:
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None - the line is valid only for Normal image
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S: - the line is valid only for SPL image
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T: - the line is valid only for TPL image
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ST: - the line is valid for SPL and TPL images
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+S: - the line is valid for Normal and SPL images
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+T: - the line is valid for Normal and TPL images
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+ST: - the line is valid for Normal, SPL and SPL images
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So, if neither CONFIG_SPL nor CONFIG_TPL is defined, the defconfig file
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has no "<condition>:" part and therefore has the same form as in Linux.
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From the example defconfig shown above, three separete configuration sets
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are generated and used for creating .config, spl/.config and tpl/.config.
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- Input for the default configuration of Normal
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CONFIG_FOO=100
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CONFIG_BAZ=y
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CONFIG_QUX=y
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CONFIG_QUUX=y
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- Input for the default configuration of SPL
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CONFIG_FOO=200
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CONFIG_BAR=y
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CONFIG_BAZ=y
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CONFIG_QUUX=y
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- Input for the default configuration of TPL
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CONFIG_FOO=300
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CONFIG_BAR=y
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CONFIG_QUX=y
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CONFIG_QUUX=y
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- savedefconfig
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This is the reverse operation of "make defconfig". If neither CONFIG_SPL
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nor CONFIG_TPL is defined in the .config file, it works like "savedefconfig"
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in Linux Kernel: creates the minimal set of config based on the .config
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and saves it into the "defconfig" file. If CONFIG_SPL (and CONFIG_TPL) is
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defined, the common lines among .config, spl/.config (and tpl/.config) are
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coalesced together with "<condition:>" prefix for each line as shown above.
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This file can be used as an input of "defconfig" target.
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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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- spl/include/generated/autoconf.h (generated by Kconfig for SPL)
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- tpl/include/generated/autoconf.h (generated by Kconfig for TPL)
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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 for Normal)
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- spl/include/config/auto.conf (generated by Kconfig for SPL)
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- tpl/include/config/auto.conf (generated by Kconfig for TPL)
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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 correspoing defconfig files
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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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