1. 前言
UBOOT版本:uboot2018.03,开发板myimx8mmek240。
2. 背景
在编译构建目标时(如 make xxx),顶层 Makefile 的 dot-config 变量值设置为 1 。 如下:
#note: 顶层Makefile
dot-config := 1
ifneq ($(filter $(no-dot-config-targets), $(MAKECMDGOALS)),)
ifeq ($(filter-out $(no-dot-config-targets), $(MAKECMDGOALS)),)
dot-config := 0
endif
endi
在顶层 Makefile 中:
#note: 顶层Makefile
KCONFIG_CONFIG ?= .config
export KCONFIG_CONFIG
ifeq ($(dot-config),1)
# Read in config
-include include/config/auto.conf (1)
# Read in dependencies to all Kconfig* files, make sure to run
# oldconfig if changes are detected.
-include include/config/auto.conf.cmd (2)
# To avoid any implicit rule to kick in, define an empty command
$(KCONFIG_CONFIG) include/config/auto.conf.cmd: ;
# If .config is newer than include/config/auto.conf, someone tinkered
# with it and forgot to run make oldconfig.
# if auto.conf.cmd is missing then we are probably in a cleaned tree so
# we execute the config step to be sure to catch updated Kconfig files
include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
$(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig
@# If the following part fails, include/config/auto.conf should be
@# deleted so "make silentoldconfig" will be re-run on the next build.
$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.autoconf || \
{ rm -f include/config/auto.conf; false; }
@# include/config.h has been updated after "make silentoldconfig".
@# We need to touch include/config/auto.conf so it gets newer
@# than include/config.h.
@# Otherwise, 'make silentoldconfig' would be invoked twice.
$(Q)touch include/config/auto.conf
#endif
其中(1)(2)两行尝试包含 auto.conf 和 auto.conf.cmd 这两个文件。由于使用 -include 进行声明,所以即使这两个文件不存在也不会报错退出,比如在第 1 次编译uboot,且已经配置好 .config 文件时,这两个文件还未生成。
假如我们已经编译好了xxx_deconfig 这个目标,那么我们会在 include/config 这个目录下看到 auto.conf 和 auto.conf.cmd 这两个文件。
从 include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd 这条规则可以知道,auto.conf 文件依赖于 $(KCONFIG_CONFIG) 和 include/config/auto.conf.cmd 。其中 $(KCONFIG_CONFIG) 变量的值就是 .config 这个配置文件。
3. 分析
# To avoid any implicit rule to kick in, define an empty command
$(KCONFIG_CONFIG) include/config/auto.conf.cmd: ;
现在仍然假设 auto.conf 和 auto.conf.cmd 还没有生成,那么由上面这条语句可以知道,该语句中的目标没有依赖,也没有生成它的规则命令,所以可想 GNU Make 本身无法生成 auto.conf.cmd 的。然后该条语句后面的一个分号表明,这两个目标被强制是最新的。所以下面标注的这几条命令得以执行:
#note: 顶层Makefile
include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
$(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig (1)
@# If the following part fails, include/config/auto.conf should be
@# deleted so "make silentoldconfig" will be re-run on the next build.
$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.autoconf || \
{ rm -f include/config/auto.conf; false; } (2)
@# include/config.h has been updated after "make silentoldconfig".
@# We need to touch include/config/auto.conf so it gets newer
@# than include/config.h.
@# Otherwise, 'make silentoldconfig' would be invoked twice.
$(Q)touch include/config/auto.conf (3)
3.1 语句 $ (Q) $(MAKE) -f $(srctree)/Makefile silentoldconfig
这里我们看到要生成一个目标 silentoldconfig ,这个目标定义在 scripts/kconfig/Makefile 中。因为这里使用 -f 选项重新指定了顶层 Makefile,而目标又是 silentoldconfig ,所以该命令最终会在顶层 Makefile 里执行:
#note: 顶层Makefile
# We need some generic definitions (do not try to remake the file).
scripts/Kbuild.includ