make xxx_deconfig过程详解
UBOOT版本：uboot2018.03，开发板myimx8mmek240。
2. 概述
Ubootb编译第一步通常是执行make xxx_config，在编译指定顶层目录生成.config文件，这种方式要求厂商提供一个基础的xxx_config文件(通常来说开发者不会通过执行make menuconfig从零开始配置，这个工作过量太大了)。本文接下来的章节主要解析这条指令背后主要做了什么。我是用的开发板执行命令为：make myimx8mmek240-8mm-2g_defconfig
3. build变量的定义
在scripts/Kbuild.include 中定义：
###
# Shorthand for $(Q)$(MAKE) -f scripts/Makefile.build obj=
# Usage:
# $(Q)$(MAKE) $(build)=dir
build := -f $(srctree)/scripts/Makefile.build obj
4. 目标%config的定义
在顶层Makefile中定义：
# We need some generic definitions (do not try to remake the file).
scripts/Kbuild.include: ;
include scripts/Kbuild.include //注意这个引用
......
config: scripts_basic outputmakefile FORCE
$(Q)$(MAKE) $(build)=scripts/kconfig $@
%config: scripts_basic outputmakefile FORCE
$(Q)$(MAKE) $(build)=scripts/kconfig $@
4.1 依赖 scripts_basic
（参考：linux内核Makefile中的变量build— 过渡篇（五））
# 顶层Makefile
# Basic helpers built in scripts/
PHONY += scripts_basic
scripts_basic:
$(Q)$(MAKE) $(build)=scripts/basic
$(Q)rm -f .tmp_quiet_recordmcount
展开变量build
# 顶层Makefile
# Basic helpers built in scripts/
PHONY += scripts_basic
scripts_basic:
$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.build obj=scripts/basic
$(Q)rm -f .tmp_quiet_recordmcount
make -f $(srctree)/scripts/Makefile.build obj=scripts/basic的解析如下：这是一种不指定目标的情况，由于未指定目标，这时会使用Makefile.build中的默认目标__build。然后更进一步，会使用$(obj)/Makefile(scripts/basic/Makefile)中定义的变量来进行目标匹配。
__build在Makefile.build中的构建规则为：
__build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) \
$(if $(KBUILD_MODULES),$(obj-m) $(modorder-target)) \
$(subdir-ym) $(always)
@:
4.1.1 语句$(if $ (KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y))
在顶层Makefile中，KBUILD_BUILTIN的定义如下：
# note：顶层Makefile
KBUILD_BUILTIN := 1
export KBUILD_MODULES KBUILD_BUILTIN
该语句展开为：
$(builtin-target) $(lib-target) $(extra-y)
（1）lib-target
# note：顶层Makefile
ifneq ($(strip $(lib-y) $(lib-m) $(lib-)),)
lib-target := $(obj)/lib.a
endif
在此语句之前obj-y := ;obj-m := ;obj-未定义。因此lib-target为空。
（2）builtin-target
# note：顶层Makefile
ifneq ($(strip $(obj-y) $(obj-m) $(obj-) $(subdir-m) $(lib-target)),)
builtin-target := $(obj)/built-in.o
endif
在此语句之前obj-y := ; obj-m := ;obj-未定义 ; subdir-m := ;并且在所包含的文件中也没有给这些变量增加值。lib-target 为空。因此builtin-target为空。
（3）extra-y未定义
综上：语句$(if $ (KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y))为空。
4.1.2 语句 $(if $ (KBUILD_MODULES),$(obj-m) $(modorder-target))*
在顶层Makefile中，KBUILD_BUILTIN的定义如下：
# note：顶层Makefile
KBUILD_MODULES :=
综上：语句$(if $ (KBUILD_MODULES),$(obj-m) $(modorder-target))为空。
4.1.3 $(subdir-ym)
在scripts/Makefile.lib中，subdir-ym的定义如下：
# note：scripts/Makefile.lib
# Subdirectories we need to descend into
subdir-ym := $(sort $(subdir-y) $(subdir-m))
......
subdir-ym := $(addprefix $(obj)/,$(subdir-ym))
subdir-y与subdir-m都为空。
综上：语句$(subdir-ym)为空。
4.1.4 $(always) 重点关注
在scripts/Makefile.build有如下定义
# note：scripts/Makefile.build
# The filename Kbuild has precedence over Makefile
kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))
kbuild-file := $(if $(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Kbuild,$(kbuild-dir)/Makefile)
include $(kbuild-file)
4.1.4.1 src的定义
在scripts/Makefile.build有如下定义
# note：scripts/Makefile.build
# Modified for U-Boot
prefix := tpl
src := $(patsubst $(prefix)/%,%,$(obj))
ifeq ($(obj),$(src))
prefix := spl
src := $(patsubst $(prefix)/%,%,$(obj))
ifeq ($(obj),$(src))
prefix := .
endif
endif
在命令make -f ./scripts/Makefile.build obj=scripts/basic我们传入了obj=scripts/basic
所以src = obj = scripts/basic, prefix := .
4.1.4.2 kbuild-dir的定义
在scripts/Makefile.build有如下定义
kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))
所以 kbuild-dir = ./scripts/basic
4.1.4.3 kbuild-file的定义
在scripts/Makefile.build有如下定义
kbuild-file := $(if $(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Kbuild,$(kbuild-dir)/Makefile)
所以kbuild-file = ./scripts/basic/Makefile
所以include $(kbuild-file)即为include ./scripts/basic/Makefile
在./scripts/basic/Makefile中：
# note：scripts/basic/Makefile
hostprogs-y := fixdep
always := $(hostprogs-y)
# fixdep is needed to compile other host programs
$(addprefix $(obj)/,$(filter-out fixdep,$(always))): $(obj)/fixdep
所以always = fixdep
4.1.4.4 又在Makefile.build中包含include scripts/Makefile.lib
而在scripts/Makefile.lib中
# note：scripts/Makefile.lib
always := $(addprefix $(obj)/,$(always))
所以最终 always = scripts/basic/fixdep 。
故__build 展开如下：
__build: scripts/basic/fixdep
@:
1. 在Makefile.build中有如下定义：
# note：scripts/Makefile.build
ifneq ($(hostprogs-y)$(hostprogs-m),)
include scripts/Makefile.host
endif
在./scripts/basic/Makefile中hostprogs-y = fixdep, 所以scripts/Makefile.host被包含进Makefile.build;
在scripts/Makefile.host中
# note：scripts/Makefile.host
__hostprogs := $(sort $(hostprogs-y) $(hostprogs-m))
hostprogs-m为空，所以__hostprogs = fixdep。
在scripts/Makefile.host中
# note：scripts/Makefile.host
# C code
# Executables compiled from a single .c file
host-csingle := $(foreach m,$(__hostprogs), \
$(if $($(m)-objs)$($(m)-cxxobjs)$($(m)-sharedobjs),,$(m)))
......
host-csingle := $(addprefix $(obj)/,$(host-csingle))
因为fixdep-objs与fixdep-cxxobjs都不存在，所以host-csingle = fixdep； 又 host-csingle := $ (addprefix $ (obj)/,$(host-csingle))，所以host-csingle = scripts/basic/fixdep。
host-csingle规则如下：
# note：scripts/Makefile.host
$(host-csingle): $(obj)/%: $(src)/%.c FORCE
$(call if_changed_dep,host-csingle)
等价于：
scripts/basic/fixdep:scripts/basic/fixdep.c FORCE
$(call if_changed_dep,host-csingle)
2. if_changed_dep在scripts/Kbuild.include中定义
# note：scripts/Kbuild.include
# Execute the command and also postprocess generated .d dependencies file.
if_changed_dep = $(if $(strip $(any-prereq) $(arg-check) ), \
@set -e; \
$(echo-cmd) $(cmd_$(1)); \
scripts/basic/fixdep $(depfile) $@ '$(make-cmd)' > $(dot-target).tmp;\
rm -f $(depfile); \
mv -f $(dot-target).tmp $(dot-target).cmd)
2.1 $(strip $(any-prereq) $(arg-check) )
(1) any-prereq在scripts/Kbuild.include中定义
# note：scripts/Kbuild.include
any-prereq = $(filter-out $(PHONY),$?) $(filter-out $(PHONY) $(wildcard $^),$^)
$ ? 表示所有比目标还要新的依赖文件;$ ^ 表示所有的依赖文件，$(filter-out $ (PHONY), $?)就是过滤到比目标还要新的依赖文件中的伪目标，即为 scripts/basic/fixdep.c， $ (filter-out $ (PHONY) $ (wildcard $ ^ ), $^)表示过滤掉所有的依赖文件中的伪目标与存在的依赖文件，这里为空，所以any-prereq = scripts/basic/fixdep.c。
(2) arg-check在scripts/Kbuild.include中定义：
# note：scripts/Kbuild.include
ifneq ($(KBUILD_NOCMDDEP),1)
arg-check = $(strip $(filter-out $(cmd_$(1)), $(cmd_$@)) \
$(filter-out $(cmd_$@), $(cmd_$(1))) )
else
arg-check = $(if $(strip $(cmd_$@)),,1)
endif
KBUILD_NOCMDDEP是在make命令行中定义，我们并没有定义，所以：
arg-check = $(strip $(filter-out $(cmd_$(1)), $(cmd_$@)) $(filter-out $(cmd_$@), $(cmd_$(1))) )
<1> $ (filter-out $ (cmd_ $ (1)), $ (cmd_ $@)) 表示过滤掉 $(cmd_ $@)中符合 $(cmd_ $(1))的项。 $(1)表示if_changed_dep函数的第一个参数host-csingle， $@表示目标文件scripts/basic/fixdep。
<2> cmd_scripts/basic/fixdep并没有定义，所以 $(filter-out $(cmd_ $(1)), $(cmd_ $@))为空；
<3> cmd_host-csingle 在Makefile.host中定义：
cmd_host-csingle = $(HOSTCC) $(hostc_flags) -o $@ $< $(HOST_LOADLIBES) $(HOSTLOADLIBES_$(@F))
所以arg-check = $ (filter-out $ (cmd_$ @), $ (cmd_$ (1))) = $ (HOSTCC) $ (hostc_flags) -o $@ $< $(HOST_LOADLIBES) $(HOSTLOADLIBES_ $(@F))
$(any-prereq) $(arg-check)都为非空，所以：
if_changed_dep = @set -e; \ /如果任何语句的执行结果不是true则应该退出
$(echo-cmd) $(cmd_$(1)); \
scripts/basic/fixdep $(depfile) $@ '$(make-cmd)' > $(dot-target).tmp;\
rm -f $(depfile); \
mv -f $(dot-target).tmp $(dot-target).cmd)
2.2 $(echo-cmd) $(cmd_ $ (1))等价于$(echo-cmd) $(cmd_host-csingle)
echo-cmd = $(if $( $(quiet)cmd_$(1)),echo ' $(call escsq, $( $(quiet)cmd_ $(1))) $(echo-why)';）
quiet=quiet_，在顶层Makefile分析过(当然如果你想看到更详细的打印，您可以通过传入V值，来改变)， $(cmd_host-csingle)上面分析过，存在，所以：
echo-cmd = echo ' $(call escsq,$(cmd_host-csingle))$(echo-why)';
在scripts/Kbuild.include中：
# Escape single quote for use in echo statements
escsq = $(subst $(squote),'\$(squote)',$1)
ifeq ($(KBUILD_VERBOSE),2)
why = \
$(if $(filter $@, $(PHONY)),- due to target is PHONY, \
$(if $(wildcard $@), \
$(if $(strip $(any-prereq)),- due to: $(any-prereq), \
$(if $(arg-check), \
$(if $(cmd_$@),- due to command line change, \
$(if $(filter $@, $(targets)), \
- due to missing .cmd file, \
- due to $(notdir $@) not in $$(targets) \
) \
) \
) \
), \
- due to target missing \
) \
)
echo-why = $(call escsq, $(strip $(why)))
endif
KBUILD_VERBOSE一般我们会采用默认值0(需要调试编译除外)，所以 echo-why 为空。
quiet_cmd_host-csingle = HOSTCC $@ //用来打印
cmd_host-csingle = '$(HOSTCC) $(hostc_flags) -o $@ $< $(HOST_LOADLIBES) $(HOSTLOADLIBES_$(@F))'
$(HOSTCC)为cc，此处不再深入解释，hostc_flags在Makefile.host中定义：
#####
# Handle options to gcc. Support building with separate output directory
_hostc_flags = $(HOSTCFLAGS) $(HOST_EXTRACFLAGS) \
$(HOSTCFLAGS_$(basetarget).o) //-Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
_hostcxx_flags = $(HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
$(HOSTCXXFLAGS_$(basetarget).o)
ifeq ($(KBUILD_SRC),) //KBUILD_SRC在make命令行定义，此处未定义
__hostc_flags = $(_hostc_flags)
__hostcxx_flags = $(_hostcxx_flags)
else
__hostc_flags = -I$(obj) $(call flags,_hostc_flags)
__hostcxx_flags = -I$(obj) $(call flags,_hostcxx_flags)
endif
hostc_flags = -Wp,-MD,$(depfile) $(__hostc_flags)
在scripts/Kbuild.include中：
comma := ,
dot-target = $(dir $@).$(notdir $@) //scripts/basic/.fixdep
depfile = $(subst $(comma),_,$(dot-target).d) //scripts/basic/.fixdep.d
hostc_flags = -Wp,-MD,scripts/basic/.fixdep.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
综上
cmd_host-csingle = cc -Wp,-MD,scripts/basic/.fixdep.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -o scripts/basic/fixdep scripts/basic/fixdep.c
所以echo-cmd 的作用是打印quiet_cmd_host-csingle（HOSTCC $ @ ）或者cmd_host-csingle（根据顶层MakefileV值决定），$ (cmd_$(1))即为执行cmd_host-csingle生成fixdep同时生成fixdep的依赖文件.fixdep.d
（2.3） scripts/basic/fixdep $(depfile) $@ ’ $(make-cmd)’ > $(dot-target).tmp
等价于：scripts/basic/fixdep scripts/basic/.fixdep.d scripts/basic/fixdep ‘cc -Wp,-MD,scripts/basic/.fixdep.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -o scripts/basic/fixdep scripts/basic/fixdep.c’ > scripts/basic/.fixdep.tmp
（2.4） rm -f $(depfile)
删除scripts/basic/.fixdep.d
（2.5） mv -f $(dot-target).tmp $(dot-target).cmd)
将scripts/basic/.fixdep.tmp重命名为scripts/basic/.fixdep.cmd
总结：生成scripts/basic/fixdep的过程中会先打印cc -Wp,-MD,scripts/basic/.fixdep.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -o scripts/basic/fixdep scripts/basic/fixdep.c同时执行该语句生成fixdep，再用fixdep生成.fixdep.cmd
4.2 依赖 outputmakefile
# 顶层Makefile
# outputmakefile generates a Makefile in the output directory, if using a
# separate output directory. This allows convenient use of make in the
# output directory.
outputmakefile:
ifneq ($(KBUILD_SRC),)
$(Q)ln -fsn $(srctree) source
$(Q)$(CONFIG_SHELL) $(srctree)/scripts/mkmakefile \
$(srctree) $(objtree) $(VERSION) $(PATCHLEVEL)
endif
上面批注已经说的很清楚了，outputmakefile 在输出目录中生成一个 Makefile，如果使用单独的输出目录。 这允许在输出目录中方便地使用 make。当KBUILD_SRC不为空时，才会编译到这里。
4.3 依赖 FORCE
# 顶层Makefile
PHONY += FORCE
FORCE:
实际上它是一个伪目标，从上面看到，FORCE 既没有依赖的规则，其底下也没有可执行的命令。如果一个规则没有命令或者依赖，而且它的目标不是一个存在的文件名，在执行此规则时，目标总会被认为是最新的。也就是说，这个规则一旦被执行，make 就认为它所表示的目标已经被更新过。当将这样的目标(FORCE)作为一个规则的依赖时(如上)，由于依赖总被认为是被更新过的，所以作为依赖所在的规则定义的命令总会被执行。
4.4 规则 $ (Q)$(MAKE) $(build)=scripts/kconfig $@
等价于：make -f $(srctree)/scripts/Makefile.build obj=scripts/kconfig myimx8mmek240-8mm-2g_defconfig
在scripts/kconfig/Makefile中(至于为什么会引用scripts/kconfig/Makefile，参见4.1小节)
# note：scripts/kconfig/Makefile
ifdef KBUILD_KCONFIG
Kconfig := $(KBUILD_KCONFIG)
else
Kconfig := Kconfig
endif
%_defconfig: $(obj)/conf
$(Q)$< $(silent) --defconfig=arch/$(SRCARCH)/configs/$@ $(Kconfig) //$< = $(obj)/conf
# Added for U-Boot (backward compatibility)
%_config: %_defconfig
@:
编译的流程为：
（1）先编译scripts/kconfig/conf可执行文件；
（2）再执行scripts/kconfig/conf --defconfig=arch/…/configs/myimx8mmek240-8mm-2g_defconfig Kconfig语句
编译打印如下
make -f ./scripts/Makefile.build obj=scripts/kconfig myimx8mmek240-8mm-2g_defconfig
cc -Wp,-MD,scripts/kconfig/.conf.o.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -I/usr/include/ncursesw -DCURSES_LOC="<curses.h>" -DNCURSES_WIDECHAR=1 -DLOCALE -c -o scripts/kconfig/conf.o scripts/kconfig/conf.c
cat scripts/kconfig/zconf.tab.c_shipped > scripts/kconfig/zconf.tab.c
cat scripts/kconfig/zconf.lex.c_shipped > scripts/kconfig/zconf.lex.c
cat scripts/kconfig/zconf.hash.c_shipped > scripts/kconfig/zconf.hash.c
cc -Wp,-MD,scripts/kconfig/.zconf.tab.o.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -I/usr/include/ncursesw -DCURSES_LOC="<curses.h>" -DNCURSES_WIDECHAR=1 -DLOCALE -Iscripts/kconfig -c -o scripts/kconfig/zconf.tab.o scripts/kconfig/zconf.tab.c
cc -o scripts/kconfig/conf scripts/kconfig/conf.o scripts/kconfig/zconf.tab.o
scripts/kconfig/conf --defconfig=arch/../configs/myimx8mmek240-8mm-2g_defconfig Kconfig
#
# configuration written to .config
#
5 总结
经过前面的分析可知：当执行make xxx_deconfig时，最终会执行以下两个语句：
（1）make -f $(srctree)/scripts/Makefile.build obj=scripts/basic
（2）make -f ./scripts/Makefile.build obj=scripts/kconfig xxx_deconfig