UBOOT版本：uboot2018.03，开发板myimx8mmek240。

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)=dirbuild := -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_basicscripts_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_basicscripts_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Makefileifneq ($(strip $(lib-y) $(lib-m) $(lib-)),) lib-target := $(obj)/lib.aendif

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Makefileifneq ($(strip $(obj-y) $(obj-m) $(obj-) $(subdir-m) $(lib-target)),) builtin-target := $(obj)/built-in.oendif

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MakefileKBUILD_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 intosubdir-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 Makefilekbuild-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-Bootprefix := tplsrc := $(patsubst $(prefix)/%,%,$(obj))ifeq ($(obj),$(src))prefix := splsrc := $(patsubst $(prefix)/%,%,$(obj))ifeq ($(obj),$(src))prefix := .endifendif

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/Makefilehostprogs-y := fixdepalways := $(hostprogs-y) # fixdep is needed to compile other host programs$(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.libalways := $(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.buildifneq ($(hostprogs-y)$(hostprogs-m),) include scripts/Makefile.hostendif

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 filehost-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)

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.includeifneq ($(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 statementsescsq = $(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) $(call flags,_hostcxx_flags)endifhostc_flags = -Wp,-MD,$(depfile) $(__hostc_flags)

在scripts/Kbuild.include中：

comma := ,dot-target = $(dir $@).$(notdir $@) //scripts/basic/.fixdepdepfile = $(subst $(comma),_,$(dot-target).d) //scripts/basic/.fixdep.dhostc_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MakefilePHONY += FORCEFORCE:

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/Makefileifdef KBUILD_KCONFIGKconfig := $(KBUILD_KCONFIG)elseKconfig := Kconfigendif %_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.oscripts/kconfig/conf --defconfig=arch/../configs/myimx8mmek240-8mm-2g_defconfig Kconfig## configuration written to .config#

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