PHONY :=
MAKE := makesrctree        := $(if $(KBUILD_SRC),$(KBUILD_SRC),$(CURDIR))
export srctree AS        = $(CROSS_COMPILE)as
LD        = $(CROSS_COMPILE)ld
CC        = $(CROSS_COMPILE)gcc
CPP        = $(CC) -E
AR        = $(CROSS_COMPILE)ar
NM        = $(CROSS_COMPILE)nmexport AS LD CC CPP AR NM include $(srctree)/scripts/Kbuild.include //该文件中定义有build变量

all: targetsrcdir := add sub main
PHONY += $(srcdir)objdir := $(srcdir)
PHONY += $(objdir)target: $(srcdir) //总目标的依赖是srcdirgcc -o target add/built-in.o sub/built-in.o main/built-in.o  //定义总目标的生成规则

$(srcdir):  //通过srcdir依次生成各个子目录中的*/built-in.o$(MAKE) $(build)=$@ clean-dirs := $(addprefix _clean_,$(objdir)) //clean-dirs其实与objdir中的目录是一样的，只是为了执行不同的命令（clean-dirs要删除目标文件），所以要clean-dirs的前边要加上前缀以与srcdir=objdir的目标区别
PHONY += $(clean-dirs) clean$(clean-dirs):$(MAKE) $(clean)=$(patsubst _clean_%,%,$@)clean: $(clean-dirs)rm -f targetclean := -f scripts/Makefile.clean obj.PHONY: $(PHONY)

# ==========================================================================
# Building
# ==========================================================================src := $(obj)PHONY := __build
__build: //Makefile.build中的总目标

include scripts/Kbuild.include# 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)  //包含子目录中的Makefile

include scripts/Makefile.libifneq ($(strip $(obj-y)),)
builtin-target := $(obj)/built-in.o
endif__build: $(builtin-target)# If the list of objects to link is empty, just create an empty built-in.o
cmd_link_o_target = $(if $(strip $(obj-y)),\$(LD) $(ld_flags) -r -o $@ $(filter $(obj-y), $^))$(builtin-target): $(obj-y)$(call if_changed,link_o_target) //调用生成库built-in.o的函数

cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $<define rule_cc_o_c$(cmd_cc_o_c)
endef# Built-in and composite module parts
$(obj)/%.o: $(src)/%.c $(call if_changed_rule,cc_o_c) //调用生成目标文件.o的函数

PHONY += FORCE
FORCE:.PHONY: $(PHONY)

build := -f scripts/Makefile.build objif_changed = $(cmd_$(1))//定义函数if_changed                                 # Usage: $(call if_changed_rule,foo)
# Will check if $(cmd_foo) or any of the prerequisites changed,
# and if so will execute $(rule_foo).
if_changed_rule = $(rule_$(1)) //定义函数if_changed_rule

c_flags        =        //定义gcc编译器选项ld_flags       = $(LDFLAGS) $(ldflags-y) //定义ld连接器选项

obj-y := $(patsubst %/, %/built-in.o, $(obj-y)) //对obj-y中的目录添加built-in.oobj-y := $(addprefix $(obj)/,$(obj-y))//再添加obj前缀

# ==========================================================================
# Cleaning up
# ==========================================================================src := $(obj)PHONY := __clean //此为Makefile.clean中的总目标

__clean: 　　 rm -f $(obj)/*.o  //删除目标文件.o

obj-y                += add.o