uboot顶层目录中的config.mk定义了确定了当前执行makefile所对应的源文件目录、目标文件目录，编译的程序编译、连接的选项，以及目标文件生成的规则等等。它被包含在顶层的makefile以及各个子目录的makefile中，可以说算是一个全局的makefile包含文件。
ifneq ($(OBJTREE),$(SRCTREE)) //判断目标目录和源文件目录是否一样，如果不一样下边的有效
ifeq ($(CURDIR),$(SRCTREE)) //判断当前目录是否是顶层目录
dir := //如果是，那么dir为空
else
dir := $(subst $(SRCTREE)/,,$(CURDIR)) //如果不是，那么dir为$(CURDIR)去掉$(SRCTREE)的部分
endif
obj := $(if $(dir),$(OBJTREE)/$(dir)/,$(OBJTREE)/) //如果dir不为空(当前目录不是顶层目录)，那么obj等于
//$(OBJTREE)/$(dir)/，否则(当前目录是顶层目录)等于$(OBJTREE)/
src := $(if $(dir),$(SRCTREE)/$(dir)/,$(SRCTREE)/) //如果dir不为空(当前目录不是顶层目录)，那么obj等于
//$(SRCTREE)/$(dir)/，否则(当前目录是顶层目录)等于$(SRCTREE)/
$(shell mkdir -p $(obj)) //创建obj目录
else
obj :=
src :=
endif
//想要了解如何实现编译生成的目标文件目录与源文件目录不同，可以参考笔者的这篇
//文章实现目标文件与源码分开的makefile测试实验，附有实验源码。
# clean the slate ... //先将这三个变量清零
PLATFORM_RELFLAGS =
PLATFORM_CPPFLAGS = //这个变量后边会被CPPFLAGS调用
PLATFORM_LDFLAGS =
#
# When cross-compiling on NetBSD, we have to define __PPC__ or else we
# will pick up a va_list declaration that is incompatible with the
# actual argument lists emitted by the compiler.
#
# [Tested on NetBSD/i386 1.5 + cross-powerpc-netbsd-1.3]
ifeq ($(ARCH),ppc)
ifeq ($(CROSS_COMPILE),powerpc-netbsd-)
PLATFORM_CPPFLAGS+= -D__PPC__
endif
ifeq ($(CROSS_COMPILE),powerpc-openbsd-)
PLATFORM_CPPFLAGS+= -D__PPC__
endif
endif
ifeq ($(ARCH),arm)
ifeq ($(CROSS_COMPILE),powerpc-netbsd-)
PLATFORM_CPPFLAGS+= -D__ARM__
endif
ifeq ($(CROSS_COMPILE),powerpc-openbsd-)
PLATFORM_CPPFLAGS+= -D__ARM__
endif
endif
ifeq ($(ARCH),blackfin)
PLATFORM_CPPFLAGS+= -D__BLACKFIN__ -mno-underscore
endif
//以上这部分，smdk2410定义的变量不满足上述条件，所以都无效
//以下这部分，根据make *_config生成的$(OBJTREE)/include/config.mk中相关变量的定义，选择适合开发板的config.mk
smdk2410生成的include/config.mk内容： ARCH = arm
CPU = arm920t
BOARD = smdk2410
SOC = s3c24x0
ifdef ARCH
sinclude $(TOPDIR)/$(ARCH)_config.mk # include architecture dependend rules
//此处包含$(TOPDIR)/$(ARCH)_config.mk；smdk2410的ARCH=ARM，所以包含源程序顶层目录中的arm_config.mk
arm_config.mk内容: PLATFORM_CPPFLAGS += -DCONFIG_ARM -D__ARM__
endif
ifdef CPU
sinclude $(TOPDIR)/cpu/$(CPU)/config.mk # include CPU specific rules
//此处包含$(TOPDIR)/cpu/$(CPU)/config.mk；smdk2410的CPU=arm920t，所以包含源程序顶层目录中的cpu/arm920t/config.mk
cpu/arm920t/config.mk内容： PLATFORM_RELFLAGS += -fno-strict-aliasing -fno-common -ffixed-r8 \
-msoft-float PLATFORM_CPPFLAGS += -march=armv4
# =========================================================================
#
# Supply options according to compiler version
#
# =========================================================================
PLATFORM_CPPFLAGS +=$(call cc-option,-mapcs-,-mabi=apcs-gnu)
PLATFORM_RELFLAGS +=$(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
endif
ifdef SOC
sinclude $(TOPDIR)/cpu/$(CPU)/$(SOC)/config.mk # include SoC specific rules
//此处包含$(TOPDIR)/cpu/$(CPU)/$(SOC)/config.mk；smdk2410的SOC=s3c24x0，所以包含源程序顶层目录中
//的cpu/arm920t/s3c24x0/config.mk，但这个目录中不存在config.mk文件，就忽略
endif
ifdef VENDOR //smdk2410的include/config.mk没有这个变量的定义
BOARDDIR = $(VENDOR)/$(BOARD)
else //所以else分支有效
BOARDDIR = $(BOARD) //BOARDDIR = $(BOARD) ；对于smdk2410，BOARDDIR = smdk2410
endif
ifdef BOARD
sinclude $(TOPDIR)/board/$(BOARDDIR)/config.mk # include board specific rules
//此处包含$(TOPDIR)/board/$(BOARDDIR)/config.mk；smdk2410的SOC=s3c24x0，所以
//包含源程序顶层目录中的board/smdk2410/config.mk
board/smdk2410/config.mk内容： TEXT_BASE = 0x33F80000
endif
#########################################################################
CONFIG_SHELL := $(shell if [ -x "$ $BASH" ]; then echo $ $BASH; \
//[ -x "$file" ] 判断$file是否存在且有可执行权限
else if [ -x /bin/bash ]; then echo /bin/bash; \
else echo sh; fi ; fi)
//定义CONFIG_SHELL变量，笔者用linux，此变量等于/bin/bash
ifeq ($(HOSTOS)-$(HOSTARCH),darwin-ppc)
//笔者用的笔记本cpu是酷睿双核，在虚拟机上搭建的linux操作系统，所以$(HOSTOS)-$(HOSTARCH)=linux-i386
HOSTCC = cc
else
HOSTCC = gcc
//else分支有效，HOSTCC = gcc
endif
HOSTCFLAGS = -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
HOSTSTRIP = strip
//定义了两个变量HOSTCFLAGS、HOSTSTRIP
#########################################################################
#
# Option checker (courtesy linux kernel) to ensure
# only supported compiler options are used
#
cc-option = $(shell if $(CC) $(CFLAGS) $(1) -S -o /dev/null -xc /dev/null \
> /dev/null 2>&1; then echo "$(1)"; else echo "$(2)"; fi ;)
//英文注释的意思好像是说检查编译选项对于当前编译器是否存在，确保仅仅只有支持的编译器选项被使用
#
# Include the make variables (CC, etc...)
#
AS = $(CROSS_COMPILE)as //定义arm-linux-as
LD = $(CROSS_COMPILE)ld //定义arm-linux-ld
CC = $(CROSS_COMPILE)gcc //定义编译所用的编译器，CROSS_COMPILE在顶层Makefile中已经定义，
//对于smdk2410开发板，这里是arm-linux-gcc
CPP = $(CC) -E //预处理后就停止，不进行编译
AR = $(CROSS_COMPILE)ar //定义arm-linux-ar
NM = $(CROSS_COMPILE)nm //定义arm-liux-nm
STRIP = $(CROSS_COMPILE)strip //定义arm-linux-strip
OBJCOPY = $(CROSS_COMPILE)objcopy //定义arm-linu-objcopy
OBJDUMP = $(CROSS_COMPILE)objdump //定义amr-linux-objdump
RANLIB = $(CROSS_COMPILE)RANLIB //定义arm-linux-RANLIB
ifneq (,$(findstring s,$(MAKEFLAGS))) //MAKEFLAGS是否包含s
ARFLAGS = cr //是
else
ARFLAGS = crv //不是，就在变量ARFLAGS上加v选项
endif
RELFLAGS= $(PLATFORM_RELFLAGS) //定义RELFLAGS
DBGFLAGS= -g # -DDEBUG //定义DBGFLAGS，即编译选项
OPTFLAGS= -Os #-fomit-frame-pointer //定义OPTFLAGS
//以下这部分是确定连接脚本
ifndef LDSCRIPT //没有定义变量 LDSCRIPT
#LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot.lds.debug
ifeq ($(CONFIG_NAND_U_BOOT),y) //也没有定义CONFIG_NAND_U_BOOT
LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot-nand.lds
else //所以分支有效
LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot.lds
//连接脚本是$(TOPDIR)/board/$(BOARDDIR)/u-boot.lds ；对于smdk2410，连接脚本是board/smdk2410/u-boot.lds
endif
endif
OBJCFLAGS += --gap-fill=0xff //定义变量OBJCFLAGS
gccincdir := $(shell $(CC) -print-file-name=include) //定义变量gccincdir
CPPFLAGS := $(DBGFLAGS) $(OPTFLAGS) $(RELFLAGS) \ //定义变量CPPFLAGS
-D__KERNEL__ -DTEXT_BASE=$(TEXT_BASE) \
ifneq ($(OBJTREE),$(SRCTREE)) //判断目标目录与源文件目录是否相同
CPPFLAGS += -I$(OBJTREE)/include2 -I$(OBJTREE)/include
//不相同CPPFLAGS要加选项 -I$(OBJTREE)/include2 -I$(OBJTREE)/include
endif
CPPFLAGS += -I$(TOPDIR)/include //相同CPPFLAGS要加选项 -I$(TOPDIR)/include
CPPFLAGS += -fno-builtin -ffreestanding -nostdinc \
-isystem $(gccincdir) -pipe $(PLATFORM_CPPFLAGS) //CPPFLAGS还要再加一些选项
ifdef BUILD_TAG //如果定义了BUILD_TAG ，smdk2410没有定义
CFLAGS := $(CPPFLAGS) -Wall -Wstrict-prototypes \
-DBUILD_TAG='"$(BUILD_TAG)"'
else //所以分支有效
CFLAGS := $(CPPFLAGS) -Wall -Wstrict-prototypes //定义变量CFLAGS
endif
# avoid trigraph warnings while parsing pci.h (produced by NIOS gcc-2.9)
# this option have to be placed behind -Wall -- that's why it is here
ifeq ($(ARCH),nios) //cpu架构不是nios，此部分无效
ifeq ($(findstring 2.9,$(shell $(CC) --version)),2.9)
CFLAGS := $(CPPFLAGS) -Wall -Wno-trigraphs
endif
endif
# $(CPPFLAGS) sets -g, which causes gcc to pass a suitable -g<format>
# option to the assembler.
AFLAGS_DEBUG := //定义变量AFLAGS_DEBUG，好像是汇编编译调试选项
# turn jbsr into jsr for m68k
ifeq ($(ARCH),m68k) //cpu架构不是m68k，此部分无效
ifeq ($(findstring 3.4,$(shell $(CC) --version)),3.4)
AFLAGS_DEBUG := -Wa,-gstabs,-S
endif
endif
AFLAGS := $(AFLAGS_DEBUG) -D__ASSEMBLY__ $(CPPFLAGS) //定义变量AFLAGS
LDFLAGS += -Bstatic -T $(LDSCRIPT) -Ttext $(TEXT_BASE) $(PLATFORM_LDFLAGS) //定义连接选项LDFLAGS
# Location of a usable BFD library, where we define "usable" as
# "built for ${HOST}, supports ${TARGET}". Sensible values are
# - When cross-compiling: the root of the cross-environment
# - Linux/ppc (native): /usr
# - NetBSD/ppc (native): you lose ... (must extract these from the
# binutils build directory, plus the native and U-Boot include
# files don't like each other)
#
# So far, this is used only by tools/gdb/Makefile.
ifeq ($(HOSTOS)-$(HOSTARCH),darwin-ppc) //$(HOSTOS)-$(HOSTARCH)=linux-i386，此部分无效
BFD_ROOT_DIR = /usr/local/tools
else
ifeq ($(HOSTARCH),$(ARCH)) //如果主机架构与目标板cpu架构相同；但是显然不一样，此部分无效
# native
BFD_ROOT_DIR = /usr
else
#BFD_ROOT_DIR = /LinuxPPC/CDK # Linux/i386
#BFD_ROOT_DIR = /usr/pkg/cross # NetBSD/i386
BFD_ROOT_DIR = /opt/powerpc //分支有效，定义变量BFD_ROOT_DIR = /opt/powerpc
endif
endif
ifeq ($(PCI_CLOCK),PCI_66M) //没有定义变量PCI_CLOCK，此部分无效
CFLAGS := $(CFLAGS) -DPCI_66M
endif
#########################################################################
export CONFIG_SHELL HPATH HOSTCC HOSTCFLAGS CROSS_COMPILE \
AS LD CC CPP AR NM STRIP OBJCOPY OBJDUMP \
MAKE
export TEXT_BASE PLATFORM_CPPFLAGS PLATFORM_RELFLAGS CPPFLAGS CFLAGS AFLAGS
//将一些列编译、连接、生成库等等的选项导出
#########################################################################
ifndef REMOTE_BUILD //看是否目标目录与源码目录不同，REMOTE_BUILD在顶层目录Makefile中定义
//目标目录与源码目录相同
%.s: %.S
$(CPP) $(AFLAGS) -o $@ $<
%.o: %.S
$(CC) $(AFLAGS) -c -o $@ $<
%.o: %.c
$(CC) $(CFLAGS) -c -o $@ $<
//定义目标文件（*.o）以及中间文件（*.s）的生成规则
else
//目标目录与源码目录不相同
$(obj)%.s: %.S
$(CPP) $(AFLAGS) -o $@ $<
$(obj)%.o: %.S
$(CC) $(AFLAGS) -c -o $@ $<
$(obj)%.o: %.c
$(CC) $(CFLAGS) -c -o $@ $<
endif
//定义目标文件（*.o）以及中间文件（*.s）的生成规则
#########################################################################
最后，附上几个比较重要变量的打印值
AFLAGS:
-D__ASSEMBLY__ -g -Os -fno-strict-aliasing -fno-common -ffixed-r8 -msoft-float -malignment-traps -D__KERNEL__ -DTEXT_BASE=0x33F80000 -I/work/u-boot-1.1./include -fno-builtin -ffreestanding -nostdinc -isystem /opt/EmbedSky/crosstools_3..5_softfloat/gcc-3.4.-glibc-2.3./arm-linux/lib/gcc/arm-linux/3.4./include -pipe -DCONFIG_ARM -D__ARM__ -march=armv4 -mapcs-
CFLAGS:
-g -Os -fno-strict-aliasing -fno-common -ffixed-r8 -msoft-float -malignment-traps -D__KERNEL__ -DTEXT_BASE=0x33F80000 -I/work/u-boot-1.1./include -fno-builtin -ffreestanding -nostdinc -isystem /opt/EmbedSky/crosstools_3..5_softfloat/gcc-3.4.-glibc-2.3./arm-linux/lib/gcc/arm-linux/3.4./include -pipe -DCONFIG_ARM -D__ARM__ -march=armv4 -mapcs- -Wall -Wstrict-prototypes