VERSION = 1
PATCHLEVEL = 3
SUBLEVEL = 4
EXTRAVERSION =
U_BOOT_VERSION = $(VERSION).$(PATCHLEVEL).$(SUBLEVEL)$(EXTRAVERSION)
uboot的版本号定义
VERSION_FILE = $(obj)include/version_autogenerated.h
这个obj定义在后面，文件是编译的时候自动生成的。
生成的内容大致为#define U_BOOT_VERSION "U-Boot 1.3.4"
HOSTARCH := $(shell uname -m |
sed -e s/i.86/i386/
-e s/sun4u/sparc64/
-e s/arm.*/arm/
-e s/sa110/arm/
-e s/powerpc/ppc/
-e s/ppc64/ppc/
-e s/macppc/ppc/)
HOSTOS := $(shell uname -s | tr '[:upper:]' '[:lower:]' |
sed -e 's/(cygwin).*/cygwin/')
export HOSTARCH HOSTOS
$(shell xxx)这个变量指用shell去执行xxx命令。|是管道，把前面的输出作为后面的输入。uname -m会得到当前计算机的cpu的版本号信息，如i686，i786之类的。sed -e s/i.86/i386/用正则表达式替换，如果存在i+任意字符+86就替换成i386，后面的同理。HOSTARCH就是编译环境（主机）的计算机的架构。
HOSTOS就是主机的操作系统，得出来的是小写。
# Allow for silent builds
ifeq (,$(findstring s,$(MAKEFLAGS)))
XECHO = echo
else
XECHO = :
endif
这段时静默编译，编译不输出
ifdef O
ifeq ("$(origin O)", "command line")
BUILD_DIR := $(O)
endif
endif
ifneq ($(BUILD_DIR),)
saved-output := $(BUILD_DIR)
# Attempt to create a output directory.
$(shell [ -d ${BUILD_DIR} ] || mkdir -p ${BUILD_DIR})
# Verify if it was successful.
BUILD_DIR := $(shell cd $(BUILD_DIR) && /bin/pwd)
$(if $(BUILD_DIR),,$(error output directory "$(saved-output)" does not exist))
endif # ifneq ($(BUILD_DIR),)
OBJTREE := $(if $(BUILD_DIR),$(BUILD_DIR),$(CURDIR))
SRCTREE := $(CURDIR)
TOPDIR := $(SRCTREE)
LNDIR := $(OBJTREE)
export TOPDIR SRCTREE OBJTREE
MKCONFIG := $(SRCTREE)/mkconfig
export MKCONFIG
ifneq ($(OBJTREE),$(SRCTREE))
REMOTE_BUILD := 1
export REMOTE_BUILD
endif
# $(obj) and (src) are defined in config.mk but here in main Makefile
# we also need them before config.mk is included which is the case for
# some targets like unconfig, clean, clobber, distclean, etc.
ifneq ($(OBJTREE),$(SRCTREE))
obj := $(OBJTREE)/
src := $(SRCTREE)/
else
obj :=
src :=
endif
export obj src
# Make sure CDPATH settings don't interfere
unexport CDPATH
定义输出目录 make O=输出目录
OBJTREE编译出的.O文件存放目录
SRCTREE源代码存放的目录
MKCONFIG 源码根目录下的config，是个脚本，后面会用到
ifeq ($(obj)include/config.mk,$(wildcard $(obj)include/config.mk))
#到处make config时间段的config文件的变量作为环境变量
# load ARCH, BOARD, and CPU configuration
include $(obj)include/config.mk
export ARCH CPU BOARD VENDOR SOC
#CROSS_COMPILE定义交叉工具链的前缀
ifndef CROSS_COMPILE
ifeq ($(HOSTARCH),$(ARCH))
CROSS_COMPILE =
else
ifeq ($(ARCH),ppc)
CROSS_COMPILE = ppc_8xx-
endif
ifeq ($(ARCH),arm)
#CROSS_COMPILE = arm-linux-
#CROSS_COMPILE = /usr/local/arm/4.4.1-eabi-cortex-a8/usr/bin/arm-linux-
#CROSS_COMPILE = /usr/local/arm/4.2.2-eabi/usr/bin/arm-linux-
CROSS_COMPILE = /usr/local/arm/arm-2009q3/bin/arm-none-linux-gnueabi-
endif
ifeq ($(ARCH),i386)
CROSS_COMPILE = i386-linux-
endif
ifeq ($(ARCH),mips)
CROSS_COMPILE = mips_4KC-
endif
ifeq ($(ARCH),nios)
CROSS_COMPILE = nios-elf-
endif
ifeq ($(ARCH),nios2)
CROSS_COMPILE = nios2-elf-
endif
ifeq ($(ARCH),m68k)
CROSS_COMPILE = m68k-elf-
endif
ifeq ($(ARCH),microblaze)
CROSS_COMPILE = mb-
endif
ifeq ($(ARCH),blackfin)
CROSS_COMPILE = bfin-uclinux-
endif
ifeq ($(ARCH),avr32)
CROSS_COMPILE = avr32-linux-
endif
ifeq ($(ARCH),sh)
CROSS_COMPILE = sh4-linux-
endif
ifeq ($(ARCH),sparc)
CROSS_COMPILE = sparc-elf-
endif # sparc
endif # HOSTARCH,ARCH
endif # CROSS_COMPILE
export CROSS_COMPILE
定义交叉工具链的前缀
# load other configuration
include $(TOPDIR)/config.mk
这个文件定义了很多变量，各个开发板的可移植就靠这个
它定义了很多cpu架构的专有config 定义了交叉工具链 以及autoconfig的CONFIG_xxxxx的配置过程中生成的宏，指导uboot编译过程。原材料是include/configs/xxx.h 里面都是宏定义，方便移植。移植的很多工作都是对这个进行配置，board/sansung/x210下还生成了一个config.mk，定义了TEXT_BASE=0xc3e00000，指定链接地址（还取决于uboot中做的虚拟地址映射）。
OBJS = cpu/$(CPU)/start.o
ifeq ($(CPU),i386)
OBJS += cpu/$(CPU)/start16.o
OBJS += cpu/$(CPU)/reset.o
endif
ifeq ($(CPU),ppc4xx)
OBJS += cpu/$(CPU)/resetvec.o
endif
ifeq ($(CPU),mpc85xx)
OBJS += cpu/$(CPU)/resetvec.o
endif
OBJS := $(addprefix $(obj),$(OBJS))
LIBS = lib_generic/libgeneric.a
LIBS += $(shell if [ -f board/$(VENDOR)/common/Makefile ]; then echo
"board/$(VENDOR)/common/lib$(VENDOR).a"; fi)
LIBS += cpu/$(CPU)/lib$(CPU).a
ifdef SOC
LIBS += cpu/$(CPU)/$(SOC)/lib$(SOC).a
endif
ifeq ($(CPU),ixp)
LIBS += cpu/ixp/npe/libnpe.a
endif
LIBS += lib_$(ARCH)/lib$(ARCH).a
LIBS += fs/cramfs/libcramfs.a fs/fat/libfat.a fs/fdos/libfdos.a fs/jffs2/libjffs2.a
fs/reiserfs/libreiserfs.a fs/ext2/libext2fs.a
LIBS += net/libnet.a
LIBS += disk/libdisk.a
LIBS += drivers/bios_emulator/libatibiosemu.a
LIBS += drivers/block/libblock.a
LIBS += drivers/dma/libdma.a
LIBS += drivers/hwmon/libhwmon.a
LIBS += drivers/i2c/libi2c.a
LIBS += drivers/input/libinput.a
LIBS += drivers/misc/libmisc.a
LIBS += drivers/mmc/libmmc.a
LIBS += drivers/mtd/libmtd.a
LIBS += drivers/mtd/nand/libnand.a
LIBS += drivers/mtd/nand_legacy/libnand_legacy.a
LIBS += drivers/mtd/onenand/libonenand.a
LIBS += drivers/mtd/ubi/libubi.a
LIBS += drivers/mtd/spi/libspi_flash.a
LIBS += drivers/net/libnet.a
LIBS += drivers/net/sk98lin/libsk98lin.a
LIBS += drivers/pci/libpci.a
LIBS += drivers/pcmcia/libpcmcia.a
LIBS += drivers/spi/libspi.a
ifeq ($(CPU),mpc83xx)
LIBS += drivers/qe/qe.a
endif
ifeq ($(CPU),mpc85xx)
LIBS += drivers/qe/qe.a
endif
LIBS += drivers/rtc/librtc.a
LIBS += drivers/serial/libserial.a
LIBS += drivers/usb/libusb.a
LIBS += drivers/video/libvideo.a
LIBS += common/libcommon.a
LIBS += libfdt/libfdt.a
LIBS += api/libapi.a
LIBS += post/libpost.a
LIBS := $(addprefix $(obj),$(LIBS))
.PHONY : $(LIBS) $(VERSION_FILE)
LIBBOARD = board/$(BOARDDIR)/lib$(BOARD).a
LIBBOARD := $(addprefix $(obj),$(LIBBOARD))
OBJS定义了所有要编译的文件的集合 LIBS定义了库
ALL += $(obj)u-boot.srec $(obj)u-boot.bin $(obj)System.map $(U_BOOT_NAND) $(U_BOOT_ONENAND) $(obj)u-boot.dis
ifeq ($(ARCH),blackfin)
ALL += $(obj)u-boot.ldr
endif
all: $(ALL)
make的目标就是all 下面的代码还有小目标，就不详写。
包含了生成的可执行文件