U-BOOT MakfeFile 分析

   最近开始想将uboot2010.03往一块开发板上，下面详细分析主Makefile文件（主要部分）。解释以红字标出，以****开头

#
# (C) Copyright 2000-2009
# Wolfgang Denk, DENX Software Engineering, wd@denx.de.
#
# See file CREDITS for list of people who contributed to this
# project.
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License as
# published by the Free Software Foundatio; either version 2 of
# the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.    See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston,
# MA 02111-1307 USA
#

VERSION = 2010
PATCHLEVEL = 03
SUBLEVEL =
EXTRAVERSION =
****uboot版本号的变量定义
ifneq "$(SUBLEVEL)" ""
U_BOOT_VERSION = $(VERSION).$(PATCHLEVEL).$(SUBLEVEL)$(EXTRAVERSION)
else
U_BOOT_VERSION = $(VERSION).$(PATCHLEVEL)$(EXTRAVERSION)
endif
**** U_BOOT_VERSION赋 值=2010.03
TIMESTAMP_FILE = $(obj)include/timestamp_autogenerated.h
**** 时间戳文件变量
VERSION_FILE = $(obj)include/version_autogenerated.h
**** 版本文件变量

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/)
**** 首先执行uname -m得到I686,通过管道传送给sed命令,然后sed命令将执行sed -e s/i.86/i386/,将I686替换成i386,最后的结是       HOSTARCH=i386.
HOSTOS := $(shell uname -s | tr '[:upper:]' '[:lower:]' | /
        sed -e 's//(cygwin/).*/cygwin/')
**** HOSTOS =Linux
# Set shell to bash if possible, otherwise fall back to sh
SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; /
    else if [ -x /bin/bash ]; then echo /bin/bash; /
    else echo sh; fi; fi)

export    HOSTARCH HOSTOS SHELL
**** HOSTARCH=i386
    HOSTOS =Linux
    SHELL =$BASH
# Deal with colliding definitions from tcsh etc.
VENDOR=

#########################################################################
# Allow for silent builds
ifeq (,$(findstring s,$(MAKEFLAGS)))
XECHO = echo
else
XECHO = :
endif
**** 选择编译的方式，允许沉默编译。findstring函数在MAKEFLAGS环境变量中查找用户是否在命令行中是否 加入-s选项
#########################################################################
#
# U-boot build supports producing a object files to the separate external
# directory. Two use cases are supported:
#
# 1) Add O= to the make command line
# 'make O=/tmp/build all'
#
# 2) Set environement variable BUILD_DIR to point to the desired location
# 'export BUILD_DIR=/tmp/build'
# 'make'
#
# The second approach can also be used with a MAKEALL script
# 'export BUILD_DIR=/tmp/build'
# './MAKEALL'
#
# Command line 'O=' setting overrides BUILD_DIR environent variable.
#
# When none of the above methods is used the local build is performed and
# the object files are placed in the source directory.
#
**** 根据解释可知，可以通过指定命令行选项和定义环境变量的方法，使得目标文件可以存放在别的的目录里面。
ifdef O
ifeq ("$(origin O)", "command line")
**** origin函数用于查询变量的出处，此处期望他在命令行中定义
BUILD_DIR := $(O)
**** 如果命令行定义O，那么BUILD_DIR就等于那个目录
endif
endif

ifneq ($(BUILD_DIR),)
saved-output := $(BUILD_DIR)

# Attempt to create a output directory.
$(shell [ -d ${BUILD_DIR} ] || mkdir -p ${BUILD_DIR})
**** 当$BUILD_DIR存在或者建立这个目录
# Verify if it was successful.
BUILD_DIR := $(shell cd $(BUILD_DIR) && /bin/pwd)
$(if $(BUILD_DIR),,$(error output directory "$(saved-output)" does not exist))
****目 录不存在则抛出错误
endif # ifneq ($(BUILD_DIR),)

OBJTREE        := $(if $(BUILD_DIR),$(BUILD_DIR),$(CURDIR))
****当BUILD_DIR存在时则赋给OBJTREE，否则OBJTREE=CURDIR （它代表当前工作目录，应该是个环境变量）
SRCTREE        := $(CURDIR)
****源代码目录是当前工作目录
TOPDIR        := $(SRCTREE)
****TOPDIR为当前源代码目录
LNDIR        := $(OBJTREE)
export    TOPDIR SRCTREE OBJTREE

MKCONFIG    := $(SRCTREE)/mkconfig
****这个脚本文件很重要
export MKCONFIG

ifneq ($(OBJTREE),$(SRCTREE))
REMOTE_BUILD    := 1
export REMOTE_BUILD
endif
****建立外部目录的部分就结束了
# $(obj) and (src) are defined in config.mk but here in main Makefile
# we also need them before config.mk is included which is the case for
# some targets like unconfig, clean, clobber, distclean, etc.
ifneq ($(OBJTREE),$(SRCTREE))
obj := $(OBJTREE)/
src := $(SRCTREE)/
else
obj :=
src :=
endif
export obj src
**** 抛出两个变量，在后面会用到，分别代表目标文件和源代码目录
# Make sure CDPATH settings don't interfere
unexport CDPATH
****这个变量不晓得干嘛的？
#########################################################################

ifeq ($(ARCH),powerpc)
ARCH = ppc
endif

# The "tools" are needed early, so put this first
# Don't include stuff already done in $(LIBS)
SUBDIRS    = tools /
      examples/standalone /
      examples/api

.PHONY : $(SUBDIRS)
**** 定义了几个伪目标，见上面的定义
ifeq ($(obj)include/config.mk,$(wildcard $(obj)include/config.mk))
**** 'wildcard' 的函 数，它有一个参数，功能是展开成一列所有符合由其参数描述的文件名，文件间以空格间隔。此句是判断$(obj)include/config.mk是否存 在。『当调用make <boardname>_config后将生成config.mk』---（来源于longhaihai'blog)
# Include autoconf.mk before config.mk so that the config options are available
# to all top level build files.  We need the dummy all: target to prevent the
# dependency target in autoconf.mk.dep from being the default.
all:
****第一个目标all
sinclude $(obj)include/autoconf.mk.dep
sinclude $(obj)include/autoconf.mk

# load ARCH, BOARD, and CPU configuration
include $(obj)include/config.mk
****这个文件由 make <boardname>_config产生，它里面定义了 ARCH CPU BOARD VENDOR SOC这几个变量 ，注意这几个变量都在那里使用了
export    ARCH CPU BOARD VENDOR SOC

# set default to nothing for native builds
ifeq ($(HOSTARCH),$(ARCH))
CROSS_COMPILE ?=
endif
****判断主机和目标机的体系结构是否一致，来决定是否交叉编译 ，此处？=是条件赋值语句，只有变量之前没定义过才赋值
# load other configuration
include $(TOPDIR)/config.mk
****重要的配置脚本总结：
     $(SRCTREE)/mkconfig
    $(TOPDIR)/config.mk
    $(obj)include/autoconf.mk.dep
   $(obj)include/autoconf.mk
  $(obj)include/config.mk
注意前面两个是在源文件目录，而后两个在目标文件目录，当然他们也可能一样
#########################################################################
# U-Boot objects....order is important (i.e. start must be first)

OBJS  = cpu/$(CPU)/start.o
ifeq ($(CPU),i386)
OBJS += cpu/$(CPU)/start16.o
OBJS += cpu/$(CPU)/resetvec.o
endif
ifeq ($(CPU),ppc4xx)
OBJS += cpu/$(CPU)/resetvec.o
endif
ifeq ($(CPU),mpc85xx)
OBJS += cpu/$(CPU)/resetvec.o
endif

OBJS := $(addprefix $(obj),$(OBJS))
****定义 OBJS，如果是cpu是arm920t，那么 OBJS = $(obj)/cpu/arm920t/start.o
LIBS  = lib_generic/libgeneric.a
LIBS += lib_generic/lzma/liblzma.a
LIBS += lib_generic/lzo/liblzo.a
LIBS += $(shell if [ -f board/$(VENDOR)/common/Makefile ]; then echo /
    "board/$(VENDOR)/common/lib$(VENDOR).a"; fi)
****-f选项表示文件是否存在
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 fs/yaffs2/libyaffs2.a /
    fs/ubifs/libubifs.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/fpga/libfpga.a
LIBS += drivers/gpio/libgpio.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/onenand/libonenand.a
LIBS += drivers/mtd/ubi/libubi.a
LIBS += drivers/mtd/spi/libspi_flash.a
LIBS += drivers/net/libnet.a
LIBS += drivers/net/phy/libphy.a
LIBS += drivers/pci/libpci.a
LIBS += drivers/pcmcia/libpcmcia.a
LIBS += drivers/power/libpower.a
LIBS += drivers/spi/libspi.a
ifeq ($(CPU),mpc83xx)
LIBS += drivers/qe/qe.a
endif
ifeq ($(CPU),mpc85xx)
LIBS += drivers/qe/qe.a
LIBS += cpu/mpc8xxx/ddr/libddr.a
LIBS += cpu/mpc8xxx/lib8xxx.a
endif
ifeq ($(CPU),mpc86xx)
LIBS += cpu/mpc8xxx/ddr/libddr.a
LIBS += cpu/mpc8xxx/lib8xxx.a
endif
LIBS += drivers/rtc/librtc.a
LIBS += drivers/serial/libserial.a
LIBS += drivers/twserial/libtws.a
LIBS += drivers/usb/gadget/libusb_gadget.a
LIBS += drivers/usb/host/libusb_host.a
LIBS += drivers/usb/musb/libusb_musb.a
LIBS += drivers/usb/phy/libusb_phy.a
LIBS += drivers/video/libvideo.a
LIBS += drivers/watchdog/libwatchdog.a
LIBS += common/libcommon.a
LIBS += libfdt/libfdt.a
LIBS += api/libapi.a
LIBS += post/libpost.a

LIBS := $(addprefix $(obj),$(LIBS))
**** LIBS,包含了后缀为.a的 文件，这种文件是在进到到各个目录执行make产生的库文件，链接时需要这些文件。链接的文件涉及到了 lib_generic目录,cpu目录，lib_arm目录（以arm为例子），fs目录，net目录，disk目录，drivers目录。从上面这段 代码就可以清楚的知道，整个u-boot究竟需要什么文件，由哪些文件组成。
**** addprefix为增加前缀函数
.PHONY : $(LIBS)  $(TIMESTAMP_FILE)  $(VERSION_FILE)
****声明伪目标，在后面的编译中用
LIBBOARD = board/$(BOARDDIR)/lib$(BOARD).a
LIBBOARD := $(addprefix $(obj),$(LIBBOARD))
****与开发板相关的库目标文件，这视乎 和 $(BOARDDIR)定义有关，好想没看到，在之前的u-boot的Makefile里面是定义为 BOARDDIR=$(BOARD)
# Add GCC lib
ifdef USE_PRIVATE_LIBGCC
ifeq ("$(USE_PRIVATE_LIBGCC)", "yes")
PLATFORM_LIBGCC = -L $(OBJTREE)/lib_$(ARCH) -lgcc
else
PLATFORM_LIBGCC = -L $(USE_PRIVATE_LIBGCC) -lgcc
endif
else
PLATFORM_LIBGCC = -L $(shell dirname `$(CC) $(CFLAGS) -print-libgcc-file-name`) -lgcc
****CC和CFLAGS都是make的隐含变量，CC表示C编译器，cflags表示执行CC时的命令行参 数。当然是交叉编译，其定义在config.mk里（根目录下），这也是一个很重要的文件，用于定义编译连接工具和选项的文件
endif
PLATFORM_LIBS += $(PLATFORM_LIBGCC)
export PLATFORM_LIBS

# Special flags for CPP when processing the linker script.
# Pass the version down so we can handle backwards compatibility
# on the fly.
LDPPFLAGS += /
    -include $(TOPDIR)/include/u-boot/u-boot.lds.h /
    $(shell $(LD) --version | /
      sed -ne 's/GNU ld version /([0-9][0-9]*/)/./([0-9][0-9]*/).*/-DLD_MAJOR=/1 -DLD_MINOR=/2/p')

ifeq ($(CONFIG_NAND_U_BOOT),y)
NAND_SPL = nand_spl
U_BOOT_NAND = $(obj)u-boot-nand.bin
endif

ifeq ($(CONFIG_ONENAND_U_BOOT),y)
ONENAND_IPL = onenand_ipl
U_BOOT_ONENAND = $(obj)u-boot-onenand.bin
ONENAND_BIN ?= $(obj)onenand_ipl/onenand-ipl-2k.bin
endif
****包含u-boot.lds.h,如果定义从Nandflash启动，就定义变量 U_BOOT_ONENAND = $( obj) u- boot- onenand. bin.
__OBJS := $(subst $(obj),,$(OBJS))
****去掉前缀 ，__OBJS = cpu/arm920t/start.o
__LIBS := $(subst $(obj),,$(LIBS))  $(subst $(obj),,$(LIBBOARD))
****去掉前缀， __LIBS= $(LIBS)+ $(LIBBOARD)， 当然是去掉前缀部分
#########################################################################
#########################################################################

# Always append ALL so that arch config.mk's can add custom ones
ALL += $(obj)u-boot.srec $(obj)u-boot.bin $(obj)System.map $(U_BOOT_NAND) $(U_BOOT_ONENAND)

all:        $(ALL)
**** 当 输入命令 make all,讲寻找到此次的伪目标all，all依赖于 $(ALL),ALL由5部分组成。那么接下去就分别对这5部分进行递归的处理。（贴出来格式变了，最好自己看源码）。看到308行，

（1）目标文件u-boot.bin依赖于 $(obj)u-boot目标，可以在344行找到；

    （1.1）$(obj)u-boot依赖于：depend  $(SUBDIRS)  $(OBJS)  $(LIBBOARD)  $(LIBS)  $(LDSCRIPT)  $(obj)u-boot.lds；

          （1.1.1）depend；402行。执行的操作为：进入到$(SUBDIRS)，生成各个子目

                  录的.depend文件。通过调用各个子目录的make _depend。

          （1.1.2）$(SUBDIRS)；363行。依赖depend。执行操作为：对各个子目录执行

                   makefile文件。

          (1.1.3) $(OBJS);354行。依赖depend。执行：

             $(MAKE) -C cpu/$(CPU) $(if $(REMOTE_BUILD),$@,$(notdir $@))

                   如果cpu为arm920t，编译文件cpu/arm920t。

          (1.1.4) $(LIBS) ;357行。依赖$(SUBDIRS).执行操作为：

                  $(MAKE) -C $(dir $(subst $(obj),,$@))，

                  dir为GNU内嵌函数为取目录操作，比如$(dir a/b/c) 结果为

                  a/b/;subst 也是GNU内嵌函数$(subst from,to,text)表示在text

                  中把form 字符串替换为字符串to。subst资料上面是这样讲的。但是

                  在此处，我认为是：$(subst text， to， from)。总之，这样理解

                  吧：在LIBS包含的n个路径下面执行make

          (1.1.5) $(LIBBOARD);360行。依赖depend，$（LIBS）。执行：

                  $(MAKE) -C $(dir $(subst $(obj),,$@))。

                  再在之前，$(LIBS)已经执行完毕，现在obj只剩下了：LIBBOAED路径

                  集合了。在这些路径集合下面执行make。

          (1.1.6) $(LDSCRIPT);336行。依赖depend。执行：

                  $(MAKE) -C $(dir $@) $(notdir $@)，notdir为取文件名函数，

                  比如，$(notdir a/my.o)结果为my.o。

                 

          (1.1.7) $(obj)u-boot.lds。369行。依赖$(LDSCRIPT).执行：

      $(CPP) $(CPPFLAGS) $(LDPPFLAGS) -ansi -D__ASSEMBLY__ -P - <$^ >$@

                  生成链接文件 。

*****上面这段文字引自：http://blog.chinaunix.net/u3/108840 /showart_2135045.html
$(obj)u-boot.hex:    $(obj)u-boot
        $(OBJCOPY) ${OBJCFLAGS} -O ihex $< $@

$(obj)u-boot.srec:    $(obj)u-boot
        $(OBJCOPY) -O srec $< $@

$(obj)u-boot.bin:    $(obj)u-boot
        $(OBJCOPY) ${OBJCFLAGS} -O binary $< $@

$(obj)u-boot.ldr:    $(obj)u-boot
        $(CREATE_LDR_ENV)
        $(LDR) -T $(CONFIG_BFIN_CPU) -c $@ $< $(LDR_FLAGS)

$(obj)u-boot.ldr.hex:    $(obj)u-boot.ldr
        $(OBJCOPY) ${OBJCFLAGS} -O ihex $< $@ -I binary

$(obj)u-boot.ldr.srec:    $(obj)u-boot.ldr
        $(OBJCOPY) ${OBJCFLAGS} -O srec $< $@ -I binary

$(obj)u-boot.img:    $(obj)u-boot.bin
        ./tools/mkimage -A $(ARCH) -T firmware -C none /
        -a $(TEXT_BASE) -e 0 /
        -n $(shell sed -n -e 's/.*U_BOOT_VERSION//p' $(VERSION_FILE) | /
            sed -e 's/"[     ]*$$/ for $(BOARD) board"/') /
        -d $< $@

$(obj)u-boot.imx:       $(obj)u-boot.bin
        $(obj)tools/mkimage -n $(IMX_CONFIG) -T imximage /
        -e $(TEXT_BASE) -d $< $@

$(obj)u-boot.kwb:       $(obj)u-boot.bin
        $(obj)tools/mkimage -n $(KWD_CONFIG) -T kwbimage /
        -a $(TEXT_BASE) -e $(TEXT_BASE) -d $< $@

$(obj)u-boot.sha1:    $(obj)u-boot.bin
        $(obj)tools/ubsha1 $(obj)u-boot.bin

$(obj)u-boot.dis:    $(obj)u-boot
        $(OBJDUMP) -d $< > $@

GEN_UBOOT = /
        UNDEF_SYM=`$(OBJDUMP) -x $(LIBBOARD) $(LIBS) | /
        sed  -n -e 's/.*/($(SYM_PREFIX)__u_boot_cmd_.*/)/-u/1/p'|sort|uniq`;/
        cd $(LNDIR) && $(LD) $(LDFLAGS) $$UNDEF_SYM $(__OBJS) /
            --start-group $(__LIBS) --end-group $(PLATFORM_LIBS) /
            -Map u-boot.map -o u-boot
$(obj)u-boot:    depend $(SUBDIRS) $(OBJS) $(LIBBOARD) $(LIBS) $(LDSCRIPT) $(obj)u-boot.lds
**** LDSCRIPT是一个和nand相关的的 u-boot.lds的脚本？
        $(GEN_UBOOT)
ifeq ($(CONFIG_KALLSYMS),y)
        smap=`$(call SYSTEM_MAP,u-boot) | /
            awk '$$2 ~ /[tTwW]/ {printf $$1 $$3 "////000"}'` ; /
        $(CC) $(CFLAGS) -DSYSTEM_MAP="/"$${smap}/"" /
            -c common/system_map.c -o $(obj)common/system_map.o
        $(GEN_UBOOT) $(obj)common/system_map.o
endif
****产 生Elf格式的U-boot，用于其它目标的转换
$(OBJS):    depend
        $(MAKE) -C cpu/$(CPU) $(if $(REMOTE_BUILD),$@,$(notdir $@))

$(LIBS):    depend $(SUBDIRS)
        $(MAKE) -C $(dir $(subst $(obj),,$@))

$(LIBBOARD):    depend $(LIBS)
        $(MAKE) -C $(dir $(subst $(obj),,$@))

$(SUBDIRS):    depend
        $(MAKE) -C $@ all

$(LDSCRIPT):    depend
        $(MAKE) -C $(dir $@) $(notdir $@)

$(obj)u-boot.lds: $(LDSCRIPT)
        $(CPP) $(CPPFLAGS) $(LDPPFLAGS) -ansi -D__ASSEMBLY__ -P - <$^ >$@

$(NAND_SPL):    $(TIMESTAMP_FILE) $(VERSION_FILE) $(obj)include/autoconf.mk
        $(MAKE) -C nand_spl/board/$(BOARDDIR) all

$(U_BOOT_NAND):    $(NAND_SPL) $(obj)u-boot.bin
        cat $(obj)nand_spl/u-boot-spl-16k.bin $(obj)u-boot.bin > $(obj)u-boot-nand.bin

$(ONENAND_IPL):    $(TIMESTAMP_FILE) $(VERSION_FILE) $(obj)include/autoconf.mk
        $(MAKE) -C onenand_ipl/board/$(BOARDDIR) all

$(U_BOOT_ONENAND):    $(ONENAND_IPL) $(obj)u-boot.bin
        cat $(ONENAND_BIN) $(obj)u-boot.bin > $(obj)u-boot-onenand.bin

$(VERSION_FILE):
        @( printf '#define U_BOOT_VERSION "U-Boot %s%s"/n' "$(U_BOOT_VERSION)" /
         '$(shell $(TOPDIR)/tools/setlocalversion $(TOPDIR))' ) > $@.tmp
        @cmp -s $@ $@.tmp && rm -f $@.tmp || mv -f $@.tmp $@

$(TIMESTAMP_FILE):
        @date +'#define U_BOOT_DATE "%b %d %C%y"' > $@
        @date +'#define U_BOOT_TIME "%T"' >> $@

gdbtools:
        $(MAKE) -C tools/gdb all || exit 1

updater:
        $(MAKE) -C tools/updater all || exit 1

env:
        $(MAKE) -C tools/env all MTD_VERSION=${MTD_VERSION} || exit 1

# Explicitly make _depend in subdirs containing multiple targets to prevent
# parallel sub-makes creating .depend files simultaneously.
depend dep:    $(TIMESTAMP_FILE) $(VERSION_FILE) $(obj)include/autoconf.mk
        for dir in $(SUBDIRS) cpu/$(CPU) $(dir $(LDSCRIPT)) ; do /
            $(MAKE) -C $$dir _depend ; done

TAG_SUBDIRS = $(SUBDIRS)
TAG_SUBDIRS += $(dir $(__LIBS))
TAG_SUBDIRS += include

tags ctags:
        ctags -w -o $(obj)ctags `find $(TAG_SUBDIRS) /
                        -name '*.[chS]' -print`

etags:
        etags -a -o $(obj)etags `find $(TAG_SUBDIRS) /
                        -name '*.[chS]' -print`
cscope:
        find $(TAG_SUBDIRS) -name '*.[chS]' -print > cscope.files
        cscope -b -q -k

SYSTEM_MAP = /
        $(NM) $1 | /
        grep -v '/(compiled/)/|/(/.o$$/)/|/( [aUw] /)/|/(/./.ng$$/)/|/(LASH[RL]DI/)' | /
        LC_ALL=C sort
$(obj)System.map:    $(obj)u-boot
        @$(call SYSTEM_MAP,$<) > $(obj)System.map

#
# Auto-generate the autoconf.mk file (which is included by all makefiles)
#
# This target actually generates 2 files; autoconf.mk and autoconf.mk.dep.
# the dep file is only include in this top level makefile to determine when
# to regenerate the autoconf.mk file.
$(obj)include/autoconf.mk.dep: $(obj)include/config.h include/common.h
    @$(XECHO) Generating $@ ; /
    set -e ; /
    : Generate the dependancies ; /
    $(CC) -x c -DDO_DEPS_ONLY -M $(HOSTCFLAGS) $(CPPFLAGS) /
        -MQ $(obj)include/autoconf.mk include/common.h > $@

$(obj)include/autoconf.mk: $(obj)include/config.h
    @$(XECHO) Generating $@ ; /
    set -e ; /
    : Extract the config macros ; /
    $(CPP) $(CFLAGS) -DDO_DEPS_ONLY -dM include/common.h | /
        sed -n -f tools/scripts/define2mk.sed > $@.tmp && /
    mv $@.tmp $@

#########################################################################
else    # !config.mk
all $(obj)u-boot.hex $(obj)u-boot.srec $(obj)u-boot.bin /
$(obj)u-boot.img $(obj)u-boot.dis $(obj)u-boot /
$(filter-out tools,$(SUBDIRS)) $(TIMESTAMP_FILE) $(VERSION_FILE) gdbtools /
updater env depend dep tags ctags etags cscope $(obj)System.map:
    @echo "System not configured - see README" >&2
    @ exit 1

tools:
    $(MAKE) -C tools
tools-all:
    $(MAKE) -C tools HOST_TOOLS_ALL=y
endif    # config.mk

.PHONY : CHANGELOG
CHANGELOG:
    git log --no-merges U-Boot-1_1_5.. | /
    unexpand -a | sed -e 's//s/s*$$//' > $@

include/license.h: tools/bin2header COPYING
     cat COPYING | gzip -9 -c | ./tools/bin2header license_gzip > include/license.h
#########################################################################

unconfig:
    @rm -f $(obj)include/config.h $(obj)include/config.mk /
        $(obj)board/*/config.tmp $(obj)board/*/*/config.tmp /
        $(obj)include/autoconf.mk $(obj)include/autoconf.mk.dep

%: %_config
    $(MAKE)


Makefile就分析到这，对 all目标建立过程这段还是分析的很粗糙，这段主要是参考网友的观点，我现在SHELL这块还很薄弱，先分析到这，以后再补充。下面分析mkconfig 文件。