uboot源代码学习-Makefile

24到29行

VERSION = 1
PATCHLEVEL = 3
SUBLEVEL = 4
EXTRAVERSION =
U_BOOT_VERSION = $(VERSION).$(PATCHLEVEL).$(SUBLEVEL)$(EXTRAVERSION)
VERSION_FILE = $(obj)include/version_autogenerated.h

这5行定义了版本号，最终版本号为U_BOOT_VERSION。version_atuogenerated.h文件是编译后生成的文件，里面就是版本号。

31到38行

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/)

$(shell xxxx)相当于在linux中输入xxxx。uname -m得到当前计算机CPU版本号。下面为uname命令的不同选项。

-a或--all：显示全部的信息；
-m或--machine：显示电脑类型；
-n或-nodename：显示在网络上的主机名称；
-r或--release：显示操作系统的发行编号；
-s或--sysname：显示操作系统名称；
-v：显示操作系统的版本；
-p或--processor：输出处理器类型或"unknown"；
-i或--hardware-platform：输出硬件平台或"unknown"；
-o或--operating-system：输出操作系统名称；
--help：显示帮助；
--version：显示版本信息。

|表示把|左边的结果作为输入输入给|右边。\是一个换行符。sed -e s/a/b/ 意思是用b来替换a。i.86中的.相当于通用符。

40到43行

HOSTOS := $(shell uname -s | tr '[:upper:]' '[:lower:]' | \
	    sed -e 's/\(cygwin\).*/cygwin/')

export	HOSTARCH HOSTOS

uname -s为显示操作系统名称。tr '[:upper:]' '[:lower:]' 表示把大写换成小写，tr是linux中的一个指令。HOSTARCH表示主机CPU架构，HOSTOS表示主机操作系统。

50到54行

ifeq (,$(findstring s,$(MAKEFLAGS)))
XECHO = echo
else
XECHO = :
endif

MAKEFLAGS表示编译时传递的选项，比如make -s，则MAKEFLAGS为-s。$(findstring <find>,<in>)为一个查找字符串函数。即输入make -s，函数会等于1，ifeq（0，1），于是执行XECHO = : ，从而实现静默编译。

58到122行

# 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.
#

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

Makefile提供了2种编译方法，原地编译与单独输出文件夹编译。有2种单独输出文件夹编译的方法。

make O=输出目录
export BUILD_DIR=输出目录

第一种方法优先级比较高。

TREE表示根目录，OBJTREE是.o文件根目录，SRCTREE是源代码目录。MKCONFIG是配置脚本。

126到182行

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

ifeq ($(obj)include/config.mk,$(wildcard $(obj)include/config.mk))

# load ARCH, BOARD, and CPU configuration
include $(obj)include/config.mk
export	ARCH CPU BOARD VENDOR SOC

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

wildcard作用是判断文件是否存在，include可以把别的makefile关联进来。这里的config.mk是编译生成的，里面是5个要素的配置信息。ARCH配置在Makefile下面，修改也在Makefile。CROSS_COMPILE用于确定交叉编译的前缀。

185行

include $(TOPDIR)/config.mk

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