1.包含主目录的config.mk

# load other configuration
include $(TOPDIR)/config.mk

在config.mk中主要做了哪些工作呢？

1.编译工具定义

#
# Include the make variables (CC, etc...)
#
AS	= $(CROSS_COMPILE)as
LD	= $(CROSS_COMPILE)ld
CC	= $(CROSS_COMPILE)gcc
CPP	= $(CC) -E
AR	= $(CROSS_COMPILE)ar
NM	= $(CROSS_COMPILE)nm
STRIP	= $(CROSS_COMPILE)strip
OBJCOPY = $(CROSS_COMPILE)objcopy
OBJDUMP = $(CROSS_COMPILE)objdump
RANLIB	= $(CROSS_COMPILE)RANLIB

2.包含开发板配置项目

ifdef	ARCH
sinclude $(TOPDIR)/$(ARCH)_config.mk	# include architecture dependend rules
endif
ifdef	CPU
sinclude $(TOPDIR)/cpu/$(CPU)/config.mk	# include  CPU	specific rules
endif
ifdef	SOC
sinclude $(TOPDIR)/cpu/$(CPU)/$(SOC)/config.mk	# include  SoC	specific rules
endif
ifdef	VENDOR
BOARDDIR = $(VENDOR)/$(BOARD)
else
BOARDDIR = $(BOARD)
endif
ifdef	BOARD
sinclude $(TOPDIR)/board/$(BOARDDIR)/config.mk	# include board specific rules
endif

在/board/100ask24x0目录下的config.mk仅仅有一行代码：

TEXT_BASE = 0x33F80000

3.设置头文件搜寻路径

ifneq ($(OBJTREE),$(SRCTREE))
CPPFLAGS += -I$(OBJTREE)/include2 -I$(OBJTREE)/include
endif

CPPFLAGS += -I$(TOPDIR)/include
CPPFLAGS += -fno-builtin -ffreestanding -nostdinc 	\
	-isystem $(gccincdir) -pipe $(PLATFORM_CPPFLAGS)

段设置了头文件的搜寻路径，添加了顶层目录下的include文件夹作为搜索路径。然后进行了一些其他设置（比如禁止标准的include路径）

这样，编译器在编译的时候就能正确的读取到include文件夹下的头文件（其实是配置阶段创建的符号链接）了

4.包含链接脚本

ifndef LDSCRIPT
#LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot.lds.debug
ifeq ($(CONFIG_NAND_U_BOOT),y)
LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot-nand.lds
else
LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot.lds
endif
endif

简单分析一下，就可以得知，我们包含的链接脚本是(TOPDIR)/board/$(BOARDDIR)/u-boot.lds。

5.设定LDFLAGS

LDFLAGS += -Bstatic -T $(LDSCRIPT) -Ttext $(TEXT_BASE) $(PLATFORM_LDFLAGS)

其中TEXT_BASE 在/board/100ask24x0目录下的config.mk中定义：

TEXT_BASE = 0x33F80000

LDSCRIPT就是(TOPDIR)/board/$(BOARDDIR)/u-boot.lds。

最终生成的LDFLAGS会在生成bin文件的时候用到。

6.导出若干换环境变量

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

7.设定自动推导规则

ifndef REMOTE_BUILD

%.s:	%.S
	$(CPP) $(AFLAGS) -o $@ $<
%.o:	%.S
	$(CC) $(AFLAGS) -c -o $@ $<
%.o:	%.c
	$(CC) $(CFLAGS) -c -o $@ $<

else

$(obj)%.s:	%.S
	$(CPP) $(AFLAGS) -o $@ $<
$(obj)%.o:	%.S
	$(CC) $(AFLAGS) -c -o $@ $<
$(obj)%.o:	%.c
	$(CC) $(CFLAGS) -c -o $@ $<
endif

makefile的自动推导规则，和顶层Makefile内的规则配合使用，就能实现众多文件的编译

u-boot主makefile分析2