Top level makefile analysis of u-boot (3)

stay u-boot Top layer of Makefile analysis （ Two ） Analysis of config.mk Generation process of , Then continue to analyze the top level Makefile

export config.mk For environment variables

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

• take include/config.mk The file is loaded in and the five internal variables are exported as environment variables

Set the cross compilation tool chain

ifndef CROSS_COMPILE
ifeq ($(HOSTARCH),$(ARCH))
CROSS_COMPILE =
else
ifeq ($(ARCH),ppc)
CROSS_COMPILE = ppc_8xx-
endif
ifeq ($(ARCH),arm)
CROSS_COMPILE = arm-none-linux-gnueabi-
endif
…………
endif   # HOSTARCH,ARCH
endif   # CROSS_COMPILE

export  CROSS_COMPILE
include $(TOPDIR)/config.mk

• This is one of the important jobs --------- Set the cross compilation tool chain
• The above code only lists the first few lines , Ellipsis is used to judge other platforms that cannot be used
• Because of what we set up earlier ARCH The value of the variable is arm, So here we find arm And modify CROSS_COMPILE A variable's value
• In actual use, it can also be modified here , But through make CROSS_COMPILE=xxx To set up the cross compilation tool chain
• export CROSS_COMPILE Variables are environment variables
• The next analysis config.mk The key part of the document

config.mk Analysis of key parts of the document

Compiler settings

AS	= $(CROSS_COMPILE)as
LD	= $(CROSS_COMPILE)ld
CC	= $(CROSS_COMPILE)gcc
CPP	= $(CC) -E
AR	= $(CROSS_COMPILE)ar
NM	= $(CROSS_COMPILE)nm
LDR	= $(CROSS_COMPILE)ldr
STRIP	= $(CROSS_COMPILE)strip
OBJCOPY = $(CROSS_COMPILE)objcopy
OBJDUMP = $(CROSS_COMPILE)objdump
RANLIB	= $(CROSS_COMPILE)RANLIB

#########################################################################

# Load generated board configuration
sinclude $(OBJTREE)/include/autoconf.mk

• the AS,LD And other variables , These values are combined with the variables of the cross compilation tool chain configured above to form a compiler file to compile the source file
• Import include/autoconf.mk file , This file is now empty , however Makefile Use sinclude To introduce , This results in no error even if the file does not exist
• include/autoconf.mk Files are generated during configuration , It is full of CONFIG_XX The macro definition at the beginning （ It can be understood as a switch ）, Used to specify uboot The trend in the compilation process , At the end of this article, there is the generation process of this file ！

Compilation options configuration

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

• According to ARCH,CPU,SOC Wait to add different compilation options , Here is arm_config.mk The content in PLATFORM_CPPFLAGS += -DCONFIG_ARM -D__ARM, Others are similar
• Be careful BOARD Under the config.mk（board/samsung/x210/config.mk） The contents of the document TEXT_BASE = 0xc3e00000, This is different from the top floor Makefile Medium x210_sd_config The link address written by the task is the same

Link script

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

• Here we have no definition CONFIG_NAND_U_BOOT Variable , therefore LDSCRIPT The value of is u-boot.lds, This is a linked script file , analysis uboot The link script file should be considered when compiling
• First put the file below , For detailed instructions, please refer to u-boot Link script post , It just says .text Section defines the running sequence of the program , therefore cpu/s5pc11x/start.o Is the first file to run

OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
/*OUTPUT_FORMAT("elf32-arm", "elf32-arm", "elf32-arm")*/
OUTPUT_ARCH(arm)
ENTRY(_start)
SECTIONS
{
    
	. = 0x00000000;

	. = ALIGN(4);
	.text      :
	{
    
	  cpu/s5pc11x/start.o	(.text)
	  cpu/s5pc11x/s5pc110/cpu_init.o	(.text)
	  board/samsung/x210/lowlevel_init.o	(.text)
          cpu/s5pc11x/onenand_cp.o      (.text)                 
          cpu/s5pc11x/nand_cp.o (.text)                     
          cpu/s5pc11x/movi.o (.text) 
          common/secure_boot.o (.text) 
	  common/ace_sha1.o (.text)
	  cpu/s5pc11x/pmic.o (.text)
	  *(.text)
	}

	. = ALIGN(4);
	.rodata : {
     *(.rodata) }

	. = ALIGN(4);
	.data : {
     *(.data) }

	. = ALIGN(4);
	.got : {
     *(.got) }

	__u_boot_cmd_start = .;
	.u_boot_cmd : {
     *(.u_boot_cmd) }
	__u_boot_cmd_end = .;

	. = ALIGN(4);
	.mmudata : {
     *(.mmudata) }

	. = ALIGN(4);
	__bss_start = .;
	.bss : {
     *(.bss) }
	_end = .;
}

Specify the link address

ifneq ($(TEXT_BASE),)
CPPFLAGS += -DTEXT_BASE=$(TEXT_BASE)
endif

ifneq ($(TEXT_BASE),)
LDFLAGS += -Ttext $(TEXT_BASE)
endif

• I've set it up here u-boot The link address specified when linking
• TEXT_BASE Variable values are in the Compilation options configuration The source is analyzed in section

Compile the file

ifndef REMOTE_BUILD

%.s:	%.S
	$(CPP) $(AFLAGS) -o [email protected] $<
%.o:	%.S
	$(CC) $(AFLAGS) -c -o [email protected] $<
%.o:	%.c
	$(CC) $(CFLAGS) -c -o [email protected] $<

else

$(obj)%.s:	%.S
	$(CPP) $(AFLAGS) -o [email protected] $<
$(obj)%.o:	%.S
	$(CC) $(AFLAGS) -c -o [email protected] $<
$(obj)%.o:	%.c
	$(CC) $(CFLAGS) -c -o [email protected] $<
endif

• Through here Makefile Automatic derivation planning compilation target file

Continue to analyze the main Makefile

#########################################################################
# 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)/reset.o
endif
ifeq ($(CPU),ppc4xx)
OBJS += cpu/$(CPU)/resetvec.o
endif
……
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

• Next, continue to analyze the main Makefile
• Pay attention to the notes ,order is important (i.e. start must be first), Order is important and start It must be the first file to run . This is similar to the link script we mentioned earlier .text The first run file defined in the section is the same .
• Next, add various runtime files

The first goal all

all:		$(ALL)

$(obj)u-boot.srec:	$(obj)u-boot
		$(OBJCOPY) ${OBJCFLAGS} -O srec $< [email protected]

$(obj)u-boot:		depend $(SUBDIRS) $(OBJS) $(LIBBOARD) $(LIBS) $(LDSCRIPT)
		UNDEF_SYM=`$(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

$(SUBDIRS):	depend $(obj)include/autoconf.mk
		#$(error SUBDIRS)
		$(MAKE) -C [email protected] all

• all yes Makefile The first goal of , So execute make The function of is equal to the execution make all
• $(ALL) The value of the variable is printed u-boot.srec u-boot.bin System.map u-boot.dis, Therefore, compiling and completing these four tasks respectively is considered to be completed
• u-boot.srec Depend on u-boot, and u-boot And depend on $(SUBDIRS) $(OBJS) $(LIBBOARD) $(LIBS) $(LDSCRIPT),SUBDIRS And depend on include/autoconf.mk file .
• Rely on it all the way , Then compile backwards
• Let's analyze include/autoconf.mk How are files generated , One is that many goals depend on this , Second, it is control u-boot Compile the switch file of the trend

autoconf.mk File generation

#
# 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 [email protected] ; \
	set -e ; \
	: Generate the dependancies ; \
	$(CC) -x c -DDO_DEPS_ONLY -M $(HOST_CFLAGS) $(CPPFLAGS) \
		-MQ $(obj)include/autoconf.mk include/common.h > [email protected]
	

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

• The above rules are used to generate autoconf.mk file
• (obj)include/autoconf.mk.dep Rules don't work
• (obj)include/autoconf.mk Rules are used to generate autoconf.mk file
• Open this chapter The first goal all In the paragraph $(SUBDIRS) The effect can be seen by the comments in the goal （ Delete first. include/autoconf.mk file ）

• here include/autoconf.mk The file has been generated , To know its complete command , You can use （ Delete first include/autoconf.mk file ）make --trace command , Press... After two seconds of operation Ctrl+z interrupt , Then look back , The content in the red box below is the complete command .
  
• The following figure is an example of the contents of this file .
  

thus , Lord Makefile Analysis completed ！