1. 前言

　UBOOT版本：uboot2018.03，开发板myimx8mmek240。

2. 概述

本文采用自顶向下的方法，从顶层目标开始到最原始的依赖。

3. u-boot编译

uboot的编译分为两步：配置、编译。

• 配置，执行make pmyimx8mmek240-8mm-2g_defconfig进行配置，在生成.config文件；
• 编译，执行make进行编译，生成u-boot*、System.map等。
  
  

4. u-boot编译过程涉及的目标分析

# 顶层Makefile
# If building an external module we do not care about the all: rule
# but instead _all depend on modules
PHONY += all                  //伪目标all
ifeq ($(KBUILD_EXTMOD),)      //假设未定义KBUILD_EXTMOD
_all: all                     //_all 是顶层Makefile第一个目标，依赖是all 
else
_all: modules
endif

......

all:		$(ALL-y) cfg     //all自身依赖于$(ALL-y)和cfg
ifeq ($(CONFIG_DM_I2C_COMPAT)$(CONFIG_SANDBOX),y)
	@echo "===================== WARNING ======================"
	@echo "This board uses CONFIG_DM_I2C_COMPAT. Please remove"
	@echo "(possibly in a subsequent patch in your series)"
	@echo "before sending patches to the mailing list."
	@echo "===================================================="
endif
	@# Check that this build does not use CONFIG options that we do not
	@# know about unless they are in Kconfig. All the existing CONFIG
	@# options are whitelisted, so new ones should not be added.
	$(call cmd,cfgcheck,u-boot.cfg)

4.1 目标$(ALL-y)

# 顶层Makefile
# Read in config
-include include/config/auto.conf

......

-include include/autoconf.mk
......

# Always append ALL so that arch config.mk's can add custom ones
ALL-y += u-boot.srec u-boot.bin u-boot.sym System.map binary_size_check

ALL-$(CONFIG_ONENAND_U_BOOT) += u-boot-onenand.bin  //include/config/auto.conf中未定义
ifeq ($(CONFIG_SPL_FSL_PBL),y)                      //include/config/auto.conf中未定义
ALL-$(CONFIG_RAMBOOT_PBL) += u-boot-with-spl-pbl.bin
else
ifneq ($(CONFIG_SECURE_BOOT), y)                   //include/config/auto.conf中未定义
# For Secure Boot The Image needs to be signed and Header must also
# be included. So The image has to be built explicitly
ALL-$(CONFIG_RAMBOOT_PBL) += u-boot.pbl
endif
endif
ALL-$(CONFIG_SPL) += spl/u-boot-spl.bin            //关注：include/config/auto.conf中有定义
ifeq ($(CONFIG_MX6)$(CONFIG_SECURE_BOOT), yy)      //include/config/auto.conf中未定义
ALL-$(CONFIG_SPL_FRAMEWORK) += u-boot-ivt.img
else
ALL-$(CONFIG_SPL_FRAMEWORK) += u-boot.img         //关注：include/config/auto.conf中有定义
endif
ALL-$(CONFIG_TPL) += tpl/u-boot-tpl.bin           //include/config/auto.conf中未定义
ALL-$(CONFIG_OF_SEPARATE) += u-boot.dtb           //关注：include/config/auto.conf中有定义
ifeq ($(CONFIG_SPL_FRAMEWORK),y)                  //关注：include/config/auto.conf中有定义
ALL-$(CONFIG_OF_SEPARATE) += u-boot-dtb.img       //关注：include/config/auto.conf中有定义
endif
ALL-$(CONFIG_OF_HOSTFILE) += u-boot.dtb           //include/config/auto.conf中未定义
ifneq ($(CONFIG_SPL_TARGET),)                    //include/config/auto.conf中未定义 
ALL-$(CONFIG_SPL) += $(CONFIG_SPL_TARGET:"%"=%)  
endif
ALL-$(CONFIG_REMAKE_ELF) += u-boot.elf           //关注：include include/autoconf.mk中有定义
ALL-$(CONFIG_EFI_APP) += u-boot-app.efi          //include/config/auto.conf中未定义
ALL-$(CONFIG_EFI_STUB) += u-boot-payload.efi     //include/config/auto.conf中未定义

ifneq ($(BUILD_ROM)$(CONFIG_BUILD_ROM),)        //include/config/auto.conf中未定义 
ALL-$(CONFIG_X86_RESET_VECTOR) += u-boot.rom    
endif

# enable combined SPL/u-boot/dtb rules for tegra
ifeq ($(CONFIG_TEGRA)$(CONFIG_SPL),yy)            //include/config/auto.conf中未定义
ALL-y += u-boot-tegra.bin u-boot-nodtb-tegra.bin
ALL-$(CONFIG_OF_SEPARATE) += u-boot-dtb-tegra