1. 前言

　UBOOT版本：uboot2018.03，开发板myimx8mmek240。

2. 概述

此文件包含在 ./Makefile 和 spl/Makefile 中。 清理状态以避免添加两次相同的标志。有些平台需要不同的 SPL 标志，这就是为什么这个文件也必须包含在 spl/Makefile 中的原因。如果我们没有SPL，构建系统会简单得多。我使用的平台有使用SPL。

3. 顶层config.mk解析

由于内容较少，直接在源代码中批注：

#
# (C) Copyright 2000-2013
# Wolfgang Denk, DENX Software Engineering, wd@denx.de.
#
# SPDX-License-Identifier:	GPL-2.0+
#
#########################################################################

# This file is included from ./Makefile and spl/Makefile.
# Clean the state to avoid the same flags added twice.
#
# (Tegra needs different flags for SPL.
#  That's the reason why this file must be included from spl/Makefile too.
#  If we did not have Tegra SoCs, build system would be much simpler...)
PLATFORM_RELFLAGS :=
PLATFORM_CPPFLAGS :=
PLATFORM_LDFLAGS :=
LDFLAGS :=
LDFLAGS_FINAL :=
OBJCOPYFLAGS :=
# clear VENDOR for tcsh
VENDOR :=
#########################################################################

ARCH := $(CONFIG_SYS_ARCH:"%"=%) //架构 arm
CPU := $(CONFIG_SYS_CPU:"%"=%)   //CPU armv8
ifdef CONFIG_SPL_BUILD           //我使用的平台有定义 -DCONFIG_SPL_BUILD
ifdef CONFIG_TEGRA
CPU := arm720t
endif
endif
BOARD := $(CONFIG_SYS_BOARD:"%"=%) //板 myimx8mm
ifneq ($(CONFIG_SYS_VENDOR),)
VENDOR := $(CONFIG_SYS_VENDOR:"%"=%) //厂商 myzr
endif
ifneq ($(CONFIG_SYS_SOC),)
SOC := $(CONFIG_SYS_SOC:"%"=%)     //SOC imx8m
endif

# Some architecture config.mk files need to know what CPUDIR is set to,
# so calculate CPUDIR before including ARCH/SOC/CPU config.mk files.
# Check if arch/$ARCH/cpu/$CPU exists, otherwise assume arch/$ARCH/cpu contains
# CPU-specific code.
CPUDIR=arch/$(ARCH)/cpu$(if $(CPU),/$(CPU),) //  arch/arm/cpu/armv8

sinclude $(srctree)/arch/$(ARCH)/config.mk	# include architecture dependend rules // arch/arm/config.mk
sinclude $(srctree)/$(CPUDIR)/config.mk		# include  CPU	specific rules //arch/arm/cpu/armv8/config.mk

ifdef	SOC
sinclude $(srctree)/$(CPUDIR)/$(SOC)/config.mk	# include  SoC	specific rules //arch/arm/cpu/armv8/imx8m/config.mk
endif
ifneq ($(BOARD),)
ifdef	VENDOR
BOARDDIR = $(VENDOR)/$(BOARD) // myzr/myimx8mm
else
BOARDDIR = $(BOARD)
endif
endif
ifdef	BOARD
sinclude $(srctree)/board/$(BOARDDIR)/config.mk	# include board specific rules //board/myzr/myimx8mm/config.mk
endif

ifdef FTRACE
PLATFORM_CPPFLAGS += -finstrument-functions -DFTRACE  //进行函数跟踪,一般不开，调试时可以打开
endif

# Allow use of stdint.h if available
//如果您的工具链可以使用 stdint.h，您可以定义它启用它的选项。 您可以在以下情况下提供选项“USE_STDINT=1”构建 U-Boot 来实现这一点。
ifneq ($(USE_STDINT),)
PLATFORM_CPPFLAGS += -DCONFIG_USE_STDINT
endif

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

RELFLAGS := $(PLATFORM_RELFLAGS)

PLATFORM_CPPFLAGS += $(RELFLAGS)
PLATFORM_CPPFLAGS += -pipe  //使用管道代替临时文件。

LDFLAGS += $(PLATFORM_LDFLAGS)
LDFLAGS_FINAL += -Bstatic

export PLATFORM_CPPFLAGS
export RELFLAGS
export LDFLAGS_FINAL
export CONFIG_STANDALONE_LOAD_ADDR

3.1 架构(ARCH)特定规则

# arch/arm/config.mk
# (C) Copyright 2000-2002
# Wolfgang Denk, DENX Software Engineering, wd@denx.de.
#
# SPDX-License-Identifier:	GPL-2.0+
#

ifndef CONFIG_STANDALONE_LOAD_ADDR
ifneq ($(CONFIG_ARCH_OMAP2PLUS),) //未定义
CONFIG_STANDALONE_LOAD_ADDR = 0x80300000
else
CONFIG_STANDALONE_LOAD_ADDR = 0xc100000
endif
endif

CFLAGS_NON_EFI := -fno-pic -ffixed-r9 -ffunction-sections -fdata-sections
//-fno-pic           : -fno-pic,-fno-PIC是同义的，生成position-dependent code
//-ffixed-r9          :生成的代码不要用寄存器r9，uboot中用来指向global data
//-ffunction-sections :把每个函数放入单独的section中，如果目标文件支持任意数量section
//-fdata-sections     :同上，只是对象是数据
CFLAGS_EFI := -fpic -fshort-wchar
//-fpic               :生成位置无关的代码,即代码无绝对跳转，都是相对跳转
//-fshort-wchar       :强制将wchar_t指定成两个字节，使用这个字段常常是因为wchar_t类型在Windows和Linux平台下字节大小的不同，但这样做只会改变代码中实现的部分，而内部库或者是第三方库中用到的接口和函数都是没有变的，仍然采用的是4字节编码
LDFLAGS_FINAL += --gc-sections
//--gc-sections       :在链接生成最终可执行文件时，如果带有-Wl,--gc-sections参数，并且之前编译目标文件时带有-ffunction-sections、-fdata-sections参数，则链接器ld不会链接未使用的函数，从而减小可执行文件大小；
PLATFORM_RELFLAGS += -ffunction-sections -fdata-sections \
		     -fno-common -ffixed-r9
PLATFORM_RELFLAGS += $(call cc-option, -msoft-float) \
      $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
//-fno-common        :它指定编译器将未初始化的全局变量放在对象文件的BSS部分。
//-f-mshort-load-bytes、-malignment-traps  :-mshort-load-bytes 在低版本gcc中使用，and -malignment-traps在高版本gcc中使用
# LLVM support
LLVMS_RELFLAGS		:= $(call cc-option,-mllvm,) \
			$(call cc-option,-target arm-none-eabi,) \
			$(call cc-option,-arm-use-movt=0,)
PLATFORM_RELFLAGS	+= $(LLVM_RELFLAGS)

PLATFORM_CPPFLAGS += -D__ARM__  //定义宏ARM

ifdef CONFIG_ARM64
PLATFORM_ELFFLAGS += -B aarch64 -O elf64-littleaarch64
else
PLATFORM_ELFFLAGS += -B arm -O elf32-littlearm
endif
//-fno-common        :放置未初始化的全局变量，这里不放到common block中，而是放到data section中
# Choose between ARM/Thumb instruction sets
ifeq ($(CONFIG_$(SPL_)SYS_THUMB_BUILD),y)
AFLAGS_IMPLICIT_IT	:= $(call as-option,-Wa$(comma)-mimplicit-it=always)
PF_CPPFLAGS_ARM		:= $(AFLAGS_IMPLICIT_IT) \
			$(call cc-option, -mthumb -mthumb-interwork,\
			$(call cc-option,-marm,)\
			$(call cc-option,-mno-thumb-interwork,)\
		)
else
PF_CPPFLAGS_ARM := $(call cc-option,-marm,) \
		$(call cc-option,-mno-thumb-interwork,)
endif
//-marm                :和-mthumb用来执行生成的代码在arm模式还是thumb模式执行
//-mno-thumb-interwork :没有ARM/Thumb之间的切换
# Only test once
ifeq ($(CONFIG_$(SPL_)SYS_THUMB_BUILD),y)
archprepare: checkthumb checkgcc6

checkthumb:
	@if test "$(call cc-name)" = "gcc" -a \
			"$(call cc-version)" -lt "0404"; then \
		echo -n '*** Your GCC does not produce working '; \
		echo 'binaries in THUMB mode.'; \
		echo '*** Your board is configured for THUMB mode.'; \
		false; \
	fi
else //判断gcc的版本是否低于6.0
archprepare: checkgcc6
endif

checkgcc6:
	@if test "$(call cc-name)" = "gcc" -a \
			"$(call cc-version)" -lt "0600"; then \
		echo '*** Your GCC is older than 6.0 and will not be supported'; \
	fi


# Try if EABI is supported, else fall back to old API,
# i. e. for example:
# - with ELDK 4.2 (EABI supported), use:
#	-mabi=aapcs-linux
# - with ELDK 4.1 (gcc 4.x, no EABI), use:
#	-mabi=apcs-gnu
# - with ELDK 3.1 (gcc 3.x), use:
#	-mapcs-32
PF_CPPFLAGS_ABI := $(call cc-option,\
			-mabi=aapcs-linux,\
			$(call cc-option,\
				-mapcs-32,\
				$(call cc-option,\
					-mabi=apcs-gnu,\
				)\
			)\
		)
PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_ARM) $(PF_CPPFLAGS_ABI)
//-mabi=aapcs-linux : 针对不同系统的调用规则生成代码

# For EABI, make sure to provide raise()
ifneq (,$(findstring -mabi=aapcs-linux,$(PLATFORM_CPPFLAGS)))
# This file is parsed many times, so the string may get added multiple
# times. Also, the prefix needs to be different based on whether
# CONFIG_SPL_BUILD is defined or not. 'filter-out' the existing entry
# before adding the correct one.
PLATFORM_LIBS := arch/arm/lib/eabi_compat.o \
	$(filter-out arch/arm/lib/eabi_compat.o, $(PLATFORM_LIBS))
endif

# needed for relocation
LDFLAGS_u-boot += -pie

/*# FIXME: binutils versions < 2.22 have a bug in the assembler where
# branches to weak symbols can be incorrectly optimized in thumb mode
# to a short branch (b.n instruction) that won't reach when the symbol
# gets preempted
#
# http://sourceware.org/bugzilla/show_bug.cgi?id=12532
#*/
ifeq ($(CONFIG_$(SPL_)SYS_THUMB_BUILD),y)
ifeq ($(GAS_BUG_12532),)
export GAS_BUG_12532:=$(shell if [ $(call binutils-version) -lt 0222 ] ; \
	then echo y; else echo n; fi)
endif
ifeq ($(GAS_BUG_12532),y)
PLATFORM_RELFLAGS += -fno-optimize-sibling-calls
endif
endif

ifneq ($(CONFIG_SPL_BUILD),y)
# Check that only R_ARM_RELATIVE relocations are generated.
ALL-y += checkarmreloc
# The movt / movw can hardcode 16 bit parts of the addresses in the
# instruction. Relocation is not supported for that case, so disable
# such usage by requiring word relocations.
PLATFORM_CPPFLAGS += $(call cc-option, -mword-relocations)
PLATFORM_CPPFLAGS += $(call cc-option, -fno-pic)
endif
//-mword-relocations : 由于使用pic时movt / movw指令会硬编码16bit的地址域，而uboot的relocation并不支持这个， 
所以arm平台使用mword-relocations来生成位置无关代码
# limit ourselves to the sections we want in the .bin.
ifdef CONFIG_ARM64
OBJCOPYFLAGS += -j .text -j .secure_text -j .secure_data -j .rodata -j .data \
		-j .u_boot_list -j .rela.dyn -j .got -j .got.plt \
		-j .binman_sym_table
else
OBJCOPYFLAGS += -j .text -j .secure_text -j .secure_data -j .rodata -j .hash \
		-j .data -j .got -j .got.plt -j .u_boot_list -j .rel.dyn \
		-j .binman_sym_table
endif

# if a dtb section exists we always have to include it
# there are only two cases where it is generated
# 1) OF_EMBEDED is turned on
# 2) unit tests include device tree blobs
OBJCOPYFLAGS += -j .dtb.init.rodata

ifdef CONFIG_EFI_LOADER
OBJCOPYFLAGS += -j .efi_runtime -j .efi_runtime_rel
endif

ifdef CONFIG_IMX_M4_BIND
OBJCOPYFLAGS += -j .firmware_image
endif

ifneq ($(CONFIG_IMX_CONFIG),)
ifdef CONFIG_SPL
ifndef CONFIG_SPL_BUILD
ALL-y += SPL
endif
else
ifeq ($(CONFIG_OF_SEPARATE),y)
ALL-y += u-boot-dtb.imx
else
ALL-y += u-boot.imx
endif
endif
ifneq ($(CONFIG_VF610),)
ALL-y += u-boot.vyb
endif
endif

EFI_LDS := elf_arm_efi.lds
EFI_CRT0 := crt0_arm_efi.o
EFI_RELOC := reloc_arm_efi.o

3.2 CPU特定规则

# arch/arm/cpu/armv8/config.mk
#
# (C) Copyright 2002
# Gary Jennejohn, DENX Software Engineering, <garyj@denx.de>
#
# SPDX-License-Identifier:	GPL-2.0+
#
PLATFORM_RELFLAGS += -fno-common -ffixed-x18

PF_NO_UNALIGNED := $(call cc-option, -mstrict-align)
PLATFORM_CPPFLAGS += $(PF_NO_UNALIGNED)
//-mstrict-align : 避免非对齐内存访问
EFI_LDS := elf_aarch64_efi.lds
EFI_CRT0 := crt0_aarch64_efi.o
EFI_RELOC := reloc_aarch64_efi.o

3.3 SoC (SoC )特定规则

我使用的板子未定义，可根据自己的需要添加。

3.4 板级(BOARD)特定规则

我使用的板子未定义，可根据自己的需要添加。

4. 总结

顶层config.mk会被 ./Makefile 和 spl/Makefile 所包含，主要内容如下：

（1）定义架构、CPU、平台、板、厂商和SOC；
（2）包含架构(ARCH)特定规则(比如：arch/arm/config.mk)；
（3）包含CPU(CPU)特定规则(比如：arch/arm/cpu/armv8/config.mk)；
（4）包含SoC (SoC )特定规则(比如：arch/arm/cpu/armv8/imx8m/config.mk)；
（5）包含板级(BOARD)特定规则(比如：board/myzr/myimx8mm/config.mk)；
（6）特定的编译选项(比如：-pipe等)；