u-boot-2018.05之make编译过程目标依赖分析

u-boot的编译分两步：
- 第一步：配置，执行make xxx_defconfig进行各项配置，生成.config文件 (u-boot-2018.05之make配置过程分析)
- 第二步：编译，执行make进行编译，生成可执行的二进制文件u-boot.*

顶层目标依赖

a). `_all`和`all`对`$(ALL-y)`的依赖

从顶层Makefile开始查找，首先找到的是_all伪目标：

# That's our default target when none is given on the command line
PHONY := _all
_all:

紧接着会对_all伪目标添加all伪目标的依赖：

PHONY += all
ifeq ($(KBUILD_EXTMOD),)
_all: all
else
_all: modules
endif

all自身依赖于$(ALL-y)

all: $(ALL-y)

b). `$(ALL-y)`对`u-boot`目标文件的依赖

$(ALL-y)定义了最终需要生成所有文件：

# 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 u-boot.cfg binary_size_check

ALL-$(CONFIG_ONENAND_U_BOOT) += u-boot-onenand.bin
ifeq ($(CONFIG_SPL_FSL_PBL),y)
ALL-$(CONFIG_RAMBOOT_PBL) += u-boot-with-spl-pbl.bin
else
ifneq ($(CONFIG_SECURE_BOOT), y)
# 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
ALL-$(CONFIG_SPL_FRAMEWORK) += u-boot.img
ALL-$(CONFIG_TPL) += tpl/u-boot-tpl.bin
ALL-$(CONFIG_OF_SEPARATE) += u-boot.dtb
ifeq ($(CONFIG_SPL_FRAMEWORK),y)
ALL-$(CONFIG_OF_SEPARATE) += u-boot-dtb.img
endif
ALL-$(CONFIG_OF_HOSTFILE) += u-boot.dtb
ifneq ($(CONFIG_SPL_TARGET),)
ALL-$(CONFIG_SPL) += $(CONFIG_SPL_TARGET:"%"=%)
endif
ALL-$(CONFIG_REMAKE_ELF) += u-boot.elf
ALL-$(CONFIG_EFI_APP) += u-boot-app.efi
ALL-$(CONFIG_EFI_STUB) += u-boot-payload.efi

ifneq ($(BUILD_ROM),)
ALL-$(CONFIG_X86_RESET_VECTOR) += u-boot.rom
endif

# enable combined SPL/u-boot/dtb rules for tegra
ifeq ($(CONFIG_TEGRA)$(CONFIG_SPL),yy)
ALL-y += u-boot-tegra.bin u-boot-nodtb-tegra.bin
ALL-$(CONFIG_OF_SEPARATE) += u-boot-dtb-tegra.bin
endif

以上的$(ALL-y)目标中看起来非常复杂，但除了第一行的通用目标外，其余目标都只在特殊条件下才生成，这里略去不提。只分析通用目标依赖：

ALL-y += u-boot.srec u-boot.bin u-boot.sym System.map u-boot.cfg binary_size_check

i. 依赖`u-boot.bin`

依赖u-boot.bin：

ifeq ($(CONFIG_OF_SEPARATE),y)
u-boot-dtb.bin: u-boot-nodtb.bin dts/dt.dtb FORCE
    $(call if_changed,cat)

u-boot.bin: u-boot-dtb.bin FORCE
    $(call if_changed,copy)
else
u-boot.bin: u-boot-nodtb.bin FORCE
    $(call if_changed,copy)
endif

如果打开了device tree支持，则有依赖关系：
u-boot.bin --> u-boot-dtb.bin --> u-boot-nodtb.bin + dts/dt.dtb
这里没有打开device tree支持，所以：
u-boot.bin --> u-boot-nodtb.bin

进一步，对于u-boot-nodtb.bin，其规则是：

u-boot-nodtb.bin: u-boot FORCE
    $(call if_changed,objcopy)
    $(call DO_STATIC_RELA,$<,$@,$(CONFIG_SYS_TEXT_BASE))
    $(BOARD_SIZE_CHECK)

ii. 依赖`System.map`

依赖System.map：

System.map: u-boot
        @$(call SYSTEM_MAP,$<) > $@

iii. 依赖`u-boot.cfg`

依赖u-boot.cfg：

u-boot.cfg: include/config.h FORCE
    $(call if_changed,cpp_cfg)

2. `u-boot`文件目标依赖

a). 依赖`u-boot`

依赖u-boot：

u-boot: $(u-boot-init) $(u-boot-main) u-boot.lds FORCE
    $(call if_changed,u-boot__)
ifeq ($(CONFIG_KALLSYMS),y)
    $(call cmd,smap)
    $(call cmd,u-boot__) common/system_map.o
endif

其中$(u-boot-init)和$(u-boot-main)分别被定义为：

u-boot-init := $(head-y)
u-boot-main := $(libs-y)

i. 依赖`$(head-y)`

$(head-y)在arch/arm/Makefile被定义为：

head-y := arch/arm/cpu/$(CPU)/start.o

ii. 依赖`$(libs-y)`

在顶层Makefile中搜索一下$(libs-y)，其被定义为各层驱动目录下build-in.o的集合：

libs-y += lib/
...此处...libs-y := $(sort $(libs-y))
u-boot-dirs := $(patsubst %/,%,$(filter %/, $(libs-y))) tools examples
libs-y      := $(patsubst %/, %/built-in.o, $(libs-y))u-boot-main := $(libs-y)

b). 依赖`u-boot.lds`

依赖u-boot.lds：

u-boot.lds: $(LDSCRIPT) prepare FORCE
    $(call if_changed_dep,cpp_lds)

其中$(LDSCRIPT)的定义如下：

# If board code explicitly specified LDSCRIPT or CONFIG_SYS_LDSCRIPT, use
# that (or fail if absent).  Otherwise, search for a linker script in a
# standard location.

ifndef LDSCRIPT
    #LDSCRIPT := $(srctree)/board/$(BOARDDIR)/u-boot.lds.debug
    ifdef CONFIG_SYS_LDSCRIPT
        # need to strip off double quotes
        LDSCRIPT := $(srctree)/$(CONFIG_SYS_LDSCRIPT:"%"=%)
    endif
endif

# If there is no specified link script, we look in a number of places for it
ifndef LDSCRIPT
    ifeq ($(wildcard $(LDSCRIPT)),)
        LDSCRIPT := $(srctree)/board/$(BOARDDIR)/u-boot.lds
    endif
    ifeq ($(wildcard $(LDSCRIPT)),)
        LDSCRIPT := $(srctree)/$(CPUDIR)/u-boot.lds
    endif
    ifeq ($(wildcard $(LDSCRIPT)),)
        LDSCRIPT := $(srctree)/arch/$(ARCH)/cpu/u-boot.lds
    endif
endif

如果没有定义LDSCRIPT和CONFIG_SYS_LDSCRIPT，则默认使用u-boot自带的lds文件。包括board/$(BOARDIDR)和$(CPUDIR)目录下定制的针对board或cpu的lds文件，如果没有定制的lds文件，则采用arch/$(ARCH)/cpu目录下默认的u-boot.lds

3. `prepare`系列目标依赖

a). `prepare`系列依赖的规则

实际上prepare是一系列prepare伪目标和动作的组合，完成编译前的准备工作：

# Things we need to do before we recursively start building the kernel
# or the modules are listed in "prepare".
# A multi level approach is used. prepareN is processed before prepareN-1.
# archprepare is used in arch Makefiles and when processed asm symlink,
# version.h and scripts_basic is processed / created.

# Listed in dependency order
PHONY += prepare archprepare prepare0 prepare1 prepare2 prepare3

# prepare3 is used to check if we are building in a separate output directory,
# and if so do:
# 1) Check that make has not been executed in the kernel src $(srctree)
prepare3: include/config/uboot.release
ifneq ($(KBUILD_SRC),)
    @$(kecho) '  Using $(srctree) as source for U-Boot'
    $(Q)if [ -f $(srctree)/.config -o -d $(srctree)/include/config ]; then \
        echo >&2 "  $(srctree) is not clean, please run 'make mrproper'"; \
        echo >&2 "  in the '$(srctree)' directory.";\
        /bin/false; \
    fi;
endif

# prepare2 creates a makefile if using a separate output directory
prepare2: prepare3 outputmakefile

prepare1: prepare2 $(version_h) $(timestamp_h) \
                   include/config/auto.conf
ifeq ($(wildcard $(LDSCRIPT)),)
    @echo >&2 "  Could not find linker script."
    @/bin/false
endif

archprepare: prepare1 scripts_basic

prepare0: archprepare FORCE
    $(Q)$(MAKE) $(build)=.

# All the preparing..
prepare: prepare0

伪目标prepare，prepare0，archprepare，prepare1，prepare2，prepare3之间的依赖如下：
<code>prepare</code>系列伪目标之间的依赖关系

b). `prepare`系列其他的依赖规则

在prepare1的依赖列表中，除了include/config/auto.conf外，还有$(version_h)和$(timestamp_h)，他们的依赖分别为：

$(version_h): include/config/uboot.release FORCE
    $(call filechk,version.h)

$(timestamp_h): $(srctree)/Makefile FORCE
    $(call filechk,timestamp.h)

这里的两个filechk函数调用会动态生成version.h和timestamp.h。

对于最里层的prepare3的依赖include/config/uboot.release，还存在下一级依赖：

# Store (new) UBOOTRELEASE string in include/config/uboot.release
include/config/uboot.release: include/config/auto.conf FORCE
    $(call filechk,uboot.release)

对于include/config/auto.conf，Makefile中有一个匹配的规则：

# If .config is newer than include/config/auto.conf, someone tinkered
# with it and forgot to run make oldconfig.
# if auto.conf.cmd is missing then we are probably in a cleaned tree so
# we execute the config step to be sure to catch updated Kconfig files
include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
    $(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig
    @# If the following part fails, include/config/auto.conf should be
    @# deleted so "make silentoldconfig" will be re-run on the next build.
    $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.autoconf || \
        { rm -f include/config/auto.conf; false; }
    @# include/config.h has been updated after "make silentoldconfig".
    @# We need to touch include/config/auto.conf so it gets newer
    @# than include/config.h.
    @# Otherwise, 'make silentoldconfig' would be invoked twice.
    $(Q)touch include/config/auto.conf

所以include/config/auto.conf依赖于$(KCONFIG_CONFIG)和include/config/auto.conf.cmd。
- $(KCONFIG_CONFIG)实际上就是.config文件；
- include/config/auto.conf.cmd是由fixdep在编译时生成的依赖文件。