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本文将继续讲解arm linux内核zImage的生成过程,内核版本号4.10。
在arch/arm/Makefile文件中,可以看到zImage 依赖于vmlinux,这里的vmlinux指的是根目录下的vmlinux。
arch/arm/Makefile:
boot := arch/arm/boot
BOOT_TARGETS = zImage Image xipImage bootpImage uImage
… …
$(BOOT_TARGETS):vmlinux
$(Q)$(MAKE) $(build)=$(boot)MACHINE=$(MACHINE) $(boot)/$@ --(1)
@$(kecho) ' Kernel: $(boot)/$@ is ready'
这里的vmlinux是编译生成的linux内核的elf文件:
#file vmlinux
vmlinux: ELF 32-bitLSB executable, ARM, version 1 (SYSV), statically linked, not stripped
其中,(build)=$(boot)被扩展为了scripts/Makefile.build obj=arch/arm/boot。
build是scripts/Kbuild.include中定义的变量:
build := -f$(srctree)/scripts/Makefile.build obj
boot := arch/arm/boot 直接指明该架构的boot文件生成路径,而MACHINE则是由用户配置来决定,嵌入式物联网更多资料企鹅意义气呜呜吧久零就易,毕竟一个ARM CPU可以和各类外设组成不同的机器架构。
ifneq ($(machine-y),)
MACHINE := arch/arm/mach-$(word 1,$(machine-y))/
else
MACHINE :=
endif
比如:
machine-$(CONFIG_ARCH_S3C64XX) := s3c6400 s3c6410
在内核配置文件.config可以找到CONFIG_ARCH_S3C64XX=y。
这样,语句(1)就可以解析成:
make -fscripts/Makefile.build obj=arch/arm/boot MACHINE=arch/arm/mach-s3c6400/arch/arm/boot/zImage
接着看scripts/Makefile.build,它包含arch/arm/boot/Makefile文件的方式有些特殊,它是从上面的命令行得到obj,然后找到对应文件夹下的Makefile并执行。
在scripts/Makefile.build的开头,src的值被赋值为arch/arm/boot:
src := $(obj)
… …
# The filename Kbuildhas precedence over Makefile
kbuild-dir := $(if$(filter /%,$(src)),$(src),$(srctree)/$(src))
kbuild-file := $(if$(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Kbuild,$(kbuild-dir)/Makefile)
include$(kbuild-file)
kbuild-file就是src指定路径下的Makefile文件,此时就是arch/arm/boot/Makefile,它包含了构建arch/arm/boot/zImage的规则。
PHONY := __build
… …
__build: $(if$(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) \
KBUILD_BUILTIN 在顶层Makefile 中被初始化为1,所以这个规则的依赖需要一个builtin-target 变量。
这个变量在scripts/Makefile.build中定义。
ifneq ($(strip$(obj-y) $(obj-m) $(obj-) $(subdir-m) $(lib-target)),)
builtin-target := $(obj)/built-in.o
endif
变量obj 就是vmlinux-dirs 变量指定的目录。所以这里会构建$(vmlinux-dirs)/built-in.o 目标,在scripts/Makefile.build文件中有这个目标的规则及命令的定义:
# If the list ofobjects to link is empty, just create an empty built-in.o
cmd_link_o_target =$(if $(strip $(obj-y)),\
$(cmd_make_builtin) $@$(filter $(obj-y), $^) \
$(cmd_secanalysis),\
$(cmd_make_empty_builtin)$@)
$(builtin-target):$(obj-y) FORCE
$(call if_changed,link_o_target)
vmlinux-dirs将在后面解释,它包含了所有由变量$(xx)代表的需要编译处理的文件夹。所有的目标文件生成规则在scripts/Makefile.build中定义如下:
# Built-in andcomposite module parts
$(obj)/%.o:$(src)/%.c $(recordmcount_source) $(objtool_obj) FORCE
$(call cmd,force_checksrc)
$(call if_changed_rule,cc_o_c)
vmlinux在生成完毕后,接着会执行make -fscripts/Makefile.build obj=arch/arm/boot MACHINE=arch/arm/mach-s3c6400/arch/arm/boot/zImage。arch/arm/boot/Makefile中定义了如下规则:
$(obj)/zImage: $(obj)/compressed/vmlinux FORCE
$(call if_changed,objcopy)
变量obj的值即是arch/arm/boot。显然zImage此时又依赖于$(obj)/compressed/vmlinux。
$(obj)/compressed/vmlinux:$(obj)/Image FORCE
$(Q)$(MAKE) $(build)=$(obj)/compressed$@
扩展开的命令如下:
make -f scripts/Makefile.buildobj=arch/arm/boot/compressed
arch/arm/boot/compressed/vmlinux
继续回到压缩vmlinux生成命令,make -f scripts/Makefile.build obj=arch/arm/boot/compressed arch/arm/boot/compressed/vmlinux。
此时obj=arch/arm/boot/compressed,所以scripts/Makefile.build会自动包含arch/arm/boot/compressed/Makefile,该文件指明了arch/arm/boot/compressed/vmlinux的生成规则。
arch/arm/boot/compressed/Makefile:
$(obj)/vmlinux:$(obj)/vmlinux.lds $(obj)/$(HEAD) $(obj)/piggy.o \
$(addprefix $(obj)/, $(OBJS))$(lib1funcs) $(ashldi3) \
$(bswapsdi2) $(efi-obj-y) FORCE
@$(check_for_multiple_zreladdr)
$(call if_changed,ld)
@$(check_for_bad_syms)
这两个规则的第一个就是把由vmlinux 进行objcopy生成的Image进行压缩生成piggy.gz,然后生成piggy.o。cmd_ld 命令在scripts/Makefile.lib 文件定义:
quiet_cmd_ld =LD $@
cmd_ld = $(LD)$(LDFLAGS) $(ldflags-y) $(LDFLAGS_$(@F)) \
$(filter-out FORCE,$^) -o $@
这里根据链接脚本arch/arm/boot/compressed/vmlinux.lds 链接生成了arch/arm/boot/compressed/vmlinux文件。然后在arch/arm/boot/Makefile规则中:
$(obj)/zImage:$(obj)/compressed/vmlinux FORCE
$(callif_changed,objcopy)
这样,经过objcopy处理后便生成了最终的zImage。