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Arm linux 内核构建(2)

已有 69 次阅读2017-11-15 11:38 |系统分类:电测仪表

本文将继续讲解arm linux内核zImage的生成过程,内核版本号4.10。

 

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

buildscripts/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。

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/zImagearch/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.ocmd_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

 


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