openwrt的编译系统是如何制作根文件系统的

答:分析以下makefile即可获取整个过程

以nxp layerscape系统的编译过程为例

1.分析target/linux/layerscape/image/Makefile的最后一句,这是一个好的开端

$(eval $(call BuildImage))

2.宏BuildImage定义在哪里呢?定义在include/image.mk中,简化后如下

define BuildImage
    ...
    $(foreach device,$(TARGET_DEVICES),$(call Device,$(device))

    install-images: kernel_prepare $(foreach fs,$(filter-out $(if $(UBIFS_OPTS),,ubifs),$(TARGET_FILESYSTEMS) $(fs-subtypes-y)),$(KDIR)/root.$(fs))
        $(foreach fs,$(TARGET_FILESYSTEMS),
                $(call Image/Build,$(fs))
        )
     ...
endef

3.在宏定义BuildImage中会由上到下根据依赖依次执行,我们首先关注

$(foreach device,$(TARGET_DEVICES),$(call Device,$(device))

4.宏Device又定义在哪里呢?同样定义在include/image.mk文件中,定义简化后如下

define Device
  ...
  $(call Device/$(if $(DUMP),Dump,Build),$(1))

endef

5.此时会执行宏Device/Dump或宏Device/Build,那么以执行宏Device/Build为例,宏Device/Build简化后如下

define Device/Build
  ...

  $$(eval $$(foreach compile,$$(COMPILE), 
    $$(call Device/Build/compile,$$(compile),$(1))))

  $$(eval $$(foreach image,$$(IMAGES), 
    $$(foreach fs,$$(filter $(TARGET_FILESYSTEMS),$$(FILESYSTEMS)), 
      $$(call Device/Build/image,$$(fs),$$(image),$(1)))))
endef

6.接着执行宏Device/Build/compile和Device/Build/image,这两个宏都定义在include/image.mk中,在Device/Build/image会做根文件系统制作方面的工作