OpenWrt kernel install分析（1）

一. 前言

        本文主要介绍OpenWrt内核的编译和安装流程的流程。

二. 流程分析

1. 在include/toplevel.mk中加入如下语句

include/toplevel.mk:
kernel_install: prepare_kernel_conf
	$(_SINGLE)$(NO_TRACE_MAKE) -C target/linux install

         此时，在OpenWrt源码的根目录下就可以执行make kernel_install编译安装内核源码了

2. 进入target/linux，BOARD=mediatek，所以进入target/linux/mediatek下，Makefile源码如下：

target/linux/Makefile:
include $(TOPDIR)/rules.mk
include $(INCLUDE_DIR)/target.mk

export TARGET_BUILD=1

prereq clean download prepare compile install oldconfig menuconfig nconfig xconfig update refresh install-image clean-linux: FORCE
	@+$(NO_TRACE_MAKE) -C $(BOARD) $@

@+$(NO_TRACE_MAKE) -C $(BOARD) $@指令展开为make -C mediatek install。注意，这里TARGET_BUILD=1了。

3. include/linux/mediatek/Makefile的源码如下：

target/linux/mediatek/Makefile:
# Copyright (c) 2015 OpenWrt.org
#
include $(TOPDIR)/rules.mk

ARCH:=arm
BOARD:=mediatek
BOARDNAME:=MediaTek Ralink ARM
SUBTARGETS:=mt7622 mt7623 mt7629 mt7986 mt7981
FEATURES:=squashfs nand ramdisk fpu

KERNEL_PATCHVER:=5.4
KERNEL_TESTING_PATCHVER:=5.4

include $(INCLUDE_DIR)/target.mk
DEFAULT_PACKAGES += \
	kmod-leds-gpio kmod-gpio-button-hotplug

$(eval $(call BuildTarget))

include/target.mk:
......
include $(INCLUDE_DIR)/kernel.mk
ifeq ($(TARGET_BUILD),1)
  include $(INCLUDE_DIR)/kernel-build.mk
  BuildTarget?=$(BuildKernel)
endif

        由上分析可知，此时BuildTarget=$(BuildKernel)。

4. make -C mediatek install命令的install目标在BuildKernel中，BuildKernel代码如下：

include/kernel-build.mk：
define BuildKernel
  $(if $(QUILT),$(Build/Quilt))
  $(if $(LINUX_SITE),$(call Download,kernel))
  $(if $(call qstrip,$(CONFIG_KERNEL_GIT_CLONE_URI)),$(call Download,git-kernel))

  # 所有命令按照串行方式执行
  .NOTPARALLEL:

  $(Kernel/Autoclean)
  $(STAMP_PREPARED): $(if $(LINUX_SITE),$(DL_DIR)/$(LINUX_SOURCE))
	-rm -rf $(KERNEL_BUILD_DIR)
	-mkdir -p $(KERNEL_BUILD_DIR)
	$(Kernel/Prepare)
	touch $$@

  $(STAMP_CONFIGURED): $(STAMP_PREPARED) $(LINUX_KCONFIG_LIST) $(TOPDIR)/.config FORCE
	$(Kernel/Configure)
	touch $$@

......
  $(LINUX_DIR)/.image: export STAGING_PREFIX=$$(STAGING_DIR_HOST)
  $(LINUX_DIR)/.image: export PKG_CONFIG_PATH=$$(STAGING_DIR_HOST)/lib/pkgconfig
  $(LINUX_DIR)/.image: export PKG_CONFIG_LIBDIR=$$(STAGING_DIR_HOST)/lib/pkgconfig
  $(LINUX_DIR)/.image: $(STAMP_CONFIGURED) $(if $(CONFIG_STRIP_KERNEL_EXPORTS),$(KERNEL_BUILD_DIR)/symtab.h) FORCE
	$(Kernel/CompileImage)
	$(Kernel/CollectDebug)
	touch $$@
	
  mostlyclean: FORCE
	$(Kernel/Clean)

......
  oldconfig menuconfig nconfig xconfig: $(STAMP_PREPARED) $(STAMP_CHECKED) FORCE
	rm -f $(LINUX_DIR)/.config.prev
	rm -f $(STAMP_CONFIGURED)
	$(LINUX_RECONF_CMD) > $(LINUX_DIR)/.config
	$(_SINGLE)$(KERNEL_MAKE) \
		$(if $(findstring Darwin,$(HOST_OS)), \
			HOST_LOADLIBES="-L$(STAGING_DIR_HOST)/lib -lncurses" \
			HOSTLDLIBS_mconf="-L$(STAGING_DIR_HOST)/lib -lncurses" \
			filechk_conf_cfg="	:" \
		) \
		YACC=$(STAGING_DIR_HOST)/bin/bison \
		$$@
	$(call LINUX_RECONF_DIFF,$(LINUX_DIR)/.config) > $(LINUX_RECONFIG_TARGET)

  install: $(LINUX_DIR)/.image
	+$(MAKE) -C image compile install TARGET_BUILD=
......
endef

        由上可知，install目标依赖于$(LINUX_DIR)/.image，$(LINUX_DIR)/.image依赖$(STAMP_CONFIGURED)，$(STAMP_CONFIGURED)依赖$(STAMP_PREPARED)。

$(STAMP_PREPARED)目标的实现如下：

define Kernel/Prepare/Default
	$(LINUX_CAT) $(DL_DIR)/$(LINUX_SOURCE) | $(TAR) -C $(KERNEL_BUILD_DIR) $(TAR_OPTIONS)
	$(Kernel/Patch)
	$(if $(QUILT),touch $(LINUX_DIR)/.quilt_used)
endef

define Kernel/Prepare
	$(call Kernel/Prepare/Default)
endef

$(STAMP_PREPARED): $(if $(LINUX_SITE),$(DL_DIR)/$(LINUX_SOURCE))
	-rm -rf $(KERNEL_BUILD_DIR)
	-mkdir -p $(KERNEL_BUILD_DIR)
	$(Kernel/Prepare)
	touch $$@

        以上代码主要工作：解压linux源码包，为源码打patch。

$(STAMP_CONFIGURED)目标的实现如下：

define Kernel/Configure/Default
	rm -f $(LINUX_DIR)/localversion
	$(LINUX_CONF_CMD) > $(LINUX_DIR)/.config.target
# copy CONFIG_KERNEL_* settings over to .config.target
	awk '/^(#[[:space:]]+)?CONFIG_KERNEL/{sub("CONFIG_KERNEL_","CONFIG_");print}' $(TOPDIR)/.config >> $(LINUX_DIR)/.config.target
	echo "# CONFIG_KALLSYMS_EXTRA_PASS is not set" >> $(LINUX_DIR)/.config.target
	echo "# CONFIG_KALLSYMS_ALL is not set" >> $(LINUX_DIR)/.config.target
	echo "CONFIG_KALLSYMS_UNCOMPRESSED=y" >> $(LINUX_DIR)/.config.target
	$(SCRIPT_DIR)/package-metadata.pl kconfig $(TMP_DIR)/.packageinfo $(TOPDIR)/.config $(KERNEL_PATCHVER) > $(LINUX_DIR)/.config.override
	$(SCRIPT_DIR)/kconfig.pl 'm+' '+' $(LINUX_DIR)/.config.target /dev/null $(LINUX_DIR)/.config.override > $(LINUX_DIR)/.config.set
	$(call Kernel/SetNoInitramfs)
	rm -rf $(KERNEL_BUILD_DIR)/modules
	cmp -s $(LINUX_DIR)/.config.set $(LINUX_DIR)/.config.prev || { \
		cp $(LINUX_DIR)/.config.set $(LINUX_DIR)/.config; \
		cp $(LINUX_DIR)/.config.set $(LINUX_DIR)/.config.prev; \
	}
	$(_SINGLE) [ -d $(LINUX_DIR)/user_headers ] || $(KERNEL_MAKE) INSTALL_HDR_PATH=$(LINUX_DIR)/user_headers headers_install
	grep '=[ym]' $(LINUX_DIR)/.config.set | LC_ALL=C sort | mkhash md5 > $(LINUX_DIR)/.vermagic
endef

define Kernel/Configure
	$(call Kernel/Configure/Default)
endef

$(STAMP_CONFIGURED): $(STAMP_PREPARED) $(LINUX_KCONFIG_LIST) $(TOPDIR)/.config FORCE
	$(Kernel/Configure)
	touch $$@

        以上代码主要工作：生成内核的.config配置文件，安装内核的头文件(user_header)。

$(LINUX_DIR)/.image目标的实现：

include/kernel-defaults.mk:
KERNEL_BUILD_DIR=build_dir/target-xxx/linux-mediatek_mt7981
define Kernel/CopyImage
	cmp -s $(LINUX_DIR)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).debug || { \
		$(KERNEL_CROSS)objcopy -O binary $(OBJCOPY_STRIP) -S $(LINUX_DIR)/vmlinux $(LINUX_KERNEL)$(1); \
		$(KERNEL_CROSS)objcopy $(OBJCOPY_STRIP) -S $(LINUX_DIR)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).elf; \
		$(CP) $(LINUX_DIR)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).debug; \
        # cp -fpR */linux-5.4.203/arch/arm64/boot//Image */linux-mediatek_mt7981/Image
		$(foreach k, \
			$(if $(KERNEL_IMAGES),$(KERNEL_IMAGES),$(filter-out vmlinux dtbs,$(KERNELNAME))), \
			$(CP) $(LINUX_DIR)/arch/$(LINUX_KARCH)/boot/$(IMAGES_DIR)/$(k) $(KERNEL_BUILD_DIR)/$(k)$(1); \
		) \
	}
endef


include/kernel-defaults.mk:
define Kernel/SetInitramfs
	rm -f $(LINUX_DIR)/.config.prev
	mv $(LINUX_DIR)/.config $(LINUX_DIR)/.config.old
	$(call Kernel/SetInitramfs/PreConfigure)
	echo 'CONFIG_INITRAMFS_ROOT_UID=$(shell id -u)' >> $(LINUX_DIR)/.config
	echo 'CONFIG_INITRAMFS_ROOT_GID=$(shell id -g)' >> $(LINUX_DIR)/.config
	echo "$(if $(CONFIG_TARGET_INITRAMFS_FORCE),CONFIG_INITRAMFS_FORCE=y,# CONFIG_INITRAMFS_FORCE is not set)" >> $(LINUX_DIR)/.config
	echo "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_NONE),CONFIG_INITRAMFS_COMPRESSION_NONE=y,# CONFIG_INITRAMFS_COMPRESSION_NONE is not set)" >> $(LINUX_DIR)/.config
	echo -e "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_GZIP),CONFIG_INITRAMFS_COMPRESSION_GZIP=y\nCONFIG_RD_GZIP=y,# CONFIG_INITRAMFS_COMPRESSION_GZIP is not set\n# CONFIG_RD_GZIP is not set)" >> $(LINUX_DIR)/.config
	echo -e "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_BZIP2),CONFIG_INITRAMFS_COMPRESSION_BZIP2=y\nCONFIG_RD_BZIP2=y,# CONFIG_INITRAMFS_COMPRESSION_BZIP2 is not set\n# CONFIG_RD_BZIP2 is not set)" >> $(LINUX_DIR)/.config
	echo -e "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_LZMA),CONFIG_INITRAMFS_COMPRESSION_LZMA=y\nCONFIG_RD_LZMA=y,# CONFIG_INITRAMFS_COMPRESSION_LZMA is not set\n# CONFIG_RD_LZMA is not set)" >> $(LINUX_DIR)/.config
	echo -e "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_LZO),CONFIG_INITRAMFS_COMPRESSION_LZO=y\nCONFIG_RD_LZO=y,# CONFIG_INITRAMFS_COMPRESSION_LZO is not set\n# CONFIG_RD_LZO is not set)" >> $(LINUX_DIR)/.config
	echo -e "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_XZ),CONFIG_INITRAMFS_COMPRESSION_XZ=y\nCONFIG_RD_XZ=y,# CONFIG_INITRAMFS_COMPRESSION_XZ is not set\n# CONFIG_RD_XZ is not set)" >> $(LINUX_DIR)/.config
	echo -e "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_LZ4),CONFIG_INITRAMFS_COMPRESSION_LZ4=y\nCONFIG_RD_LZ4=y,# CONFIG_INITRAMFS_COMPRESSION_LZ4 is not set\n# CONFIG_RD_LZ4 is not set)" >> $(LINUX_DIR)/.config
	echo -e "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_ZSTD),CONFIG_INITRAMFS_COMPRESSION_ZSTD=y\nCONFIG_RD_STD=y,# CONFIG_INITRAMFS_COMPRESSION_ZSTD is not set\n# CONFIG_RD_ZSTD is not set)" >> $(LINUX_DIR)/.config
  endef

include/kernel-defaults.mk:
define Kernel/Configure/Initramfs
	$(call Kernel/SetInitramfs)
endef

define Kernel/CompileImage/Initramfs
	$(call Kernel/Configure/Initramfs)
#	$(CP) $(GENERIC_PLATFORM_DIR)/other-files/init $(TARGET_DIR)/init
	$(LN) usr/bin/gateway $(TARGET_DIR)/init
	$(if $(SOURCE_DATE_EPOCH),touch -hcd "@$(SOURCE_DATE_EPOCH)" $(TARGET_DIR)/init)
	rm -rf $(KERNEL_BUILD_DIR)/linux-$(LINUX_VERSION)/usr/initramfs_data.cpio*
	+$(KERNEL_MAKE) $(if $(KERNELNAME),$(KERNELNAME),all) modules
    # cp -fpR */linux-5.4.203/arch/arm64/boot//Image */linux-mediatek_mt7981/Image-initramfs
	$(call Kernel/CopyImage,-initramfs)
endef

define Kernel/CompileImage/Default
    # TARGET_DIR=build_dir/target-xxx/root-mediatek
	rm -f $(TARGET_DIR)/init
    # KERNEL_MAKE=$(MAKE) -j12 -C $(LINUX_DIR) $(KERNEL_MAKE_FLAGS)
    # KERNELNAME=Image dtbs，
	+$(KERNEL_MAKE) $(if $(KERNELNAME),$(KERNELNAME),all) modules
	$(call Kernel/CopyImage)
endef

define Kernel/CompileImage
	$(call Kernel/CompileImage/Default)
	$(call Kernel/CompileImage/Initramfs)
endef

$(LINUX_DIR)/.image: $(STAMP_CONFIGURED) $(if $(CONFIG_STRIP_KERNEL_EXPORTS),$(KERNEL_BUILD_DIR)/symtab.h) FORCE
	$(Kernel/CompileImage)
	$(Kernel/CollectDebug)
	touch $$@

三. 总结

        至此，install目标的$(LINUX_DIR)/.image的依赖执行完毕，主要做了如下事情：

1. 解压内核，为内核打patch，安装内核头文件(user_header)。

2. 配置内核，主要是通过拷贝target/linux下的config-xxx文件到build_dir下的内核源码路径。

3. 编译生成内核镜像，拷贝到对应目录。