转载:http://blog.csdn.net/wavemcu/article/details/7270439
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作者:EasyWave
时间:2012.02.18
类别:linux驱动开发 声明:转载,请保留链接
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内核编译完成后会生成zImage内核镜像文件。zImage是如何解压的呢?本文将结合关键代码,讲解zImage的解压过程。还是先来看看zImage的组成吧。在内核编译完成后会在arch/arm/boot/下生成zImage。
在arch/arm/boot/Makefile中,如下代码:
# # arch/arm/boot/Makefile # # This file is included by the global makefile so that you can add your own # architecture-specific flags and dependencies. # # This file is subject to the terms and conditions of the GNU General Public # License. See the file "COPYING" in the main directory of this archive # for more details. # # Copyright (C) 1995-2002 Russell King # MKIMAGE := $(srctree)/scripts/mkuboot.sh ifneq ($(MACHINE),) include $(srctree)/$(MACHINE)/Makefile.boot endif # Note: the following conditions must always be true: # ZRELADDR == virt_to_phys(PAGE_OFFSET + TEXT_OFFSET) # PARAMS_PHYS must be within 4MB of ZRELADDR # INITRD_PHYS must be in RAM ZRELADDR := $(zreladdr-y) PARAMS_PHYS := $(params_phys-y) INITRD_PHYS := $(initrd_phys-y) export ZRELADDR INITRD_PHYS PARAMS_PHYS targets := Image zImage xipImage bootpImage uImage ifeq ($(CONFIG_XIP_KERNEL),y) $(obj)/xipImage: vmlinux FORCE $(call if_changed,objcopy) @echo ‘ Kernel: $@ is ready (physical address: $(CONFIG_XIP_PHYS_ADDR))‘ $(obj)/Image $(obj)/zImage: FORCE @echo ‘Kernel configured for XIP (CONFIG_XIP_KERNEL=y)‘ @echo ‘Only the xipImage target is available in this case‘ @false else $(obj)/xipImage: FORCE @echo ‘Kernel not configured for XIP (CONFIG_XIP_KERNEL!=y)‘ @false $(obj)/Image: vmlinux FORCE $(call if_changed,objcopy) @echo ‘ Kernel: $@ is ready‘ $(obj)/compressed/vmlinux: $(obj)/Image FORCE $(Q)$(MAKE) $(build)=$(obj)/compressed $@ $(obj)/zImage: $(obj)/compressed/vmlinux FORCE $(call if_changed,objcopy) @echo ‘ Kernel: $@ is ready‘ endif quiet_cmd_uimage = UIMAGE $@ cmd_uimage = $(CONFIG_SHELL) $(MKIMAGE) -A arm -O linux -T kernel -C none -a $(LOADADDR) -e $(STARTADDR) -n ‘Linux-$(KERNELRELEASE)‘ -d {1}lt; $@ ifeq ($(CONFIG_ZBOOT_ROM),y) $(obj)/uImage: LOADADDR=$(CONFIG_ZBOOT_ROM_TEXT) else $(obj)/uImage: LOADADDR=$(ZRELADDR) endif $(obj)/uImage: STARTADDR=$(LOADADDR) $(obj)/uImage: $(obj)/zImage FORCE $(call if_changed,uimage) @echo ‘ Image $@ is ready‘ $(obj)/bootp/bootp: $(obj)/zImage initrd FORCE $(Q)$(MAKE) $(build)=$(obj)/bootp $@ @: $(obj)/bootpImage: $(obj)/bootp/bootp FORCE $(call if_changed,objcopy) @echo ‘ Kernel: $@ is ready‘ PHONY += initrd FORCE initrd: @test "$(INITRD_PHYS)" != "" || (echo This machine does not support INITRD; exit -1) @test "$(INITRD)" != "" || (echo You must specify INITRD; exit -1) install: $(obj)/Image $(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) $(obj)/Image System.map "$(INSTALL_PATH)" zinstall: $(obj)/zImage $(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) $(obj)/zImage System.map "$(INSTALL_PATH)" zi: $(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) $(obj)/zImage System.map "$(INSTALL_PATH)" i: $(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) $(obj)/Image System.map "$(INSTALL_PATH)" subdir- := bootp compressed
我们将上面的部分内容抓取出来,独立来分析,这样就不会被其它的代码干扰到,见下面的代码:
$(obj)/Image: vmlinux
FORCE
$(call if_changed,objcopy)
@echo ‘ Kernel: $@ is
ready‘
$(obj)/zImage:
$(obj)/compressed/vmlinux FORCE
$(call if_changed,objcopy)
@echo ‘
Kernel: $@ is ready‘
由此可见,zImage是由arch/arm/boot/compressed/vmlinux二进制化得到的,与此同时在arch/armboot/compressed/Makefile中:
$(obj)/vmlinux: $(obj)/vmlinux.lds
$(obj)/$(HEAD) $(obj)/piggy.$(suffix_y).o \
$(addprefix $(obj)/, $(OBJS))
$(lib1funcs) FORCE
$(call if_changed,ld)
@:
$(obj)/piggy.$(suffix_y): $(obj)/../Image FORCE
$(call
if_changed,$(suffix_y))
$(obj)/piggy.$(suffix_y).o:
$(obj)/piggy.$(suffix_y) FORCE
其中Image是由内核顶层目录下的vmlinux二进制化后得到的[见上面蓝色加粗部分代码],其中在arch/armboot/compressed/Makefile链接选项中有个
–fpic参数:
EXTRA_CFLAGS :=
-fpic -fno-builtin
EXTRA_AFLAGS := -Wa,-march=all
总结一下zImage的组成,它是由一个压缩后的内核piggy.o,连接上一段初始化及解压功能的代码(head.o misc.o),组成的。见下面的代码:
# # linux/arch/arm/boot/compressed/Makefile # # create a compressed vmlinuz image from the original vmlinux # ...................... ...................... # # We now have a PIC decompressor implementation. Decompressors running # from RAM should not define ZTEXTADDR. Decompressors running directly # from ROM or Flash must define ZTEXTADDR (preferably via the config) # FIXME: Previous assignment to ztextaddr-y is lost here. See SHARK ifeq ($(CONFIG_ZBOOT_ROM),y) ZTEXTADDR := $(CONFIG_ZBOOT_ROM_TEXT) ZBSSADDR := $(CONFIG_ZBOOT_ROM_BSS) else ZTEXTADDR := 0 ZBSSADDR := ALIGN(8) endif SEDFLAGS = s/TEXT_START/$(ZTEXTADDR)/;s/BSS_START/$(ZBSSADDR)/ suffix_$(CONFIG_KERNEL_GZIP) = gzip suffix_$(CONFIG_KERNEL_LZO) = lzo suffix_$(CONFIG_KERNEL_LZMA) = lzma targets := vmlinux vmlinux.lds piggy.$(suffix_y) piggy.$(suffix_y).o font.o font.c head.o misc.o $(OBJS) # Make sure files are removed during clean extra-y += piggy.gzip piggy.lzo piggy.lzma lib1funcs.S
targets :=
vmlinux vmlinux.lds \
piggy.$(suffix_y) piggy.$(suffix_y).o \
font.o
font.c head.o misc.o $(OBJS)
那么内核是从什么地方开始运行的呢?这个要看lds文件。其实zImage的生成经历了两次链接过程:一是顶层vmlinux的生成,在arch/arm
/boot/vmlinux.lds(这个lds文件是由arch/arm/kernel/vmlinux.lds.S生成的)中;另一次是arch
/arm/boot/compressed/vmlinux的生成,在arch/arm/boot/compressed/vmlinux.lds.in
中。zImage的入口点应该由arch/arm/boot/compressed/vmlinux.lds.in决定。从中可以看出入口点为‘_start’
/* * linux/arch/arm/boot/compressed/vmlinux.lds.in * * Copyright (C) 2000 Russell King * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation. */ OUTPUT_ARCH(arm) ENTRY(_start) SECTIONS { /DISCARD/ : { *(.ARM.exidx*) *(.ARM.extab*) /* * Discard any r/w data - this produces a link error if we have any, * which is required for PIC decompression. Local data generates * GOTOFF relocations, which prevents it being relocated independently * of the text/got segments. */ *(.data) } . = TEXT_START; _text = .; .text : { _start = .; *(.start) *(.text) *(.text.*) *(.fixup) *(.gnu.warning) *(.rodata) *(.rodata.*) *(.glue_7) *(.glue_7t) *(.piggydata) . = ALIGN(4); } _etext = .; /* Assume size of decompressed image is 4x the compressed image */ _image_size = (_etext - _text) * 4; _got_start = .; .got : { *(.got) } _got_end = .; .got.plt : { *(.got.plt) } _edata = .; . = BSS_START; __bss_start = .; .bss : { *(.bss) } _end = .; . = ALIGN(8); /* the stack must be 64-bit aligned */ .stack : { *(.stack) } .stab 0 : { *(.stab) } .stabstr 0 : { *(.stabstr) } .stab.excl 0 : { *(.stab.excl) } .stab.exclstr 0 : { *(.stab.exclstr) } .stab.index 0 : { *(.stab.index) } .stab.indexstr 0 : { *(.stab.indexstr) } .comment 0 : { *(.comment) } }
在arch/arm/boot/compressed/head.S中找到入口点。
看看head.S会做些什么样的工作:[下面是从网络上摘录]
1: 对于各种Arm
CPU的DEBUG输出设定,通过定义宏来统一操作;
2: 设置kernel开始和结束地址,保存architecture ID;
3:
如果在ARM2以上的CPU中,用的是普通用户模式,则升到超级用户模式,然后关中断
4: 分析LC0结构delta
offset,判断是否需要重载内核地址(r0存入偏移量,判断r0是否为零)。
5: 需要重载内核地址,将r0的偏移量加到BSS region和GOT
table中的每一项。
对于位置无关的代码,程序是通过GOT表访问全局数据目标的,也就是说GOT表中中记录的是全局数据目标的绝对地址,所以其中的每一项也需要重载。
6: 清空bss堆栈空间r2-r3
7: 建立C程序运行需要的缓存
8:
这时r2是缓存的结束地址,r4是kernel的最后执行地址,r5是kernel境象文件的开始地址
9:
用文件misc.c的函数decompress_kernel(),解压内核于缓存结束的地方(r2地址之后)。