关键词：Image，zImage，piggy，gzip，lzo，xz，lzma等等。

1. zImage的压缩

zImage是Image经过压缩后，加上解压缩头后生成的文件：Image -> piggy_data -> piggy.o ->vmlinux(compressed) -> zImage。

Image经过压缩生成piggy_data：

cmd_arch/arm/boot/compressed/piggy_data := { cat arch/arm/boot/compressed/../Image | lzop -9; printf \\070\\266\\126\\000; } > arch/arm/boot/compressed/piggy_data

piggy.o包含了piggy_data数据：

/* SPDX-License-Identifier: GPL-2.0 */
    .section .piggydata,#alloc
    .globl    input_data
input_data:
    .incbin    "arch/arm/boot/compressed/piggy_data"
    .globl    input_data_end
input_data_end:

cpmpressed/vmlinux包含了piggy.o：

cmd_arch/arm/boot/compressed/vmlinux := arm-none-linux-gnueabihf-ld  -EL   --defsym _kernel_bss_size=100900 --no-undefined -X
 -T arch/arm/boot/compressed/vmlinux.lds
 arch/arm/boot/compressed/head.o
 arch/arm/boot/compressed/piggy.o
 arch/arm/boot/compressed/misc.o
 arch/arm/boot/compressed/decompress.o
 arch/arm/boot/compressed/string.o
 arch/arm/boot/compressed/hyp-stub.o
 arch/arm/boot/compressed/lib1funcs.o
 arch/arm/boot/compressed/ashldi3.o
 arch/arm/boot/compressed/bswapsdi2.o
 -o arch/arm/boot/compressed/vmlinux

从compressed/vmlinux.lds可以看出piggydata段放在rodata之后：

OUTPUT_ARCH(arm)
ENTRY(_start)
SECTIONS
{
  /DISCARD/ : {
    *(.ARM.exidx*)
    *(.ARM.extab*)
    *(.data)
  }
  . = 0;
  _text = .;
  .text : {
    _start = .;
    *(.start)
    *(.text)
    *(.text.*)
    *(.fixup)
    *(.gnu.warning)
    *(.glue_7t)
    *(.glue_7)
  }
  .table : ALIGN(4) {
    _table_start = .;
    LONG((2))
    LONG((0x5a534c4b))
    LONG((__piggy_size_addr - _start))
    LONG((_kernel_bss_size))
    LONG(0)
    _table_end = .;
  }
  .rodata : {
    *(.rodata)
    *(.rodata.*)
    *(.data.rel.ro)
  }
  .piggydata : {
    *(.piggydata)
    __piggy_size_addr = . - 4;
  }
  . = ALIGN(4);
  _etext = .;
  .got.plt : { *(.got.plt) }
  _got_start = .;
  .got : { *(.got) }
  _got_end = .;
  .pad : { BYTE(0); . = ALIGN(8); }
  _edata = .;
  .image_end (NOLOAD) : {
    _edata_real = .;
  }
  _magic_sig = (0x016f2818);
  _magic_start = (_start);
  _magic_end = (_edata);
  _magic_table = (_table_start - _start);
  . = ALIGN(8);
  __bss_start = .;
  .bss : { *(.bss) }
  _end = .;
  . = ALIGN(8);
  .stack : { *(.stack) }
...
}
ASSERT(_edata_real == _edata, "error: zImage file size is incorrect");

从compressed/vmlinux中拷贝zImage：

cmd_arch/arm/boot/zImage := arm-none-linux-gnueabihf-objcopy -O binary -R .comment -S  arch/arm/boot/compressed/vmlinux arch/arm/boot/zImage

2. zImage的解压

 由compressed/vmlinux.lds可知，zImage启动从arch/arm/boot/compressed/head.S中start开始：

start
　　->__hyp_stub_install
　　->not_angel
　　->restart
　　->wont_overwrite
　　　　->not_relocated--清空bss段，并配置好C执行环境。r0-kernel输出起始地址；r1/r2-分配内存的其实和结束地址；r3-架构类型ID。
　　　　->decompress_kernel--对内核进行解压。
　　　　　　->arch_decomp_setup
　　　　　　->do_decompress
　　　　　　　　->__decompress
　　　　　　　　　　->unlzo--调用对应的解压缩算法进行解压。
      ->cache_clean_flush--刷新并清空cache。
      ->cache_off--关闭cache。
      ->__enter_kernel--准备好寄存器跳转到解压后的内核。

decompress_kernel是解压Image入口：

• output_start是指向zImage起始地址+TEXT_OFFSET的地址，对于0x80000000来说即为0x80008000。
• free_mem_ptr_p和free_mem_ptr_end_p指向分配的一段64KB空间起始和结束地址，给compressed中malloc/free使用。
• arch_id为架构ID，此为0x80004000。

void
decompress_kernel(unsigned long output_start, unsigned long free_mem_ptr_p,
        unsigned long free_mem_ptr_end_p,
        int arch_id)
{
    int ret;

    output_data        = (unsigned char *)output_start;
    free_mem_ptr        = free_mem_ptr_p;
    free_mem_end_ptr    = free_mem_ptr_end_p;
    __machine_arch_type    = arch_id;

    arch_decomp_setup();

    putstr("Uncompressing Linux...");
    ret = do_decompress(input_data, input_data_end - input_data,
                output_data, error);--input_data和input_data_end实在piggy.S中定义的piggy_data起始和结束地址。此处将piggy_data解压到TEXT_OFFSET之下那个的地方。
    if (ret)
        error("decompressor returned an error");
    else
        putstr(" done, booting the kernel.\n");
}

__enter_kernel未进入内核做准备，然后跳转到解压结果首地址。

r0为0,；r1架构编号；r2位atags指针。r4保存解压后内核跳转地址，最后跳转到解压后的内核。

__enter_kernel:
        mov    r0, #0            @ must be 0
        mov    r1, r7            @ restore architecture number
        mov    r2, r8            @ restore atags pointer
 ARM(        mov    pc, r4        )    @ call kernel
 M_CLASS(    add    r4, r4, #1    )    @ enter in Thumb mode for M class
 THUMB(        bx    r4        )    @ entry point is always ARM for A/R classes

3. 不同算法性能对比

在一FPGA环境下测试不同解压算法的耗时：