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2008-10-14 17:14:18

一、BOOTLOADER启动过程

1.建立和初始化RAM

要求:必须

功能:探测所有的RAM位置和大小,并对RAM进行初始化。

2.初始化一个串口。

要求:可选,建议

功能:Bootloader应该初始化并启动一个串口。这可以让内核的串口驱动自动探测哪个串口作为内核的控制台。另外也可以通过给内核传递“console=”参数完成此工作。

3.检测机器的系统结构。

要求:必须

功能:Bootloader应该通过某种方法探测机器类型,最后传递给内核一个MACH_TYPE_xxx值,这些值参看linux/arch/arm/tools/mach-types

4.建立内核的tagged list

要求:必须

功能:Bootloader必须创建和初始化内核的tagged list。一个合法的tagged list开始于ATAG_CORE并结束于ATAG_NONEATAG_CORE tag可以为空。一个空的ATAG_CORE tagsize字段设为“2(0x00000002)ATAG_NONE size字段必须设为“0”。tagged list可以有任意多的tagBootloader必须至少传递系统内存的大小和位置,以及根文件系统的位置,一个最小化的tagged list应该像如下:

        +-----------+

base -> | ATAG_CORE |  |

        +-----------+  |

        | ATAG_MEM  |  | increasing address

        +-----------+  |

        | ATAG_NONE |  |

        +-----------+  v

tagged list应该放在内核解压时和initrd”bootp”程序都不会覆盖的内存区域。建议放在RAM的起始的16K大小的地方。

5.调用内核镜像。

要求:必须

功能:可以从flash调用内核,也可以从系统RAM中调用内核。对于后者需要注意,内核使用内核镜像以下的16K内存作为页表,建议把内核起始放在RAM32K处。无论是哪种方法,如下条件必须满足:

- CPU register settings

  r0 = 0,

  r1 = machine type number discovered in (3) above.

  r2 = physical address of tagged list in system RAM.

- CPU mode

  All forms of interrupts must be disabled (IRQs and FIQs)

  The CPU must be in SVC mode.  (A special exception exists for Angel)

- Caches, MMUs

  The MMU must be off.

  Instruction cache may be on or off.

  Data cache must be off.

- The boot loader is expected to call the kernel image by jumping

  directly to the first instruction of the kernel image.

 

二、ARM LINUX启动

LINUX启动后执行的第一个文件是arch/arm/kernel下的head-($PROCESSOR).S文件,PROCESSOR是由include/linux/autoconf.h#define CONFIG_CPU_32 1得到了在arch/arm/Makefile中通过

ifeq ($(CONFIG_CPU_32),y)

PROCESSOR = armv

TEXTADDR = 0xC0008000

LDSCRIPT = arch/arm/vmlinux-armv.lds.in

endif

得到的。

但是我看了一下linux 2.6.25.3内核中没有这个东西。

但我还是先分析一下arch/arm/kernel/head-armv.S这个文件:

后悔了,看来不太可行。为什么大家都说文件中有stext呢?我怎么就没看到??

 

终于找到了一个网站讨论2.6内核的。

ARM Linux Boot Sequence

The following traces the Linux boot sequence for ARM-based systems in the 2.6.18 kernel. It looks at just the earliest stages of the boot process, until the generic non-processor-specific start_kernel function is called. The line numbers of each statement are in parenthese at the end of the line; the kernel source itself can be conveniently browsed on the .

zImage decompression

  • arch/arm/boot/compressed/head.S: start (108)
    • First code executed, jumped to by the bootloader, at label "start" (108)
    • save contents of registers r1 and r2 in r7 and r8 to save off architecture ID and atags pointer passed in by bootloader (118)
    • execute arch-specific code (inserted at 146)
      • arch/arm/boot/compressed/head-xscale.S or other arch-specific code file
      • added to build in arch/arm/boot/compressed/Makefile
      • linked into head.S by linker section declaration:  .section “start”
      • flush cache, turn off cache and MMU
    • load registers with stored parameters (152)
      • sp = stack pointer for decompression code (152)
      • r4 = zreladdr = kernel entry point physical address
    • check if running at link address, and fix up global offset table if not (196)
    • zero decompression bss (205)
    • call cache_on to turn on cache (218)
      • defined at arch/arm/boot/compressed/head.S (320)
      • call call_cache_fn to turn on cache as appropriate for processor variant
        • defined at arch/arm/boot/compressed/head.S (505)
        • walk through proc_types list (530) until find corresponding processor
        • call cache-on function in list item corresponding to processor (511)
          • for ARMv5tej core, cache_on function is __armv4_mmu_cache_on (417)
            • call setup_mmu to set up initial page tables since MMU must be on for cache to be on (419)
            • turn on cache and MMU (426)
    • check to make sure won't overwrite image during decompression; assume not for this trace (232)
    • call decompress_kernel to decompress kernel to RAM (277)
    • branch to call_kernel (278)
      • call cache_clean_flush to flush cache contents to RAM (484)
      • call cache_off to turn cache off as expected by kernel initialization routines (485)
      • jump to start of kernel in RAM (489)
        • jump to address in r4 = zreladdr from previous load
          • zreladdr = ZRELADDR = zreladdr-y
          • zreladdr-y specified in arch/arm/mach-vx115/Makefile.boot

 

ARM-specific kernel code

  • arch/arm/kernel/head.S: stext (72)
    • call __lookup_processor_type (76)
      • defined in arch/arm/kernel/head-common.S (146)
      • search list of supported processor types __proc_info_begin (176)
        • kernel may be built to support more than one processor type
        • list of proc_info_list structs 
          • defined in arch/arm/mm/proc-arm926.S (467) and other corresponding proc-*.S files
          • linked into list by section declaration:  .section ".proc.info.init"
      • return pointer to proc_info_list struct corresponding to processor if found, or loop in error if not
    • call __lookup_machine_type (79)
      • defined in arch/arm/kernel/head-common.S (194)
      • search list of supported machines (boards)
        • kernel may be built to support more than one board
        • list of machine_desc structs 
          • machine_desc struct for boards defined in board-specific file vx115_vep.c
          • linked into list by section declaration that's part of MACHINE_DESC macro
      • return pointer to machine_desc struct corresponding to machine (board)
    • call __create_page_tables to set up initial MMU tables (82)
    • set lr to __enable_mmu, r13 to address of __switch_data (91, 93)
      • lr and r13 used for jumps after the following calls
      • __switch_data defined in arch/arm/kernel/head-common.S (15)
    • call the __cpu_flush function pointer in the previously returned proc_info_list struct (94)
      • offset is #PROCINFO_INITFUNC into struct
      • this function is __arm926_setup for the ARM 926EJ-S, defined in arch/arm/mm/proc-arm926.S (392)
        • initialize caches, writebuffer
        • jump to lr, previously set to address of __enable_mmu
    • __enable_mmu (147)
      • set page table pointer (TTB) in MMU hardware so it knows where to start page-table walks (167)
      • enable MMU so running with virtual addresses (185)
      • jump to r13, previously set to address of __switch_data, whose first field is address of __mmap_switched
        • __switch_data defined in arch/arm/kernel/head-common.S (15)

 

  • arch/arm/kernel/head-common.S: __mmap_switched (35)
    • copy data segment to RAM (39)
    • zero BSS (45)
    • branch to start_kernel (55)

 

Processor-independent kernel code

  • init/main.c: start_kernel (456)
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