ARM Linux Boot Sequence

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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 theLinux Cross-Reference website.

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)