ARM Linux boot sequence

ARM Linux boot sequenceThe 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 Linux 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 'toverwrite 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-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 cballs
    • _ Switch_data defined in arch/ARM/kernel/head-common.S (15)
  • Call the _ cpu_flush function pointer in the previusly 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)