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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