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 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'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)