ARM LINUX启动过程

xchunhua

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_NONE。ATAG_CORE tag可以为空。一个空的ATAG_CORE tag的size字段设为“2”(0x00000002)。ATAG_NONE 的size字段必须设为“0”。tagged list可以有任意多的tag。Bootloader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的”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RAM的32K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-($PROCESSOR).S文件，PROCESSOR是由include/linux/autoconf.h中#define CONFIG_CPU_32 1得到了在arch/arm/Makefile中通过
ifeq ($(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？？

http://gicl.cs.drexel.edu/people/sevy/linux/ARM_Linux_boot_sequence.html
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
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)

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