基於ARM 的Linux 的啟動分析報告——ARM+Linux的啟動分析（3）

(9) 調用檔案misc.c的函數decompress_kernel()，解壓核心於緩衝結束的地
方(r2地址之後)。此時各寄存器值有如下變化：
r0為解壓後kernel的大小
r4為kernel執行時的地址
r5為解壓後kernel的起始地址
r6為CPU類型值(processor ID)
r7為系統類別型值(architecture ID)
(10) 將reloc_start代碼拷貝之kernel之後(r5+r0之後)，首先清除緩衝，而
後執行reloc_start。
(11) reloc_start將r5開始的kernel重載於r4地址處。
(12) 清除cache內容，關閉cache，將r7中architecture ID賦於r1，執行
r4開始的kernel代碼。
5) 我們在核心啟動的開始都會看到這樣的輸出
Uncompressing Linux...done, booting the kernel.
這也是由decompress_kernel函數內部輸出的，它調用了putc()輸出字串，
putc是在#/include/asm-arm/arch-pxa/uncompress.h中實現的。執行完解壓過
程，再返回到#/arch/arm/boot/compressed/head.S中，啟動核心：
call_kernel: bl cache_clean_flush
bl cache_off
mov r0, #0
mov r1, r7 @ restore architecture number
mov pc, r4 @ call kernel
6) 執行zImage 鏡像，到start_kernel( )
整個arm linux核心的啟動可分為三個階段：第一階段主要是進行cpu和
體繫結構的檢查、cpu本身的初始化以及頁表的建立等；第一階段的初始化是從
核心入口（ENTRY(stext)）開始到start_kernel前結束。這一階段的代碼
在/arch/arm/kernel/head.S中。/arch/arm/kernel/head.S用彙編程式碼完成，
是核心最先執行的一個檔案。這一段彙編代碼的主要作用，是檢查cpu
id，architecture number，初始化頁表、cpu、bbs等操作，並跳到
start_kernel函數。它在執行前，處理器的狀態應滿足：
r 0 - should be 0
r1 - unique architecture number
MMU - off
I-cache - on or off
D-cache – off
a) 流程圖

b) 代碼詳細注釋
#/arch/arm/kernel/head.S
/*
* swapper_pg_dir is the virtual address of the initial page table.
* We place the page tables 16K below KERNEL_RAM_VADDR. Therefore, we
* must make sure that KERNEL_RAM_VADDR is correctly set. Currently,
we *expect the least significant 16 bits to be 0x8000, but we could
probably relax this *restriction to KERNEL_RAM_VADDR >= PAGE_OFFSET +
0x4000.
*/
#if (KERNEL_RAM_VADDR & 0xffff) != 0x8000
#error KERNEL_RAM_VADDR must start at 0xXXXX8000
#endif
.globl swapper_pg_dir
.equ swapper_pg_dir, KERNEL_RAM_VADDR - 0x4000
.macro pgtbl, rd
ldr /rd, =(KERNEL_RAM_PADDR - 0x4000)
.endm
/*
* Since the page table is closely related to the kernel start
address, we
* can convert the page table base address to the base address of the
section
* containing both.
*/
.macro krnladr, rd, pgtable, rambase
bic /rd, /pgtable, #0x000ff000
.endm
/*
/*
* Kernel startup entry point.
* ---------------------------
*
* This is normally called from the decompressor code. The
requirements
* are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0,
* r1 = machine nr, r2 = atags pointer.
*
* See linux/arch/arm/tools/mach-types for the complete list of
machine
* numbers for r1.
*/
.section ".text.head", "ax"
.type stext, %function
ENTRY(stext) //核心進入點
msr cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE
//程式狀態，禁止FIQ、IRQ，設定Supervisor模式。0b11010011
mrc p15, 0, r9, c0, c0 @ get processor id
bl __lookup_processor_type @ r5=procinfo r9=cupid //跳轉到
判斷
//cpu類型，尋找啟動並執行cpu的id值和此linux編譯支援的id值是否有相
等
movs r10, r5 @ invalid processor (r5=0)?
beq __error_p @ yes, error 'p'
bl __lookup_machine_type @ r5=machinfo
//跳轉到判斷體系類型，看r1寄存器的architecture number值是否支援。
movs r8, r5 @ invalid machine (r5=0)?
beq __error_a @ yes, error 'a'
bl __vet_atags
bl __create_page_tables //建立核心頁表
/*
* The following calls CPU specific code in a position independent
* manner. See arch/arm/mm/proc-*.S for details. r10 = base of
* xxx_proc_info structure selected by __lookup_machine_type
* above. On return, the CPU will be ready for the MMU to be
* turned on, and r0 will hold the CPU control register value.
*/
ldr r13, __switch_data @ address to jump to after
@ mmu has been enabled
adr lr, __enable_mmu @ return (PIC) address //lr=0xc0028054
add pc, r10, #PROCINFO_INITFUNC
@ initialise processor //r10：pointer to processor
structure
#/arch/arm/kernel/head-common.S
*/
#define ATAG_CORE 0x54410001
#define ATAG_CORE_SIZE ((2*4 + 3*4) >> 2)
.type __switch_data, %object
__switch_data:
.long __mmap_switched
.long __data_loc @ r4
.long __data_start @ r5
.long __bss_start @ r6