U-Boot源代码阅读笔记（一） —— 对start.S的分析
本文主要分析与U-Boot启动过程相关的汇编代码cpu/pxa/start.S，目标平台以PXA270为例。 系统启动执行的第一条指令 /* armboot - Startup Code for XScale */
.globl _start
_start: b reset /*跳转到reset标号执行*/设置cpu为superviser模式 reset:
mrs r0,cpsr /* 读cpsr寄存器状态 */
bic r0,r0,#0x1f /* 位清除，清除0x1f对应的位 */
orr r0,r0,#0x13 /* 设置M=10011，superviser 模式 */
msr cpsr,r0 /* 写cpsr寄存器 */
bl cpu_init_crit /* 跳转到cpu_init_crit执行系统的关键初始化 */
cpu_init_crit:
/* mask all IRQs */
ldr r0, IC_BASE /* r0 <- Interrupt-Controller base address */
mov r1, #0x00
str r1, [r0, #ICMR_OFFSET] /* ICMR寄存器清零，屏蔽所有的中断 */
/*
* before relocating, we have to setup RAM timing
* because memory timing is board-dependend, you will
* find a lowlevel_init.S in your board directory.
*/
bl lowlevel_init /* 跳转到lowlevel_init执行SDRAM相关的初始化，参见对lowlevel_init.S的分析 */
#if defined(CFG_CPUSPEED)
/*
* 系统频率的计算方法如下：
* Turbo-mode frequency (T) = 13-MHz processor-oscillator frequency * L * N
* Run-mode frequency (R) = 13-MHz processor-oscillator frequency * L
* System-bus frequency = 13-MHz processor-oscillator frequency * L / B,
* where B = 1 (when in fast-bus mode) or B = 2 (when not in fast-bus mode)
* For CCCR[A] = 0 :
* Memory-controller frequency = 13-MHz processor-oscillator frequency * L / M,
* where M = 1 (L = 2-10), M = 2 (L = 11-20), or M = 4 (L = 21-31)
* LCD frequency = 13-MHz processor-oscillator frequency * L / K,
* where K = 1 (L = 2-7), K = 2 (L = 8-16), or K = 4 (L = 17-31)
* For CLKCFG[ B] = 0 and CCCR[A] = 1 :
* Memory-controller frequency = 13-MHz processor-oscillator frequency * L / 2
* LCD frequency = 13-MHz processor-oscillator frequency * L / K,
* where K = 1 (L = 2-7), K = 2 (L = 8-16), or K = 4 (L = 17-31)
* For CLKCFG[ B] = 1 and CCCR[A] = 1 :
* Memory-controller frequency = 13-MHz processor-oscillator frequency * L
* LCD frequency = 13-MHz processor-oscillator frequency * L / K,
* where K = 1 (L = 2-7), K = 2 (L = 8-16), or K = 4 (L = 17-31)
*/
/* set clock speed */
ldr r0, =CC_BASE
ldr r1, cpuspeed
str r1, [r0, #CCCR_OFFSET] /* Core Clock Configuration Register */
ldr r1, [r0, #CCCR_OFFSET] /* read back to make sure write action completed */
mov ip, lr /* 临时寄存器 <- 链接寄存器*/
bl xlli_freq_change /* 跳到标号xlli_freq_change执行 */
mov lr, ip
/* Before enabling fast bus mode, must make sure Memory Controller is configured properly */
#if defined(CONFIG_SYSBUS_208)
mov r0, #0x0B /* Fast bus | Freq. Change | Turbo mode */
#else
mov r0, #0x03 /* ~Fast bus | Freq. Change | Turbo mode */
#endif
mcr p14, 0, r0, c6, c0, 0 /* Write CCLKCFG */
#if defined(CONFIG_MEMC_BS) /* memory controller buffer strength control */
ldr r0, =MEMC_BASE
ldr r1, =CFG_BSCNTR0_VAL
str r1, [r0, #BSCNTR0_OFFSET]
ldr r1, =CFG_BSCNTR1_VAL
str r1, [r0, #BSCNTR1_OFFSET]
ldr r1, =CFG_BSCNTR2_VAL
str r1, [r0, #BSCNTR2_OFFSET]
ldr r1, =CFG_BSCNTR3_VAL
str r1, [r0, #BSCNTR3_OFFSET]
ldr r1, [r0, #BSCNTR3_OFFSET] /* Make sure value is written */
#endif
setspeed_done:
#endif
.global normal_boot
normal_boot:
/* Memory interfaces are working. Disable MMU and enable I-cache. */
ldr r0, =0x2001 /* enable access to all coproc. */
mcr p15, 0, r0, c15, c1, 0 /* enable access to CP13 CP0*/
CPWAIT /* wait for effect */
mcr p15, 0, r0, c7, c10, 4 /* drain the write & fill buffers */
CPWAIT
mcr p15, 0, r0, c7, c7, 0 /* flush Icache, Dcache and BTB */
CPWAIT
mcr p15, 0, r0, c8, c7, 0 /* flush instuction and data TLBs */
CPWAIT
/* Enable the Icache */
mrc p15, 0, r0, c1, c0, 0
orr r0, r0, #0x1800 /* Instruction Cache Enable, Branch Target Buffer Enable */
mcr p15, 0, r0, c1, c0, 0
CPWAIT
@ mov pc, lr
b cpu_init_done /* call return back */
/* Frequency Change Sequence from BLOB */
xlli_freq_change:
mrc p14, 0, r0, c6, c0, 0 /* Get present status (preserve Turbo and Fast Bus bits) */
orr r0, r0, #2 /* Set the F bit, Frequency Change, A change sequence is initiated when F is set. */
mcr p14, 0, r0, c6, c0, 0 /* initiate the frequency change sequence */
/*
* If the clock frequency is chaged, the MDREFR Register must be rewritten, even
* if it's the same value. This will result in a refresh being performed and the
* refresh counter being reset to the reset interval. (Section 13.10.3, pg 13-17 of EAS)
*/
ldr r4, =MEMC_BASE /* Get memory controller base address */
ldr r1, [r4, #MDREFR_OFFSET] /* Get the current state of MDREFR */
str r1, [r4, #MDREFR_OFFSET] /* Re-write this value */
mov pc, lr /* return to calling routine */
/* RS: ??? */
.macro CPWAIT /* Canonical method to wait for CP15 update */
mrc p15,0,r0,c2,c0,0 /* arbitrary read of CP15 */
mov r0,r0 /* nop, wait for it */
sub pc,pc,#4 /* branch to next instruction */
/* At this point, any previous CP15 writes are guaranteed to have taken effect. */
.endm
系统初始化完成 cpu_init_done:
#ifndef CONFIG_SKIP_RELOCATE_UBOOT
relocate: /* relocate U-Boot to RAM */
adr r0, _start /* r0 <- current position of code */
ldr r1, _TEXT_BASE /* test if we run from flash or RAM */
cmp r0, r1 /* don't reloc during debug */
beq stack_setup
ldr r2, _armboot_start
ldr r3, _bss_start
sub r2, r3, r2 /* r2 <- size of armboot */
add r2, r0, r2 /* r2 <- source end address */
copy_loop:
ldmia r0!, {r3-r10} /* copy from source address [r0] */
stmia r1!, {r3-r10} /* copy to target address [r1] */
cmp r0, r2 /* until source end addreee [r2] */
ble copy_loop
#endif /* CONFIG_SKIP_RELOCATE_UBOOT */
/* Set up the stack */
stack_setup:
ldr r0, _TEXT_BASE /* upper 128 KiB: relocated uboot */
sub r0, r0, #CFG_MALLOC_LEN /* malloc area */
sub r0, r0, #CFG_GBL_DATA_SIZE /* bdinfo */
#ifdef CONFIG_USE_IRQ
sub r0, r0, #(CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ)
#endif
sub sp, r0, #12 /* leave 3 words for abort-stack , sp —— 栈指针寄存器 */
clear_bss:
ldr r0, _bss_start /* find start of bss segment */
ldr r1, _bss_end /* stop here */
mov r2, #0x00000000 /* clear */
clbss_l:str r2, [r0] /* clear loop... */
add r0, r0, #4
cmp r0, r1
ble clbss_l
跳转到start_armboot执行
ldr pc, _start_armboot /* 跳转到start_armboot执行， 参见对lib_arm/board.c的分析 */
_start_armboot: .word start_armboot
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