uboot入口boot flow（ARMv8）

uboot入口boot flow（ARMv8）

uboot entry point是_start，这个在u-boot.lds里有define

_start里执行b reset跳转到reset label处执行

19 .globl    _start
20 _start:
21 #if defined(CONFIG_LINUX_KERNEL_IMAGE_HEADER)
22 #include <asm/boot0-linux-kernel-header.h>
23 #elif defined(CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK)
24 /*
25  * Various SoCs need something special and SoC-specific up front in
26  * order to boot, allow them to set that in their boot0.h file and then
27  * use it here.
28  */
29 #include <asm/arch/boot0.h>
30 #else
31     b    reset
32 #endif

u-boot-2019.04\arch\arm\cpu\armv8\u-boot.lds

OUTPUT_ARCH(aarch64)
ENTRY(_start)
SECTIONS

reset label里会调用switch_el、lowlevel_init（define本文件，start.S），然后bl _main

104     switch_el x1, 3f, 2f, 1f
105 3:    set_vbar vbar_el3, x0
106     mrs    x0, scr_el3
107     orr    x0, x0, #0xf            /* SCR_EL3.NS|IRQ|FIQ|EA */
108     msr    scr_el3, x0
109     msr    cptr_el3, xzr            /* Enable FP/SIMD */
110 #ifdef COUNTER_FREQUENCY
111     ldr    x0, =COUNTER_FREQUENCY
112     msr    cntfrq_el0, x0            /* Initialize CNTFRQ */
113 #endif
114     b    0f
115 2:    set_vbar    vbar_el2, x0
116     mov    x0, #0x33ff
117     msr    cptr_el2, x0            /* Enable FP/SIMD */
118     b    0f
119 1:    set_vbar    vbar_el1, x0
120     mov    x0, #3 << 20
121     msr    cpacr_el1, x0            /* Enable FP/SIMD */
122 0:
...
148     /* Processor specific initialization */
149     bl    lowlevel_init
...
168 master_cpu:
169     bl    _main

_main()主要所做的事情如下：

1. 设置relocation前的sp（初始stack pointer），此时的sp将是这一阶段运行的stack，然后调用board_init_f设置relocation相关的gd成员（gd->relocaddr/gd->reloc_off/gd->start_addr_sp/gd->new_gd）

2. 设置relocation的sp（gd->start_addr_sp），此时的sp将是之后uboot运行的stack（top），之后调用relocate_code完成uboot relocation

3. 调用board_init_r，第一参数的值是gd指针，第二个参数为gd->relocaddr

66 ENTRY(_main)
79     ldr    x0, =(CONFIG_SYS_INIT_SP_ADDR)
80 #endif
81     bic    sp, x0, #0xf    /* 16-byte alignment for ABI compliance */
82     mov    x0, sp
83     bl    board_init_f_alloc_reserve
84     mov    sp, x0
85     /* set up gd here, outside any C code */
86     mov    x18, x0
87     bl    board_init_f_init_reserve
88 
89     mov    x0, #0
90     bl    board_init_f
...
92 #if !defined(CONFIG_SPL_BUILD)
93 /*
94  * Set up intermediate environment (new sp and gd) and call
95  * relocate_code(addr_moni). Trick here is that we'll return
96  * 'here' but relocated.
97  */
98     ldr    x0, [x18, #GD_START_ADDR_SP]    /* x0 <- gd->start_addr_sp */
99     bic    sp, x0, #0xf    /* 16-byte alignment for ABI compliance */
100     ldr    x18, [x18, #GD_NEW_GD]        /* x18 <- gd->new_gd */
102     adr    lr, relocation_return
110     /* Add in link-vs-relocation offset */
111     ldr    x9, [x18, #GD_RELOC_OFF]    /* x9 <- gd->reloc_off */
112     add    lr, lr, x9    /* new return address after relocation */
113     ldr    x0, [x18, #GD_RELOCADDR]    /* x0 <- gd->relocaddr */
114     b    relocate_code
115 
116 relocation_return:
140 /*
141  * Clear BSS section
142  */
143     ldr    x0, =__bss_start        /* this is auto-relocated! */
144     ldr    x1, =__bss_end            /* this is auto-relocated! */
145 clear_loop:
146     str    xzr, [x0], #8
147     cmp    x0, x1
148     b.lo    clear_loop
149 
150     /* call board_init_r(gd_t *id, ulong dest_addr) */
151     mov    x0, x18                /* gd_t */
152     ldr    x1, [x18, #GD_RELOCADDR]    /* dest_addr */
153     b    board_init_r            /* PC relative jump */
154 
155     /* NOTREACHED - board_init_r() does not return */
156 #endif
157 
158 ENDPROC(_main)