uboot 的 relocate_code 问题

a314825348

在分析imx6ull的uboot启动流程时，有几个问题麻烦大家帮忙看一下，谢谢。

1. uboot在执行完 relocate_code 后，此时DDR里面是不是有两份uboot镜像了？
2. uboot在执行完 relocate_code 后，又重新设置了向量表，然后清除了bss段，接着跳转到了 board_init_r 进行外设的初始化。在这里执行的 board_init_r  从哪里可以看出来运行的是DDR中 relocate_code 后的uboot，而不是iROM加载的那份uboot呢？
下面是 arch\arm\lib\crt0.S 的源码

1. /* SPDX-License-Identifier: GPL-2.0+ */
   
2. /*
   
3. *  crt0 - C-runtime startup Code for ARM U-Boot
   
4. *
   
5. *  Copyright (c) 2012  Albert ARIBAUD <albert.u.boot@aribaud.net>
   
6. */
   
7. 
   
8. #include <config.h>
   
9. #include <asm-offsets.h>
   
10. #include <linux/linkage.h>
    
11. #ifdef CONFIG_CPU_V7M
    
12. #include <asm/armv7m.h>
    
13. #endif
    
14. 
    
15. /*
    
16. * This file handles the target-independent stages of the U-Boot
    
17. * start-up where a C runtime environment is needed. Its entry point
    
18. * is _main and is branched into from the target's start.S file.
    
19. *
    
20. * _main execution sequence is:
    
21. *
    
22. * 1. Set up initial environment for calling board_init_f().
    
23. *    This environment only provides a stack and a place to store
    
24. *    the GD ('global data') structure, both located in some readily
    
25. *    available RAM (SRAM, locked cache...). In this context, VARIABLE
    
26. *    global data, initialized or not (BSS), are UNAVAILABLE; only
    
27. *    CONSTANT initialized data are available. GD should be zeroed
    
28. *    before board_init_f() is called.
    
29. *
    
30. * 2. Call board_init_f(). This function prepares the hardware for
    
31. *    execution from system RAM (DRAM, DDR...) As system RAM may not
    
32. *    be available yet, , board_init_f() must use the current GD to
    
33. *    store any data which must be passed on to later stages. These
    
34. *    data include the relocation destination, the future stack, and
    
35. *    the future GD location.
    
36. *
    
37. * 3. Set up intermediate environment where the stack and GD are the
    
38. *    ones allocated by board_init_f() in system RAM, but BSS and
    
39. *    initialized non-const data are still not available.
    
40. *
    
41. * 4a.For U-Boot proper (not SPL), call relocate_code(). This function
    
42. *    relocates U-Boot from its current location into the relocation
    
43. *    destination computed by board_init_f().
    
44. *
    
45. * 4b.For SPL, board_init_f() just returns (to crt0). There is no
    
46. *    code relocation in SPL.
    
47. *
    
48. * 5. Set up final environment for calling board_init_r(). This
    
49. *    environment has BSS (initialized to 0), initialized non-const
    
50. *    data (initialized to their intended value), and stack in system
    
51. *    RAM (for SPL moving the stack and GD into RAM is optional - see
    
52. *    CONFIG_SPL_STACK_R). GD has retained values set by board_init_f().
    
53. *
    
54. * 6. For U-Boot proper (not SPL), some CPUs have some work left to do
    
55. *    at this point regarding memory, so call c_runtime_cpu_setup.
    
56. *
    
57. * 7. Branch to board_init_r().
    
58. *
    
59. * For more information see 'Board Initialisation Flow in README.
    
60. */
    
61. 
    
62. /*
    
63. * entry point of crt0 sequence
    
64. */
    
65. 
    
66. ENTRY(_main)
    
67. 
    
68. /*
    
69. * Set up initial C runtime environment and call board_init_f(0).
    
70. */
    
71. 
    
72. #if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_STACK)
    
73.         ldr        r0, =(CONFIG_SPL_STACK)
    
74. #else
    
75.         ldr        r0, =(CONFIG_SYS_INIT_SP_ADDR)
    
76. #endif
    
77.         bic        r0, r0, #7        /* 8-byte alignment for ABI compliance */
    
78.         mov        sp, r0
    
79.         bl        board_init_f_alloc_reserve        // 为malloc和gd_data留出空间
    
80.         mov        sp, r0
    
81.         /* set up gd here, outside any C code */
    
82.         mov        r9, r0       
    
83.         bl        board_init_f_init_reserve        // 初始化gd结构体
    
84. 
    
85.         mov        r0, #0
    
86.         bl        board_init_f
    
87. 
    
88. #if ! defined(CONFIG_SPL_BUILD)
    
89. 
    
90. /*
    
91. * Set up intermediate environment (new sp and gd) and call
    
92. * relocate_code(addr_moni). Trick here is that we'll return
    
93. * 'here' but relocated.
    
94. */
    
95. 
    
96.         ldr        r0, [r9, #GD_START_ADDR_SP]        /* sp = gd->start_addr_sp */
    
97.         bic        r0, r0, #7        /* 8-byte alignment for ABI compliance */
    
98.         mov        sp, r0
    
99.         ldr        r9, [r9, #GD_BD]                /* r9 = gd->bd */
    
100.         sub        r9, r9, #GD_SIZE                /* new GD is below bd */
     
101. 
     
102.         adr        lr, here
     
103.         ldr        r0, [r9, #GD_RELOC_OFF]                /* r0 = gd->reloc_off */
     
104.         add        lr, lr, r0
     
105. #if defined(CONFIG_CPU_V7M)
     
106.         orr        lr, #1                                /* As required by Thumb-only */
     
107. #endif
     
108.         ldr        r0, [r9, #GD_RELOCADDR]                /* r0 = gd->relocaddr */
     
109.         b        relocate_code
     
110. here:
     
111. /*
     
112. * now relocate vectors
     
113. */
     
114. 
     
115.         bl        relocate_vectors
     
116. 
     
117. /* Set up final (full) environment */
     
118. 
     
119.         bl        c_runtime_cpu_setup        /* we still call old routine here */
     
120. #endif
     
121. #if !defined(CONFIG_SPL_BUILD) || defined(CONFIG_SPL_FRAMEWORK)
     
122. # ifdef CONFIG_SPL_BUILD
     
123.         /* Use a DRAM stack for the rest of SPL, if requested */
     
124.         bl        spl_relocate_stack_gd
     
125.         cmp        r0, #0
     
126.         movne        sp, r0
     
127.         movne        r9, r0
     
128. # endif
     
129.         ldr        r0, =__bss_start        /* this is auto-relocated! */
     
130. 
     
131. #ifdef CONFIG_USE_ARCH_MEMSET
     
132.         ldr        r3, =__bss_end                /* this is auto-relocated! */
     
133.         mov        r1, #0x00000000                /* prepare zero to clear BSS */
     
134. 
     
135.         subs        r2, r3, r0                /* r2 = memset len */
     
136.         bl        memset
     
137. #else
     
138.         ldr        r1, =__bss_end                /* this is auto-relocated! */
     
139.         mov        r2, #0x00000000                /* prepare zero to clear BSS */
     
140. 
     
141. clbss_l:cmp        r0, r1                        /* while not at end of BSS */
     
142. #if defined(CONFIG_CPU_V7M)
     
143.         itt        lo
     
144. #endif
     
145.         strlo        r2, [r0]                /* clear 32-bit BSS word */
     
146.         addlo        r0, r0, #4                /* move to next */
     
147.         blo        clbss_l
     
148. #endif
     
149. 
     
150. #if ! defined(CONFIG_SPL_BUILD)
     
151.         bl coloured_LED_init
     
152.         bl red_led_on
     
153. #endif
     
154.         /* call board_init_r(gd_t *id, ulong dest_addr) */
     
155.         mov     r0, r9                  /* gd_t */
     
156.         ldr        r1, [r9, #GD_RELOCADDR]        /* dest_addr */
     
157.         /* call board_init_r */
     
158. #if CONFIG_IS_ENABLED(SYS_THUMB_BUILD)
     
159.         ldr        lr, =board_init_r        /* this is auto-relocated! */
     
160.         bx        lr
     
161. #else
     
162.         ldr        pc, =board_init_r        /* this is auto-relocated! */
     
163. #endif
     
164.         /* we should not return here. */
     
165. #endif
     
166. 
     
167. ENDPROC(_main)
     

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a314825348

解析参考文章https://blog.csdn.net/skyflying2012/article/details/37660265
从测试中可以看出，ARM对全局变量和函数均是采用相对寻址的方式进行寻址，uboot中rel.dyn段存储了全局变量和函数的地址，通过下面这段代码，将rel.dyn段进行重定位

1.         /*
   
2.          * fix .rel.dyn relocations
   
3.          */
   
4.         ldr        r2, =__rel_dyn_start        /* r2 <- SRC &__rel_dyn_start */
   
5.         ldr        r3, =__rel_dyn_end        /* r3 <- SRC &__rel_dyn_end */
   
6. fixloop:
   
7.         ldmia        r2!, {r0-r1}                /* (r0,r1) <- (SRC location,fixup) */
   
8.         and        r1, r1, #0xff
   
9.         cmp        r1, #R_ARM_RELATIVE
   
10.         bne        fixnext
    
11. 
    
12.         /* relative fix: increase location by offset */
    
13.         add        r0, r0, r4
    
14.         ldr        r1, [r0]
    
15.         add        r1, r1, r4
    
16.         str        r1, [r0]
    
17. fixnext:
    
18.         cmp        r2, r3
    
19.         blo        fixloop

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重定位后，当调用下面这行代码时，将board_init_r函数的地址放入PC指针中，就跳转到了board_init_r函数处去执行。在完成rel.dyn段的重定位后，board_init_r函数的地址已经被定位到了新的位置，所以会执行重定位后的代码

1. ldr        pc, =board_init_r        /* this is auto-relocated! */

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