Uboot启动流程分析(二)

1、前言

在前面的文章Uboot启动流程分析(一)https://www.cnblogs.com/Cqlismy/p/12000889.html中，已经简单地分析了low_level_init函数，其调用流程如下：

save_boot_params_ret
    |
    cpu_init_crit
    |        　　|
    |        　　lowlevel_init
    |        　　|
    |        　　s_init
    |
    _main

接下来，则继续往下分析_main函数。

2、_main函数

在save_boot_params_ret的最后，会运行bl _main这句代码，Uboot则将会跳转到_main函数中去运行，该函数的定义在arch/arm/lib/crt0.S文件中，_main函数的功能已经在文件中注释得很清楚了，先来看看_main函数实现的功能是什么，注释如下：

/*
 * This file handles the target-independent stages of the U-Boot
 * start-up where a C runtime environment is needed. Its entry point
 * is _main and is branched into from the target's start.S file.
 *
 * _main execution sequence is:
 *
 * 1. Set up initial environment for calling board_init_f().
 *    This environment only provides a stack and a place to store
 *    the GD ('global data') structure, both located in some readily
 *    available RAM (SRAM, locked cache...). In this context, VARIABLE
 *    global data, initialized or not (BSS), are UNAVAILABLE; only
 *    CONSTANT initialized data are available. GD should be zeroed
 *    before board_init_f() is called.
 *
 * 2. Call board_init_f(). This function prepares the hardware for
 *    execution from system RAM (DRAM, DDR...) As system RAM may not
 *    be available yet, , board_init_f() must use the current GD to
 *    store any data which must be passed on to later stages. These
 *    data include the relocation destination, the future stack, and
 *    the future GD location.
 *
 * 3. Set up intermediate environment where the stack and GD are the
 *    ones allocated by board_init_f() in system RAM, but BSS and
 *    initialized non-const data are still not available.
 *
 * 4a.For U-Boot proper (not SPL), call relocate_code(). This function
 *    relocates U-Boot from its current location into the relocation
 *    destination computed by board_init_f().
 *
 * 4b.For SPL, board_init_f() just returns (to crt0). There is no
 *    code relocation in SPL.
 *
 * 5. Set up final environment for calling board_init_r(). This
 *    environment has BSS (initialized to 0), initialized non-const
 *    data (initialized to their intended value), and stack in system
 *    RAM (for SPL moving the stack and GD into RAM is optional - see
 *    CONFIG_SPL_STACK_R). GD has retained values set by board_init_f().
 *
 * 6. For U-Boot proper (not SPL), some CPUs have some work left to do
 *    at this point regarding memory, so call c_runtime_cpu_setup.
 *
 * 7. Branch to board_init_r().
 *
 * For more information see 'Board Initialisation Flow in README.
 */

从注释中，我们可以大概知道_main函数的执行顺序为：

先设置用于调用board_init_f()函数的初始环境，该环境仅仅是提供了堆栈和存储位置GD('global data')结构，两者都是位于可以使用的RAM(SRAM，locked cache...)中，在调用board_init_f()函数前，GD应该被清0；

Uboot启动流程分析(二)

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