(am335x) u-boot启动过程分析之概述

左手的ㄟ右手 2021-11-29 23:04 670阅读 0赞

***芯片到uboot启动流程 ：ROM → MLO(SPL)→ uboot.img***

***第一级bootloader:引导加载程序，板子上电后会自动执行这些代码，如启动方式（SDcard、SPI、NOR），为启动介质加载驱动，加载第二级bootloader到 ISRAM，然后跳转至第二级bootloader。这些代码存放在 176KB 的 ROM 中.***

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***第二级bootloader:MLO(SPL)，主要是cpu相关的初始化，关看门狗，关中断，设置CPU时钟频率、速度、加载uboot.img、dts等操作，然后会跳转到第三级bootloader。MLO文件应该会被映射到 64 KB的 Internal SRAM 中.***

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***第三级bootloader:u-boot.img, C代码的入口. 板级相关的初始化：uart，网卡，中断，gpio引脚等，加载内核和设备树到ddr,并跳转到内核入口。***

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其中第一级 bootloader 是板子固化的，第二级和第三级是通过编译 uboot 所得的。

这里先整体介绍u-boot的整体功能，接下来再具体展开分析。  
***执行流程如下：***

### SPL(MLO): ###

*  **1. reset; (@arch/arm/cpu/armv7/start.s)**
    
     *  \-> @ disable interrupts (FIQ and IRQ);
     *  \-> @ set the cpu to SVC32 mode;
 *  **2. cpu\_init\_cp15: (@arch/arm/cpu/armv7/start.s)**
    
     *  \-> @ setup CP15 registers (cache, MMU, TLBs);
     *  \-> @ Invalidate L1 I/D;
     *  \-> @ disable MMU stuff and caches
 *  **3. cpu\_init\_crit: (@arch/arm/cpu/armv7/start.s)**
    
     *  \-> go setup pll,mux,memory;
     *  \-> ***lowlevel\_init: (@arch/arm/cpu/armv7/lowlevel\_init.s)***
     *  \----> @ Setup a temporary stack
     *  \----> @ Set up global data
     *  \----> **s\_init: (@arch/arm/cpu/armv7/am33xx/board.c)**
     *  \-------> watchdog\_disable
     *  \-------> timer\_init
     *  \-------> set\_uart\_mux\_conf
     *  \-------> setup\_clocks\_for\_console
     *  \-------> uart\_soft\_reset
     *  \-------> preloader\_console\_init
     *  \-------> rtc32k\_enable
     *  \-------> `prcm_init`
     *  \-------> `sdram_init`
 *  **4. \_*main*: (@arch/arm/lib/crt0.s)**
    
     *  \-> @ Set up initial C runtime environment and call board\_init\_f;
     *  \-> **board\_init\_f:(@arch/arm/lib/spl.c)**
     *  \----> @ call board\_init\_r;
     *  \----> **board\_init\_r:(@common/spl/spl.c)**
     *  \-------> spl\_xxx\_load\_image :根据不同的启动方式加载 u-boot 镜像
     *  \-------> jump\_to\_image\_no\_args:进入u-boot代码运行

### U-BOOT: ###

*  **1. reset; (@arch/arm/cpu/armv7/start.s)**
    
     *  \-> @ set SVC mode;
     *  \-> @ disable FIQ and IRQ;
 *  **２. \_*main*: (@arch/arm/lib/crt0.s)**
    
     *  \-> @ Set up initial C runtime environment and call board\_init\_f;
     *  \-> **board\_init\_f:(@arch/arm/lib/board.c)**
     *  \----> **init\_sequence:(@arch/arm/lib/board.c)**;
     *  \-> **relocate\_code:(@arch/arm/lib/relocate.S)**
     *  \----> @ 代码重定位;
     *  \-> **board\_init\_r:(@arch/arm/lib/board.c)**
     *  \----> enable\_caches;
     *  \----> **board\_init:(@board/ti/am335x/board.c)**;
     *  \-------> ＠ 根据配置初始化各种外设;
     *  \----> **main\_loop:(@common/main.c)**;

转自：https://www.jianshu.com/p/6c85f3c05b95

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