【OK210试用体验】u-boot篇 -- SPL移植

2015-8-26 20:51:15

h1654155865.6393 ( 楼主 ) 2015-8-26 20:51:15 只看该作者 2841 | 0 倒序浏览

本帖最后由 q15920078530 于 2015-8-28 20:25 编辑

【OK210试用体验】u-boot篇 -- SPL移植

在S5PV210的启动流程中可以知道，iROM运行后会先后拷贝BL1和BL2到内存中运行。BL1阶段除了完成基本的硬件初始化，还完成了BL2拷贝到内存中运行的重要任务。在u-boot中，其提供了SPL架构，来更好地完成BL2的加载。

SPL，secondary program loader，中文翻译为第二阶段编程加载器。在u-boot**/doc目录下，有README.SPL专门说明SPL。

重点文件及函数：start.S、lowlevel_init、board_init_f、board_init_r、_main。

移植准备

要移植SPL，首先要先分析一下u-boot的走向。在逻辑程序编写部分上，我们总是习惯用start.S来作为起始部分代码，u-boot其实也不例外。S5PV210的可以在**/arch/arm/cpu/armv7 找到 start.S。代码的内存安排可以看链接脚本：**/arch/arm/cpu/u-boot.lds 和 **/arch/arm/cpu/u-boot-spl.lds 。不管是u-boot-spl.lds还是u-boot.lds ，起始入口都是ENTRY(_start)。所以要分析详细思路，还是从 start.S开始。

根据提示可以更方便地看懂汇编代码，没有仔细学过ARM汇编的可以参考《GNU assembly》和ARM汇编指令详解等资料。有了提示就很方便，不看代码看注释也可快速浏览知道代码做了哪些事。

在cpu_init_crit中调用了 lowlevel_init，lowlevel_init位于 **/board/samsung/smdkv210/lowlevel_init.S，其进行了系统时钟、串口、内存等的底层初始化。

判断完CONFIG_SKIP_LOWLEVEL_INIT是否定义，执行相关命令后跳转到 _main，_main位于**/arch/arm/lib/crt0.S。crt0是 C Run-time。

在crt0.S中有列出_main函数的工作流程：

* 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.

*

* 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.

*

* (the following applies only to non-SPL builds)

*

* 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.

*

* 4. Call relocate_code(). This function relocates U-Boot from its

* current location into the relocation destination computed by

* board_init_f().

*

* 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. GD has retained values set by board_init_f(). Some CPUs

* have some work left to do at this point regarding memory, so

* call c_runtime_cpu_setup.

*

* 6. Branch to board_init_r().
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