RISC-V UEFI 架构支持详解，第 1 部分 - OpenSBI/U-Boot/UEFI 简介

Qemu (both RV32 & RV64) for the following bootflow
   OpenSBI->U-Boot->Linux
   EDK2->Linux

HiFive unleashed using (RV64) for the following bootflow
   OpenSBI->U-Boot->Linux
   EDK2->Linux

_start:
	/* Find preferred boot HART id */  		# 判断 HART id
	MOV_3R	s0, a0, s1, a1, s2, a2d
	call	fw_boot_hart
	add	a6, a0, zero
	MOV_3R	a0, s0, a1, s1, a2, s2
	li	a7, -1
	beq	a6, a7, _try_lottery
	/* Jump to relocation wait loop if we are not boot hart */
	bne	a0, a6, _wait_relocate_copy_done
_try_lottery:

	/* Reset all registers for boot HART */         # 清除寄存器值
	li	ra, 0
	call	_reset_regs

	/* Zero-out BSS */				# 清除 BSS 段
	lla	s4, _bss_start
	lla	s5, _bss_end
_bss_zero:
	REG_S	zero, (s4)
	add	s4, s4, __SIZEOF_POINTER__
	blt	s4, s5, _bss_zero

	/* Setup temporary stack */			# 设置 SP 栈指针
	lla	s4, _fw_end
	li	s5, (SBI_SCRATCH_SIZE * 2)
	add	sp, s4, s5

	/*
	 * Initialize platform
	 * Note: The a0 to a4 registers passed to the
	 * firmware are parameters to this function.
	 */
	MOV_5R	s0, a0, s1, a1, s2, a2, s3, a3, s4, a4
	call	fw_platform_init  			# 读取设备树中的设备信息

        # FDT 重定位
	/*
	 * Relocate Flatened Device Tree (FDT)
	 * source FDT address = previous arg1
	 * destination FDT address = next arg1
	 *
	 * Note: We will preserve a0 and a1 passed by
	 * previous booting stage.
	 */

        ...

	/* Initialize SBI runtime */
	csrr	a0, CSR_MSCRATCH
	call	sbi_init 		# 底层关键初始化结束，跳转到 sbi_init
        ...

static void __noreturn init_coldboot(struct sbi_scratch *scratch, u32 hartid)
{
        ...

        # 初始化动态加载的镜像的模块
        /* Note: This has to be second thing in coldboot init sequence */
        rc = sbi_domain_init(scratch, hartid);

        ...
        # 平台的早期初始化
        rc = sbi_platform_early_init(plat, TRUE);

        # 控制台初始化，从这里开始，就可以使用串口输出
        rc = sbi_console_init(scratch);

        # 核间中断初始化
        rc = sbi_ipi_init(scratch, TRUE);

        # MMU 的 TLB 表的初始化
        rc = sbi_tlb_init(scratch, TRUE);

        # Timer 初始化
        rc = sbi_timer_init(scratch, TRUE);
                sbi_printf("%s: PMP configure failed (error %d)\n",
                           __func__, rc);
                sbi_hart_hang();
        }

        # 准备下一级的 Bootloader
        sbi_hsm_prepare_next_jump(scratch, hartid);

static void __noreturn init_coldboot(struct sbi_scratch *scratch, u32 hartid)
{
    sbi_hart_switch_mode(hartid, scratch->next_arg1, scratch->next_addr,
                              scratch->next_mode, FALSE);
}

+--------+----------------+----------------+----------+
| Boot   | Terminology #1 | Terminology #2 | Actual   |
| stage  |                |                | program  |
| number |                |                | name     |
+--------+----------------+----------------+----------+
| 1      |  Primary       |  -             | ROM code |
|        |  Program       |                |          |
|        |  Loader        |                |          |
|        |                |                |          |
| 2      |  Secondary     |  1st stage     | U-Boot   |
|        |  Program       |  bootloader    | SPL      |
|        |  Loader (SPL)  |                |          |
|        |                |                |          |
| 3      |  -             |  2nd stage     | U-Boot   |
|        |                |  bootloader    |          |
|        |                |                |          |
| 4      |  -             |  -             | kernel   |
|        |                |                |          |
+--------+----------------+----------------+----------+

static const init_fnc_t init_sequence_f[] = {
        ...
	setup_mon_len,
	initf_bootstage,	/* uses its own timer, so does not need DM */
	event_init,
        ...
	setup_spl_handoff,
	arch_cpu_init,		/* basic arch cpu dependent setup */
	mach_cpu_init,		/* SoC/machine dependent CPU setup */
	initf_dm,
        ...
	env_init,		/* initialize environment */
	init_baud_rate,		/* initialze baudrate settings */
	serial_init,		/* serial communications setup */
	console_init_f,		/* stage 1 init of console */
	display_options,	/* say that we are here */
	display_text_info,	/* show debugging info if required */
	checkcpu,
        ...
	INIT_FUNC_WATCHDOG_RESET
	/*
	 * Now that we have DRAM mapped and working, we can
	 * relocate the code and continue running from DRAM.
	 *
	 * Reserve memory at end of RAM for (top down in that order):
	 *  - area that won't get touched by U-Boot and Linux (optional)
	 *  - kernel log buffer
	 *  - protected RAM
	 *  - LCD framebuffer
	 *  - monitor code
	 *  - board info struct
	 */
	setup_dest_addr,
        ...
	do_elf_reloc_fixups,
        ...
	clear_bss,
};

void board_init_f(ulong boot_flags)
{
        ...
	if (initcall_run_list(init_sequence_f))
		hang();
        ...
}

static init_fnc_t init_sequence_r[] = {
	initr_trace,
        ...
	initr_caches,			# 使能 cache 接口
	initr_reloc_global_data,   	# 设置 monitor_flash_len
        ...
	initr_barrier,
	initr_malloc,
	log_init,
	initr_bootstage,	/* Needs malloc() but has its own timer */
        ...
	board_init,	/* Setup chipselects */
        ...
	pci_init,
        ...
	cpu_init_r,
        ...
};

# board_init_r 主要执行 init_sequence_r 中一系列初始化函数

void board_init_r(gd_t *new_gd, ulong dest_addr)
{
        ...
	if (initcall_run_list(init_sequence_r))
		hang();
        ...
}

void main_loop(void)
{
	const char *s;

	bootstage_mark_name(BOOTSTAGE_ID_MAIN_LOOP, "main_loop");

        # 设置 U-boot 的版本号

	if (IS_ENABLED(CONFIG_VERSION_VARIABLE))
		env_set("ver", version_string);  /* set version variable */

        # 命令初始化，初始化 shell 相关的变量
	cli_init();

        # 获取环境变量 perboot 的内容
	if (IS_ENABLED(CONFIG_USE_PREBOOT))
		run_preboot_environment_command();

        ...

        # 读取环境变量 bootdelay 和 bootcmd 的内容
        # 然后将 bootdelay 的值赋值给全局变量 stored_bootdelay

	s = bootdelay_process();
	if (cli_process_fdt(&s))
		cli_secure_boot_cmd(s);

        # 检查倒计时是否结束

	autoboot_command(s);

        # 命令处理函数，负责接收好处理输入的命令
	cli_loop();
	panic("No CLI available");
}