啟動流程和上篇博文介紹的rtems類似

首先是start.s：

在這裡選擇arm的arm7作為例子分析，程式碼：/arch/arm/cpu/armv7/start.s

/*
 * armboot - Startup Code for OMAP3530/ARM Cortex CPU-core
 *
 * Copyright (c) 2004	Texas Instruments <[email protected]>
 *
 * Copyright (c) 2001	Marius Gröger <[email protected]>
 * Copyright (c) 2002	Alex Züpke <
 
[email protected]>
 * Copyright (c) 2002	Gary Jennejohn <[email protected]>
 * Copyright (c) 2003	Richard Woodruff <[email protected]>
 * Copyright (c) 2003	Kshitij <[email protected]>
 * Copyright (c) 2006-2008 Syed Mohammed Khasim <[email protected]>
 *
 * SPDX-License-Identifier:	GPL-2.0+
 */

#include <asm-offsets.h>
#include <config.h>
#include <asm/system.h>
#include <linux/linkage.h>

/*************************************************************************
 *
 * Startup Code (reset vector)                      //這是真正的啟動程式碼
 *
 * Do important init only if we don't start from memory!
 * Setup memory and board specific bits prior to relocation.
 * Relocate armboot to ram. Setup stack.
 *
 *************************************************************************/

	.globl	reset
	.globl	save_boot_params_ret

reset:
	/* Allow the board to save important registers */
	b	save_boot_params
save_boot_params_ret:
	/*
	 * disable interrupts (FIQ and IRQ), also set the cpu to SVC32 mode,
	 * except if in HYP mode already
	 */
	mrs	r0, cpsr
	and	r1, r0, #0x1f		@ mask mode bits
	teq	r1, #0x1a		@ test for HYP mode
	bicne	r0, r0, #0x1f		@ clear all mode bits
	orrne	r0, r0, #0x13		@ set SVC mode
	orr	r0, r0, #0xc0		@ disable FIQ and IRQ
	msr	cpsr,r0

/*
 * Setup vector:
 * (OMAP4 spl TEXT_BASE is not 32 byte aligned.
 * Continue to use ROM code vector only in OMAP4 spl)
 */
#if !(defined(CONFIG_OMAP44XX) && defined(CONFIG_SPL_BUILD))
	/* Set V=0 in CP15 SCTLR register - for VBAR to point to vector */
	mrc	p15, 0, r0, c1, c0, 0	@ Read CP15 SCTLR Register
	bic	r0, #CR_V		@ V = 0
	mcr	p15, 0, r0, c1, c0, 0	@ Write CP15 SCTLR Register

	/* Set vector address in CP15 VBAR register */
	ldr	r0, =_start
	mcr	p15, 0, r0, c12, c0, 0	@Set VBAR
#endif

	/* the mask ROM code should have PLL and others stable */
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
	bl	cpu_init_cp15
	bl	cpu_init_crit
#endif

	bl	_main

/*------------------------------------------------------------------------------*/

ENTRY(c_runtime_cpu_setup)
/*
 * If I-cache is enabled invalidate it
 */
#ifndef CONFIG_SYS_ICACHE_OFF
	mcr	p15, 0, r0, c7, c5, 0	@ invalidate icache
	mcr     p15, 0, r0, c7, c10, 4	@ DSB
	mcr     p15, 0, r0, c7, c5, 4	@ ISB
#endif

	bx	lr

ENDPROC(c_runtime_cpu_setup)

/*************************************************************************
 *
 * void save_boot_params(u32 r0, u32 r1, u32 r2, u32 r3)
 *	__attribute__((weak));
 *
 * Stack pointer is not yet initialized at this moment
 * Don't save anything to stack even if compiled with -O0
 *
 *************************************************************************/
ENTRY(save_boot_params)
	b	save_boot_params_ret		@ back to my caller
ENDPROC(save_boot_params)
	.weak	save_boot_params

/*************************************************************************
 *
 * cpu_init_cp15
 *
 * Setup CP15 registers (cache, MMU, TLBs). The I-cache is turned on unless
 * CONFIG_SYS_ICACHE_OFF is defined.
 *
 *************************************************************************/
ENTRY(cpu_init_cp15)
	/*
	 * Invalidate L1 I/D
	 */
	mov	r0, #0			@ set up for MCR
	mcr	p15, 0, r0, c8, c7, 0	@ invalidate TLBs
	mcr	p15, 0, r0, c7, c5, 0	@ invalidate icache
	mcr	p15, 0, r0, c7, c5, 6	@ invalidate BP array
	mcr     p15, 0, r0, c7, c10, 4	@ DSB
	mcr     p15, 0, r0, c7, c5, 4	@ ISB

	/*
	 * disable MMU stuff and caches
	 */
	mrc	p15, 0, r0, c1, c0, 0
	bic	r0, r0, #0x00002000	@ clear bits 13 (--V-)
	bic	r0, r0, #0x00000007	@ clear bits 2:0 (-CAM)
	orr	r0, r0, #0x00000002	@ set bit 1 (--A-) Align
	orr	r0, r0, #0x00000800	@ set bit 11 (Z---) BTB
#ifdef CONFIG_SYS_ICACHE_OFF
	bic	r0, r0, #0x00001000	@ clear bit 12 (I) I-cache
#else
	orr	r0, r0, #0x00001000	@ set bit 12 (I) I-cache
#endif
	mcr	p15, 0, r0, c1, c0, 0

#ifdef CONFIG_ARM_ERRATA_716044
	mrc	p15, 0, r0, c1, c0, 0	@ read system control register
	orr	r0, r0, #1 << 11	@ set bit #11
	mcr	p15, 0, r0, c1, c0, 0	@ write system control register
#endif

#if (defined(CONFIG_ARM_ERRATA_742230) || defined(CONFIG_ARM_ERRATA_794072))
	mrc	p15, 0, r0, c15, c0, 1	@ read diagnostic register
	orr	r0, r0, #1 << 4		@ set bit #4
	mcr	p15, 0, r0, c15, c0, 1	@ write diagnostic register
#endif

#ifdef CONFIG_ARM_ERRATA_743622
	mrc	p15, 0, r0, c15, c0, 1	@ read diagnostic register
	orr	r0, r0, #1 << 6		@ set bit #6
	mcr	p15, 0, r0, c15, c0, 1	@ write diagnostic register
#endif

#ifdef CONFIG_ARM_ERRATA_751472
	mrc	p15, 0, r0, c15, c0, 1	@ read diagnostic register
	orr	r0, r0, #1 << 11	@ set bit #11
	mcr	p15, 0, r0, c15, c0, 1	@ write diagnostic register
#endif
#ifdef CONFIG_ARM_ERRATA_761320
	mrc	p15, 0, r0, c15, c0, 1	@ read diagnostic register
	orr	r0, r0, #1 << 21	@ set bit #21
	mcr	p15, 0, r0, c15, c0, 1	@ write diagnostic register
#endif

	mov	r5, lr			@ Store my Caller
	mrc	p15, 0, r1, c0, c0, 0	@ r1 has Read Main ID Register (MIDR)
	mov	r3, r1, lsr #20		@ get variant field
	and	r3, r3, #0xf		@ r3 has CPU variant
	and	r4, r1, #0xf		@ r4 has CPU revision
	mov	r2, r3, lsl #4		@ shift variant field for combined value
	orr	r2, r4, r2		@ r2 has combined CPU variant + revision

#ifdef CONFIG_ARM_ERRATA_798870
	cmp	r2, #0x30		@ Applies to lower than R3p0
	bge	skip_errata_798870      @ skip if not affected rev
	cmp	r2, #0x20		@ Applies to including and above R2p0
	blt	skip_errata_798870      @ skip if not affected rev

	mrc	p15, 1, r0, c15, c0, 0  @ read l2 aux ctrl reg
	orr	r0, r0, #1 << 7         @ Enable hazard-detect timeout
	push	{r1-r5}			@ Save the cpu info registers
	bl	v7_arch_cp15_set_l2aux_ctrl
	isb				@ Recommended ISB after l2actlr update
	pop	{r1-r5}			@ Restore the cpu info - fall through
skip_errata_798870:
#endif

#ifdef CONFIG_ARM_ERRATA_801819
	cmp	r2, #0x24		@ Applies to lt including R2p4
	bgt	skip_errata_801819      @ skip if not affected rev
	cmp	r2, #0x20		@ Applies to including and above R2p0
	blt	skip_errata_801819      @ skip if not affected rev
	mrc	p15, 0, r0, c0, c0, 6	@ pick up REVIDR reg
	and	r0, r0, #1 << 3		@ check REVIDR[3]
	cmp	r0, #1 << 3
	beq	skip_errata_801819	@ skip erratum if REVIDR[3] is set

	mrc	p15, 0, r0, c1, c0, 1	@ read auxilary control register
	orr	r0, r0, #3 << 27	@ Disables streaming. All write-allocate
					@ lines allocate in the L1 or L2 cache.
	orr	r0, r0, #3 << 25	@ Disables streaming. All write-allocate
					@ lines allocate in the L1 cache.
	push	{r1-r5}			@ Save the cpu info registers
	bl	v7_arch_cp15_set_acr
	pop	{r1-r5}			@ Restore the cpu info - fall through
skip_errata_801819:
#endif

#ifdef CONFIG_ARM_ERRATA_454179
	cmp	r2, #0x21		@ Only on < r2p1
	bge	skip_errata_454179

	mrc	p15, 0, r0, c1, c0, 1	@ Read ACR
	orr	r0, r0, #(0x3 << 6)	@ Set DBSM(BIT7) and IBE(BIT6) bits
	push	{r1-r5}			@ Save the cpu info registers
	bl	v7_arch_cp15_set_acr
	pop	{r1-r5}			@ Restore the cpu info - fall through

skip_errata_454179:
#endif

#ifdef CONFIG_ARM_ERRATA_430973
	cmp	r2, #0x21		@ Only on < r2p1
	bge	skip_errata_430973

	mrc	p15, 0, r0, c1, c0, 1	@ Read ACR
	orr	r0, r0, #(0x1 << 6)	@ Set IBE bit
	push	{r1-r5}			@ Save the cpu info registers
	bl	v7_arch_cp15_set_acr
	pop	{r1-r5}			@ Restore the cpu info - fall through

skip_errata_430973:
#endif

#ifdef CONFIG_ARM_ERRATA_621766
	cmp	r2, #0x21		@ Only on < r2p1
	bge	skip_errata_621766

	mrc	p15, 0, r0, c1, c0, 1	@ Read ACR
	orr	r0, r0, #(0x1 << 5)	@ Set L1NEON bit
	push	{r1-r5}			@ Save the cpu info registers
	bl	v7_arch_cp15_set_acr
	pop	{r1-r5}			@ Restore the cpu info - fall through

skip_errata_621766:
#endif

	mov	pc, r5			@ back to my caller
ENDPROC(cpu_init_cp15)

#ifndef CONFIG_SKIP_LOWLEVEL_INIT
/*************************************************************************
 *
 * CPU_init_critical registers
 *
 * setup important registers
 * setup memory timing
 *
 *************************************************************************/
ENTRY(cpu_init_crit)
	/*
	 * Jump to board specific initialization...
	 * The Mask ROM will have already initialized
	 * basic memory. Go here to bump up clock rate and handle
	 * wake up conditions.
	 */
	b	lowlevel_init		@ go setup pll,mux,memory
ENDPROC(cpu_init_crit)
#endif
 

這部分主要是彙編編寫。

然後進入main_loop函式中：

檔案：common/main.c

void main_loop(void)
{
	const char *s;

	bootstage_mark_name(BOOTSTAGE_ID_MAIN_LOOP, "main_loop");

#ifndef CONFIG_SYS_GENERIC_BOARD
	puts("Warning: Your board does not use generic board. Please read\n");
	puts("doc/README.generic-board and take action. Boards not\n");
	puts("upgraded by the late 2014 may break or be removed.\n");
#endif

	modem_init();
#ifdef CONFIG_VERSION_VARIABLE
	setenv("ver", version_string);  /* set version variable */
#endif /* CONFIG_VERSION_VARIABLE */

	cli_init();

	run_preboot_environment_command();

#if defined(CONFIG_UPDATE_TFTP)
	update_tftp(0UL, NULL, NULL);
#endif /* CONFIG_UPDATE_TFTP */

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

	autoboot_command(s);

	cli_loop();
}
 

其中bootdelay_process函式是為了獲取各種bootcmd環境變數：

檔案：autoboot.c

const char *bootdelay_process(void)
{
	char *s;
	int bootdelay;
#ifdef CONFIG_BOOTCOUNT_LIMIT
	unsigned long bootcount = 0;
	unsigned long bootlimit = 0;
#endif /* CONFIG_BOOTCOUNT_LIMIT */

#ifdef CONFIG_BOOTCOUNT_LIMIT
	bootcount = bootcount_load();
	bootcount++;
	bootcount_store(bootcount);
	setenv_ulong("bootcount", bootcount);
	bootlimit = getenv_ulong("bootlimit", 10, 0);
#endif /* CONFIG_BOOTCOUNT_LIMIT */

	s = getenv("bootdelay");
	bootdelay = s ? (int)simple_strtol(s, NULL, 10) : CONFIG_BOOTDELAY;

#ifdef CONFIG_OF_CONTROL
	bootdelay = fdtdec_get_config_int(gd->fdt_blob, "bootdelay",
			bootdelay);
#endif

	debug("### main_loop entered: bootdelay=%d\n\n", bootdelay);

#if defined(CONFIG_MENU_SHOW)
	bootdelay = menu_show(bootdelay);
#endif
	bootretry_init_cmd_timeout();

#ifdef CONFIG_POST
	if (gd->flags & GD_FLG_POSTFAIL) {
		s = getenv("failbootcmd");
	} else
#endif /* CONFIG_POST */
#ifdef CONFIG_BOOTCOUNT_LIMIT
	if (bootlimit && (bootcount > bootlimit)) {
		printf("Warning: Bootlimit (%u) exceeded. Using altbootcmd.\n",
		       (unsigned)bootlimit);
		s = getenv("altbootcmd");
	} else
#endif /* CONFIG_BOOTCOUNT_LIMIT */
		s = getenv("bootcmd");

	process_fdt_options(gd->fdt_blob);
	stored_bootdelay = bootdelay;

	return s;
}

該函式首先是獲取了bootdelay，將其作為按鍵觸發時間限制，這個待會討論。函式最後，s是獲得了bootcmd變數的資料，然後將其返回。

返回到main_loop函式。接下去執行autoboot_command函式：

檔案：autoboot.c

void autoboot_command(const char *s)
{
	debug("### main_loop: bootcmd=\"%s\"\n", s ? s : "<UNDEFINED>");

	if (stored_bootdelay != -1 && s && !abortboot(stored_bootdelay)) {
#if defined(CONFIG_AUTOBOOT_KEYED) && !defined(CONFIG_AUTOBOOT_KEYED_CTRLC)
		int prev = disable_ctrlc(1);	/* disable Control C checking */
#endif

		run_command_list(s, -1, 0);

#if defined(CONFIG_AUTOBOOT_KEYED) && !defined(CONFIG_AUTOBOOT_KEYED_CTRLC)
		disable_ctrlc(prev);	/* restore Control C checking */
#endif
	}

#ifdef CONFIG_MENUKEY
	if (menukey == CONFIG_MENUKEY) {
		s = getenv("menucmd");
		if (s)
			run_command_list(s, -1, 0);
	}
#endif /* CONFIG_MENUKEY */
}

該函式首先判斷bootdelay，利用一個判斷式：if (stored_bootdelay != -1 && s && !abortboot(stored_bootdelay))
該判斷式中呼叫了abortboot函式：

static int abortboot(int bootdelay)
{
#ifdef CONFIG_AUTOBOOT_KEYED
	return abortboot_keyed(bootdelay);
#else
	return abortboot_normal(bootdelay);
#endif
}

這裡沒有定義CONFIG_AUTOBOOT_KEYED，這個巨集定義的意思是定義了指定按鍵，我們是按任意鍵停止boot，所以沒有定義，執行abortboot_normal函式。

abortboot_normal：

static int abortboot_normal(int bootdelay)
{
	int abort = 0;
	unsigned long ts;

#ifdef CONFIG_MENUPROMPT
	printf(CONFIG_MENUPROMPT);
#else
	if (bootdelay >= 0)
		printf("Hit any key to stop autoboot: %2d ", bootdelay);
#endif

#if defined CONFIG_ZERO_BOOTDELAY_CHECK
	/*
	 * Check if key already pressed
	 * Don't check if bootdelay < 0
	 */
	if (bootdelay >= 0) {
		if (tstc()) {	/* we got a key press	*/
			(void) getc();  /* consume input	*/
			puts("\b\b\b 0");
			abort = 1;	/* don't auto boot	*/
		}
	}
#endif

	while ((bootdelay > 0) && (!abort)) {
		--bootdelay;
		/* delay 1000 ms */
		ts = get_timer(0);
		do {
			if (tstc()) {	/* we got a key press	*/
				abort  = 1;	/* don't auto boot	*/
				bootdelay = 0;	/* no more delay	*/
# ifdef CONFIG_MENUKEY
				menukey = getc();
# else
				(void) getc();  /* consume input	*/
# endif
				break;
			}
			udelay(10000);
		} while (!abort && get_timer(ts) < 1000);

		printf("\b\b\b%2d ", bootdelay);
	}

	putc('\n');

#ifdef CONFIG_SILENT_CONSOLE
	if (abort)
		gd->flags &= ~GD_FLG_SILENT;
#endif

	return abort;
}

這裡判斷如果bootdelay是大於0的，也就是定義了bootdelay時間，那麼列印Hit any key to stop autoboot:，提示敲擊任意鍵即可終止，然後函式開始檢測有無按鍵按下：

if (tstc()) {	/* we got a key press	*/ 如果有按鍵按下
				abort  = 1;	/* don't auto boot	*/ 將abort置為1
				bootdelay = 0;                      //將bootdelay設定為0

最後函式返回abort。

然後回到autoboot_command函式的判斷式中，如果按鍵沒有按下，也就是abort函式返回0，判斷式條件成立，將禁用ctrl+c按鍵，並呼叫run_command_list函式，該函式執行從環境變數讀取的bootcmd的值，也就是s。

該函式在cli.c檔案中：

int run_command_list(const char *cmd, int len, int flag)
{
	int need_buff = 1;
	char *buff = (char *)cmd;	/* cast away const */
	int rcode = 0;

	if (len == -1) {
		len = strlen(cmd);
#ifdef CONFIG_SYS_HUSH_PARSER
		/* hush will never change our string */
		need_buff = 0;
#else
		/* the built-in parser will change our string if it sees \n */
		need_buff = strchr(cmd, '\n') != NULL;
#endif
	}
	if (need_buff) {
		buff = malloc(len + 1);
		if (!buff)
			return 1;
		memcpy(buff, cmd, len);
		buff[len] = '\0';
	}
#ifdef CONFIG_SYS_HUSH_PARSER
	rcode = parse_string_outer(buff, FLAG_PARSE_SEMICOLON);
#else
	/*
	 * This function will overwrite any \n it sees with a \0, which
	 * is why it can't work with a const char *. Here we are making
	 * using of internal knowledge of this function, to avoid always
	 * doing a malloc() which is actually required only in a case that
	 * is pretty rare.
	 */
	rcode = cli_simple_run_command_list(buff, flag);
	if (need_buff)
		free(buff);
#endif

	return rcode;
}

該函式通過parse_string_outer函式呼叫了bush_shell的命令直譯器parse_stream_outer函式來解釋bootcmd的命令，而環境變數bootcmd的啟動命令用來設定linux必要的啟動環境，然後載入和啟動核心。

回到上面，如果有按鍵進行干預，那麼判讀式不成立，跳出執行menucmd命令：

#ifdef CONFIG_MENUKEY
	if (menukey == CONFIG_MENUKEY) {
		s = getenv("menucmd");
		if (s)
			run_command_list(s, -1, 0);
	}

s = getenv("menucmd");是用來獲取在uboot命令列下輸入的命令，menucmd就是儲存命令的環境變數。然後同樣執行run_command_list進行命令解釋，然後跳回到main_loop函式，執行最後的cli_loop函式，也就是進入了uboot命令列模式。

以上就是uboot的啟動過程的分析。

接下來簡單說說uboot對於網絡卡驅動的結構：

首先是最底層的驅動檔案，在這裡選擇cpsw.c驅動檔案進行分析，這是ti公司處理器所獨創的一種驅動方式：

檔案：driver/net/cpsw.c

這裡麵包含了cpsw的傳送，接受，初始化，註冊等。

int cpsw_register(struct cpsw_platform_data *data)
{
	struct cpsw_priv	*priv;
	struct cpsw_slave	*slave;
	void			*regs = (void *)data->cpsw_base;
	struct eth_device	*dev;

	dev = calloc(sizeof(*dev), 1);
	if (!dev)
		return -ENOMEM;

	priv = calloc(sizeof(*priv), 1);
	if (!priv) {
		free(dev);
		return -ENOMEM;
	}

	priv->data = *data;
	priv->dev = dev;

	priv->slaves = malloc(sizeof(struct cpsw_slave) * data->slaves);
	if (!priv->slaves) {
		free(dev);
		free(priv);
		return -ENOMEM;
	}

	priv->host_port		= data->host_port_num;
	priv->regs		= regs;
	priv->host_port_regs	= regs + data->host_port_reg_ofs;
	priv->dma_regs		= regs + data->cpdma_reg_ofs;
	priv->ale_regs		= regs + data->ale_reg_ofs;
	priv->descs		= (void *)regs + data->bd_ram_ofs;

	int idx = 0;

	for_each_slave(slave, priv) {
		cpsw_slave_setup(slave, idx, priv);
		idx = idx + 1;
	}

	strcpy(dev->name, "cpsw");
	dev->iobase	= 0;
	dev->init	= cpsw_init;
	dev->halt	= cpsw_halt;
	dev->send	= cpsw_send;
	dev->recv	= cpsw_recv;
	dev->priv	= priv;

	eth_register(dev);

	cpsw_mdio_init(dev->name, data->mdio_base, data->mdio_div);
	priv->bus = miiphy_get_dev_by_name(dev->name);
	for_active_slave(slave, priv)
		cpsw_phy_init(dev, slave);

	return 1;
}

然後是smsc的一個註冊函式，註冊具體的乙太網晶片，這裡選擇lan8710：

檔案：net/phy/smsc.c

static struct phy_driver lan8710_driver = {
	.name = "SMSC LAN8710/LAN8720",
	.uid = 0x0007c0f0,
	.mask = 0xffff0,
	.features = PHY_BASIC_FEATURES,
	.config = &genphy_config_aneg,
	.startup = &genphy_startup,
	.shutdown = &genphy_shutdown,
};

int phy_smsc_init(void)
{
	phy_register(&lan8710_driver);
	phy_register(&lan911x_driver);
	phy_register(&lan8700_driver);

	return 0;
}

phy_smsc_init函式包含在phy_init()函式中，然後phy_init()函式包含在miiphy_init函式中，位於miiphyinit.c，然後該函式包含在eth_common_init函式中，位於eth.c檔案中，然後該函式包含在eth_initialize函式中，位於eth.c中。然後該函式包含在board_eth_init函式中，位於board.c檔案中，然後在uboot初始化時，呼叫board_eth_init函式從而完成網路驅動的註冊和初始化。