kernel启动分析

• • kernel启动分析
    • 一、链接脚本
    • 二、head.S
      • 1.汇编阶段
        • 内核运行的虚拟地址与物理地址
        • 重要注释
        • 设置CPU工作模式
        • 汇编阶段
          • __lookup_processor_type
          • __lookup_machine_type
          • __vet_atags
          • __create_page_tables
          • __switch_data -- __mmap_switched
        • C语言阶段
          • banner
          • setup_arch
          • rest_init
        • 最终阶段启动
          • init进程
    • 三、cmdline
      • 物理存储
      • 网络存储
    • 四、mach

//	内核运行的虚拟地址				0xC0008000
#define KERNEL_RAM_VADDR	(PAGE_OFFSET + TEXT_OFFSET)
//	内核运行的物理地址				0x30008000
#define KERNEL_RAM_PADDR	(PHYS_OFFSET + TEXT_OFFSET)

/*
 * Kernel startup entry point.
 * ---------------------------
 *
 * This is normally called from the decompressor code.  The requirements
 * are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0,
 * r1 = machine nr, r2 = atags pointer.
 *
 * This code is mostly position independent, so if you link the kernel at
 * 0xc0008000, you call this at __pa(0xc0008000).
 *
 * See linux/arch/arm/tools/mach-types for the complete list of machine
 * numbers for r1.
 *
 * We're trying to keep crap to a minimum; DO NOT add any machine specific
 * crap here - that's what the boot loader (or in extreme, well justified
 * circumstances, zImage) is for.
 */

	__HEAD
ENTRY(stext)
	setmode	PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode
						@ and irqs disabled

/*
 * Read processor ID register (CP#15, CR0), and look up in the linker-built
 * supported processor list.  Note that we can't use the absolute addresses
 * for the __proc_info lists since we aren't running with the MMU on
 * (and therefore, we are not in the correct address space).  We have to
 * calculate the offset.
 * 读取处理器ID，然后在链接器内置的处理器列表中寻找该ID
 *	r9 = cpuid
 * Returns:
 *	r3, r4, r6 corrupted
 *	r5 = proc_info pointer in physical address space
 *	r9 = cpuid (preserved)
 */

/*
 * Lookup machine architecture in the linker-build list of architectures.
 * Note that we can't use the absolute addresses for the __arch_info
 * lists since we aren't running with the MMU on (and therefore, we are
 * not in the correct address space).  We have to calculate the offset.
 * 在内置的架构列表中，寻找机器架构码
 *  r1 = machine architecture number
 * Returns:
 *  r3, r4, r6 corrupted
 *  r5 = mach_info pointer in physical address space
 */

/* 
 * Determine validity of the r2 atags pointer.  The heuristic requires
 * that the pointer be aligned, in the first 16k of physical RAM and
 * that the ATAG_CORE marker is first and present.  Future revisions
 * of this function may be more lenient with the physical address and
 * may also be able to move the ATAGS block if necessary.
 * 确认atags中tag的有效性
 * r8  = machinfo
 *
 * Returns:
 *  r2 either valid atags pointer, or zero
 *  r5, r6 corrupted
 */

/*
 * The following fragment of code is executed with the MMU on in MMU mode,
 * and uses absolute addresses; this is not position independent.
 * 下面的代码时在MMU模式下操作MMU，是非PIC码
 *  r0  = cp#15 control register
 *  r1  = machine ID
 *  r2  = atags pointer
 *  r9  = processor ID
 */

	str	r9, [r4]			@ Save processor ID
	str	r1, [r5]			@ Save machine type
	str	r2, [r6]			@ Save atags pointer
	bic	r4, r0, #CR_A			@ Clear 'A' bit
	stmia	r7, {r0, r4}			@ Save control register values	

#define	KERN_EMERG	"<0>"	/* system is unusable			*/
#define	KERN_ALERT	"<1>"	/* action must be taken immediately	*/
#define	KERN_CRIT	"<2>"	/* critical conditions			*/
#define	KERN_ERR	"<3>"	/* error conditions			*/
#define	KERN_WARNING	"<4>"	/* warning conditions			*/
#define	KERN_NOTICE	"<5>"	/* normal but significant condition	*/
#define	KERN_INFO	"<6>"	/* informational			*/
#define	KERN_DEBUG	"<7>"	/* debug-level messages			*/

static const struct machine_desc __mach_desc_SMDKV210	
 __used							
 __attribute__((__section__(".arch.info.init"))) = {	
	.nr		= MACH_TYPE_SMDKV210,		
	.name		= "SMDKV210",
	.phys_io	= S3C_PA_UART & 0xfff00000,
	.io_pg_offst	= (((u32)S3C_VA_UART) >> 18) & 0xfffc,
	.boot_params	= S5P_PA_SDRAM + 0x100,
	.init_irq	= s5pv210_init_irq,
	.map_io		= smdkv210_map_io,
	.init_machine	= smdkv210_machine_init,
	.timer		= &s5p_systimer,
};

static noinline void __init_refok rest_init(void)
	__releases(kernel_lock)
{
	int pid;

	rcu_scheduler_starting();
	/*
	 * We need to spawn init first so that it obtains pid 1, however
	 * the init task will end up wanting to create kthreads, which, if
	 * we schedule it before we create kthreadd, will OOPS.
	 */
	kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
	numa_default_policy();
	pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
	rcu_read_lock();
	kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
	rcu_read_unlock();
	complete(&kthreadd_done);
	unlock_kernel();

	/*
	 * The boot idle thread must execute schedule()
	 * at least once to get things moving:
	 */
	init_idle_bootup_task(current);
	preempt_enable_no_resched();
	schedule();
	preempt_disable();

	/* Call into cpu_idle with preempt disabled */
	cpu_idle();
}

	/* Open the /dev/console on the rootfs, this should never fail */
	if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
		printk(KERN_WARNING "Warning: unable to open an initial console.
");

	(void) sys_dup(0);
	(void) sys_dup(0);

	/*
	 * check if there is an early userspace init.  If yes, let it do all
	 * the work
	 */

	if (!ramdisk_execute_command)
		ramdisk_execute_command = "/init";

	if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {
		ramdisk_execute_command = NULL;
		prepare_namespace();
	}

	/*
	 * We try each of these until one succeeds.
	 *
	 * The Bourne shell can be used instead of init if we are
	 * trying to recover a really broken machine.
	 */
	if (execute_command) {
		run_init_process(execute_command);
		printk(KERN_WARNING "Failed to execute %s.  Attempting "
					"defaults...
", execute_command);
	}
	
	/*
	 *	按顺序一次寻找init程序
	 */
	run_init_process("/sbin/init");
	run_init_process("/etc/init");
	run_init_process("/bin/init");
	run_init_process("/bin/sh");

	panic("No init found.  Try passing init= option to kernel. "
	      "See Linux Documentation/init.txt for guidance.");

console=ttySAC2,115200 
root=/dev/mmcblk0p2 rw 
init=/linuxrc 
rootfstype=ext3

root=/dev/nfs 
nfsroot=192.168.1.20:/root/s3c2440/build_rootfs/aston_rootfs 
ip=192.168.1.99:192.168.1.20:192.168.1.0:255.255.255.0::eth0:off  
init=/linuxrc 
console=ttySAC2,115200