Arm linux 內(nèi)核移植及系統(tǒng)初始化過(guò)程分析

1. 內(nèi)核移植2. 涉及文件分布介紹
2.1. 內(nèi)核移植2.2. 涉及的頭文件
/linux-2.6.18.8/include
[root@localhost include]# tree -L 1
.
|-- Kbuild
|-- acpi
|-- asm -> asm-arm
|-- asm-alpha
|-- asm-arm ------------------------------->(1)
|-- asm-sparc
|-- asm-sparc64
|-- config
|-- keys
|-- linux ------------------------------->(2)
|-- math-emu
|-- media
|-- mtd
|-- net
|-- pcmcia
|-- rdma
|-- rxrpc
|-- scsi
|-- sound
`-- video

內(nèi)核移植過(guò)程中涉及到的頭文件包括處理器相關(guān)的頭文件(1)和處理器無(wú)關(guān)的頭文件(2)。

2.3. 內(nèi)核移植2.4. 涉及的源文件
/linux-2.6.18.8/arch/arm
[root@localhost arm]# tree -L 1
.
|-- Kconfig
|-- Kconfig-nommu
|-- Kconfig.debug
|-- Makefile
|-- boot ------------------------------->(2)
|-- common
|-- configs
|-- kernel ------------------------------->(3)
|-- lib
|-- mach-at91rm9200
……
|-- mach-omap1
|-- mach-omap2
|-- mach-realview
|-- mach-rpc
|-- mach-s3c2410 ------------------------------->(4)
|-- mach-sa1100
|-- mach-versatile
|-- mm ------------------------------->(5)
|-- nwfpe
|-- oprofile
|-- plat-omap
|-- tools ------------------------------->(1)
`-- vfp

(1)
/linux-2.6.18.8/arch/arm/tools
[root@localhost tools]# tree -L 1
.
|-- Makefile
|-- gen-mach-types
`-- mach-types

Mach-types 文件定義了不同系統(tǒng)平臺(tái)的系統(tǒng)平臺(tái)號(hào)。移植linux內(nèi)核到新的平臺(tái)上需要對(duì)新的平臺(tái)登記系統(tǒng)平臺(tái)號(hào)。

Mach-types文件格式如下：
# machine_is_xxx CONFIG_xxxx MACH_TYPE_xxx number
s3c2410 ARCH_S3C2410 S3C2410 182
smdk2410 ARCH_SMDK2410 SMDK2410 193

之所以需要這些信息，是因?yàn)槟_本文件linux/arch/arm/tools/gen-mach-types需要linux/arch/tools/mach-types來(lái)產(chǎn)生linux/include/asm-arm/mach-types.h文件，該文件中設(shè)置了一些宏定義，需要這些宏定義來(lái)為目標(biāo)系統(tǒng)選擇合適的代碼。

(2)
linux-2.6.18.8/arch/arm/boot/compressed
[root@localhost compressed]# tree -L 1
.
|-- Makefile
|-- Makefile.debug
|-- big-endian.S
|-- head-at91rm9200.S
|-- head.S
|-- ll_char_wr.S
|-- misc.c
|-- ofw-shark.c
|-- piggy.S
`-- vmlinux.lds.in

Head.s 是內(nèi)核映像的入口代碼，是自引導(dǎo)程序。自引導(dǎo)程序包含一些初始化程序，這些程序都是體系結(jié)構(gòu)相關(guān)的。在對(duì)系統(tǒng)作完初始化設(shè)置工作后，調(diào)用misc.c文件中的decompress_kernel()函數(shù)解壓縮內(nèi)核映像到指定的位置，然后跳轉(zhuǎn)到kernel的入口地址。

Vmlinux.lds.in用來(lái)生成內(nèi)核映像的內(nèi)存配置文件。

(3)
linux-2.6.18.8/arch/arm/kernel
[root@localhost kernel]# tree -L 1
.
|-- Makefile
|-- apm.c
|-- armksyms.c
|-- arthur.c
|-- asm-offsets.c
|-- bios32.c
|-- calls.S
|-- dma.c
|-- ecard.c
|-- entry-armv.S
|-- entry-common.S
|-- entry-header.S
|-- fiq.c
|-- head-common.S
|-- head-nommu.S
|-- head.S
|-- init_task.c
|-- io.c
|-- irq.c
|-- isa.c
|-- module.c
|-- process.c
|-- ptrace.c
|-- ptrace.h
|-- semaphore.c
|-- setup.c
|-- smp.c
|-- sys_arm.c
|-- time.c
|-- traps.c
`-- vmlinux.lds.S

內(nèi)核入口處也是由一段匯編語(yǔ)言實(shí)現(xiàn)的，由head.s和head-common.s兩個(gè)文件組成。
Head.s 是內(nèi)核的入口文件， 在head.s的末尾處 #include "head-common.S"。 經(jīng)過(guò)一系列的初始化后，跳轉(zhuǎn)到linux-2.6.18.8/init/main.c中的start_kernel()函數(shù)中，開(kāi)始內(nèi)核的基本初始化過(guò)程。

/linux-2.6.18.8/init
[root@localhost init]# tree
.
|-- Kconfig
|-- Makefile
|-- calibrate.c
|-- do_mounts.c
|-- do_mounts.h
|-- do_mounts_initrd.c
|-- do_mounts_md.c
|-- do_mounts_rd.c
|-- initramfs.c
|-- main.c
`-- version.c

(4)
/linux-2.6.18.8/arch/arm/mach-s3c2410
[root@localhost mach-s3c2410]# tree -L 1
.
|-- Kconfig
|-- Makefile
|-- Makefile.boot
|-- bast-irq.c
|-- bast.h
|-- clock.c
|-- clock.h
|-- common-smdk.c
|-- common-smdk.h
|-- cpu.c
|-- cpu.h
|-- devs.c
|-- devs.h
|-- dma.c
|-- gpio.c
|-- irq.c
|-- irq.h
|-- mach-anubis.c
|-- mach-smdk2410.c
|-- pm-simtec.c
|-- pm.c
|-- pm.h
|-- s3c2400-gpio.c
|-- s3c2400.h
|-- s3c2410-clock.c
|-- s3c2410-gpio.c
|-- s3c2410.c
|-- s3c2410.h
|-- sleep.S
|-- time.c
|-- usb-simtec.c
`-- usb-simtec.h

這個(gè)目錄中的文件都是板級(jí)相關(guān)的，其中比較重要是如下幾個(gè)：
linux/arch/arm/mach-s3c2410/cpu.c
linux/arch/arm/mach-s3c2410/common-smdk.c
linux/arch/arm/mach-s3c2410/devs.c
linux/arch/arm/mach-s3c2410/mach-smdk2410.c
linux/arch/arm/mach-s3c2410/Makefile.boot
linux/arch/arm/mach-s3c2410/s3c2410.c

3. 處理器和設(shè)備4.
這里主要介紹處理器和設(shè)備的描述和操作過(guò)程。設(shè)備描述在linux/arch/arm/mach-s3c2410/devs.c和linux/arch/arm/mach-s3c2410/common-smdk.c中實(shí)現(xiàn)。最后以nand flash為例具體介紹。
4.1. 處理器、設(shè)備4.2. 描述
設(shè)備描述主要兩個(gè)結(jié)構(gòu)體完成：struct resource和struct platform_device。
先來(lái)看看著兩個(gè)結(jié)構(gòu)體的定義：
struct resource {
resource_size_t start;
resource_size_t end;
const char *name;
unsigned long flags;
struct resource *parent, *sibling, *child;
};

Resource結(jié)構(gòu)體主要是描述了設(shè)備在系統(tǒng)中的起止地址、名稱、標(biāo)志以及為了鏈?zhǔn)矫枋龇奖阒赶虮窘Y(jié)構(gòu)體類(lèi)型的指針。Resource定義的實(shí)例將被添加到platform_device結(jié)構(gòu)體對(duì)象中去。

struct platform_device {
const char * name;
u32 id;
struct device dev;
u32 num_resources;
struct resource * resource;
};

Platform_device結(jié)構(gòu)體包括結(jié)構(gòu)體的名稱、ID號(hào)、平臺(tái)相關(guān)的信息、設(shè)備的數(shù)目以及上面定義的resource信息。Platform_device結(jié)構(gòu)對(duì)象將被直接通過(guò)設(shè)備操作函數(shù)注冊(cè)導(dǎo)系統(tǒng)中去。具體注冊(cè)和注銷(xiāo)過(guò)程在下一節(jié)介紹。

4.3. 處理器、設(shè)備4.4. 操作
(1) int platform_device_register(struct platform_device * pdev)； 注冊(cè)設(shè)備
(2) void platform_device_unregister(struct platform_device * pdev)； 注銷(xiāo)設(shè)備
(3) int platform_add_devices(struct platform_device devs, int num)；添加設(shè)備，通過(guò)調(diào)用上面兩個(gè)函數(shù)實(shí)現(xiàn)。
4.5. 添加Nand flash設(shè)備4.6.
下面以nand flash 設(shè)備的描述為例，具體介紹下設(shè)備的描述和注冊(cè)過(guò)程。

// resource結(jié)構(gòu)體實(shí)例s3c_nand_resource 對(duì)nand flash 控制器描述，包括控制器的起止地址和標(biāo)志。
static struct resource s3c_nand_resource[] = {
[0] = {
.start = S3C2410_PA_NAND,
.end = S3C2410_PA_NAND + S3C24XX_SZ_NAND - 1,
.flags = IORESOURCE_MEM,
}
};

//platform_device結(jié)構(gòu)體實(shí)例s3c_device_nand定義了設(shè)備的名稱、ID號(hào)并把resource對(duì)象作為其成員之一。
struct platform_device s3c_device_nand = {
.name = "s3c2410-nand",
.id = -1,
.num_resources = ARRAY_SIZE(s3c_nand_resource),
.resource = s3c_nand_resource,
};

// nand flash 的分區(qū)情況，由mtd_partition結(jié)構(gòu)體定義。
static struct mtd_partition smdk_default_nand_part[] = {
[0] = {
.name = "Boot Agent",
.size = SZ_16K,
.offset = 0,
},
[1] = {
.name = "S3C2410 flash partition 1",
.offset = 0,
.size = SZ_2M,
},
[2] = {
.name = "S3C2410 flash partition 2",
.offset = SZ_4M,
.size = SZ_4M,
},
[3] = {
.name = "S3C2410 flash partition 3",
.offset = SZ_8M,
.size = SZ_2M,
},
[4] = {
.name = "S3C2410 flash partition 4",
.offset = SZ_1M * 10,
.size = SZ_4M,
},
[5] = {
.name = "S3C2410 flash partition 5",
.offset = SZ_1M * 14,
.size = SZ_1M * 10,
},
[6] = {
.name = "S3C2410 flash partition 6",
.offset = SZ_1M * 24,
.size = SZ_1M * 24,
},
[7] = {
.name = "S3C2410 flash partition 7",
.offset = SZ_1M * 48,
.size = SZ_16M,
}
};

static struct s3c2410_nand_set smdk_nand_sets[] = {
[0] = {
.name = "NAND",
.nr_chips = 1,
.nr_partitions = ARRAY_SIZE(smdk_default_nand_part),
.partitions = smdk_default_nand_part,
},
};

/* choose a set of timings which should suit most 512Mbit
* chips and beyond.
*/

static struct s3c2410_platform_nand smdk_nand_info = {
.tacls = 20,
.twrph0 = 60,
.twrph1 = 20,
.nr_sets = ARRAY_SIZE(smdk_nand_sets),
.sets = smdk_nand_sets,
};

/* devices we initialise */
// 最后將nand flash 設(shè)備加入到系統(tǒng)即將注冊(cè)的設(shè)備集合中。
static struct platform_device __initdata *smdk_devs[] = {
&s3c_device_nand,
&smdk_led4,
&smdk_led5,
&smdk_led6,
&smdk_led7,
};

然后通過(guò)smdk_machine_init()函數(shù)，調(diào)用設(shè)備添加函數(shù)platform_add_devices(smdk_devs, ARRAY_SIZE(smdk_devs)) 完成設(shè)備的注冊(cè)。具體過(guò)程參見(jiàn)系統(tǒng)初始化的相關(guān)部分。
5. 系統(tǒng)初始化
5.1. 系統(tǒng)初始化的主干線
Start_kernel() èsetup_arch() èreset_init() è kernel_thread(init …) è init() è do_basic_setup() èdriver_init() è do_initcall()

Start_kernel()函數(shù)負(fù)責(zé)初始化內(nèi)核各個(gè)子系統(tǒng)，最后調(diào)用reset_init()，啟動(dòng)一個(gè)叫做init的內(nèi)核線程，繼續(xù)初始化。Start_kernel()函數(shù)在init/main.c中實(shí)現(xiàn)。

asmlinkage void __init start_kernel(void)
{
char * command_line;
extern struct kernel_param __start___param[], __stop___param[];

smp_setup_processor_id();

/*
* Need to run as early as possible, to initialize the
* lockdep hash:
*/
lockdep_init();

local_irq_disable();
early_boot_irqs_off();
early_init_irq_lock_class();

/*
* Interrupts are still disabled. Do necessary setups, then
* enable them
*/
lock_kernel();
boot_cpu_init();
page_address_init();
printk(KERN_NOTICE);
printk(linux_banner);
setup_arch(&command_line);
//setup processor and machine and destinate some pointers for do_initcalls() functions
// for example init_machine pointer is initialized with smdk_machine_init() function , and //init_machine() function is called by customize_machine(), and the function is processed by //arch_initcall(fn). Therefore smdk_machine_init() is issured. by edwin
setup_per_cpu_areas();
smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */

/*
* Set up the scheduler prior starting any interrupts (such as the
* timer interrupt). Full topology setup happens at smp_init()
* time - but meanwhile we still have a functioning scheduler.
*/
sched_init();
/*
* Disable preemption - early bootup scheduling is extremely
* fragile until we cpu_idle() for the first time.
*/
preempt_disable();
build_all_zonelists();
page_alloc_init();
printk(KERN_NOTICE "Kernel command line: %s/n", saved_command_line);
parse_early_param();
parse_args("Booting kernel", command_line, __start___param,
__stop___param - __start___param,
&unknown_bootoption);
sort_main_extable();
unwind_init();
trap_init();
rcu_init();
init_IRQ();
pidhash_init();
init_timers();
hrtimers_init();
softirq_init();
timekeeping_init();
time_init();
profile_init();
if (!irqs_disabled())
printk("start_kernel(): bug: interrupts were enabled early/n");
early_boot_irqs_on();
local_irq_enable();

/*
* HACK ALERT! This is early. Were enabling the console before
* weve done PCI setups etc, and console_init() must be aware of
* this. But we do want output early, in case something goes wrong.
*/
console_init();
if (panic_later)
panic(panic_later, panic_param);

lockdep_info();

/*
* Need to run this when irqs are enabled, because it wants
* to self-test [hard/soft]-irqs on/off lock inversion bugs
* too:
*/
locking_selftest();

#ifdef CONFIG_BLK_DEV_INITRD
if (initrd_start && !initrd_below_start_ok &&
initrd_start < min_low_pfn << PAGE_SHIFT) {
printk(KERN_CRIT "initrd overwritten (0x%08lx < 0x%08lx) - "
"disabling it./n",initrd_start,min_low_pfn << PAGE_SHIFT);
initrd_start = 0;
}
#endif
vfs_caches_init_early();
cpuset_init_early();
mem_init();
kmem_cache_init();
setup_per_cpu_pageset();
numa_policy_init();
if (late_time_init)
late_time_init();
calibrate_delay();
pidmap_init();
pgtable_cache_init();
prio_tree_init();
anon_vma_init();
#ifdef CONFIG_X86
if (efi_enabled)
efi_enter_virtual_mode();
#endif
fork_init(num_physpages);
proc_caches_init();
buffer_init();
unnamed_dev_init();
key_init();
security_init();
vfs_caches_init(num_physpages);
radix_tree_init();
signals_init();
/* rootfs populating might need page-writeback */
page_writeback_init();
#ifdef CONFIG_PROC_FS
proc_root_init();
#endif
cpuset_init();
taskstats_init_early();
delayacct_init();

check_bugs();

acpi_early_init(); /* before LAPIC and SMP init */

/* Do the rest non-__inited, were now alive */
rest_init();
}

分析start_kernel()源碼， 其中setup_arch() 和 reset_init()是兩個(gè)比較關(guān)鍵的函數(shù)。下面將具體分析這兩個(gè)函數(shù)。
5.2. setup_arch()函數(shù)分析
首先我們來(lái)分析下setup_arch()函數(shù)。
Setup_arch()函數(shù)主要工作是安裝cpu和machine，并為start_kernel()后面的初始化函數(shù)指針指定值。
其中setup_processor()函數(shù)調(diào)用linux/arch/arm/kernel/head_common.S 中的lookup_processor_type函數(shù)查詢處理器的型號(hào)并安裝。

Setup_machine()函數(shù)調(diào)用inux/arch/arm/kernel/head_common.S 中的lookup_machine_type(__machine_arch_type)函數(shù)根據(jù)體系結(jié)構(gòu)號(hào)__machine_arch_type，在__arch_info_begin和__arch_info_end段空間查詢體系結(jié)構(gòu)。問(wèn)題是__machine_arch_type是在什么時(shí)候賦的初值？__arch_info_begin和__arch_info_end段空間到底放的是什么內(nèi)容？
__machine_arch_type是一個(gè)全局變量，在linux/boot/decompress/misc.c的解壓縮函數(shù)中得以賦值。
decompress_kernel(ulg output_start, ulg free_mem_ptr_p, ulg free_mem_ptr_end_p, int arch_id)
{
__machine_arch_type = arch_id;
}

__arch_info_begin和__arch_info_end段空間到底放的內(nèi)容由鏈接器決定，存放是.arch.info.init段的內(nèi)容。這個(gè)段是通過(guò)段屬性__attribute__指定的。Grep一下.arch.info.init 得到./include/asm/mach/arch.h:53: __attribute__((__section__(".arch.info.init"))) = { / 在linux/include/asm-arm/mach/arch.h 中發(fā)現(xiàn)MACHINE_START宏定義。

#define MACHINE_START(_type,_name) /
static const struct machine_desc __mach_desc_##_type /
__attribute_used__ /
__attribute__((__section__(".arch.info.init"))) = { /
.nr = MACH_TYPE_##_type, /
.name = _name,

#define MACHINE_END /
};

inux/arch/arm/mach-s3c2410/mach-smdk2410.c中對(duì).arch.info.init段的初始化如下。
MACHINE_START(SMDK2410, "SMDK2410") /* @TODO: request a new identifier and switch
* to SMDK2410 */
/* Maintainer: Jonas Dietsche */
.phys_io = S3C2410_PA_UART,
.io_pg_offst = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
.boot_params = S3C2410_SDRAM_PA + 0x100,
.map_io = smdk2410_map_io,
.init_irq = s3c24xx_init_irq,
.init_machine = smdk_machine_init,
.timer = &s3c24xx_timer,
MACHINE_END

由此可見(jiàn)在.arch.info.init段內(nèi)存放了__desc_mach_desc_SMDK2410結(jié)構(gòu)體。初始化了相應(yīng)的初始化函數(shù)指針。問(wèn)題又來(lái)了， 這些初始化指針函數(shù)是什么時(shí)候被調(diào)用的呢？
分析發(fā)現(xiàn)，不一而同。
如s3c24xx_init_irq()函數(shù)是通過(guò)start_kernel()里的init_IRQ()函數(shù)調(diào)用init_arch_irq()實(shí)現(xiàn)的。因?yàn)樵贛ACHINE_START結(jié)構(gòu)體中 .init_irq = s3c24xx_init_irq，而在setup_arch()函數(shù)中init_arch_irq = mdesc->init_irq， 所以調(diào)用init_arch_irq()就相當(dāng)于調(diào)用了s3c24xx_init_irq()。
又如smdk_machine_init()函數(shù)的初始化。在MACHINE_START結(jié)構(gòu)體中，函數(shù)指針賦值，.init_machine = smdk_machine_init。而init_machine()函數(shù)被linux/arch/arm/kernel/setup.c文件中的customize_machine()函數(shù)調(diào)用并被arch_initcall(Fn)宏處理，arch_initcall(customize_machine)。 被arch_initcall(Fn)宏處理過(guò)函數(shù)將linux/init/main.c
do_initcalls()函數(shù)調(diào)用。 具體參看下邊的部分。

void __init setup_arch(char cmdline_p)
{
struct tag *tags = (struct tag *)&init_tags;
struct machine_desc *mdesc;
char *from = default_command_line;

setup_processor();
mdesc = setup_machine(machine_arch_type);//machine_arch_type =SMDK2410 by edwin
machine_name = mdesc->name;

if (mdesc->soft_reboot)
reboot_setup("s");

if (mdesc->boot_params)
tags = phys_to_virt(mdesc->boot_params);

/*
* If we have the old style parameters, convert them to
* a tag list.
*/
if (tags->hdr.tag != ATAG_CORE)
convert_to_tag_list(tags);
if (tags->hdr.tag != ATAG_CORE)
tags = (struct tag *)&init_tags;

if (mdesc->fixup)
mdesc->fixup(mdesc, tags, &from, &meminfo);

if (tags->hdr.tag == ATAG_CORE) {
if (meminfo.nr_banks != 0)
squash_mem_tags(tags);
parse_tags(tags);
}

init_mm.start_code = (unsigned long) &_text;
init_mm.end_code = (unsigned long) &_etext;
init_mm.end_data = (unsigned long) &_edata;
init_mm.brk = (unsigned long) &_end;

memcpy(saved_command_line, from, COMMAND_LINE_SIZE);
saved_command_line[COMMAND_LINE_SIZE-1] = /0;
parse_cmdline(cmdline_p, from);
paging_init(&meminfo, mdesc);
request_standard_resources(&meminfo, mdesc);

#ifdef CONFIG_SMP
smp_init_cpus();
#endif

cpu_init();

/*
* Set up various architecture-specific pointers
*/
init_arch_irq = mdesc->init_irq;
system_timer = mdesc->timer;
init_machine = mdesc->init_machine;

#ifdef CONFIG_VT
#if defined(CONFIG_VGA_CONSOLE)
conswitchp = &vga_con;
#elif defined(CONFIG_DUMMY_CONSOLE)
conswitchp = &dummy_con;
#endif
#endif
}
5.3. rest_init()函數(shù)分析
下面我們來(lái)分析下rest_init()函數(shù)。
Start_kernel()函數(shù)負(fù)責(zé)初始化內(nèi)核各子系統(tǒng)，最后調(diào)用reset_init()，啟動(dòng)一個(gè)叫做init的內(nèi)核線程，繼續(xù)初始化。在init內(nèi)核線程中，將執(zhí)行下列init()函數(shù)的程序。Init()函數(shù)負(fù)責(zé)完成根文件系統(tǒng)的掛接、初始化設(shè)備驅(qū)動(dòng)程序和啟動(dòng)用戶空間的init進(jìn)程等重要工作。

static void noinline rest_init(void)
__releases(kernel_lock)
{
kernel_thread(init, NULL, CLONE_FS | CLONE_SIGHAND);
numa_default_policy();
unlock_kernel();

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

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

static int init(void * unused)
{
lock_kernel();
/*
* init can run on any cpu.
*/
set_cpus_allowed(current, CPU_MASK_ALL);
/*
* Tell the world that were going to be the grim
* reaper of innocent orphaned children.
*
* We dont want people to have to make incorrect
* assumptions about where in the task array this
* can be found.
*/
child_reaper = current;

smp_prepare_cpus(max_cpus);

do_pre_smp_initcalls();

smp_init();
sched_init_smp();

cpuset_init_smp();

/*
* Do this before initcalls, because some drivers want to access
* firmware files.
*/
populate_rootfs(); //掛接根文件系統(tǒng)

do_basic_setup(); //初始化設(shè)備驅(qū)動(dòng)程序

/*
* check if there is an early userspace init. If yes, let it do all
* the work //啟動(dòng)用戶空間的init進(jìn)程
*/

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();
}

/*
* Ok, we have completed the initial bootup, and
* were essentially up and running. Get rid of the
* initmem segments and start the user-mode stuff..
*/
free_initmem();
unlock_kernel();
mark_rodata_ro();
system_state = SYSTEM_RUNNING;
numa_default_policy();

if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
printk(KERN_WARNING "Warning: unable to open an initial console./n");

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

if (ramdisk_execute_command) {
run_init_process(ramdisk_execute_command);
printk(KERN_WARNING "Failed to execute %s/n",
ramdisk_execute_command);
}

/*
* 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.../n", execute_command);
}
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.");
}

5.3.1. 掛接根文件系統(tǒng)
Linux/init/ramfs.c
void __init populate_rootfs(void)
{
char *err = unpack_to_rootfs(__initramfs_start,
__initramfs_end - __initramfs_start, 0);
if (err)
panic(err);
#ifdef CONFIG_BLK_DEV_INITRD
if (initrd_start) {
#ifdef CONFIG_BLK_DEV_RAM
int fd;
printk(KERN_INFO "checking if image is initramfs...");
err = unpack_to_rootfs((char *)initrd_start,
initrd_end - initrd_start, 1);
if (!err) {
printk(" it is/n");
unpack_to_rootfs((char *)initrd_start,
initrd_end - initrd_start, 0);
free_initrd();
return;
}
printk("it isnt (%s); looks like an initrd/n", err);
fd = sys_open("/initrd.image", O_WRONLY|O_CREAT, 0700);
if (fd >= 0) {
sys_write(fd, (char *)initrd_start,
initrd_end - initrd_start);
sys_close(fd);
free_initrd();
}
#else
printk(KERN_INFO "Unpacking initramfs...");
err = unpack_to_rootfs((char *)initrd_start,
initrd_end - initrd_start, 0);
if (err)
panic(err);
printk(" done/n");
free_initrd();
#endif
}
#endif
}

5.3.2. 初始化設(shè)備5.3.3. 驅(qū)動(dòng)程序
linux/init/main.c
static void __init do_basic_setup(void)
{
/* drivers will send hotplug events */
init_workqueues();
usermodehelper_init();
driver_init(); /* 初始化驅(qū)動(dòng)程序模型。調(diào)用驅(qū)動(dòng)初始化函數(shù)初始化子系統(tǒng)。 */

#ifdef CONFIG_SYSCTL
sysctl_init();
#endif

do_initcalls();
}

linux/init/main.c
extern initcall_t __initcall_start[], __initcall_end[];

static void __init do_initcalls(void)
{
initcall_t *call;
int count = preempt_count();

for (call = __initcall_start; call < __initcall_end; call++) {
char *msg = NULL;
char msgbuf[40];
int result;

if (initcall_debug) {
printk("Calling initcall 0x%p", *call);
print_fn_descriptor_symbol(": %s()",
(unsigned long) *call);
printk("/n");
}

result = (*call)();

……
……
……
}

/* Make sure there is no pending stuff from the initcall sequence */
flush_scheduled_work();
}
分析上面一段代碼可以看出，設(shè)備的初始化是通過(guò)do_basic_setup()函數(shù)調(diào)用do_initcalls()函數(shù)，實(shí)現(xiàn)__initcall_start, __initcall_end段之間的指針函數(shù)執(zhí)行的。而到底是那些驅(qū)動(dòng)函數(shù)怎么會(huì)被集中到這個(gè)段內(nèi)的呢？我們知道系統(tǒng)內(nèi)存空間的分配是由鏈接器ld讀取鏈接腳本文件決定。鏈接器將同樣屬性的文件組織到相同的段里面去，如所有的.text段都被放在一起。在鏈接腳本里面可以獲得某塊內(nèi)存空間的具體地址。我們來(lái)看下linux-2.6.18.8/arch/arm/kernel/vmlinux.lds.S文件。由于文件過(guò)長(zhǎng)，只貼出和__initcall_start, __initcall_end相關(guān)的部分。
__initcall_start = .;
*(.initcall1.init)
*(.initcall2.init)
*(.initcall3.init)
*(.initcall4.init)
*(.initcall5.init)
*(.initcall6.init)
*(.initcall7.init)
__initcall_end = .;
從腳本文件中我們可以看出， 在__initcall_start, __initcall_end之間放置的是屬行為(.initcall*.init)的函數(shù)數(shù)據(jù) 。在linux/include/linux/init.h文件中可以知道，(.initcall*.init)屬性是由__define_initcall(level, fn)宏設(shè)定的。

#define __define_initcall(level,fn) /
static initcall_t __initcall_##fn __attribute_used__ /
__attribute__((__section__(".initcall" level ".init"))) = fn

#define core_initcall(fn) __define_initcall("1",fn)
#define postcore_initcall(fn) __define_initcall("2",fn)
#define arch_initcall(fn) __define_initcall("3",fn)
#define subsys_initcall(fn) __define_initcall("4",fn)
#define fs_initcall(fn) __define_initcall("5",fn)
#define device_initcall(fn) __define_initcall("6",fn)
#define late_initcall(fn) __define_initcall("7",fn)
#define __initcall(fn) device_initcall(fn)

由此可以判斷，所有的設(shè)備驅(qū)動(dòng)函數(shù)都必然通過(guò)*_initcall(fn)宏的處理。以此為入口，可以查詢所有的設(shè)備驅(qū)動(dòng)。
core_initcall(fn)
static int __init consistent_init(void) linux/arch/arm/mm/consistent.c
static int __init v6_userpage_init(void) linux/arch/arm/mm/copypage-v6.c
static int __init init_dma(void) linux/arch/arm/kernel/dma.c
static int __init s3c2410_core_init(void) linux/arch/arm/mach-s3c2410/s3c2410.c

postcore_initcall(fn)
static int ecard_bus_init(void) linux/arch/arm/kernel/ecard.c

arch_initcall(fn)
static __init int bast_irq_init(void) linux/arch/arm/mach-s3c2410/bast-irq.c
static int __init s3c_arch_init(void) linux/arch/arm/mach-s3c2410/cpu.c
static __init int pm_simtec_init(void) linux/arch/arm/mach-s3c2410/pm-simtec.c
static int __init customize_machine(void) linux/arch/arm/kernel/setup.c

subsys_initcall(fn)
static int __init ecard_init(void) linux/arch/arm/kernel/ecard.c
int __init scoop_init(void) linux/arch/arm/common/scoop.c
static int __init topology_init(void) linux/arch/arm/kernel/setup.c

fs_initcall(fn)
static int __init alignment_init(void) linux/arch/arm/mm/alignment.c

device_initcall(fn)
static int __init leds_init(void) linux/arch/arm/kernel/time.c
static int __init timer_init_sysfs(void) linux/arch/arm/kernel/time.c

late_initcall(fn)
static int __init crunch_init(void) arch/arm/kernel/crunch.c
static int __init arm_mrc_hook_init(void) linux/arch/arm/kernel/traps.c

5.3.4. 啟動(dòng)用戶空間的程序
略