rk3188--4.android用initrd文件系统启动流程

在init/intramfs.c中

static int __init populate_rootfs(void)
{
    unpack_to_rootfs(__initramfs_start, __initramfs_size);  //1. initramfs的解压
    if (initrd_start) {
        unpack_to_rootfs((char *)initrd_start, initrd_end - initrd_start); //2.initrd的解压
        free_initrd();
    }
}
rootfs_initcall(populate_rootfs);   //这个相当于module_init在系统初始化时会调用

1. initramfs的解压
unpack_to_rootfs(__initramfs_start, __initramfs_size);
rootfs_initcall(populate_rootfs);
    --> populate_rootfs
        --> unpack_to_rootfs
在init/initramfs.c中

static char * __init unpack_to_rootfs(char *buf, unsigned len)
{
    int i;
    int written, res;
    decompress_fn decompress;
    const char *compress_name;
    static __initdata char msg_buf[64];
    header_buf = kmalloc(110, GFP_KERNEL);
    symlink_buf = kmalloc(PATH_MAX + N_ALIGN(PATH_MAX) + 1, GFP_KERNEL);
    name_buf = kmalloc(N_ALIGN(PATH_MAX), GFP_KERNEL);
    state = Start;
    this_header = 0;
    message = NULL;
    while (!message && len) {
        loff_t saved_offset = this_header;
        //如果开头以字符'0'开始,说明这是cpio格式的ram disk,不用解压直接用复制
        if (*buf == '0' && !(this_header & 3)) {
            state = Start;
            written = write_buffer(buf, len);
            buf += written;
            len -= written;
            continue;
        }
    }
    dir_utime();
    kfree(name_buf);
    kfree(symlink_buf);
    kfree(header_buf);
    return message;
}

在initramfs.cpio中打包了3个文件(2个目录 1个字符设备文件):
        /dev   目录
               /dev/console 文件
        /root    目录

dir /dev 0755 0 0
nod /dev/console 0600 0 0 c 5 1
dir /root 0700 0 0

下面来看一下它们是如何依次解出来的:
    buf=__initramfs_start, len=__initramfs_size

static int __init write_buffer(char *buf, unsigned len)
{
    count = len;
    victim = buf;

    while (!actions[state]())
        ;
    return len - count;
}

1.1 do_start
因为在initramfs.cpio的文件长度都为0,所以没有do_copy的过程
write_buffer
    --> do_start
在init/initramfs.c中

static int __init do_start(void)
{
    //实际作用是将collect指针移动到打包的cpio每一个文件头处
    read_into(header_buf, 110, GotHeader);
    return 0;
}

注意:这个名虽然叫read_into而且第一个参数又是buf,但实际上这个buf没有起到任何作用

static void __init read_into(char *buf, unsigned size, enum state next)
{
    if (count >= size) {
        collected = victim;
        eat(size);
        state = next;      //下一步要执行do_header
    }
}

1.2 do_header解析110字节的头
write_buffer
    --> do_start
        --> do_header

static int __init do_header(void)
{
    if (memcmp(collected, "070707", 6)==0) {
        error("incorrect cpio method used: use -H newc option");
        return 1;
    }
    if (memcmp(collected, "070701", 6)) {
        error("no cpio magic");
        return 1;
    }
    parse_header(collected);      //从101个字节的头中解析出inod mode uid gid等
    next_header = this_header + N_ALIGN(name_len) + body_len;   //移到下一个文件的头处
    next_header = (next_header + 3) & ~3;              //cpio的头部都是4字节对齐的
    state = SkipIt;
    if (name_len <= 0 || name_len > PATH_MAX)
        return 0;
    if (S_ISLNK(mode)) {
        if (body_len > PATH_MAX)
            return 0;
        collect = collected = symlink_buf;
        remains = N_ALIGN(name_len) + body_len;
        next_state = GotSymlink;
        state = Collect;
        return 0;
    }
    //注意下面这个 !body_len,目录的body_len为0设备文件的body_len也为0
    //所以这儿代表的是,所有非链接文件
    if (S_ISREG(mode) || !body_len)
        read_into(name_buf, N_ALIGN(name_len), GotName);  //这个实际的作用是,将指针移动到下一个文件的头处
    return 0;                                             //并将状态改为GotName,即要调用do_name
}

1.3 do_name建立目录文件
write_buffer
    --> do_start
        --> do_header
            --> do_name
进行到此处,系统中己存在/与/root两个目录(都是虚拟的),此时再把打包在cpio里面的文件解析到系统的相应位置上.

static int __init do_name(void)
{
    state = SkipIt;
    next_state = Reset;
    if (strcmp(collected, "TRAILER!!!") == 0) {   //判断是不是结尾
        free_hash();
        return 0;
    }
    clean_path(collected, mode);         //把原先有的路径去掉, 相当于rmdir /dev 或 rm /dev/console
    if (S_ISREG(mode)) {
        int ml = maybe_link();
        if (ml >= 0) {
            int openflags = O_WRONLY|O_CREAT;
            if (ml != 1)
                openflags |= O_TRUNC;
            wfd = sys_open(collected, openflags, mode);    //如果是普通文件打开sys_open

            if (wfd >= 0) {
                sys_fchown(wfd, uid, gid);                 //设置权限等
                sys_fchmod(wfd, mode);
                if (body_len)
                    sys_ftruncate(wfd, body_len);
                vcollected = kstrdup(collected, GFP_KERNEL);
                state = CopyFile;                         //最后调用do_copy将文件内容复制过来
            }
        }
    } else if (S_ISDIR(mode)) {                      // 以/dev为例
        sys_mkdir(collected, mode);                  // 创建 /dev目录
        sys_chown(collected, uid, gid);              // 设置所有者
        sys_chmod(collected, mode);                  // 设置权限
        dir_add(collected, mtime);                   // 更改/dev目录的mtime
    } else if (S_ISBLK(mode) || S_ISCHR(mode) || S_ISFIFO(mode) || S_ISSOCK(mode)) {
        if (maybe_link() == 0) {                     // 以/dev/console为例
            sys_mknod(collected, mode, rdev);        // 创建 /dev/console结点
            sys_chown(collected, uid, gid);          // 设置所有者
            sys_chmod(collected, mode);              // 设置权限
            do_utime(collected, mtime);              // 更改时间戳
        }
    }
    return 0;
}

1.3 do_skip
write_buffer
    --> do_start
        --> do_header
            --> do_name
                --> do_skip

static int __init do_skip(void)
{
    if (this_header + count < next_header) {
        dbmsg();
        eat(count);
        return 1;
    } else {
        dbmsg();
        eat(next_header - this_header);
        state = next_state;
        return 0;
    }
}

1.5 do_reset
write_buffer
    --> do_start
        --> do_header
            --> do_name
                --> do_skip
                    --> do_reset

static int __init do_reset(void)
{
    dbmsg();
    while(count && *victim == '\0')
        eat(1);
    if (count && (this_header & 3))
        error("broken padding");
    return 1;
}

2.initrd的解压
2.1 initrd的起始地址的获取
make menuconfig中
General setup  --->
    [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support

start_kernel
    -->setup_arch
在arch/arm/kernel/setup.c中

void __init setup_arch(char **cmdline_p)
{
    struct machine_desc *mdesc;
    mdesc = setup_machine_fdt(__atags_pointer);
    if (!mdesc)
        mdesc = setup_machine_tags(machine_arch_type);  //读取内核参数
    //uboot的参数: init=/init initrd=0x62000000,0x00130000
    //指定了initrd在内存的起始地址0x62000000,长度0x130000
    parse_early_param();
    arm_memblock_init(&meminfo, mdesc); //将物理地址转为虚地址
}

start_kernel
    -->setup_arch
        --> arm_memblock_init
在arch/arm/mm/init.c中

void __init arm_memblock_init(struct meminfo *mi, struct machine_desc *mdesc)
{
#ifdef CONFIG_BLK_DEV_INITRD
    if (phys_initrd_size) {
        memblock_reserve(phys_initrd_start, phys_initrd_size);
        initrd_start = __phys_to_virt(phys_initrd_start);     //将物理地址0x62000000转为虚地址
        initrd_end = initrd_start + phys_initrd_size;         //end地址+size=0x00130000
    }
#endif
}

unpack_to_rootfs((char *)initrd_start, initrd_end - initrd_start);
其中initrd_start是uboot传入的参数0x62000000的虚地址
里面的内容是烧入板子的boot.img去掉头8字节与尾4个字节,即out/target/product/rk3188/ramdisk.img
注: boot.img的生成
    目录out/target/product/rk30sdk/root存在
        a.将root下的每个文件加上cpio头+每个文件的内容,打包成cpios格式
        b. 将这个cpio文件用gzip压缩后写到文件ramdisk.img中
        c. mkkrnlimg会对ramdisk.img加上8个字节的头标志,尾部加上4个字节
2.2 解压并释放initrd中的文件目录
rootfs_initcall(populate_rootfs);
    --> populate_rootfs
        --> unpack_to_rootfs
在init/initramfs.c中

static char * __init unpack_to_rootfs(char *buf, unsigned len)
{
    int i;
    int written, res;
    decompress_fn decompress;
    const char *compress_name;
    static __initdata char msg_buf[64];
    header_buf = kmalloc(110, GFP_KERNEL);
    symlink_buf = kmalloc(PATH_MAX + N_ALIGN(PATH_MAX) + 1, GFP_KERNEL);
    name_buf = kmalloc(N_ALIGN(PATH_MAX), GFP_KERNEL);

    if (!header_buf || !symlink_buf || !name_buf)
        panic("can't allocate buffers");

    state = Start;
    this_header = 0;
    message = NULL;
    while (!message && len) {
        loff_t saved_offset = this_header;
        if (*buf == '0' && !(this_header & 3)) {
            //不是cpio格式,zip压缩过的开头不为字符'0'
            continue;
        }
        this_header = 0;
        //以开头的0x1f, 0x8b判断是zip压缩的,找到gunzip
        decompress = decompress_method(buf, len, &compress_name);
        //调用压缩函数进行解压缩,解压后调用flush_buffer拷贝到各个目录
        decompress(buf, len, NULL, flush_buffer, NULL, &my_inptr, error);
        this_header = saved_offset + my_inptr;
        buf += my_inptr;
        len -= my_inptr;
    }
    dir_utime();
    kfree(name_buf);
    kfree(symlink_buf);
    kfree(header_buf);
    return message;
}

do_start
do_header
do_name
do_copy
do_utime
do_skip
do_reset
这儿的wite_buffer,比initramfs的write_buffer多了一个do_copy的过程
因为initramfs中只有名,没有数据.initrd有数据,所以需要将数据复制过去.

static int __init flush_buffer(void *bufv, unsigned len)
{
    char *buf = (char *) bufv;
    int written;
    int origLen = len;
    if (message)
        return -1;
    while ((written = write_buffer(buf, len)) < len && !message) {
        char c = buf[written];
        if (c == '0') {
            buf += written;
            len -= written;
            state = Start;
        } else if (c == 0) {
            buf += written;
            len -= written;
            state = Reset;
        } else
            error("junk in compressed archive");
    }
    return origLen;
}