Linux在打开字符设备文件的时候都干了哪些工作

首先，我们需要明确以下，外设都是放在/dev/目录下的，存储其的文件系统是基于内存的，初始化具体文件系统的inode是通过init_special_inode这个函数来实现的：

void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev)
{
    inode->i_mode = mode;
    if (S_ISCHR(mode)) {
        inode->i_fop = &def_chr_fops;
        inode->i_rdev = rdev;
    } else if (S_ISBLK(mode)) {
        inode->i_fop = &def_blk_fops;
        inode->i_rdev = rdev;
    } else if (S_ISFIFO(mode))
        inode->i_fop = &pipefifo_fops;
    else if (S_ISSOCK(mode))
        ;    /* leave it no_open_fops */
    else
        printk(KERN_DEBUG "init_special_inode: bogus i_mode (%o) for"
                 " inode %s:%lu\n", mode, inode->i_sb->s_id,
                  inode->i_ino);
}

const struct file_operations def_chr_fops = {
    .open = chrdev_open,
    .llseek = noop_llseek,
};

这样，当我们想要打开一个字符设备的时候，实际上调用的就是chrdev_open函数了，内核会先找到inode对应的设备号，然后根据之前介绍的cdev_map这个哈希表，找到对应的kobject, 根据kobject可以找到字符设备的描述cdev，同时，它会把这个cdev设置到inode对应的结构里面（下次open就不用再去走kobject->cdev这条线了），并且，会把inode放进字符设备列表里面，然后就是设置file_operations那一套了（这个才是写字符驱动的时候自己写的那部分）

/*
 * Called every time a character special file is opened
 */
static int chrdev_open(struct inode *inode, struct file *filp)
{
    const struct file_operations *fops;
    struct cdev *p;
    struct cdev *new = NULL;
    int ret = 0;

    spin_lock(&cdev_lock);
    p = inode->i_cdev;
    if (!p) {
        struct kobject *kobj;
        int idx;
        spin_unlock(&cdev_lock);
        kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);
        if (!kobj)
            return -ENXIO;
        new = container_of(kobj, struct cdev, kobj);
        spin_lock(&cdev_lock);
        /* Check i_cdev again in case somebody beat us to it while
           we dropped the lock. */
        p = inode->i_cdev;
        if (!p) {
            inode->i_cdev = p = new;
            list_add(&inode->i_devices, &p->list);
            new = NULL;
        } else if (!cdev_get(p))
            ret = -ENXIO;
    } else if (!cdev_get(p))
        ret = -ENXIO;
    spin_unlock(&cdev_lock);
    cdev_put(new);
    if (ret)
        return ret;

    ret = -ENXIO;
    fops = fops_get(p->ops);
    if (!fops)
        goto out_cdev_put;

    replace_fops(filp, fops);
    if (filp->f_op->open) {
        ret = filp->f_op->open(inode, filp);
        if (ret)
            goto out_cdev_put;
    }

    return 0;

 out_cdev_put:
    cdev_put(p);
    return ret;
}

其中，inode里面关于这部分的结构代码如下：

struct inode {
    umode_t            i_mode;
    unsigned short        i_opflags;
    kuid_t            i_uid;
    kgid_t            i_gid;
    unsigned int        i_flags;
...
    dev_t            i_rdev;
    loff_t            i_size;
    struct timespec64    i_atime;
    struct timespec64    i_mtime;
    struct timespec64    i_ctime;
    spinlock_t        i_lock;    /* i_blocks, i_bytes, maybe i_size */
...
    struct list_head    i_devices;
    union {
        struct pipe_inode_info    *i_pipe;
        struct block_device    *i_bdev;
        struct cdev        *i_cdev;
        char            *i_link;
        unsigned        i_dir_seq;
    };
...
    void            *i_private; /* fs or device private pointer */
} __randomize_layout;