字符设备 和 input 设备--文件如何变成设备

  Linux操作系统把设备划分为字符设备和块设备【网络设备是单独一种类型】。

  很多情况下把字符设备 当 系统控制的一种手段，通过字符设备的I/O control 函数与内涵 进行交换数据。 实际上，Linux内核系统很多时候是把字符设备当做一个框架来使用。

5.1 文件如何变成设备

  回顾aufs系统，通过aufs_get_inode为每个文件创建它的inode对象。对文件和目录都有各自的文件操作函数 和 inode 操作函数。默认情况下，我们使用init_special_inode 函数给对象赋值。

5.1.1 init_special_inode函数

  通过init_special_inode函数使文件变成设备，字符设备 和 块设备 开始浮出水面，函数代码如下

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_rev = rdev;

   } else if ( S_ISBLK(mode)) {

    inode->i_fop = & def_blk_fops;

    inode->i_rdev = rdev;

  }else if  ( IS_ISFIFO(mode) )

    inode->i_fop = &def_fifo_fops;

  else if ( S_ISSOCK(mode))

    inode->i_fop = &bad_sock_fops;

  esle

    printk(KERN_DEBUG "init_special_inode: bogus i_mode (%o ) \n", mode );

}

  这段代码很简单，

如果字符设备，文件操作符指针赋值为def_chr_fops; 如果是块设备，赋值为def_blk_fops。同时inode的i_rdev被赋值为 rdev ，rdev是由主设备号 和次设备号 生成的设备号

  通过init_special_inode函数，inode的文件函数指针被 i_fop 替换，从此inode不在是普通的文件inode， 而是分别代表 字符设备、块设备、 fifo 和 socket的特殊inode。

def_chr_fops 定义如下，在fs/Char_dev.c文件中

/*
 * Dummy default file-operations: the only thing this does
 * is contain the open that then fills in the correct operations
 * depending on the special file...
 */
const struct file_operations def_chr_fops = {
	.open = chrdev_open,
	.llseek = noop_llseek,
};

def_blk_fops定义如下，在fs/Block_dev.c 

const struct file_operations def_blk_fops = {
	.open		= blkdev_open,
	.release	= blkdev_close,
	.llseek		= block_llseek,
	.read		= do_sync_read,
	.write		= do_sync_write,
  	.aio_read	= generic_file_aio_read,
	.aio_write	= blkdev_aio_write,
	.mmap		= generic_file_mmap,
	.fsync		= blkdev_fsync,
	.unlocked_ioctl	= block_ioctl,
#ifdef CONFIG_COMPAT
	.compat_ioctl	= compat_blkdev_ioctl,
#endif
	.splice_read	= generic_file_splice_read,
	.splice_write	= generic_file_splice_write,
};

def_fifo_fops定义如下，在fs/Fifo.c

/*
 * Dummy default file-operations: the only thing this does
 * is contain the open that then fills in the correct operations
 * depending on the access mode of the file...
 */
const struct file_operations def_fifo_fops = {
	.open		= fifo_open,	/* will set read_ or write_pipefifo_fops */
	.llseek		= noop_llseek,
};

bad_sock_fops定义如下，在net/Socket.c

const struct file_operations bad_sock_fops = {
	.owner = THIS_MODULE,
	.open = sock_no_open,
	.llseek = noop_llseek,
};

5.1.2 def_chr_fops结构

  def_chr_fops结构如下，代码清单：

const struct file_oprations def_chr_fops = {

  .open = chrdev_open,

};

/*  Called every time a character special file is opened */

每次called------字符特殊文件打开

int chrdev_open( struct inode * inode , struct file * filp)

{

  struct cdev *p;

  struct cdev *next = NULL;

  int ret = 0;

  spin_lock(&cdev_lock);

  p = inode->i_cdev;

  /* 如果字符设不存在 */

  if ( !p ) {

    sgruct kobject * kobj;

    int idx;

    /* 通过kobj_lookup 查找字符设备的kobj结构 */

    kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);

    if ( !kobj )    

      return -ENXIO;

    /* 调用container方法， 获得 cdev对象 */

    new = container_of(kobj, struct cdev, kobj);

    spin_lock(&cdev_lock);

    /*  再次检查 p  */

    p = inode->i_cdev;

    if ( !p ) {

      /* 赋值，inode的字符设备指针指向发现的字符设备  */

      inode->i_cdev = p = new ;

      inode->i_cindex = idx;

      list_add(&inode->i_devices, &p->list);

      new = NULL;

    }else if (!cdev_get(p))

      ret = -ENXIO;

    spin_unlock(&cdev_lock);

    cdev_put( new );

    if (ret)

      return ret;

    /* 获得设备的函数指针，对input设备来说，就是input_fops  */

      flip->f_op = fops_get( p->ops );

      if ( !filp->f_op ) {

        cdev_put(p);

        return -ENXIO;

      }

    /* 这个open函数就是 input 设备的input_open_file */

      if (filp->f_op->open ) {

        /* 大内核锁 */

        lock_kernel();

        ret = filp->f_op->open(inode, filp);

        unlock_kernel();

      }

      if ( ret )

        cdev_put( p );

      return ret;

  }

}

  chrdev_open 函数，每次打开字符设备的时候，都调用这个函数。首先根据设备号调用kobj_lookup搜索注册的字符设备对象，如果找到字符设备，执行字符设备的open函数，找不到则返回错误。

  Linux系统提供 mknod 程序，使用这个程序用户可以根据主从设备创建特殊文件，比如字符设备文件 或者 块设备文件。 从内核角度分析，mknod为特殊文件创建了一个inode结构和 dentry 结构，inode结构的成员包含主从设备 和设备类型，然后调用init_special_inode函数为设备文件的inode设置不同的函数指针。打开设备文件时，通过chrdev_open函数真正调用设备驱动本身的open函数，前提是已经注册了设备驱动函数。

kobj_lookup 函数如下：

struct kobject *kobj_lookup(struct kobj_map *domain, dev_t dev, int *index)
{
	struct kobject *kobj;
	struct probe *p;
	unsigned long best = ~0UL;

retry:
	mutex_lock(domain->lock);

        //  从链表头开始查找
	for (p = domain->probes[MAJOR(dev) % 255]; p; p = p->next) {
		struct kobject *(*probe)(dev_t, int *, void *);
		struct module *owner;
		void *data;

		if (p->dev > dev || p->dev + p->range - 1 < dev)
			continue;
		if (p->range - 1 >= best)
			break;
		if (!try_module_get(p->owner))
			continue;
		owner = p->owner;
		data = p->data;
		probe = p->get;
		best = p->range - 1;
		*index = dev - p->dev;
		if (p->lock && p->lock(dev, data) < 0) {
			module_put(owner);
			continue;
		}
		mutex_unlock(domain->lock);
		kobj = probe(dev, index, data);
		/* Currently ->owner protects _only_ ->probe() itself. */
		module_put(owner);
		if (kobj)
			return kobj;
		goto retry;
	}
	mutex_unlock(domain->lock);
	return NULL;
}

ENXIO定义：

#define	EPERM		 1	/* Operation not permitted */
#define	ENOENT		 2	/* No such file or directory */
#define	ESRCH		 3	/* No such process */
#define	EINTR		 4	/* Interrupted system call */
#define	EIO		 5	/* I/O error */
#define	ENXIO		 6	/* No such device or address */
#define	E2BIG		 7	/* Argument list too long */
#define	ENOEXEC		 8	/* Exec format error */
#define	EBADF		 9	/* Bad file number */
#define	ECHILD		10	/* No child processes */
#define	EAGAIN		11	/* Try again */
#define	ENOMEM		12	/* Out of memory */
#define	EACCES		13	/* Permission denied */
#define	EFAULT		14	/* Bad address */
#define	ENOTBLK		15	/* Block device required */
#define	EBUSY		16	/* Device or resource busy */
#define	EEXIST		17	/* File exists */
#define	EXDEV		18	/* Cross-device link */
#define	ENODEV		19	/* No such device */
#define	ENOTDIR		20	/* Not a directory */
#define	EISDIR		21	/* Is a directory */
#define	EINVAL		22	/* Invalid argument */
#define	ENFILE		23	/* File table overflow */
#define	EMFILE		24	/* Too many open files */
#define	ENOTTY		25	/* Not a typewriter */
#define	ETXTBSY		26	/* Text file busy */
#define	EFBIG		27	/* File too large */
#define	ENOSPC		28	/* No space left on device */
#define	ESPIPE		29	/* Illegal seek */
#define	EROFS		30	/* Read-only file system */
#define	EMLINK		31	/* Too many links */
#define	EPIPE		32	/* Broken pipe */
#define	EDOM		33	/* Math argument out of domain of func */
#define	ERANGE		34	/* Math result not representable */

5.3 input设备架构

5.3.1 注册input设备的驱动

5.3.2 匹配input管理的设备和驱动

5.3.3 注册input设备