简单字符设备驱动代码＋注释(ldd3第三章例子)

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/*
* main.c -- the bare scull char module
*
* 此代码为ldd3第三章字符设备驱动的例子，自己加了些注释;希望可以和更多有着同样兴趣的朋友们一块学习讨论。
* 哪有注释的不对的地方请发mail给我，或留言；
*
* author : liyangth@gmail.com
*
* date: 2007-04-17
*
* Note：注释的每一个关键的段都以[tag00]作了标签，大家可以按照tag的顺序阅读；
* e.g: 搜索 "Tag000"
*/

#include <linux/config.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/init.h>

#include <linux/kernel.h>    /* printk() */
#include <linux/slab.h>        /* kmalloc() */
#include <linux/fs.h>        /* everything... */
#include <linux/errno.h>    /* error codes */
#include <linux/types.h>    /* size_t */
#include <linux/proc_fs.h>
#include <linux/fcntl.h>    /* O_ACCMODE */
#include <linux/seq_file.h>    /* seq_file used to debug */
#include <linux/cdev.h>

#include <asm/system.h>        /* cli(), *_flags */
#include <asm/uaccess.h>    /* copy_*_user */

#include "scull.h"        /* local definitions */

/*
* Our parameters which can be set at load time.
*/

int scull_major =   SCULL_MAJOR;
int scull_minor =   0;
int scull_nr_devs = SCULL_NR_DEVS;    /* number of bare scull devices */
int scull_quantum = SCULL_QUANTUM;
int scull_qset =    SCULL_QSET;

/*
* 模块参数，可在模块转载时赋值，很灵活方便；
* e.g:
*         insmod scull.ko scull_major=111 scull_nr_devs=3 scull_quantum=1000
*
*[形参说明]
* 1 -- 变量名；
* 2 -- 变量类型；
* 3 -- sysfs入口项的访问许可掩码（一般用S_IRUGO就成）；
*/
module_param(scull_major, int, S_IRUGO);
module_param(scull_nr_devs, int, S_IRUGO);
module_param(scull_quantum, int, S_IRUGO);
module_param(scull_qset, int, S_IRUGO);

MODULE_AUTHOR("Alessandro Rubini, Jonathan Corbet");
MODULE_LICENSE("Dual BSD/GPL");

struct scull_dev *scull_devices;    /* allocated in scull_init_module */
/* Note: 不要把它理解成一个指向scull_dev结构的指针, 它其实是一个scull_dev结构数组,等待下面kmalloc分配多个我们scull设备空间 */

/*
* Empty out the scull device; 就像销毁链表,和理解如何编写一个字符驱动没有关系,可以不看;
*
* must be called with the device semaphore held. 要注意一下了,肯定是要同步的;
*
*/
int scull_trim(struct scull_dev *dev)
{
    struct scull_qset *next, *dptr;
    int qset = dev->qset;   /* "dev" is not-null */
    int i;

    for (dptr = dev->data; dptr; dptr = next) { /* all the list items */
        if (dptr->data) {
            for (i = 0; i < qset; i++)
                kfree(dptr->data[i]);
            kfree(dptr->data);
            dptr->data = NULL;
        }
        next = dptr->next;
        kfree(dptr);
    }
    dev->size = 0;
    dev->quantum = scull_quantum;
    dev->qset = scull_qset;
    dev->data = NULL;
    return 0;
}

//Start: [Tag003] proc的实现,debug driver 的重要手段;
#ifdef SCULL_DEBUG /* use proc only if debugging */
//这个是老方法实现的proc，我们在这重点看新的方法的实现；
/*
* The proc filesystem: function to read and entry
*/

int scull_read_procmem(char *buf, char **start, off_t offset,
                   int count, int *eof, void *data)
{
    int i, j, len = 0;
    int limit = count - 80; /* Don't print more than this */

    for (i = 0; i < scull_nr_devs && len <= limit; i++) {
        struct scull_dev *d = &scull_devices[i];
        struct scull_qset *qs = d->data;
        if (down_interruptible(&d->sem))
            return -ERESTARTSYS;
        len += sprintf(buf+len," Device %i: qset %i, q %i, sz %li ",
                i, d->qset, d->quantum, d->size);
        for (; qs && len <= limit; qs = qs->next) { /* scan the list */
            len += sprintf(buf + len, "  item at %p, qset at %p ",
                    qs, qs->data);
            if (qs->data && !qs->next) /* dump only the last item */
                for (j = 0; j < d->qset; j++) {
                    if (qs->data[j])
                        len += sprintf(buf + len,
                                "    % 4i: %8p ",
                                j, qs->data[j]);
                }
        }
        up(&scull_devices[i].sem);
    }
    *eof = 1;
    return len;
}

/*
* For now, the seq_file implementation will exist in parallel.  The
* older read_procmem function should maybe go away, though.
*/
//下面的是用新方法实现的;
/*
  * seq_file接口假定我们正在创建的虚拟文件要顺序
  * 遍历一个项目序列，而这些项目正是必须要返回给用户空间的；
  *
  * 为使用seq_file，我们必须建立一个简单的"迭代器(iterator)"对象，该
  * 对象用来表示项目序列中的位置，每前进一步，该对象输出序列
  * 中的一个项目.
  *
*/

/*下面建立4个迭代器对象; */

/*
* <IN> struct seq_file *s :大多数情况可以忽略。
* <IN> loff_t *pos: 是一个整数的位置值，表明读取的位置;
*                因为seq_file的实现通常都要遍历一个项目序列，所以，
*                此位置通常被解释为指向下一个项目的游标(cursor);
*
*[Note]:
*        我们的这个驱动程序将每一个设备当作序列中的一个项目，
*        这样，pos我们就可以作为scull_devices数组的索引;
*/
static void *scull_seq_start(struct seq_file *s, loff_t *pos)
{
    if (*pos >= scull_nr_devs)
        return NULL;   /* No more to read */
    return scull_devices + *pos;
}

/*
  * 用途:将迭代器移动到下一个位置(项目);
  * <IN> v: 是先前对start或next调用所返回的迭代器;
*/
static void *scull_seq_next(struct seq_file *s, void *v, loff_t *pos)
{
    (*pos)++;
    if (*pos >= scull_nr_devs)
        return NULL;
    return scull_devices + *pos;
}

/*
* 在内核使用完迭代器后，会调用stop方法来清除;
*/
static void scull_seq_stop(struct seq_file *s, void *v)
{
    /* Actually, there's nothing to do here */
}

/*
  * 在start()和stop()的调用之间内核会调用show()来将
  * 实际的数据输出到用户空间；
  *
  * 该方法应该为迭代器V所指向的项目建立输出；
  * 用于输出的函数不能用printk,要用针对seq_file输出提供的一组特殊函数;
  * seq_printf -- 类似C函数sprintf();
  * seq_putc / seq_puts -- 类似C函数putc,puts.
*/
static int scull_seq_show(struct seq_file *s, void *v)
{
    struct scull_dev *dev = (struct scull_dev *) v;
    struct scull_qset *d;
    int i;

    if (down_interruptible(&dev->sem))
        return -ERESTARTSYS;
    seq_printf(s, " Device %i: qset %i, q %i, sz %li ",
            (int) (dev - scull_devices), dev->qset,
            dev->quantum, dev->size);
    for (d = dev->data; d; d = d->next) { /* scan the list */
        seq_printf(s, "  item at %p, qset at %p ", d, d->data);
        if (d->data && !d->next) /* dump only the last item */
            for (i = 0; i < dev->qset; i++) {
                if (d->data[i])
                    seq_printf(s, "    % 4i: %8p ",
                            i, d->data[i]);
            }
    }
    up(&dev->sem);
    return 0;
}

/*
* 最后将这组函数打包并和/proc中的某个文件连接起来；
*/

/*
  *首先，填充seq_operations结构体;
*/
static struct seq_operations scull_seq_ops = {
    .start = scull_seq_start,
    .next  = scull_seq_next,
    .stop  = scull_seq_stop,
    .show  = scull_seq_show
};

/*
* Now to implement the /proc file we need only make an open
* method which sets up the sequence operators.
*/
static int scull_proc_open(struct inode *inode, struct file *file)
{
    /*
     * seq_open的调用将file结构和我们上面定义的那组迭代器操作方法
     * 连接在一起;
    */
    return seq_open(file, &scull_seq_ops);
}

/*
* Create a set of file operations for our proc file.
* open是我们惟一要实现的文件操作;
*/
static struct file_operations scull_proc_ops = {
    .owner   = THIS_MODULE,
    .open    = scull_proc_open,
    .read    = seq_read,
    .llseek  = seq_lseek,
    .release = seq_release
};


/*
* Actually create (and remove) the /proc file(s).
*/
//分别用新老方法实现了二个proc文件
static void scull_create_proc(void)
{
    struct proc_dir_entry *entry;
    /*old function*/
    create_proc_read_entry("scullmem", 0 /* default mode */,
            NULL /* parent dir */, scull_read_procmem,
            NULL /* client data */);

    /* New function
      * <IN>    name -- 文件名;
      * <IN>    mode -- 访问保护掩码; "0" -- 表示系统默认值;
      * <IN>    parent--父目录;    "NULL"  -- 表示"scullseq"文件将创建在/proc的根目录下;
     */
    entry = create_proc_entry("scullseq", 0, NULL);
    if (entry)
        entry->proc_fops = &scull_proc_ops;
}

static void scull_remove_proc(void)
{
    /* no problem if it was not registered */
    remove_proc_entry("scullmem", NULL /* parent dir */);
    remove_proc_entry("scullseq", NULL);
}

#endif /* SCULL_DEBUG */
//End

/* 开始实现对设备操作的方法集了,关键!!! */
/*
* Open and close
*/
//[Tag004]
/*
open应完成的工作有：
    1.检查设备特定的错误（诸如设备未就绪或类似的硬件问题）
    2.如果设备是首次打开，则对其进行初始化；
    3.如有必要，更新f_op指针；
    4.分配并填写filp->private_data；（在这里我们只实现这项即可）
*/

/*
[形参说明]
    struct inode *inode -- 用它的i_cdev成员得到dev;
    struct file *filp -- 将得到的dev存放到他的成员private_data中；
*/
int scull_open(struct inode *inode, struct file *filp)
{
    struct scull_dev *dev; /* device information */

    dev = container_of(inode->i_cdev, struct scull_dev, cdev);
    /*
    [说明]
        1.我们要填充的应该是我们自己的特殊设备，而不是钳在他里面的字符设备结构；
        2.inode结构的i_cdev成员这能提供基本字符设备结构；
        3.这里利用了定义在<linux/kernel.h>中的宏来实现通过cdev得到dev;
    */

    /*
    以后read , write ,等操作的实现中就靠他来得到dev了；
    */
    filp->private_data = dev; /* for other methods */


    /* now trim to 0 the length of the device if open was write-only */
    if ( (filp->f_flags & O_ACCMODE) == O_WRONLY) {
        if (down_interruptible(&dev->sem))
            return -ERESTARTSYS;
        scull_trim(dev); /* ignore errors */
        up(&dev->sem);
    }
    return 0;          /* success */
}

/* close device file, in here we do nothing */
/*
* [Tag005]
* close应完成的工作有：
*    1.释放由open分配的，保存在filp->private_data中的所有内容；
*  2.在最后一次关闭操作时关闭设备；
* [注意：]并不是每次的close系统调用都会去调用到release. 在open时，也仅在open时才会创建
* 一个新的数据结构；在fork, dup时只是增加了这个结构中维护的一个引用计数；
* 所以当这个引用计数为0时，调用的close才意味着要释放设备数据结构，此时release才会被调用；
*/
int scull_release(struct inode *inode, struct file *filp)
{
    return 0;
}

/*
* Follow the list
*
* 第一次调用时用于创建链表；
* 然后就是找到第n个节点；
* 对编写驱动程序关系不大；
*/
struct scull_qset *scull_follow(struct scull_dev *dev, int n)
{
    struct scull_qset *qs = dev->data;

        /* Allocate first qset explicitly if need be */
    if (! qs) {
        qs = dev->data = kmalloc(sizeof(struct scull_qset), GFP_KERNEL);
        if (qs == NULL)
            return NULL;  /* Never mind */
        memset(qs, 0, sizeof(struct scull_qset));
    }

    /* Then follow the list */
    while (n--) {
        if (!qs->next) {
            qs->next = kmalloc(sizeof(struct scull_qset), GFP_KERNEL);
            if (qs->next == NULL)
                return NULL;  /* Never mind */
            memset(qs->next, 0, sizeof(struct scull_qset));
        }
        qs = qs->next;
        continue;
    }
    return qs;
}

/*[Tag006]
* Data management: read and write
* [read和write的参数]
*        1] filp -- 文件指针；用它的成员filp->private_data得到dev;
*         2] buf -- 都是来自用户空间的指针；
*      3] count -- 缓冲区大小；(希望传输的字节数目)
*        4] f_pos -- 指向一个长偏移量对象的指针，这个对象指明了用户在文件中进行存取
*            操作的位置；
*
*[返回值]
*         1]如果返回值等于count，则完成了所请求数目的字节传输；
*        2]如果返回值是正，但小于count,则继续读或写余下的数据；
*        3]如果为0，则证明已经到了文件尾；
*        4]如果为负，则发生了错误。会返回一个错误码，该值指明了发生了什么错误。
*             错误码在<linux/errno.h>中定义；
*            例如：-EINTR (系统调用被中断)
*                  -EFAULT (无效地址)
*/

ssize_t scull_read(struct file *filp, char __user *buf, size_t count,
                loff_t *f_pos)
{
    struct scull_dev *dev = filp->private_data;
    struct scull_qset *dptr;    /* the first listitem */
    int quantum = dev->quantum, qset = dev->qset;
    int itemsize = quantum * qset; /* how many bytes in the listitem */
    int item, s_pos, q_pos, rest;
    ssize_t retval = 0;

    if (down_interruptible(&dev->sem))    /* 信号灯减1, 用于与write操作同步 */
        return -ERESTARTSYS;
    if (*f_pos >= dev->size) //操作位置到文件尾，或超出文件尾了
        goto out;
    if (*f_pos + count > dev->size) //在当前位置所要读的数目超过文件尾了
        count = dev->size - *f_pos;    //减小这次的期望读取数目

    /* find listitem, qset index, and offset in the quantum */
    item = (long)*f_pos / itemsize; //确定是哪个链表项下，即哪个节点下；
    rest = (long)*f_pos % itemsize; //在这个链表项的什么位置（偏移量），用于下面找qset索引和偏移量；
    s_pos = rest / quantum;        //在这个节点里**data这个指针数组的第几行；
     q_pos = rest % quantum; //在这行，即这个量子里的偏移量；

    /* follow the list up to the right position (defined elsewhere) */
    dptr = scull_follow(dev, item);  //找到这个链表项

    if (dptr == NULL || !dptr->data || ! dptr->data[s_pos])
        goto out; /* don't fill holes */

//以一个量子为单位传，简化了代码；
    /* read only up to the end of this quantum */
    if (count > quantum - q_pos)
        count = quantum - q_pos;

/*
* 上面为这步准备了具体在哪个链表项的指针数组的第几行的第几列（即dptr->data[s_pos] + q_pos）
* 从这个位置的内核态的buf中拷给用户态
*/

//关键一步，将数据拷给用户空间
    if (copy_to_user(buf, dptr->data[s_pos] + q_pos, count)) {
        retval = -EFAULT;
        goto out;
    }
    *f_pos += count; //更新文件指针
    retval = count;

  out:
    up(&dev->sem);
    return retval;
}

//与read的实现类似
ssize_t scull_write(struct file *filp, const char __user *buf, size_t count,
                loff_t *f_pos)
{
    struct scull_dev *dev = filp->private_data;
    struct scull_qset *dptr;
    int quantum = dev->quantum, qset = dev->qset;
    int itemsize = quantum * qset;
    int item, s_pos, q_pos, rest;
    ssize_t retval = -ENOMEM; /* value used in "goto out" statements */

    if (down_interruptible(&dev->sem))
        return -ERESTARTSYS;

    /* find listitem, qset index and offset in the quantum */
    item = (long)*f_pos / itemsize;
    rest = (long)*f_pos % itemsize;
    s_pos = rest / quantum; q_pos = rest % quantum;

    /* follow the list up to the right position */
    dptr = scull_follow(dev, item);
    if (dptr == NULL)
        goto out;
    if (!dptr->data) {
        dptr->data = kmalloc(qset * sizeof(char *), GFP_KERNEL);
        if (!dptr->data)
            goto out;
        memset(dptr->data, 0, qset * sizeof(char *));
    }
    if (!dptr->data[s_pos]) {
        dptr->data[s_pos] = kmalloc(quantum, GFP_KERNEL);
        if (!dptr->data[s_pos])
            goto out;
    }
    /* write only up to the end of this quantum */
    if (count > quantum - q_pos)
        count = quantum - q_pos;

    if (copy_from_user(dptr->data[s_pos]+q_pos, buf, count)) {
        retval = -EFAULT;
        goto out;
    }
    *f_pos += count;
    retval = count;

        /* update the size */
    if (dev->size < *f_pos)
        dev->size = *f_pos;

  out:
    up(&dev->sem);
    return retval;
}

/*
* The ioctl() implementation
* 编写设备驱动最常用的接口!
* 用于对设备不同功能的实现；
* kernel 空间和用户空间交换数据-- 用最后一个参数arg,可以值传递或指针传递；
* 如果要传递多数据(结构体)用指针；
*/

int scull_ioctl(struct inode *inode, struct file *filp,
                 unsigned int cmd, unsigned long arg)
{

    int err = 0, tmp;
    int retval = 0;

    /*
      * 命令(cmd)参数有效性的检查;
      * 1.check幻数；
      * 2. check command length;
      * 3. check 传输数据方向；
    */
    /*
     * extract the type and number bitfields, and don't decode
     * wrong cmds: return ENOTTY (inappropriate ioctl) before access_ok()
     */
    if (_IOC_TYPE(cmd) != SCULL_IOC_MAGIC) return -ENOTTY;
    if (_IOC_NR(cmd) > SCULL_IOC_MAXNR) return -ENOTTY;

    /*
     * the direction is a bitmask, and VERIFY_WRITE catches R/W
     * transfers. `Type' is user-oriented, while
     * access_ok is kernel-oriented, so the concept of "read" and
     * "write" is reversed
     */
    if (_IOC_DIR(cmd) & _IOC_READ)
        err = !access_ok(VERIFY_WRITE, (void __user *)arg, _IOC_SIZE(cmd));
    else if (_IOC_DIR(cmd) & _IOC_WRITE)
        err =  !access_ok(VERIFY_READ, (void __user *)arg, _IOC_SIZE(cmd));
    if (err) return -EFAULT;

    switch(cmd) {

      case SCULL_IOCRESET:
        scull_quantum = SCULL_QUANTUM;
        scull_qset = SCULL_QSET;
        break;

      case SCULL_IOCSQUANTUM: /* Set: arg points to the value(通过指针方式和用户空间传递数据)*/
        if (! capable (CAP_SYS_ADMIN))
            return -EPERM;

        retval = __get_user(scull_quantum, (int __user *)arg);
        break;

      case SCULL_IOCTQUANTUM: /* Tell: arg is the value(通过值传递的方式从用户空间得到数据) */
        if (! capable (CAP_SYS_ADMIN))
            return -EPERM;
        scull_quantum = arg;
        break;

      case SCULL_IOCGQUANTUM: /* Get(站在用户的角度讲): arg is pointer to result( 通过指针方式将数据传给用户空间)*/
        retval = __put_user(scull_quantum, (int __user *)arg);
        break;

      case SCULL_IOCQQUANTUM: /* Query: return it (it's positive) */
        return scull_quantum;    /* 值传递方式传数据到用户空间 */

      case SCULL_IOCXQUANTUM: /* eXchange: use arg as pointer */
        if (! capable (CAP_SYS_ADMIN))
            return -EPERM;
        tmp = scull_quantum;
        retval = __get_user(scull_quantum, (int __user *)arg);
        if (retval == 0)
            retval = __put_user(tmp, (int __user *)arg);
        break;

      case SCULL_IOCHQUANTUM: /* sHift: like Tell + Query */
        if (! capable (CAP_SYS_ADMIN))
            return -EPERM;
        tmp = scull_quantum;
        scull_quantum = arg;
        return tmp;

      case SCULL_IOCSQSET:
        if (! capable (CAP_SYS_ADMIN))
            return -EPERM;
        retval = __get_user(scull_qset, (int __user *)arg);
        break;

      case SCULL_IOCTQSET:
        if (! capable (CAP_SYS_ADMIN))
            return -EPERM;
        scull_qset = arg;
        break;

      case SCULL_IOCGQSET:
        retval = __put_user(scull_qset, (int __user *)arg);
        break;

      case SCULL_IOCQQSET:
        return scull_qset;

      case SCULL_IOCXQSET:
        if (! capable (CAP_SYS_ADMIN))
            return -EPERM;
        tmp = scull_qset;
        retval = __get_user(scull_qset, (int __user *)arg);
        if (retval == 0)
            retval = put_user(tmp, (int __user *)arg);
        break;

      case SCULL_IOCHQSET:
        if (! capable (CAP_SYS_ADMIN))
            return -EPERM;
        tmp = scull_qset;
        scull_qset = arg;
        return tmp;

        /*
         * The following two change the buffer size for scullpipe.
         * The scullpipe device uses this same ioctl method, just to
         * write less code. Actually, it's the same driver, isn't it?
         */

      case SCULL_P_IOCTSIZE:
        scull_p_buffer = arg;
        break;

      case SCULL_P_IOCQSIZE:
        return scull_p_buffer;

      default:  /* redundant, as cmd was checked against MAXNR */
        return -ENOTTY;
    }
    return retval;

}

/*
* The "extended" operations -- only seek
*/

loff_t scull_llseek(struct file *filp, loff_t off, int whence)
{
    struct scull_dev *dev = filp->private_data;
    loff_t newpos;

    switch(whence) {
      case 0: /* SEEK_SET */
        newpos = off;
        break;

      case 1: /* SEEK_CUR */
        newpos = filp->f_pos + off;
        break;

      case 2: /* SEEK_END */
        newpos = dev->size + off;
        break;

      default: /* can't happen */
        return -EINVAL;
    }
    if (newpos < 0) return -EINVAL;
    filp->f_pos = newpos;
    return newpos;
}

//[Tag007]将这组操作打包为一个对象；
struct file_operations scull_fops = {
    .owner =    THIS_MODULE,
    .llseek =   scull_llseek,
    .read =     scull_read,
    .write =    scull_write,
    .ioctl =    scull_ioctl,
    .open =     scull_open,
    .release =  scull_release,
};

/*
* Finally, the module stuff
*/

//[Tag008]模块卸载或goto fail时；
/*
* The cleanup function is used to handle initialization failures as well.
* Thefore, it must be careful to work correctly even if some of the items
* have not been initialized
*/
void scull_cleanup_module(void)
{
    int i;
    dev_t devno = MKDEV(scull_major, scull_minor);

    /* Get rid of our char dev entries */
    if (scull_devices) {
        for (i = 0; i < scull_nr_devs; i++) {
            scull_trim(scull_devices + i);
            cdev_del(&scull_devices[i].cdev);    //[???]是一个内核函数么？
        }
        kfree(scull_devices);
    }

#ifdef SCULL_DEBUG /* use proc only if debugging */
    scull_remove_proc();
#endif

    /* cleanup_module is never called if registering failed */
    unregister_chrdev_region(devno, scull_nr_devs);

    /* and call the cleanup functions for friend devices */
    scull_p_cleanup();
    scull_access_cleanup();

}

/* [Tag002]
    这里主要干了2件事;
    在内核内部使用struct cdev结构来表示字符设备;
    [1]在这里因为我们将cdev结构嵌入到自己的scull_dev设备下了,所以我们用下面这个方法来
    初始化已分配的结构;
    cdev_init(&dev->cdev, &scull_fops);

    [2]告诉内核我们新结构的信息;
*/
/*
* Set up the char_dev structure for this device.
*/
static void scull_setup_cdev(struct scull_dev *dev, int index)
{
    int err, devno = MKDEV(scull_major, scull_minor + index);

   // [1]
    cdev_init(&dev->cdev, &scull_fops);    /* 初始化, 字符设备和给它一组在它上面操作的方法集 */

    /* 填充基本字符设备的成员 */
    dev->cdev.owner = THIS_MODULE;        //模块计数
    dev->cdev.ops = &scull_fops;        //附上一组操作自己的方法集

//    [2]
    err = cdev_add (&dev->cdev, devno, 1);
    /*
    函数说明:
        cdev -- 字符设备的结构指针,我们就是要把他告诉给内核;
        devno -- 设备编号,用MKDEV利用全局的主设备号和次设备号生成的;
        1    -- 是应该和该设备关联的设备编号的数量, 一般情况下都为1;
            一般我们都是一个设备编号对应一个设备;
    */
    /*
    注意:
        在调用cdev_add后,我们的设备就被添加到系统了,他"活"了. 附加的操作集也就可以被内核调用了
        ,因此,在驱动程序还没有完全准备好处理设备上的操作时,就不能调用cdev_add!
    */
    /* Fail gracefully if need be */
    if (err)
        printk(KERN_NOTICE "Error %d adding scull%d", err, index);
}

/*[Tag000]
* 当模块加载时，调用；但是为什么要放在最后来实现他呢，看到Tag002时，你应该就明白了；
*/
int scull_init_module(void)
{
    int result, i;
    dev_t dev = 0;

/* [Tag001] */
/* [1]分配设备编号 */
/*
* Get a range of minor numbers to work with, asking for a dynamic
* major unless directed otherwise at load time.
*/
    if (scull_major) {     /* 预先自己指定了主设备号 */
        dev = MKDEV(scull_major, scull_minor); /* 利用主设备号,找到设备编号给方法1用 */
        result = register_chrdev_region(dev, scull_nr_devs, "scull");
    } else {        /* 动态自己生成设备编号,然后再利用设备编号得到主设备号;
                        记住如果用这个方法那么就要后建设备文件了,因为不能提前知道主号
                        当然也可以利用ldd3书中提供的脚本,巨方便&&通用 */
        result = alloc_chrdev_region(&dev, scull_minor, scull_nr_devs,
                "scull");
        scull_major = MAJOR(dev);
    }
    if (result < 0) {
        printk(KERN_WARNING "scull: can't get major %d ", scull_major);
        return result;
    }

    /*[2]设备对象实例化*/
        /*
     * allocate the devices -- we can't have them static, as the number
     * can be specified at load time
     */
    scull_devices = kmalloc(scull_nr_devs * sizeof(struct scull_dev), GFP_KERNEL);
    if (!scull_devices) {
        result = -ENOMEM;
        goto fail;  /* Make this more graceful */
    }
    memset(scull_devices, 0, scull_nr_devs * sizeof(struct scull_dev));

/* [3]在这里初始化设备用了2.6的新方法,在scull_setup_cdev里完成 */
        /* Initialize each device. */
    for (i = 0; i < scull_nr_devs; i++) {
        scull_devices[i].quantum = scull_quantum;    /* 可以根据自己insmod时传参
                                                        来自己改变量子和量子集(指针数组)的大小 */
        scull_devices[i].qset = scull_qset;
        init_MUTEX(&scull_devices[i].sem);
        scull_setup_cdev(&scull_devices[i], i);    /* 在分别完主设备编号后goto Tag002 设备注册 */
    }

        /* At this point call the init function for any friend device */
    dev = MKDEV(scull_major, scull_minor + scull_nr_devs);
    /*
      * 下面二个是另外二个driver，提供了更高级的操作
      * 阻塞式的read,write的实现，异步通知的实现；
      * 对设备打开权限控制的实现；
    */
    dev += scull_p_init(dev);
    dev += scull_access_init(dev);

#ifdef SCULL_DEBUG /* only when debugging */
    scull_create_proc();
#endif

    return 0; /* succeed */

  fail:
    scull_cleanup_module();
    return result;
}

module_init(scull_init_module);        //insmod
module_exit(scull_cleanup_module);    //rmmod

scull.h

/*
* scull.h -- definitions for the char module
*
* Copyright (C) 2001 Alessandro Rubini and Jonathan Corbet
* Copyright (C) 2001 O'Reilly & Associates
*
* The source code in this file can be freely used, adapted,
* and redistributed in source or binary form, so long as an
* acknowledgment appears in derived source files.  The citation
* should list that the code comes from the book "Linux Device
* Drivers" by Alessandro Rubini and Jonathan Corbet, published
* by O'Reilly & Associates.   No warranty is attached;
* we cannot take responsibility for errors or fitness for use.
*
* $Id: scull.h,v 1.15 2004/11/04 17:51:18 rubini Exp $
*/

#ifndef _SCULL_H_
#define _SCULL_H_

#include <linux/ioctl.h> /* needed for the _IOW etc stuff used later */

/*
* Macros to help debugging
*/

#undef PDEBUG             /* undef it, just in case */
#ifdef SCULL_DEBUG
#  ifdef __KERNEL__
     /* This one if debugging is on, and kernel space */
#    define PDEBUG(fmt, args...) printk( KERN_DEBUG "scull: " fmt, ## args)
#  else
     /* This one for user space */
#    define PDEBUG(fmt, args...) fprintf(stderr, fmt, ## args)
#  endif
#else
#  define PDEBUG(fmt, args...) /* not debugging: nothing */
#endif

#undef PDEBUGG
#define PDEBUGG(fmt, args...) /* nothing: it's a placeholder */

#ifndef SCULL_MAJOR
#define SCULL_MAJOR 0   /* dynamic major by default */
#endif

#ifndef SCULL_NR_DEVS
#define SCULL_NR_DEVS 4    /* scull0 through scull3 */
#endif

#ifndef SCULL_P_NR_DEVS
#define SCULL_P_NR_DEVS 4  /* scullpipe0 through scullpipe3 */
#endif

/*
* The bare device is a variable-length region of memory.
* Use a linked list of indirect blocks.
*
* "scull_dev->data" points to an array of pointers, each
* pointer refers to a memory area of SCULL_QUANTUM bytes.
*
* The array (quantum-set) is SCULL_QSET long.
*/
#ifndef SCULL_QUANTUM
#define SCULL_QUANTUM 4000        /* 每个指针(量子)指向一个4000字节的区域 */
#endif

#ifndef SCULL_QSET
#define SCULL_QSET    1000        /* 一个有1000个(量子)的指针数组 */
#endif

/*
* The pipe device is a simple circular buffer. Here its default size
*/
#ifndef SCULL_P_BUFFER
#define SCULL_P_BUFFER 4000
#endif

/*
* Representation of scull quantum sets.
* 一个链表项
*/
struct scull_qset {
    void **data;
    struct scull_qset *next;    /* 下一个链表节点（链表项） */
};

/* 我们自己的设备(包含了基本的cdev字符设备结构) */
struct scull_dev {
    struct scull_qset *data;  /* Pointer to first quantum set (链表的头)*/
    int quantum;              /* the current quantum size */
    int qset;                 /* the current array size */
    unsigned long size;       /* amount of data stored here (保存在其中的数据总量)*/
    unsigned int access_key;  /* used by sculluid and scullpriv */
    struct semaphore sem;     /* mutual exclusion semaphore     */
    struct cdev cdev;      /* Char device structure        */
};

/*
* Split minors in two parts
*/
#define TYPE(minor)    (((minor) >> 4) & 0xf)    /* high nibble */
#define NUM(minor)    ((minor) & 0xf)        /* low  nibble */

/*
* The different configurable parameters
*/
extern int scull_major;     /* main.c */
extern int scull_nr_devs;
extern int scull_quantum;
extern int scull_qset;

extern int scull_p_buffer;    /* pipe.c */

/*
* Prototypes for shared functions
*/

int     scull_p_init(dev_t dev);
void    scull_p_cleanup(void);
int     scull_access_init(dev_t dev);
void    scull_access_cleanup(void);

int     scull_trim(struct scull_dev *dev);

ssize_t scull_read(struct file *filp, char __user *buf, size_t count,
                   loff_t *f_pos);
ssize_t scull_write(struct file *filp, const char __user *buf, size_t count,
                    loff_t *f_pos);
loff_t  scull_llseek(struct file *filp, loff_t off, int whence);
int     scull_ioctl(struct inode *inode, struct file *filp,
                    unsigned int cmd, unsigned long arg);

/*
* Ioctl definitions
*/

/* Use 'k' as magic number */
#define SCULL_IOC_MAGIC  'k'
/* Please use a different 8-bit number in your code */

#define SCULL_IOCRESET    _IO(SCULL_IOC_MAGIC, 0)

/*
* S means "Set" through a ptr,
* T means "Tell" directly with the argument value
* G means "Get": reply by setting through a pointer
* Q means "Query": response is on the return value
* X means "eXchange": switch G and S atomically
* H means "sHift": switch T and Q atomically
*/
#define SCULL_IOCSQUANTUM _IOW(SCULL_IOC_MAGIC,  1, int)
#define SCULL_IOCSQSET    _IOW(SCULL_IOC_MAGIC,  2, int)
#define SCULL_IOCTQUANTUM _IO(SCULL_IOC_MAGIC,   3)
#define SCULL_IOCTQSET    _IO(SCULL_IOC_MAGIC,   4)
#define SCULL_IOCGQUANTUM _IOR(SCULL_IOC_MAGIC,  5, int)
#define SCULL_IOCGQSET    _IOR(SCULL_IOC_MAGIC,  6, int)
#define SCULL_IOCQQUANTUM _IO(SCULL_IOC_MAGIC,   7)
#define SCULL_IOCQQSET    _IO(SCULL_IOC_MAGIC,   8)
#define SCULL_IOCXQUANTUM _IOWR(SCULL_IOC_MAGIC, 9, int)
#define SCULL_IOCXQSET    _IOWR(SCULL_IOC_MAGIC,10, int)
#define SCULL_IOCHQUANTUM _IO(SCULL_IOC_MAGIC,  11)
#define SCULL_IOCHQSET    _IO(SCULL_IOC_MAGIC,  12)

/*
* The other entities only have "Tell" and "Query", because they're
* not printed in the book, and there's no need to have all six.
* (The previous stuff was only there to show different ways to do it.
*/
#define SCULL_P_IOCTSIZE _IO(SCULL_IOC_MAGIC,   13)
#define SCULL_P_IOCQSIZE _IO(SCULL_IOC_MAGIC,   14)
/* ... more to come */

#define SCULL_IOC_MAXNR 14

#endif /* _SCULL_H_ */