字符设备----设备号

1. 主次设备号：
    Linux Kernel用主设备号用来标示驱动程序，使用次设备号来标示设备文件所表示的具体设备。

2. 内部表示：
    dev_t 类型用来表示设备号。
    
    typedef __kernel_dev_t        dev_t;        //
    typedef __u32 __kernel_dev_t;            //实际上就是一个平台无关的32位整形

3. 操作：
    3.1 MAJOR(dev_t dev)
        返回主设备号
    3.2 MINOR(dev_t dev)
        返回次设备号
    3.3 MKDVE(int major, int minor)
        通过两个表示主次设备号的int构造dev_t类型的变量
    3.4 分配和释放设备号
        3.4.1 固定分配
            /** fs/char_dev.c
             * register_chrdev_region() - register a range of device numbers
             * @from: the first in the desired range of device numbers; must include
             *        the major number.
             * @count: the number of consecutive device numbers required
             * @name: the name of the device or driver.
             *
             * Return value is zero on success, a negative error code on failure.
             */
            int register_chrdev_region(dev_t from, unsigned count, const char *name)
            由于是自己只定设备号，有可能会引发设备号冲突问题。

        3.4.2 由OS自动分配    
            /**
             * alloc_chrdev_region() - register a range of char device numbers
             * @dev: output parameter for first assigned number
             * @baseminor: first of the requested range of minor numbers
             * @count: the number of minor numbers required
             * @name: the name of the associated device or driver
             *
             * Allocates a range of char device numbers.  The major number will be
             * chosen dynamically, and returned (along with the first minor number)
             * in @dev.  Returns zero or a negative error code.
             */
            int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
                        const char *name)
            建议使用的分配设备号的方法，因为是有OS自动分配主设备号。
        3.4.2 释放设备号    
            /**
             * unregister_chrdev_region() - return a range of device numbers
             * @from: the first in the range of numbers to unregister
             * @count: the number of device numbers to unregister
             *
             * This function will unregister a range of @count device numbers,
             * starting with @from.  The caller should normally be the one who
             * allocated those numbers in the first place...
             */
            void unregister_chrdev_region(dev_t from, unsigned count)
            释放设备号。
4. 实验：准备设备号&创建字符设备设备节点
    4.1 申请&释放设备号
scull.c:
/*
 * =====================================================================================
 *
 *       Filename:  cdev_num
 *
 *    Description:  This file is used for testing char device number allocation
 *
 *        Version:  1.0
 *        Created:  03/13/2011 09:18:06 PM
 *       Revision:  none
 *       Compiler:  gcc-4.4.5
 *
 *         Author:  ville lee (vl), [e-mail]
 *        Company:  BUAA
 *
 * =====================================================================================
 */
#include
#include
#include         /* printk */
#include         /* MAJOR, MINOR */
#include         /* dev_t */
#include             /* alloc_chrdev_region, unregister_chrdev_region */

/* add the header file above if needed*/

int major;
int minor;
int count = 4;
dev_t dev_num;

/*
 * ===  FUNCTION  ======================================================================
 *         Name:  module_init
 *  Description: 
 * =====================================================================================
 */
static int __init cdev_num_init(void)
{
    /* allocate the char device number */
    int result = alloc_chrdev_region(&dev_num, 0, 4, "scull");

    if (result < 0) {
        printk(KERN_WARNING "char device number allocation fails\n");
        return -1;
    }
    major = MAJOR(dev_num);
    minor = MINOR(dev_num);
    printk(KERN_ALERT "The name of the char device is %s\nThe major number is %d\nThe minor number is %d\n"
            , "scull", major, minor);
    printk(KERN_ALERT "===========================================================\n");
    return 0;
}        /* -----  end of static function module_init  ----- */

/*
 * ===  FUNCTION  ======================================================================
 *         Name:  module_exit
 *  Description: 
 * =====================================================================================
 */
static void __exit cdev_num_exit(void)
{
    unregister_chrdev_region(dev_num, 4);
    printk(KERN_ALERT "===========================================================\n");
}        /* -----  end of static function module_exit  ----- */




MODULE_LICENSE("GPL");
MODULE_AUTHOR("ville lee");
MODULE_DESCRIPTION("add your description");
MODULE_VERSION("version 1.0");

module_init(cdev_num_init);
module_exit(cdev_num_exit);
===========================================
这时候，查看/proc/devices会发现多出了scull0-scull3
也就是说明了，只要申请设备号，那么就会在/proc/devices文件中添加设备号，
无论是否创建设备节点，也就表明proc文件系统和dev文件系统并不存在完全的依赖关系。
    4.2 加载脚本和卸载脚本
scull_load.sh
#!/bin/bash
# Description:
#            This file is used for load the module and create device nodes
module="scull"        # module or device name
device="scull"
mode="664"            # node access mode for user space

insmod ./$module.ko $* || exit 1

rm -f /dev/${device}[0-3]

major=$(awk "\$2==\"$module\" {print \$1}" /proc/devices)

# create the device node
mknod /dev/${device}0 c $major 0
mknod /dev/${device}1 c $major 1
mknod /dev/${device}2 c $major 2
mknod /dev/${device}3 c $major 3

# allocate proper access-mode, and change the group attributes
# in my fedora 13, there is wheel group but no staff group
group="wheel"

chgrp $group /dev/${device}[0-3]
chmod $mode /dev/${device}[0-3]
=================================
scull_unload.sh
#!/bin/bash
module="scull"
device="scull"

rmmod $module $* || exit 1

rm -f /dev/${device}[0-3]

echo "module $scull and the refered device node has been removed!"