上面红框框起来的部分就是设备号，前面是major，后面是minor。 major号表示设备所使用的驱动，而minor号则表示具体的设备。在上图中，tty的驱动都是driver 4，而利用minor号区别不同的tty设备。 另外，通过/proc/devices文件也可以看到设备所使用的驱动，即major号：
linux-a21w:/dev # cat /proc/devices
Character devices:
1 mem
4 /dev/vc/0
4 tty
4 ttyS
5 /dev/tty
5 /dev/console
5 /dev/ptmx
7 vcs
......
关于dev_t，major和minor号定义如下（kernel版本是4.0）:
/* <linux/types.h>: */
typedef __u32 __kernel_dev_t;
typedef __kernel_dev_t dev_t;
/* <linux/kdev_t.h> */
#define MINORBITS 20
#define MINORMASK ((1U << MINORBITS) - 1)
#define MAJOR(dev) ((unsigned int) ((dev) >> MINORBITS))
#define MINOR(dev) ((unsigned int) ((dev) & MINORMASK))
#define MKDEV(ma,mi) (((ma) << MINORBITS) | (mi))
dev_t占32 bit长，其中高12位是major，低20位是minor。
获取设备号的两种方法：
（1）预先指定设备号：
int register_chrdev_region(dev_t from, unsigned count, const char *name)
from包含major和minor，通常情况下minor指定为0。count指定连续设备号的数量，name指定设备的名字。register_chrdev_region实现如下：
/**
* 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)
{
struct char_device_struct *cd;
dev_t to = from + count;
dev_t n, next;
for (n = from; n < to; n = next) {
next = MKDEV(MAJOR(n)+1, 0);
if (next > to)
next = to;
cd = __register_chrdev_region(MAJOR(n), MINOR(n),
next - n, name);
if (IS_ERR(cd))
goto fail;
}
return 0;
fail:
to = n;
for (n = from; n < to; n = next) {
next = MKDEV(MAJOR(n)+1, 0);
kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
}
return PTR_ERR(cd);
}
可以看到register_chrdev_region即是把from开始连续count个设备号（dev_t类型，包含major和minor）都注册。
举个例子（/drivers/tty/tty_io.c）：
register_chrdev_region(MKDEV(TTYAUX_MAJOR, 1), 1, "/dev/console")
（2）动态分配设备号（推荐使用）：
int alloc_chrdev_region(dev_t *dev, unsigned int firstminor, unsigned int count, char *name);
dev是传出参数，为动态获得的设备号；firstminor指定第一个minor；count和name同register_chrdev_region的参数定义。alloc_chrdev_region实现如下：
/**
* 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)
{
struct char_device_struct *cd;
cd = __register_chrdev_region(0, baseminor, count, name);
if (IS_ERR(cd))
return PTR_ERR(cd);
*dev = MKDEV(cd->major, cd->baseminor);
return 0;
}
举个例子（/drivers/watchdog/watchdog_dev.c）：
alloc_chrdev_region(&watchdog_devt, 0, MAX_DOGS, "watchdog");
释放设备号：
void unregister_chrdev_region(dev_t first, unsigned int count);
