1.申请设备号：

之前用的是register_chrdev(LED_MAJOR, LED_NAME, &led_fops);手动申请很不方便

使用alloc_chrdev_region函数申请设备号，手动申请的话要先查询是否有空余的设备号，很不方便，用此函数内核会自动将将空余设备号给你，释放设备号用unregister_chrdev_region

如果指定主设备号则

用register_chrdev_region函数，需要用MKDEV构建完整的dev_t，卸载也用unregister_chrdev_region

2.代码：

驱动代码：

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/io.h>
#include <linux/cdev.h>

#define NEWCHRLED_NAME "newchrled"
#define NEWCHRLED_COUNT 1

// 实际地址
#define PMU_GRF_BASE						      (0xFDC20000)
#define PMU_GRF_GPIO0C_IOMUX_L				(PMU_GRF_BASE + 0x0010)
#define PMU_GRF_GPIO0C_DS_0					  (PMU_GRF_BASE + 0X0090)
#define GPIO0_BASE						        (0xFDD60000)
#define GPIO0_SWPORT_DR_H				      (GPIO0_BASE + 0X0004)
#define GPIO0_SWPORT_DDR_H				    (GPIO0_BASE + 0X000C)

#define LEDOPEN 1
#define LEDCLOSE 0

/* 映射后的寄存器虚拟地址指针 */
static void __iomem *PMU_GRF_GPIO0C_IOMUX_L_PI;
static void __iomem *PMU_GRF_GPIO0C_DS_0_PI;
static void __iomem *GPIO0_SWPORT_DR_H_PI;
static void __iomem *GPIO0_SWPORT_DDR_H_PI;


// led gpio初始化操作
void gpio_init(void){
  u32 val = 0;
  // 设置GPIO0_c0为GPIO功能
  val = readl(PMU_GRF_GPIO0C_IOMUX_L_PI);
  val &= ~(0x7 << 0); //最低三位置0
  val |= ((0x7 << 16) | (0x0 << 0)); // 16 17 18位置1其他不变，bit2:0：0，用作GPIO0_C0
  writel(val, PMU_GRF_GPIO0C_IOMUX_L_PI);

  // 设置GPIO_C0驱动能力为level5
  val = readl(PMU_GRF_GPIO0C_DS_0_PI);
  val &= ~(0x3f << 0);  // 0 ~ 5置0
  val |= ((0x3f << 16) | (0x3f << 0)); // 16 ~ 21置1，0~5置1同时用作GPIO0c0
  writel(val, PMU_GRF_GPIO0C_DS_0_PI);

  // 设置GPIOO0_c0为输出
  val = readl(GPIO0_SWPORT_DDR_H_PI);
  val &= ~(0x1 << 0); // 0置0
  val |= ((0x1 << 16) | (0x1 << 0)); // 16置1,0置1
  writel(val, GPIO0_SWPORT_DDR_H_PI);

  // 设置GPIO_c0为低电平，关闭LED
  val = readl(GPIO0_SWPORT_DR_H_PI);
  val &= ~(0x1 << 0);
  val |= ((0x1 << 16) | (0x0 << 0));
  writel(val, GPIO0_SWPORT_DR_H_PI);
}

// 开关
void led_switch(int status){
  u32 val = 0;

  if(status == LEDOPEN){
  // 开灯
  val = readl(GPIO0_SWPORT_DR_H_PI);
  val &= ~(0X1 << 0); /* bit0 清零*/
  val |= ((0X1 << 16) | (0X1 << 0));	/* bit16 置1，允许写bit0，
                         bit0，高电平*/ 
  writel(val, GPIO0_SWPORT_DR_H_PI);  

  }else if(status == LEDCLOSE){

  // 关灯
  val = readl(GPIO0_SWPORT_DR_H_PI);
  val &= ~(0X1 << 0); /* bit0 清零*/
  val |= ((0X1 << 16) | (0X0 << 0));	/* bit16 置1，允许写bit0，
                         bit0，低电平	*/
  writel(val, GPIO0_SWPORT_DR_H_PI); 

  }

}

// 真实物理地址映射虚拟内存函数
void led_remap(void){

  PMU_GRF_GPIO0C_IOMUX_L_PI = ioremap(PMU_GRF_GPIO0C_IOMUX_L, 4);
	PMU_GRF_GPIO0C_DS_0_PI = ioremap(PMU_GRF_GPIO0C_DS_0, 4);
	GPIO0_SWPORT_DR_H_PI = ioremap(GPIO0_SWPORT_DR_H, 4);
	GPIO0_SWPORT_DDR_H_PI = ioremap(GPIO0_SWPORT_DDR_H, 4);

}

// 释放
void led_releaseMap(void){
  // 取消地址映射
  iounmap(PMU_GRF_GPIO0C_IOMUX_L_PI);
  iounmap(PMU_GRF_GPIO0C_DS_0_PI);
  iounmap(GPIO0_SWPORT_DR_H_PI);
  iounmap(GPIO0_SWPORT_DDR_H_PI);
}


/*LED 设备结构体*/
struct newchrled_dev{
  struct cdev cdev; // 字符设备
  dev_t devid; // 设备号
  int major;  // 主设备号
  int minor; // 次设备号
};

struct newchrled_dev newchrled; // led设备


static int newchrled_open(struct inode* inode, struct file* filp){
  return 0;
}
static int newchrled_release(struct inode* inode, struct file* filp){
  return 0;
}
static ssize_t newchrled_write(struct file* filp, const char __user* buf, size_t count, loff_t* ppos){
  int ret;
  unsigned char databuf[1];
  unsigned char state;

  ret = copy_from_user(databuf, buf, count);
  if(ret < 0){
    printk("kernel write failed\r\n");
    return -EFAULT;
  }

  state = databuf[0];

  if(state == LEDOPEN){
    led_switch(LEDOPEN);
  }else if(state == LEDCLOSE){
    led_switch(LEDCLOSE);
  }

  return 0;
}


static const struct file_operations newchrled_fops = {
  .owner = THIS_MODULE,
  .write = newchrled_write,
  .open = newchrled_open,
  .release = newchrled_release,
};

/*入口*/
static int __init newchrled_init(void){
  int ret =  0;
  printk("new chrled init\r\n");
  /*初始化虚拟内存，初始化gpio*/
  led_remap();
  gpio_init();
  // led初始化
  if(newchrled.major){ // 给定主设备号
    newchrled.devid = MKDEV(newchrled.major, 0); // 次设备号从零开始
    ret = register_chrdev_region(newchrled.devid, NEWCHRLED_COUNT, NEWCHRLED_NAME);  // 注册一个

  }else{ // 自己申请

    ret = alloc_chrdev_region(&newchrled.devid, 0, NEWCHRLED_COUNT, NEWCHRLED_NAME); //由0开始申请1个
    newchrled.major = MAJOR(newchrled.devid);
    newchrled.minor = MINOR(newchrled.devid);
  }
  if(ret < 0){
    printk("newchrled chrdev_region err\r\n");
    return -1;
  }
  printk("newchrled major = %d, minor= %d\r\n", newchrled.major, newchrled.minor);
  newchrled.cdev.owner = THIS_MODULE;
  cdev_init(&newchrled.cdev, &newchrled_fops);  // 初始化完
  ret = cdev_add(&newchrled.cdev, newchrled.devid, NEWCHRLED_COUNT);
  return 0;
}

/*出口*/
static void __exit newchrled_exit(void){
  printk("new chrled exit\r\n");
  led_releaseMap();
  // 删除字符设备
  cdev_del(&newchrled.cdev);
  // 注销设备号
  unregister_chrdev_region(newchrled.devid, NEWCHRLED_COUNT);
} 

/* 注册驱动和卸载驱动*/

module_init(newchrled_init);
module_exit(newchrled_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("narnat");

核心主要在这个部分：

  if(newchrled.major){ // 给定主设备号
    newchrled.devid = MKDEV(newchrled.major, 0); // 次设备号从零开始
    ret = register_chrdev_region(newchrled.devid, NEWCHRLED_COUNT, NEWCHRLED_NAME);  // 注册一个

  }else{ // 自己申请

    ret = alloc_chrdev_region(&newchrled.devid, 0, NEWCHRLED_COUNT, NEWCHRLED_NAME); //由0开始申请1个
    newchrled.major = MAJOR(newchrled.devid);
    newchrled.minor = MINOR(newchrled.devid);
  }
  if(ret < 0){
    printk("newchrled chrdev_region err\r\n");
    return -1;
  }

如果给定了主设备号则根据设备号注册一个MKDEV是获取设备号，注册设备用设备号，否则向内核申请

alloc_chrdev_region(&newchrled.devid, 0, NEWCHRLED_COUNT, NEWCHRLED_NAME)是申请设备号0

假设alloc_chrdev_region 分配的主设备号是 250，firstminor = 0，NEWCHRLED_COUNT = 3，那么分配的设备号范围是：
主设备号：250
次设备号：0、1、2

cdev_add(&newchrled.cdev, newchrled.devid, NEWCHRLED_COUNT);才是注册一个字符设备