在Linux驱动程序中，可以使用等待队列（wait queue）来实现阻塞进程的唤醒。等待队列可以用来同步对系统资源的访问。

1.定义和初始化队列头
wait_queue_head_t wqh;
init_waitqueue_head(wait_queue_head_t *wqh);
2.定义和初始化等待队列
DECLARE_WAITQUEUE(name, tsk);
3.添加、移除等待队列
add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);
remove_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);将等待队列wait添加到等待队列头q执行的等待队列链表中，或者从中删除。
4.等待事件
wait_event(queue, condition);当condition为真时，立即返回；否则进程进入TASK_UNINTERRUPTIBLE类型的睡眠状态，并挂在queue指定的等待队列头上。
add_wait_queue(queue, conditon);当condition为真时，立即返回；否则进程进入TASK_INTERRUPTIBLE类型的睡眠状态，并挂在queue指定的等待队列头上。
5.唤醒队列
wake_up(wait_queue_head_t *queue);
wake_up_interruptible(wait_queue_head_t *queue);唤醒由queue指向的等待队列头链表中所有等待队列对应的进程。
6.在等待队列中睡眠
sleep_on(wait_queue_head_t *q);让进程进入不可中断的睡眠，并将它放入等待队列
interruptible_sleep_on(wait_queue_head_t *q);让进程进入不可中断的睡眠，并将它进入等待队列。

内核等待队列一般使用方法：
a.定义和初始化等待队列，将进程状态改变，并将等待队列添加到等待队列数据链中
b.改变进程状态的方法：调用set_current_state(state_value)函数、调用set_task_state(task, state_value)函数、直接采用current->state = TASK_INTERRUPTIBLE,类似于赋值语句。
c.通过schedule()调用放弃cpu，调度其他进程执行
d.进程被其它地方唤醒，将等待队列移出等待队列头指向的数据链

/**
*Copyright (c) 2013.TianYuan
*All rights reserved.
*
*文件名称: char_device_driver13.c
*文件标识: 等待队列
*
*当前版本：1.0
*作者：wuyq 
*
*取代版本：xxx
*原作者：xxx
*完成日期：2013-11-29
*/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/device.h>
#include <linux/slab.h>
#include <asm/uaccess.h>

#include <asm/gpio.h>
#include <plat/gpio-cfg.h>
#include <linux/spinlock_types.h>
#include <linux/sched.h>


MODULE_LICENSE("GPL");
#define CDD_MAJOR	200//cat /proc/devices找一个尚未使用的
#define CDD_MINOR	0
#define CDD_COUNT	2
dev_t dev = 0;
u32 cdd_major = 0;
u32 cdd_minor = 0;

struct class *dev_class = NULL;
#define BUF_SIZE	100
struct cdd_cdev{
	struct cdev cdev;
	struct device *dev_device;
	u8 led;
	
	char kbuf[BUF_SIZE];
	
	u32 data_len;//记录缓冲区中已经写入数据的长度
	//定义等待队列头
	wait_queue_head_t wqh;
};

struct cdd_cdev *cdd_cdevp = NULL;

unsigned long led_gpio_table[2] = {
	S5PV210_GPC1(3),//数字
	S5PV210_GPC1(4),
};

int cdd_open(struct inode* inode, struct file *filp)
{
	struct cdd_cdev *pcdevp = NULL;
	printk("enter cdd_open!\n");

	pcdevp = container_of(inode->i_cdev, struct cdd_cdev, cdev);
	printk("led = %d\n", pcdevp->led);
	
	/*获取信号量*/
	//down获取信号量不成功，会导致进程睡眠（第3个进程的时候）
	//down(&pcdevp->sem_open);
	if(down_interruptible(&pcdevp->sem_open)<0){
		return -1;
	}
	filp->private_data = pcdevp;
	//申请gpio管脚
	gpio_request(led_gpio_table[0], "GPC1_3");
	gpio_request(led_gpio_table[1], "GPC1_4");
	
	return 0;
}

int cdd_read(struct file *filp, char __user *buf, size_t count, loff_t *offset)
{
	int ret = 0;
	u32 pos = *offset;
	u32 cnt = count;
	
	struct cdd_cdev *cdevp = filp->private_data;
#if 0
	//定义并初始化一个等待队列
	DECLARE_WAITQUEUE(wq, current);
	//将等待队列添加到wqh指向的链表
	add_wait_queue(&pcdevp->wqh, &wq);
	//判断设备有没有数据供用户空间读,假设led不为0，表示有数据供用户空间读取
	if(pcdevp->led == 0){
		printk("no data for reading! sleep...\n");
		//设置当前线程为睡眠状态
		set_current_state(TASK_INTERRUPTIBLE);
		schedule();//内核调度cpu的算法
		printk("have data for reading!\n");
	}
	//从指定的链表中删除等待队列
	remove_wait_queue(&pcdevp->wqh, &wq);
#endif
	wait_event_interruptible(&pcdevp->wqh, pcdevp->led != 0); 
	
	//printk("enter cdd_read!\n");
	if(cnt > (cdevp->data_len-pos) ){
		cnt = cdevp->data_len - pos;
	}
	
	ret = copy_to_user(buf, cdevp->kbuf+pos, cnt);
	//printk("kernel kbuf content:%s\n", cdevp->kbuf);
	*offset += cnt;
	
	pcdevp->led = 0;
	
	return ret;
}

int cdd_write(struct file *filp, const char __user *buf, size_t count, loff_t *offset)
{
	int ret = 0;
	struct cdd_cdev *cdevp = filp->private_data;
	u32 pos = *offset;
	u32 cnt = count;
	
	//printk("enter cdd_write!\n");
	if(cnt > (BUF_SIZE - pos) ){
		cnt = BUF_SIZE - pos;
	}
	ret = copy_from_user(cdevp->kbuf+pos, buf, cnt);
	*offset += cnt;
	if(*offset > cdevp->data_len){
		cdevp->data_len = *offset;
	}
	pcdevp->led = 1;
	//唤醒等待队列头中的一个等待队列
	wake_up_interruptible(&pcdevp->wqh);
	
	
	return ret;
}

int cdd_ioctl(struct inode *inode, struct file *filp, unsigned int cmd, unsigned long data)
{
	//printk("enter cdd_ioctl!\n");
	switch(cmd){
		case 1://点亮灯
			//设置管脚为输出功能
			//参数：1.要设置的管脚编号2.默认的输出值 0低电平1高电平
			gpio_direction_output(led_gpio_table[data], 0);
			//禁止内部上拉
			s3c_gpio_setpull(led_gpio_table[data], SEC_GPIO_PULL_NONE);
			//设置输出值
			gpio_set_value(led_gpio_table[data], 1);
			
			break;
		case 0://熄灭灯
			
			//设置管脚为输出功能
			//参数：1.要设置的管脚编号2.默认的输出值 0低电平1高电平
			gpio_direction_output(led_gpio_table[data], 0);
			//禁止内部上拉
			s3c_gpio_setpull(led_gpio_table[data], SEC_GPIO_PULL_NONE);
			//设置输出值
			gpio_set_value(led_gpio_table[data], 0);
			
			break;
		default:
			return -EINVAL;
	}
	
	
	return 0;
}

int cdd_release(struct inode *inode, struct file *filp)
{
	struct cdd_cdev *pcdevp = filp->private_data;
	printk("enter cdd_release!\n");
	
	gpio_free(led_gpio_table[0]);
	gpio_free(led_gpio_table[1]);
	up(&pcdevp->sem_open);
	return 0;
}

loff_t cdd_llseek(struct file *filp, loff_t offset, int whence)
{
	struct cdd_cdev *pcdevp = filp->private_data;
	loff_t newpos = 0;
	switch(whence){
		case SEEK_SET:
			newpos = offset;
			break;
		case SEEK_CUR:
			newpos = filp->f_pos + offset;
			break;
		case SEEK_END:
			newpos = pcdevp->data_len + offset;
			break;
		default:
			return -EINVAL;//无效的参数
	}
	
	if( newpos<0 || newpos>= BUF_SIZE ){
		return -EINVAL;
	}
	filp->f_pos = newpos;
	return newpos;
}

struct file_operations cdd_fops = {
	.owner = THIS_MODULE,
	.open = cdd_open,
	.read = cdd_read,
	.write = cdd_write,
	.ioctl = cdd_ioctl,
	.release = cdd_release,
	.llseek = cdd_llseek,
	};

int __init cdd_init(void)
{
	int ret = 0;
	int i = 0;
	
	if(cdd_major){
		dev = MKDEV(CDD_MAJOR, CDD_MINOR);//生成设备号
		//注册设备号;1、要注册的起始设备号2、连续注册的设备号个数3、名字
		ret = register_chrdev_region(dev, CDD_COUNT, "cdd_demo");
	}else{
		// 动态分配设备号
		ret = alloc_chrdev_region(&dev, cdd_minor, CDD_COUNT, "cdd_demo02");
	}
	
	if(ret < 0){
		printk("register_chrdev_region failed!\n");
		goto failure_register_chrdev;
	}
	//获取主设备号
	cdd_major = MAJOR(dev);
	printk("cdd_major = %d\n", cdd_major);
	
	cdd_cdevp = kzalloc(sizeof(struct cdd_cdev)*CDD_COUNT, GFP_KERNEL);
	if(IS_ERR(cdd_cdevp)){
		printk("kzalloc failed!\n");
		goto failure_kzalloc;
	}
	/*创建设备类*/
	dev_class = class_create(THIS_MODULE, "cdd_class");
	if(IS_ERR(dev_class)){
		printk("class_create failed!\n");
		goto failure_dev_class;
	}
	for(i=0; i<CDD_COUNT; i++){
		/*初始化cdev*/
		cdev_init(&(cdd_cdevp[i].cdev), &cdd_fops);
		/*添加cdev到内核*/
		cdev_add(&(cdd_cdevp[i].cdev), dev+i, 1);
		
		/* “/dev/xxx” */
		device_create(dev_class, NULL, dev+i, NULL, "cdd%d", i);
		
		cdd_cdevp[i].led = i;
		//初始化等待队列头
		init_waitqueue_head(&cdd_cdevp[i].wqh);
	}
	
	return 0;
failure_dev_class:
	kfree(cdd_cdevp);
failure_kzalloc:
	unregister_chrdev_region(dev, CDD_COUNT);
failure_register_chrdev:
	return ret;
}

void __exit cdd_exit(void)
{
/*逆序消除*/
	int i = 0;
	for(; i < CDD_COUNT; i++){
		device_destroy(dev_class, dev+i);
		cdev_del(&(cdd_cdevp[i].cdev));
		//cdev_del(&((cdd_cdevp+i)->cdev));
	}
	class_destroy(dev_class);
	kfree(cdd_cdevp);
	unregister_chrdev_region(dev, CDD_COUNT);
	
}	

module_init(cdd_init);
module_exit(cdd_exit);

/**
*Copyright (c) 2013.TianYuan
*All rights reserved.
*
*文件名称: char_device_driver13_test0.c
*文件标识: 此程序运行r，此时发生阻塞，进入等待
*
*当前版本：1.0
*作者：wuyq 
*
*取代版本：xxx
*原作者：xxx
*完成日期：2013-11-29
*/
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>

/*手工创建设备节点文件
mknod /dev/cdd c 248 0
*/
int fd = 0;
char rbuf[100];
char wbuf[100] = "nihao!\n";


int main()
{
	char ch;
	
	fd = open("/dev/cdd0", O_RDWR);
	if(fd < 0){
		printf("open failed!\n");
		return -1;
	}
	printf("open successed fd = %d\n", fd);
	while(1)
	{
		printf("starting to test /dev/cdd...\n");
		ch = getchar();
		getchar();//取走回车
		if(ch == ‘q‘){
			break;
		}
		switch(ch){
			case ‘r‘:
				memset(rbuf, 0, 100);//清空
				read(fd, rbuf, 3);
				printf("user space from kernel: %s\n", rbuf);
				break;
			case ‘w‘:
				write(fd, wbuf, strlen(wbuf) );
				break;
			case ‘o‘:
				ioctl(fd, 0, 0);
				break;
			case ‘O‘:
				ioctl(fd, 1, 0);
				break;
			case ‘p‘:
				ioctl(fd, 0, 1);
				break;
			case ‘P‘:
				ioctl(fd, 1, 1);
				break;
			case ‘l‘:
				lseek(fd, 0, SEEK_SET);//移动的文件的开头
				break;
				
			default:
				break;
		}
		sleep(1);
	}
	
	close(fd);
	return 0;
}

/**
*Copyright (c) 2013.TianYuan
*All rights reserved.
*
*文件名称: char_device_driver13_test1.c
*文件标识: test0 和 test1两个测试：此程序执行w，观察阻塞的r进程被唤醒，完成操作。
*
*当前版本：1.0
*作者：wuyq 
*
*取代版本：xxx
*原作者：xxx
*完成日期：2013-11-29
*/
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>

/*手工创建设备节点文件
mknod /dev/cdd c 248 0
*/
int fd = 0;
char rbuf[100];
char wbuf[100] = "nihao!\n";


int main()
{
	char ch;
	
	fd = open("/dev/cdd1", O_RDWR);
	if(fd < 0){
		printf("open failed!\n");
		return -1;
	}
	printf("open successed fd = %d\n", fd);
	while(1)
	{
		printf("starting to test /dev/cdd...\n");
		ch = getchar();
		getchar();//取走回车
		if(ch == ‘q‘){
			break;
		}
		switch(ch){
			case ‘r‘:
				memset(rbuf, 0, 100);//清空
				read(fd, rbuf, 3);
				printf("user space from kernel: %s\n", rbuf);
				break;
			case ‘w‘:
				write(fd, wbuf, strlen(wbuf) );
				break;
			case ‘o‘:
				ioctl(fd, 0, 0);
				break;
			case ‘O‘:
				ioctl(fd, 1, 0);
				break;
			case ‘p‘:
				ioctl(fd, 0, 1);
				break;
			case ‘P‘:
				ioctl(fd, 1, 1);
				break;
			case ‘l‘:
				lseek(fd, 0, SEEK_SET);//移动的文件的开头
				break;
				
			default:
				break;
		}
		sleep(1);
	}
	
	close(fd);
	return 0;
}

linux驱动开发--字符设备：内核等待队列