返回描述设备是否可读或可写的掩码

poll_table数据类型定义
在<linux/poll.h>中

typedef struct poll_table_struct{
	poll_queue_proc qproc;
}poll_table;
typedef void(*poll_queue_proc)(struct file *, wait_queue_head_t *, struct poll_table_struct *);

void poll_wait(struct file *filp, wait_queue_head_t *wait_address, poll_table *p);
filp:执行操作的设备文件指针
wait_address:设备驱动的等待队列头
p:内核传递的轮询表指针

poll操作返回值
通常返回下列定义“或”的结果
POLLIN 设备可无阻塞读
POLLOUT 设别可无阻塞写
POLLRDNORM 数据可读
POLLWRNORM 数据可写
设备可读通常返回： POLLIN | POLLRDNORM
设备可写通常返回： POLLOUT | POLLWRNORM

poll操作一般结构

xxx_poll()
{
unsigned int mask = 0;//定义返回值
...
//调用poll_wait()把进程添加到轮询表
...
if(device is ready for read){
mask = POLLIN | POLLRDNORM;
}
...
return mask;
}

/**
*Copyright (c) 2013.TianYuan
*All rights reserved.
*
*文件名称: cdd_poll.c
*文件标识: 按键1的中断
*
*当前版本：1.0
*作者：wuyq 
*
*取代版本：xxx
*原作者：xxx
*完成日期：2013-12-05
*/
#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>
#include <linux/poll.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	10
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_quue_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;
}

unsigned int cdd_poll(struct file *filp, struct poll_table_struct *wait)
{
	unsigned int mask = 0;
	struct cdd_cdev pcdevp = filp->private_data;
	printk("enter cdd_poll!\n");
	poll_wait(filp, &pcdep->wqh, wait);
	//led不为0即可读
	if(pcdevp->led){
		mask = POLLIN | POLLRDNORM;
	}
	return mask;
}

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,
	.poll = cdd_poll,
};

irqreturn_t cdd_isr(int irq, void *dev_id)
{
	printk("occur up key press or release!\n");
	return IRQ_HANDLED;
}

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);
	}
	
	//注册中断
	request_irq(IRQ_EINT0, cdd_isr, IRQF_TRIGGER_FALLING  | IRQF_TRIGGER_RISING, "interrupt_demo", NULL);
	
	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: test_select.c
*文件标识: select
*
*当前版本：1.0
*作者：wuyq 
*
*取代版本：xxx
*原作者：xxx
*完成日期：2013-12-05
*/
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

int fd0 = 0;
int fd1 = 0;
int fd2 = 0 ;
fd_set rfds;// 读文件描述符集合
int ret = 0;
struct timeval tv;

int main()
{
	fd0 = open("/dev/cdd0", O_RDWR);
	fd1 = open("/dev/cdd1", O_RDWR);
	fd2 = open("/dev/cdd2", O_RDWR);
	while(1){
		//清空rfds
		FD_ZERO(&rfds);
		//将fdn加入到frds集合
		FD_SET(fd0, &rfds);
		FD_SET(fd1, &rfds);
		FD_SET(fd2, &rfds);
		
		//超时时间5s
		tv.tv_sec = 5;
		tv.tv_usec = 0;
		
		ret = select(fd2+1, &rfds, NULL, NULL, &tv);
		if(ret < 0){
			printf("select failed!\n");
			return -1
		}else if(ret = 0){
			printf("select timeout!\n");
		}else{
			if(FD_ISSET(fd0, &rfds)){
				printf("/dev/cdd0 is ready for reading!\n");
			}
			if(FD_ISSET(fd1, &rfds)){
				printf("/dev/cdd1 is ready for reading!\n");
			}
			if(FD_ISSET(fd2, &rfds)){
				printf("/dev/cdd2 is ready for reading!\n");
			}
		}
		sleep(1);
	}
	
	return 0;
}

linux驱动开发--字符设备：设备轮询操作select/poll