一般来说上层调用poll()，到了内核层会调用sys_poll()，类似的还有open->sys_open、read->sys_read()，所以类比于open和read，驱动程序中也有一个poll调用。
之前用了轮询和中断的方式来实现，现在用poll机制来实现。poll机制有个好处就是能超时处理，当超过一定的时间后就能自动返回。下面我们看一下poll机制的流程是什么样的。
下面的源码是基于Linux 2.6版本的源码：
app: poll(struct pollfd fd[], nfds_t nfds, int timeout);
|
\|/
*********************
kernel:
sys_poll:
do_sys_poll(ufds, nfds, &timeout_jiffies);
poll_initwait(&table); //从名字可以看出是初始化等待
init_poll_funcptr(&pwq->pt, __pollwait);
pt->qproc = qproc; 相当于 table->pt->qproc = __pollwait
do_poll(nfds, head, &table, timeout);
for (;;)
{
for (; pfd != pfd_end; pfd++) { //poll机制可以查询多个驱动程序
if (do_pollfd(pfd, pt)) --> 调用驱动程序中的poll函数：mask = file->f_op->poll(file, pwait); return mask;
//驱动中会调用poll_wait()，然后会调用到下面的函数
p->qproc(filp, wait_address, p);
相当于
__pollwait(filp, &button_waitq, p)
init_waitqueue_entry(&entry->wait, current);//current就指向我们的当前进程
add_wait_queue(&button_waitq, &entry->wait);//把当前进程加入等待button_waitq队列
{
count++; //如果do_pollfd返回1，这里才加一
pt = NULL;
}
}
//break的条件：count非0，超时，有信号在等待处理
if (count || !*timeout || signal_pending(current))
break;
//开始休眠__timeout这么多时间
__timeout = schedule_timeout(__timeout);
}根据上面的流程分析，其实当我们的驱动程序中的poll调用poll_wait()时，就是把我们的进程挂到休眠队列中，当有中断来的时候才从休眠队列中唤醒我们的进程。下面就看一下我们的驱动程序怎么实现利用poll机制来监控按键事件。
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/delay.h>
#include <linux/irq.h>
#include <asm/uaccess.h>
#include <asm/irq.h>
#include <asm/io.h>
#include <asm/arch/regs-gpio.h>
#include <asm/hardware.h>
#include <linux/poll.h>
static int major;
static struct class *forthdrv_class;
static struct class_device *forthdrv_class_dev;
volatile unsigned long *gpfcon = NULL;
volatile unsigned long *gpgcon = NULL;
volatile unsigned long *gpfdat = NULL;
volatile unsigned long *gpgdat = NULL;
static DECLARE_WAIT_QUEUE_HEAD(button_waitq);
/*中断事件标识，中断服务程序将它置为1，forth_drv_read将它清零*/
static volatile int ev_press = 0;
struct pin_desc{
unsigned int pin;
unsigned int key_val;
};
/*键值:按下时，0x01 0x02 0x03 0x04 */
/*键值:松开时，0x81 0x82 0x83 0x84 */
static unsigned char key_val;
struct pin_desc pins_desc[4] = {
{S3C2410_GPG0, 0x01},
{S3C2410_GPG2, 0x02},
{S3C2410_GPG3, 0x03},
{S3C2410_GPG11, 0x04},
};
static irqreturn_t buttons_irq(int irq, void *dev_id)
{
struct pin_desc *pindesc = (struct pin_desc *)dev_id;
unsigned int pinval;
pinval = s3c2410_gpio_getpin(pindesc->pin);
if (pinval)
{
/*松开*/
key_val = 0x80 | pindesc->key_val;
}
else
{
/*按下*/
key_val = pindesc->key_val;
}
ev_press = 1; //表示中断发生了
wake_up_interruptible(&button_waitq);
return IRQ_RETVAL(IRQ_HANDLED);
}
static unsigned forth_drv_poll(struct file *file, poll_table *wait)
{
unsigned int mask = 0;
poll_wait(file, &button_waitq, wait); //不会立刻休眠，先加入休眠队列
if (ev_press)
mask |= POLLIN | POLLRDNORM;
return mask;
}
static int forth_drv_open(struct inode *inode, struct file *file)
{
//GPF0,2配置为输入引脚
//GPG3,11配置为输入引脚
request_irq(IRQ_EINT0, buttons_irq, IRQT_BOTHEDGE, "S2", &pins_desc[0]);
request_irq(IRQ_EINT2, buttons_irq, IRQT_BOTHEDGE, "S3", &pins_desc[1]);
request_irq(IRQ_EINT11, buttons_irq, IRQT_BOTHEDGE, "S4", &pins_desc[2]);
request_irq(IRQ_EINT19, buttons_irq, IRQT_BOTHEDGE, "S5", &pins_desc[3]);
return 0;
}
static int forth_drv_read(struct file *file, const __user *buf, size_t count, loff_t *ppos)
{
if (count != 1)
return -EINVAL;
/*如果没有按键动作，就休眠，
*当ev_press为0就进入休眠
*/
wait_event_interruptible(button_waitq, ev_press);
/*如果没有按键动作，返回键值*/
copy_to_user(buf, &key_val, 1);
ev_press = 0;
return 1;
}
static int forth_drv_close(struct inode *inode, struct file *file)
{
free_irq(IRQ_EINT0, &pins_desc[0]);
free_irq(IRQ_EINT2, &pins_desc[1]);
free_irq(IRQ_EINT11, &pins_desc[2]);
free_irq(IRQ_EINT19, &pins_desc[3]);
}
static struct file_operations forth_fops = {
.owner = THIS_MODULE,
.open = forth_drv_open,
.read = forth_drv_read,
.release = forth_drv_close,
.poll = forth_drv_poll,
};
static int forth_drv_init(void)
{
major = register_chrdev(0, "forth_drv", &forth_fops);
forthdrv_class = class_create(THIS_MODULE, "forth_drv");
forthdrv_class_dev = class_device_create(forthdrv_class, NULL, MKDEV(major, 0), NULL, "buttons");
gpfcon = (volatile unsigned long *)ioremap(0x56000050, 16);
gpfdat = gpfcon + 1;
gpgcon = (volatile unsigned long *)ioremap(0x56000060, 16);
gpgdat = gpgcon + 1;
return 0;
}
void forth_drv_exit(void)
{
unregister_chrdev(major, "forth_drv");
class_device_unregister(forthdrv_class_dev);
class_destroy(forthdrv_class);
iounmap(gpfcon);
iounmap(gpgcon);
}
module_init(forth_drv_init);
module_exit(forth_drv_exit);
MODULE_LICENSE("GPL");
从上面的代码可以看出来，大部分和我上一个利用中断方式的检测按键的代码差不多。这里主要实现了forth_drv_poll()，并且在file_operations中多了一个poll接口，就是这么简单，就能给上层应用通过调用poll()函数来实现超时操作。这里要注意的是，由文章开头分析的，当我们的驱动程序中的poll函数返回非0才表示有按键事件发生，如果没有按键来临的时候，我们需要返回0。
下面我们给出上层的测试函数的代码：
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <poll.h>
int main(int argc, char **argv)
{
unsigned char key_val;
int fd;
int ret;
struct pollfd fds[1]; //这里只检测一个文件
fd = open("/dev/buttons", O_RDWR);
if (fd < 0)
{
printf("open fd failed\n");
return 0;
}
fds[0].fd = fd;
fds[0].events = POLLIN; //POLLIN :There is data to read.
while (1)
{
//int poll(struct pollfd *fds, nfds_t nfds, int timeout);
ret = poll(fds, 1, 5000); // 5000ms
if (0 == ret)
{
printf("time out\n");
}
else
{
read(fd, &key_val, 1);
printf("key_val = 0x%x\n", key_val);
}
}
return 1;
}当poll返回0就代表没有按键事件发生，否则就代表按了按键，并且返回了数据。而且这种机制占用的资源也不多：
PID PPID USER STAT VSZ %MEM %CPU COMMAND
802 772 0 R 3096 5% 0% top
771 1 0 S 15984 26% 0% /opt/Qtopia/bin/qpe
786 771 0 S N 11492 19% 0% /opt/Qtopia/bin/quicklauncher
785 771 0 S < 9044 15% 0% /opt/Qtopia/bin/qss
772 1 0 S 3096 5% 0% -sh
1 0 0 S 3092 5% 0% init
801 772 0 S 1312 2% 0% ./forthDrvTest