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字符设备驱动程序之poll机制

当应用程序调用poll时,会相应的调用内核空间的sys_poll
sys_poll
  do_sys_poll
    poll_initwait(&table)
    void poll_initwait(struct poll_wqueues *pwq)
      init_poll_funcptr(&pwq->pt, __pollwait);//下面这个函数为该函数的具体实现
      static inline void init_poll_funcptr(poll_table *pt, poll_queue_proc qproc)
      {
        pt->qproc = qproc;//此时就相当于:pt->qproc = __pollwait,所以指针
        //qproc就指向了函数__pollwait
      }
      //接下来看一下__pollwait的具体实现:
      static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,poll_table *p)
      {
        struct poll_table_entry *entry = poll_get_entry(p);
        if (!entry)
        return;
        get_file(filp);
        entry->filp = filp;
        entry->wait_address = wait_address;
        init_waitqueue_entry(&entry->wait, current);
        add_wait_queue(wait_address, &entry->wait);
      }
  从上面的分析可知,poll_initwait(&table)函数只实现了将当前进程加入到wait_address队列中
  do_poll(nfds, head, &table, timeout),会进入一个死循环,在死循环里调用了do_pollfd(pfd, pt)
    do_pollfd(pfd, pt)
      mask = file->f_op->poll(file, pwait);//unsigned int (*poll) (struct file *, struct poll_table_struct *);
      在驱动程序中的file_operations结构体中,定义了.poll=fifth_drv_poll, 从而使得poll指针指向了函数fifth_drv_poll,
      所以在执行poll函数时,实际上是调用了驱动程序中的fifth_drv_poll这个函数。在fifth_drv_poll这个函数中,会
      调用poll_wait(file, &button_waitq, wait),看一下该函数的具体实现:
      void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
      {
        if (p && wait_address)
        p->qproc(filp, wait_address, p);//在上面已经分析过,p->qproc指向函数__pollwait。
        因此这条语句相当于执行__pollwait(filp, wait_address, p),从上面的分析可以看出,将当前进程加入到
        wait_address等待队列中。
      }
  注意:在我们的驱动程序中会有一个返回值,假设为mask1.那么此时mask=mask1,而mask又将返回给do_pollfd(pfd, pt)这个函数。内核中有这样一段程序:
          for(;;)
            {
              if (do_pollfd(pfd, pt)) {
                count++;
                pt = NULL;
            }如果mask非零,count++,表示按键按下。

            /*若count不为0或者超时或者有事件发生则会跳出死循环*/
            if (count || !*timeout || signal_pending(current))
            break;
            /*若count为0且未超时且无事件发生则会休眠__timeout时间*/
            schedule_timeout(__timeout);当休眠到timeout这段时间之后,还是没有任何事件发生,会再次进入循环中
            ,当到达if (count || !*timeout || signal_pending(current))时,会跳出循环。
}

应用程序:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <poll.h>
#include <signal.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>

int fd;

void my_signal_fun(int signum)
{
  unsigned char key_val;
  read(fd, &key_val, 1);
  printf("key_val: 0x%x\n", key_val);
}

int main(int argc, char **argv)
{
  unsigned char key_val;
  int ret;
  int Oflags;

  signal(SIGIO, my_signal_fun);

  fd = open("/dev/buttons", O_RDWR);
  if (fd < 0)
  {
    printf("can‘t open!\n");
  }

  fcntl(fd, F_SETOWN, getpid());

  Oflags = fcntl(fd, F_GETFL);

  fcntl(fd, F_SETFL, Oflags | FASYNC);


  while (1)
  {
    sleep(1000);
  }

  return 0;
}

 驱动程序:

#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 struct class *fifthdrv_class;
static struct class_device *fifthdrv_class_dev;

volatile unsigned long *gpfcon;
volatile unsigned long *gpfdat;

volatile unsigned long *gpgcon;
volatile unsigned long *gpgdat;


static DECLARE_WAIT_QUEUE_HEAD(button_waitq);

/* 中断事件标志, 中断服务程序将它置1,fifth_drv_read将它清0 */
static volatile int ev_press = 0;

static struct fasync_struct *button_async;


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_GPF0, 0x01},
  {S3C2410_GPF2, 0x02},
  {S3C2410_GPG3, 0x03},
  {S3C2410_GPG11, 0x04},
};


/*
* 确定按键值
*/
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); /* 唤醒休眠的进程 */

kill_fasync (&button_async, SIGIO, POLL_IN);

return IRQ_RETVAL(IRQ_HANDLED);
}

static int fifth_drv_open(struct inode *inode, struct file *file)
{
  /* 配置GPF0,2为输入引脚 */
  /* 配置GPG3,11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;
}

ssize_t fifth_drv_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
{
  if (size != 1)
  return -EINVAL;

  /* 如果没有按键动作, 休眠 */
  wait_event_interruptible(button_waitq, ev_press);

  /* 如果有按键动作, 返回键值 */
  copy_to_user(buf, &key_val, 1);
  ev_press = 0;

  return 1;
}


int fifth_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]);
  return 0;
}

static unsigned fifth_drv_poll(struct file *file, poll_table *wait)
{
  unsigned int mask = 0;
  poll_wait(file, &button_waitq, wait); // 不会立即休眠

  if (ev_press)
  mask |= POLLIN | POLLRDNORM;

  return mask;
}

static int fifth_drv_fasync (int fd, struct file *filp, int on)
{
  printk("driver: fifth_drv_fasync\n");
  return fasync_helper (fd, filp, on, &button_async);
}


static struct file_operations sencod_drv_fops = {
  .owner = THIS_MODULE, /* 这是一个宏,推向编译模块时自动创建的__this_module变量 */
  .open = fifth_drv_open,
  .read = fifth_drv_read,
  .release = fifth_drv_close,
  .poll = fifth_drv_poll,
  .fasync = fifth_drv_fasync,
};


int major;
static int fifth_drv_init(void)
{
  major = register_chrdev(0, "fifth_drv", &sencod_drv_fops);

  fifthdrv_class = class_create(THIS_MODULE, "fifth_drv");

  fifthdrv_class_dev = class_device_create(fifthdrv_class, NULL, MKDEV(major, 0), NULL, "buttons"); /* /dev/buttons */

  gpfcon = (volatile unsigned long *)ioremap(0x56000050, 16);
  gpfdat = gpfcon + 1;

  gpgcon = (volatile unsigned long *)ioremap(0x56000060, 16);
  gpgdat = gpgcon + 1;

  return 0;
}

static void fifth_drv_exit(void)
{
  unregister_chrdev(major, "fifth_drv");
  class_device_unregister(fifthdrv_class_dev);
  class_destroy(fifthdrv_class);
  iounmap(gpfcon);
  iounmap(gpgcon);
  return 0;
}


module_init(fifth_drv_init);

module_exit(fifth_drv_exit);

MODULE_LICENSE("GPL");

 

字符设备驱动程序之poll机制