设备驱动中的阻塞与非阻塞I/O----globalfifo驱动

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本文介绍了一个字符设备驱动程序globalfifo的例子，解释了阻塞与非阻塞I/O的概念及其在Linux设备驱动中的实现。通过具体的代码示例，展示了如何使用信号量、等待队列和poll机制来实现这两种I/O模式。

设备驱动中的阻塞与非阻塞I/O----globalfifo驱动

2008-10-19 19:32

/********************************************************************
    A globalfifo driver as an example of char device drivers
    This example is to introduce poll,blocking and non-blocking access

    The initial developer of the original code is Baohua Song
    <author@linuxdriver.cn>. All Rights Reserved.
*********************************************************************/
#include <linux/module.h>
#include <linux/types.h>
#include <linux/fs.h>
#include <linux/errno.h>
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <asm/io.h>
#include <asm/system.h>
#include <asm/uaccess.h>
#include <linux/poll.h>

#define GLOBALFIFO_SIZE    0x1000    /*全局fifo最大4K字节*/
#define FIFO_CLEAR 0x1 /*清0全局内存的长度*/
#define GLOBALFIFO_MAJOR 253    /*预设的globalfifo的主设备号*/

static int globalfifo_major = GLOBALFIFO_MAJOR;
/*globalfifo设备结构体*/
struct globalfifo_dev
{
struct cdev cdev; /*cdev结构体*/
unsigned int current_len;    /*fifo有效数据长度*/
unsigned char mem[GLOBALFIFO_SIZE]; /*全局内存*/
struct semaphore sem; /*并发控制用的信号量*/
wait_queue_head_t r_wait; /*阻塞读用的等待队列头*/
wait_queue_head_t w_wait; /*阻塞写用的等待队列头*/
};

struct globalfifo_dev *globalfifo_devp; /*设备结构体指针*/
/*文件打开函数*/
int globalfifo_open(struct inode *inode, struct file *filp)
{
/*将设备结构体指针赋值给文件私有数据指针*/
filp->private_data = globalfifo_devp;
return 0;
}
/*文件释放函数*/
int globalfifo_release(struct inode *inode, struct file *filp)
{
return 0;
}

/* ioctl设备控制函数 */
static int globalfifo_ioctl(struct inode *inodep, struct file *filp, unsigned
int cmd, unsigned long arg)
{
struct globalfifo_dev *dev = filp->private_data;/*获得设备结构体指针*/

switch (cmd)
{
    case FIFO_CLEAR:
        down(&dev->sem); //获得信号量
      dev->current_len = 0;
      memset(dev->mem,0,GLOBALFIFO_SIZE);
      up(&dev->sem); //释放信号量

      printk(KERN_INFO "globalfifo is set to zero/n");
      break;

    default:
      return - EINVAL;
}
return 0;
}

static unsigned int globalfifo_poll(struct file *filp, poll_table *wait)
{
unsigned int mask = 0;
struct globalfifo_dev *dev = filp->private_data; /*获得设备结构体指针*/

down(&dev->sem);

poll_wait(filp, &dev->r_wait, wait);
poll_wait(filp, &dev->w_wait, wait);
/*fifo非空*/
if (dev->current_len != 0)
{
    mask |= POLLIN | POLLRDNORM; /*标示数据可获得*/
}
/*fifo非满*/
if (dev->current_len != GLOBALFIFO_SIZE)
{
    mask |= POLLOUT | POLLWRNORM; /*标示数据可写入*/
}

up(&dev->sem);
return mask;
}

/*globalfifo读函数*/
static ssize_t globalfifo_read(struct file *filp, char __user *buf, size_t count,
loff_t *ppos)
{
int ret;
struct globalfifo_dev *dev = filp->private_data; //获得设备结构体指针
DECLARE_WAITQUEUE(wait, current); //定义等待队列

down(&dev->sem); //获得信号量
add_wait_queue(&dev->r_wait, &wait); //进入读等待队列头

/* 等待FIFO非空 */
while (dev->current_len == 0)
{
    if (filp->f_flags &O_NONBLOCK)
    {
      ret = - EAGAIN;
      goto out;
    }
    __set_current_state(TASK_INTERRUPTIBLE); //改变进程状态为睡眠
    up(&dev->sem);

    schedule(); //调度其他进程执行
    if (signal_pending(current))
    //如果是因为信号唤醒
    {
      ret = - ERESTARTSYS;
      goto out2;
    }

    down(&dev->sem);
}

/* 拷贝到用户空间 */
if (count > dev->current_len)
    count = dev->current_len;

if (copy_to_user(buf, dev->mem, count))
{
    ret = - EFAULT;
    goto out;
}
else
{
    memcpy(dev->mem, dev->mem + count, dev->current_len - count); //fifo数据前移
    dev->current_len -= count; //有效数据长度减少
    printk(KERN_INFO "read %d bytes(s),current_len:%d/n", count, dev->current_len);

    wake_up_interruptible(&dev->w_wait); //唤醒写等待队列

    ret = count;
}
out: up(&dev->sem); //释放信号量
out2:remove_wait_queue(&dev->w_wait, &wait); //从附属的等待队列头移除
set_current_state(TASK_RUNNING);
return ret;
}

/*globalfifo写操作*/
static ssize_t globalfifo_write(struct file *filp, const char __user *buf,
size_t count, loff_t *ppos)
{
struct globalfifo_dev *dev = filp->private_data; //获得设备结构体指针
int ret;
DECLARE_WAITQUEUE(wait, current); //定义等待队列

down(&dev->sem); //获取信号量
add_wait_queue(&dev->w_wait, &wait); //进入写等待队列头

/* 等待FIFO非满 */
while (dev->current_len == GLOBALFIFO_SIZE)
{
    if (filp->f_flags &O_NONBLOCK)
    //如果是非阻塞访问
    {
      ret = - EAGAIN;
      goto out;
    }
    __set_current_state(TASK_INTERRUPTIBLE); //改变进程状态为睡眠
    up(&dev->sem);

    schedule(); //调度其他进程执行
    if (signal_pending(current))
    //如果是因为信号唤醒
    {
      ret = - ERESTARTSYS;
      goto out2;
    }

    down(&dev->sem); //获得信号量
}

/*从用户空间拷贝到内核空间*/
if (count > GLOBALFIFO_SIZE - dev->current_len)
    count = GLOBALFIFO_SIZE - dev->current_len;

if (copy_from_user(dev->mem + dev->current_len, buf, count))
{
    ret = - EFAULT;
    goto out;
}
else
{
    dev->current_len += count;
    printk(KERN_INFO "written %d bytes(s),current_len:%d/n", count, dev
      ->current_len);

    wake_up_interruptible(&dev->r_wait); //唤醒读等待队列

    ret = count;
}

out: up(&dev->sem); //释放信号量
out2:remove_wait_queue(&dev->w_wait, &wait); //从附属的等待队列头移除
set_current_state(TASK_RUNNING);
return ret;
}

/*文件操作结构体*/
static const struct file_operations globalfifo_fops =
{
.owner = THIS_MODULE,
.read = globalfifo_read,
.write = globalfifo_write,
.ioctl = globalfifo_ioctl,
.poll = globalfifo_poll,
.open = globalfifo_open,
.release = globalfifo_release,
};

/*初始化并注册cdev*/
static void globalfifo_setup_cdev(struct globalfifo_dev *dev, int index)
{
int err, devno = MKDEV(globalfifo_major, index);

cdev_init(&dev->cdev, &globalfifo_fops);
dev->cdev.owner = THIS_MODULE;
dev->cdev.ops = &globalfifo_fops;
err = cdev_add(&dev->cdev, devno, 1);
if (err)
    printk(KERN_NOTICE "Error %d adding LED%d", err, index);
}

/*设备驱动模块加载函数*/
int globalfifo_init(void)
{
int ret;
dev_t devno = MKDEV(globalfifo_major, 0);

/* 申请设备号*/
if (globalfifo_major)
    ret = register_chrdev_region(devno, 1, "globalfifo");
else /* 动态申请设备号 */
{
    ret = alloc_chrdev_region(&devno, 0, 1, "globalfifo");
    globalfifo_major = MAJOR(devno);
}
if (ret < 0)
    return ret;
/* 动态申请设备结构体的内存*/
globalfifo_devp = kmalloc(sizeof(struct globalfifo_dev), GFP_KERNEL);
if (!globalfifo_devp)    /*申请失败*/
{
    ret = - ENOMEM;
    goto fail_malloc;
}

memset(globalfifo_devp, 0, sizeof(struct globalfifo_dev));

globalfifo_setup_cdev(globalfifo_devp, 0);

init_MUTEX(&globalfifo_devp->sem);   /*初始化信号量*/
init_waitqueue_head(&globalfifo_devp->r_wait); /*初始化读等待队列头*/
init_waitqueue_head(&globalfifo_devp->w_wait); /*初始化写等待队列头*/

return 0;

fail_malloc: unregister_chrdev_region(devno, 1);
return ret;
}

/*模块卸载函数*/
void globalfifo_exit(void)
{
cdev_del(&globalfifo_devp->cdev);   /*注销cdev*/
kfree(globalfifo_devp);     /*释放设备结构体内存*/
unregister_chrdev_region(MKDEV(globalfifo_major, 0), 1); /*释放设备号*/
}

MODULE_AUTHOR("Song Baohua");
MODULE_LICENSE("Dual BSD/GPL");

module_param(globalfifo_major, int, S_IRUGO);

module_init(globalfifo_init);
module_exit(globalfifo_exit);

验证读写:
启动两个进程
cat /dev/globalfifo&后台执行
echo 'Test ' /dev/globalfifo 前台执行
每当echo写入一串数据,cat进程就立即把数据显示出来

验证轮询:
/*======================================================================
    A test program in userspace
    This example is to introduce the ways to use "select"
     and driver poll

    The initial developer of the original code is Baohua Song
    <author@linuxdriver.cn>. All Rights Reserved.
======================================================================*/
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/time.h>

#define FIFO_CLEAR 0x1
#define BUFFER_LEN 20
main()
{
int fd, num;
char rd_ch[BUFFER_LEN];
fd_set rfds,wfds;

/*以非阻塞方式打开/dev/globalmem设备文件*/
fd = open("/dev/globalfifo", O_RDONLY | O_NONBLOCK);
if (fd != - 1)
{
    /*FIFO清0*/
    if (ioctl(fd, FIFO_CLEAR, 0) < 0)
    {
      printf("ioctl command failed/n");
    }
    while (1)
    {
      FD_ZERO(&rfds);
      FD_ZERO(&wfds);
      FD_SET(fd, &rfds);//FD_SET将一个文件描述符加入文件描述符集合中
      FD_SET(fd, &wfds);

      select(fd + 1, &rfds, &wfds, NULL, NULL);
      /*数据可获得*/
      if (FD_ISSET(fd, &rfds))
      {
          printf("Poll monitor:can be read/n");
      }
      /*数据可写入*/
      if (FD_ISSET(fd, &wfds))
      {
          printf("Poll monitor:can be written/n");
      }
    }
}
else
{
    printf("Device open failure/n");
}
}
FIFO为空时不断的输出can be write
echo写入一些数据时输出can be read and can be write.写满后只输出can be read