linux高级字符设备驱动之 三 阻塞型字符设备驱动

1、阻塞型字符设备驱动的功能

      当一个设备无法立刻满足用户的读写请求时应当如何处理? 例如:调用read时没有数据可读, 但以后可能会有;或者一个进程试图向设备写入数据,但是设备暂时没有准备好接收数据。应用程序通常不关心这种问题,应用程序只是调用 read 或 write 并得到返回值。驱动程序应当(缺省地)阻塞进程,使它进入睡眠,直到请求可以得到满足。

2、阻塞方式

      1）在阻塞型驱动程序中,Read实现方式如下:如果进程调用read,但设备没有数据或数据不足,进程阻塞。当新数据到达后,唤醒被阻塞进程。

      2）在阻塞型驱动程序中,Write实现方式如下:如果进程调用了write,但设备没有足够的空间供其写入数据,进程阻塞。当设备中的数据被读走后,缓冲区中空出部分空间,则唤醒进程。

3、非阻塞方式

      阻塞方式是文件读写操作的默认方式,但应用程序员可通过使用O_NONBLOCK标志来人为的设置读写操作为非阻塞方式(该标志定义在<linux/fcntl.h>中,在打开文件时指定)。

      如果设置了O_NONBLOCK标志,read和write的行为是不同的。如果进程在没有数据就绪时调用了read,或者在缓冲区没有空间时调用了write,系统只是简单地返回-EAGAIN,而不会阻塞进程。

4、实例分析

        程序实现的功能当进程读文件时，没有数据可读，则该进程阻塞。

        1）memdev.h源代码

#ifndef _MEMDEV_H_

#define _MEMDEV_H_

#ifndef MEMDEV_MAJOR

#define MEMDEV_MAJOR 0  

#endif

#ifndef MEMDEV_NR_DEVS

#define MEMDEV_NR_DEVS 2   

#endif

#ifndef MEMDEV_SIZE

#define MEMDEV_SIZE 4096

#endif

struct mem_dev                                     

{                                                        

  char *data;                      

  unsigned long size;

  wait_queue_head_t inq;  

};

#endif

        2）阻塞型字符驱动memdev.c

#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 "memdev.h"

static mem_major = MEMDEV_MAJOR;

bool have_data = false;

module_param(mem_major, int, S_IRUGO);

struct mem_dev *mem_devp;

struct cdev cdev;

int mem_open(struct inode *inode, struct file *filp)

{

    struct mem_dev *dev;

    int num = MINOR(inode->i_rdev);

    if (num >= MEMDEV_NR_DEVS)

            return -ENODEV;

    dev = &mem_devp[num];

    filp->private_data = dev;

    return 0;

}

int mem_release(struct inode *inode, struct file *filp)

{

  return 0;

}

static ssize_t mem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos)

{

  unsigned long p =  *ppos;

  unsigned int count = size;

  int ret = 0;

  struct mem_dev *dev = filp->private_data;

  if (p >= MEMDEV_SIZE)

    return 0;

  if (count > MEMDEV_SIZE - p)

    count = MEMDEV_SIZE - p;

while (!have_data)

{

        if (filp->f_flags & O_NONBLOCK)

            return -EAGAIN;

       wait_event_interruptible(dev->inq,have_data);

}

  if (copy_to_user(buf, (void*)(dev->data + p), count))

  {

    ret =  - EFAULT;

  }

  else

  {

    *ppos += count;

    ret = count;

    printk(KERN_INFO "read %d bytes(s) from %d\n", count, p);

  }

  have_data = false;

  return ret;

}

static ssize_t mem_write(struct file *filp, const char __user *buf, size_t size, loff_t *ppos)

{

  unsigned long p =  *ppos;

  unsigned int count = size;

  int ret = 0;

  struct mem_dev *dev = filp->private_data;

  if (p >= MEMDEV_SIZE)

    return 0;

  if (count > MEMDEV_SIZE - p)

    count = MEMDEV_SIZE - p;

  if (copy_from_user(dev->data + p, buf, count))

    ret =  - EFAULT;

  else

  {

    *ppos += count;

    ret = count;

    printk(KERN_INFO "written %d bytes(s) from %d\n", count, p);

  }

  have_data = true;

    wake_up(&(dev->inq));

  return ret;

}

static loff_t mem_llseek(struct file *filp, loff_t offset, int whence)

{

    loff_t newpos;

    switch(whence) {

      case 0:

        newpos = offset;

        break;

      case 1:

        newpos = filp->f_pos + offset;

        break;

      case 2:

        newpos = MEMDEV_SIZE -1 + offset;

        break;

      default:

        return -EINVAL;

    }

    if ((newpos<0) || (newpos>MEMDEV_SIZE))

        return -EINVAL;

    filp->f_pos = newpos;

    return newpos;

}

static const struct file_operations mem_fops =

{

  .owner = THIS_MODULE,

  .llseek = mem_llseek,

  .read = mem_read,

  .write = mem_write,

  .open = mem_open,

  .release = mem_release,

};

static int memdev_init(void)

{

  int result;

  int i;

  dev_t devno = MKDEV(mem_major, 0);

  if (mem_major)

    result = register_chrdev_region(devno, 2, "memdev");

  else 

  {

    result = alloc_chrdev_region(&devno, 0, 2, "memdev");

    mem_major = MAJOR(devno);

  }  

  if (result < 0)

    return result;

  cdev_init(&cdev, &mem_fops);

  cdev.owner = THIS_MODULE;

  cdev.ops = &mem_fops;

  cdev_add(&cdev, MKDEV(mem_major, 0), MEMDEV_NR_DEVS);

  mem_devp = kmalloc(MEMDEV_NR_DEVS * sizeof(struct mem_dev), GFP_KERNEL);

  if (!mem_devp)   

  {

    result =  - ENOMEM;

    goto fail_malloc;

  }

  memset(mem_devp, 0, sizeof(struct mem_dev));

  for (i=0; i < MEMDEV_NR_DEVS; i++)

  {

        mem_devp[i].size = MEMDEV_SIZE;

        mem_devp[i].data = kmalloc(MEMDEV_SIZE, GFP_KERNEL);

        memset(mem_devp[i].data, 0, MEMDEV_SIZE);

     init_waitqueue_head(&(mem_devp[i].inq));

  }

  return 0;

  fail_malloc:

  unregister_chrdev_region(devno, 1);

  return result;

}

static void memdev_exit(void)

{

  cdev_del(&cdev);  

  kfree(mem_devp);    

  unregister_chrdev_region(MKDEV(mem_major, 0), 2);

}

MODULE_AUTHOR("yinjiabin");

MODULE_LICENSE("GPL");

module_init(memdev_init);

module_exit(memdev_exit);

3）测试程序源码app-read.c

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <sys/ioctl.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <fcntl.h>

#include <sys/select.h>

#include <sys/time.h>

#include <errno.h>

int main()

{

    int fd;

    fd_set rds;

    int ret;

    char Buf[128];

    strcpy(Buf,"memdev is char dev!");

    printf("BUF: %s\n",Buf);

    fd = open("/dev/memdev0",O_RDWR);

    FD_ZERO(&rds);

    FD_SET(fd, &rds);

    strcpy(Buf,"Buf is NULL!");

    printf("Read BUF1: %s\n",Buf);

    ret = select(fd + 1, &rds, NULL, NULL, NULL);

    if (ret < 0)  

    {

        printf("select error!\n");

        exit(1);

    }

    if (FD_ISSET(fd, &rds))

        read(fd, Buf, sizeof(Buf));            

    printf("Read BUF2: %s\n",Buf);

    close(fd);

    return 0;    

}