Linux 设备驱动轮询编程

Linux系统下网络模型数select最为常用，当然，select只是检测文件系统数据状态，并不只局限于网络编程，select的功能需要底层驱动提供支持，其中核心应用即为等待队列，其他模型，如poll和epoll，对驱动来说并无区别，驱动只是返回数据状态而已。驱动支持select，需要实现file_operations结构中的poll函数指针，其实现也非常简单，只是poll_wait函数的调用，原型如下：unsigned int （*poll）（struct file *, struct poll_table_struct *）;void poll_wait（struct file *filp, wait_queue_head_t *queue, poll_table * wait）;需要注意的是poll_wait函数不会像它名字一样处于wait状态，仅供上层查询之用。其编程基本框架也比较固定，现修改simple等待队列驱动之中如下：unsigned int simple_poll （struct file * filp, struct poll_table_struct * wait）{unsigned int mask = 0;poll_wait（filp, &simple_queue, wait）;if （len > 0）{mask |= POLLIN | POLLRDNORM;}return mask;}在每次read之后都把len赋值为0，调用write把len赋值为数据长，poll中只需判断len，如果len大于0，则返回可读，整理之后，整个程序代码如下：#include <linux/init.h>#include <linux/module.h>#include <linux/fs.h>#include <linux/types.h>#include <linux/cdev.h>#include <linux/mm.h>#include <linux/sched.h>#include <asm/io.h>#include <asm/uaccess.h>#include <asm/system.h>#include <linux/device.h>#include <linux/poll.h>dev_t devno;struct class * simple_class;static struct cdev cdev;wait_queue_head_t simple_queue;char test_data[255];int len = 0;unsigned int simple_poll （struct file * filp, struct poll_table_struct * wait）{unsigned int mask = 0;poll_wait（filp, &simple_queue, wait）;if （len > 0）{mask |= POLLIN | POLLRDNORM;}return mask;}ssize_t simple_read（struct file * pfile,char __user * buf, size_t size, loff_t * ppos）{int ret = len;len = 0;if （copy_to_user（buf, test_data, ret））return -EFAULT;elsereturn ret;}ssize_t simple_write（struct file * pfile, const char __user * buf, size_t count, loff_t * ppos）{if （count > 255）{return -EFAULT;}if （！copy_from_user（test_data, buf, count））{len = count;wake_up（&simple_queue）;}return len;}int simple_open（struct inode * pnode, struct file * pfile）{printk（KERN_INFO "open simple "）;return 0;}int simple_release（struct inode * pnode, struct file * pfile）{printk（KERN_INFO "close simple "）;return 0;}static struct file_operations simple_op = {.owner = THIS_MODULE,.read = simple_read,.open = simple_open,.release = simple_release,.write = simple_write,.poll = simple_poll,};static int __init initialization（void）{int result;result = alloc_chrdev_region（&devno, 0, 1, "simple"）;if （result < 0）return result;cdev_init（&cdev, &simple_op）;result = cdev_add（&cdev, devno, 1）;simple_class = class_create（THIS_MODULE, "simple"）;device_create（simple_class, NULL, devno, NULL, "simple"）;printk（KERN_INFO " init simple "）;init_waitqueue_head（&simple_queue）;return result;}static void __exit cleanup（void）{device_destroy（simple_class, devno）;class_destroy（simple_class）;cdev_del（&cdev）;unregister_chrdev_region（devno, 1）;printk（KERN_INFO " cleanup simple "）;}module_init（initialization）;module_exit（cleanup）;MODULE_AUTHOR（"alloc cppbreak@gmail.com"）;MODULE_DESCRIPTION（"A simple linux kernel module"）;MODULE_VERSION（"V0.1"）;MODULE_LICENSE（"Dual BSD/GPL"）;