字符设备驱动第八课----异步通知（信号驱动IO）

概述

类比运用程序中的kill-----signal，在运用程序中常常一个进程用kill(pid，sig)向另一
进程发信号，另一个进程用signal(sig,handler)绑定相应的处理函数，实现了异步通知。

今天要讲的就是：运用程序要读，但它并不知道啥时候有东西可读，用read（）
阻塞去读显然效率不高，read()配合IO多路复用非阻塞一直在那里轮询的话效率也不好。
这里采用的办法是：驱动层有数据可读的时候kill一个SIGIO信号给运用层，
运用层收到SIGIO信号后调用预先绑定好的处理函数把数据读走。

若还迷糊概念，请看看这位大神的清晰讲解:
信号驱动IO与异步通知

<include/linux/fs.h>

struct file_operations {
    int (*open) (struct inode *, struct file *);
    int (*flush) (struct file *, fl_owner_t id);
    int (*release) (struct inode *, struct file *);
    int (*fsync) (struct file *, loff_t, loff_t, int datasync);       //用于异步通知
    ...
}

<include/linux/fs.h>

struct fasync_struct {
    spinlock_t      fa_lock;
    int         magic;
    int         fa_fd;
    struct fasync_struct    *fa_next; /* singly linked list */
    struct file     *fa_file;
    struct rcu_head     fa_rcu;
};

/*
* 功能： 得到异步通知结构。根据mod,将异步通知结构体加入链表
* 运用程序端调用这些函数改变标志和owner时就调用了这个底层实现。
*    fcntl(STDIN_FILENO,F_SETOWN,getpid());
*    oflags = fcntl(STDIN_FILENO,F_GETFL);
*    fctcl(STDIN_FILENO,F_SETFL,oflags | FASYNC);
* 输入参数:  fd:             文件描述符
*          filp：          file结构体指针
* 输出参数：fapp：得到的异步通知结构体          
*/
int fasync_helper(int fd, struct file * filp, int on, struct fasync_struct **fapp)

/*
 * 功能：通过异步通知结构发信号，此信号发出时运用端就会收到SGIO信号，就会回调预先绑定的处理函数。
 * 参数：struct fasync_struct **fp:  异步通知结构
 *      int signo:                  信号（SIGIO）
 *      int events:                 事件：POLLIN、POLLOUT
 */
void kill_fasync(struct fasync_struct **fp, int sig, int band)

范例：

1.驱动端：

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>

#include <linux/cdev.h>
#include <linux/fs.h>
#include <linux/errno.h>

#include <asm/current.h>
#include <linux/sched.h>

#include <linux/uaccess.h>
#include <linux/poll.h>

#include <asm/atomic.h>
#include <linux/mutex.h>

#include <linux/wait.h>

#include <linux/device.h>
static struct class *cls = NULL;

static int major = 0;
static int minor = 0;
const  int count = 6;

#define DEVNAME "demo"

static struct cdev *demop = NULL;
static atomic_t tv;
static wait_queue_head_t wq;

static struct fasync_struct *fasync = NULL;//定义异步通知结构体

#define KMAX 1024
static char kbuf[KMAX];
static int counter = 0;

//打开设备
static int demo_open(struct inode *inode, struct file *filp)
{
    //get major and minor from inode
    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d
",
        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);

    if(!atomic_dec_and_test(&tv)){
        atomic_inc(&tv);
        return -EBUSY;
    }

    memset(kbuf, 0, KMAX);
    counter = 0;

    return 0;
}

//关闭设备
static int demo_release(struct inode *inode, struct file *filp)
{
    //get major and minor from inode
    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d
",
        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);

    atomic_inc(&tv);
    return 0;
}

//读设备
//ssize_t read(int fd, void *buf, size_t count)
static ssize_t demo_read(struct file *filp, char __user *buf, size_t size, loff_t *offset)
{
    int err = 0;
    struct inode *inode = filp->f_path.dentry->d_inode;  //获取文件的inod号
    //get major and minor from inode
    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d
",
        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);

    if(!counter){
        if(filp->f_flags & O_NONBLOCK){
            return -EAGAIN;
        }

        err = wait_event_interruptible(wq, (0 != counter));//睡在条件上的等待队列
        if(err){                                            //没东西可读，睡
            return err;
        }
    }

    if(counter < size){
        size = counter;
    }

    if(copy_to_user(buf, kbuf, size)){
        return -EAGAIN;
    }

    counter = 0;

    return size;
}

//写设备
static ssize_t demo_write(struct file *filp, const char __user *buf, size_t size, loff_t *offset)
{
    struct inode *inode = filp->f_path.dentry->d_inode;
    //get major and minor from inode
    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d
",
        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);

    if(size > KMAX){
        return -ENOMEM;
    }

    if(copy_from_user(kbuf, buf, size)){
        return -EAGAIN;
    }

    counter = size;
    wake_up_interruptible(&wq);//广播唤醒等待队列
    kill_fasync(&fasync, SIGIO, POLLIN);//向fasync结构体发信号,
    //与fasync关联的进程(通过fcntl(...,pid)系列函数关联)就会收到SIGIO信号

    return size;
}

/* IO多路复用支持*/
static unsigned int demo_poll(struct file *filp, struct poll_table_struct *pts)
{
    unsigned int mask = 0;
    struct inode *inode = filp->f_path.dentry->d_inode;
    //get major and minor from inode
    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d
",
        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);

    poll_wait(filp, &wq, pts);//io多路复用支持,只有等待队列中有就绪的事件才会往下走，否则阻塞

    if(counter){
        mask = (POLLIN | POLLRDNORM);//返回，告诉运用层的poll函数：就绪事件是输入事件
    }

    return mask;
}


/*异步通知接口函数，应用层调fcntl()时调到此函数*/
static int demo_fasync(int fd, struct file *filp, int mode)
{
    struct inode *inode = filp->f_path.dentry->d_inode;
    //get major and minor from inode
    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d
",
        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);

    return fasync_helper(fd, filp, mode, &fasync);//根据mod,将异步通知结构体加入链表
                                                //或者从链表中移除。得到信息并填充到fasync结构体中
}

static struct file_operations fops = {
    .owner  = THIS_MODULE,
    .open   = demo_open,
    .release= demo_release,
    .read   = demo_read,
    .write  = demo_write,
    .poll   = demo_poll,
    .fasync = demo_fasync,
};

static int __init demo_init(void)
{
    dev_t devnum;
    int ret, i;

    struct device *devp = NULL;

    //get command and pid
    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d
",
        current->comm, current->pid, __FILE__, __func__, __LINE__);

    //1. alloc cdev obj
    demop = cdev_alloc();
    if(NULL == demop){
        return -ENOMEM;
    }

    //2. init cdev obj
    cdev_init(demop, &fops);

    ret = alloc_chrdev_region(&devnum, minor, count, DEVNAME);
    if(ret){
        goto ERR_STEP;
    }
    major = MAJOR(devnum);

    //3. register cdev obj
    ret = cdev_add(demop, devnum, count);
    if(ret){
        goto ERR_STEP1;
    }

    cls = class_create(THIS_MODULE, DEVNAME);
    if(IS_ERR(cls)){
        ret = PTR_ERR(cls);
        goto ERR_STEP1;
    }

    for(i = minor; i < (count+minor); i++){
        devp = device_create(cls, NULL, MKDEV(major, i), NULL, "%s%d", DEVNAME, i);
        if(IS_ERR(devp)){
            ret = PTR_ERR(devp);
            goto ERR_STEP2;
        }
    }

    // init atomic_t
    atomic_set(&tv, 1);

    init_waitqueue_head(&wq);//初始化等待队列

    //get command and pid
    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d - ok.
",
        current->comm, current->pid, __FILE__, __func__, __LINE__);
    return 0;

ERR_STEP2:
    for(--i; i >= minor; i--){
        device_destroy(cls, MKDEV(major, i));
    }
    class_destroy(cls);

ERR_STEP1:
    unregister_chrdev_region(devnum, count);

ERR_STEP:
    cdev_del(demop);

    //get command and pid
    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d - fail.
",
        current->comm, current->pid, __FILE__, __func__, __LINE__);
    return ret;
}

static void __exit demo_exit(void)
{
    int i;
    //get command and pid
    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d - leave.
",
        current->comm, current->pid, __FILE__, __func__, __LINE__);

    for(i=minor; i < (count+minor); i++){
        device_destroy(cls, MKDEV(major, i));
    }
    class_destroy(cls);

    unregister_chrdev_region(MKDEV(major, minor), count);
    cdev_del(demop);
}

module_init(demo_init);
module_exit(demo_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Farsight");
MODULE_DESCRIPTION("Demo for kernel module");

2.运用程序端：

#include <stdio.h>
#include <string.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <poll.h>
#include <signal.h>

#include <errno.h>

int fd = -1;

void handler(int sig)//信号处理函数，与signal绑定的
{
    struct pollfd pfd = {
        .fd = fd,
        .events = POLLIN,
    };

    int ret = poll(&pfd, 1, ~0);//监控pfd，最大文件描述符1，永不超时
    if(0 >= ret){               //没一个就绪则阻塞，只要其中有任意一个就绪就往下走
        perror("poll");
        return;
    }

#define MAX 1024
    char buf[MAX];
    memset(buf, 0, MAX); 
    if(0 > read(fd, buf, MAX)){
        perror("read");
    }else{
        printf("RD: %s
", buf);
    }
}

int main(int num, char *argv[])
{
    if(2 != num){
        printf("Usage: %s /dev/devfile
", argv[0]);
        return -1;
    }

    fd = open(argv[1], O_RDWR|O_NONBLOCK);
    if(0 > fd){
        printf("pid = %d, %s
", getpid(), (char *)strerror(errno));
        return -1;
    }

    signal(SIGIO, handler);//绑定信号处理函数

    fcntl(fd, F_SETOWN, getpid());//关联收发，设置对应文件的拥有者是本进程，这样接下来才能进行信号的收发

    int flag = fcntl(fd, F_GETFL);//读取对应文件描述符上的flg信息
    flag  |= O_ASYNC;
    fcntl(fd, F_SETFL, flag);     //设置对应文件描述符上的flg信息,使其支持异步通知
                                  //这个函数实质上最终调用的是操作方法集中的.fasync标准接口，对应到驱动层中的相应函数
    while(1){
        printf("---------w: 1----------
");
    #define MAX 1024
        char buf[MAX];
        fgets(buf, MAX, stdin);         
        write(fd, buf, strlen(buf));
    }

    close(fd);

    return 0;
}