linux设备驱动开发之设备驱动的异步通知与异步IO

异步通知概念

当设备就绪的时候，主动通知应用程序。

设备驱动支持异步通知代码

index 452df8f..b0a1038 100644
@@ -40,6 +40,7 @@ struct globalfifo_dev
     struct mutex mutex;
     wait_queue_head_t r_wait;
     wait_queue_head_t w_wait;
+    struct fasync_struct *async_queue;
 };
 static struct globalfifo_dev *globalfifo_devp = NULL;
 
@@ -270,6 +271,11 @@ static ssize_t globalfifo_write (struct file *filep, const char __user *buf, siz
 #ifdef globalfifo_debug
     printk(KERN_DEBUG "globalfifo_write = %s\n", dev->mem);
 #endif
+        if (dev->async_queue)  // post readable signal when async
+        {
+            kill_fasync(&dev->async_queue, SIGIO, POLL_IN);
+            printk("%s kill SIGIO\n", __func__);
+        }
     }
 
 out:
@@ -347,8 +353,16 @@ static int globalfifo_open (struct inode *inode, struct file *filep)
     return 0;
 }
 
+static int globalfifo_fasync (int fd, struct file *filep, int mode)
+{
+    struct globalfifo_dev *dev = filep->private_data;
+    return fasync_helper(fd, filep, mode, &dev->async_queue);
+}
+
 static int globalfifo_release (struct inode *inode, struct file *filep)
 {
+    // remove async function form the async queue
+    globalfifo_fasync(-1, filep, 0);
     return 0;
 }
 
@@ -362,6 +376,7 @@ static  struct file_operations chrdev_file_operations =
     .open = globalfifo_open,
     .release = globalfifo_release,
     .poll = globalfifo_poll,
+    .fasync = globalfifo_fasync,  // binging the interface @fasync
 };
 
 /**

增加异步IO应用层编码

/**
 ** This file is part of the LinuxTrainningCompany project.
 ** Copyright (C) duanzhonghuan Co., Ltd.
 ** All Rights Reserved.
 ** Unauthorized copying of this file, via any medium is strictly prohibited
 ** Proprietary and confidential
 **
 ** Written by duanzhonghuan <15818411038@163.com>, 2019/5/18
 **/

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


void signal_handle(int signo)
{
    printf("Have caught the Signal: %d\n", signo);
    exit(0);
}

int main(void)
{
    int fd, oflags;
    fd = open("/dev/globalfifo", O_RDWR);
    if (fd != -1)
    {
        // if the signal @SIGIO is captured,
        // the function of @signal_handle is called
        signal(SIGIO, signal_handle);
        // set one of the @fd owners as the process
        fcntl(fd, F_SETOWN, getpid());
        // receive the asynchronous sinals
        oflags = fcntl(fd, F_GETFL);
        fcntl(fd, F_SETFL, oflags | O_ASYNC);
        while(1)
        {
            sleep(1);
        }
    }
    else
    {
        printf("open /dev/globalfifo failed \n");
    }
#if 0
    signal(SIGINT, signal_handle);
    signal(SIGTERM, signal_handle);
    while(1) {}
#endif
    return 0;
}

由此可见,为了能在用户空间中处理一个设备释放的信号,它必须完成3项工作。
1)通过F_SETOWN IO控制命令设置设备文件的拥有者为本进程,这样从设备驱动发出的信号才能被本进程接收到。
2)通过F_SETFL IO控制命令设置设备文件以支持FASYNC,即异步通知模式。
3)通过signal()函数连接信号和信号处理函数。

AIO

	ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
	ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
	int (*aio_fsync) (struct kiocb *, int datasync);

有关AIO的介绍可参考Asynchronous I/O and vectored operations

总结

1. 使用信号可以实现设备驱动与用户程序之间的异步通知。我们利用这个特性与设备驱动进行通信。
2. 关于AIO，drivers/char/mem.c里实现的null、zero驱动是通过AIO机制实现的。