Linux设备驱动学习(4) 阻塞式PIPE内存“设备” 休眠+异步通知 scullpipe

r囧r小猫 2022-06-13 07:56 185阅读 0赞

这是scull类设备的阻塞方式，之前的scull设备中的读进程若读到内存空间无数据，则立即返回。  
scullpipe包含两个等待队列和一个缓冲区。  
写进程在“前面”写，读进程在“后面”读。  
在scull\_pipe设备中，缓冲区为空，读进程会阻塞自己，直到缓冲区内有数据后由内核唤醒，读完数据后唤醒所有写进程。  
写进程也相对应，当缓冲区满的时候阻塞，有空闲空间则被唤醒，写完数据后异步通知读进程。

/*
     * pipe.c -- fifo driver for scull
     *
     * Copyright (C) 2001 Alessandro Rubini and Jonathan Corbet
     * Copyright (C) 2001 O'Reilly & Associates
     *
     * The source code in this file can be freely used, adapted,
     * and redistributed in source or binary form, so long as an
     * acknowledgment appears in derived source files.  The citation
     * should list that the code comes from the book "Linux Device
     * Drivers" by Alessandro Rubini and Jonathan Corbet, published
     * by O'Reilly & Associates.   No warranty is attached;
     * we cannot take responsibility for errors or fitness for use.
     *
     */
     
    #include <linux/module.h>
    #include <linux/moduleparam.h>
    
    #include <linux/kernel.h>	/* printk(), min() */
    #include <linux/slab.h>		/* kmalloc() */
    #include <linux/fs.h>		/* everything... */
    #include <linux/proc_fs.h>
    #include <linux/errno.h>	/* error codes */
    #include <linux/types.h>	/* size_t */
    #include <linux/fcntl.h>
    #include <linux/poll.h>
    #include <linux/cdev.h>
    #include <asm/uaccess.h>
    
    #include "scull.h"		/* local definitions */
    
    struct scull_pipe {
            wait_queue_head_t inq, outq;       /* read and write queues *///inq是读设备的进程的等待队列，outq是写设备的进程的等待队列
            char *buffer, *end;                /* begin of buf, end of buf */
            int buffersize;                    /* used in pointer arithmetic */缓冲区大小
            char *rp, *wp;                     /* where to read, where to write *///读指针和写指针
            int nreaders, nwriters;            /* number of openings for r/w *///读写的进程数目
            struct fasync_struct *async_queue; /* asynchronous readers *///用于通知读进程的异步结构体
            struct semaphore sem;              /* mutual exclusion semaphore */
            struct cdev cdev;                  /* Char device structure */
    };
    
    /* parameters */
    static int scull_p_nr_devs = SCULL_P_NR_DEVS;	/* number of pipe devices */
    int scull_p_buffer =  SCULL_P_BUFFER;	/* buffer size */
    dev_t scull_p_devno;			/* Our first device number */
    
    module_param(scull_p_nr_devs, int, 0);	/* FIXME check perms */
    module_param(scull_p_buffer, int, 0);
    
    static struct scull_pipe *scull_p_devices;
    
    static int scull_p_fasync(int fd, struct file *filp, int mode);
    static int spacefree(struct scull_pipe *dev);
    /*
     * Open and close
     */
    
    
    static int scull_p_open(struct inode *inode, struct file *filp)//打开文件
    {
    	struct scull_pipe *dev;
    
    	dev = container_of(inode->i_cdev, struct scull_pipe, cdev);
    	filp->private_data = dev;
    
    	if (down_interruptible(&dev->sem))
    		return -ERESTARTSYS;
    	if (!dev->buffer) {
    		/* allocate the buffer *///若缓冲区不存在，则分配缓冲区，由第一个打开文件的进程执行
    		dev->buffer = kmalloc(scull_p_buffer, GFP_KERNEL);
    		if (!dev->buffer) {
    			up(&dev->sem);
    			return -ENOMEM;
    		}
    	}
    	dev->buffersize = scull_p_buffer;
    	dev->end = dev->buffer + dev->buffersize;//设置尾指针
    	dev->rp = dev->wp = dev->buffer; /* rd and wr from the beginning */
    
    	/* use f_mode,not  f_flags: it's cleaner (fs/open.c tells why) */
    	if (filp->f_mode & FMODE_READ)//读进程个数
    		dev->nreaders++;
    	if (filp->f_mode & FMODE_WRITE)//写进程个数
    		dev->nwriters++;
    	up(&dev->sem);
    
    	return nonseekable_open(inode, filp);//通知内核，设备不支持llseek操作，默认内核允许设备定位lseek
    }
    
    
    
    static int scull_p_release(struct inode *inode, struct file *filp)
    {
    	struct scull_pipe *dev = filp->private_data;
    
    	/* remove this filp from the asynchronously notified filp's */
    	scull_p_fasync(-1, filp, 0);//将此文件描述符(进程)从异步通知描述符集合中删除
    	down(&dev->sem);
    	if (filp->f_mode & FMODE_READ)
    		dev->nreaders--;
    	if (filp->f_mode & FMODE_WRITE)
    		dev->nwriters--;
    	if (dev->nreaders + dev->nwriters == 0) {
    		kfree(dev->buffer);//设备上没有读写进程了则释放设备文件
    		dev->buffer = NULL; /* the other fields are not checked on open */
    	}
    	up(&dev->sem);
    	return 0;
    }
    
    
    /*
     * Data management: read and write
     */
    
    static ssize_t scull_p_read (struct file *filp, char __user *buf, size_t count,
                    loff_t *f_pos)
    {
    	struct scull_pipe *dev = filp->private_data;
    
    	if (down_interruptible(&dev->sem))
    		return -ERESTARTSYS;
    
    	while (dev->rp == dev->wp) { /* nothing to read */
    		up(&dev->sem); /* release the lock */
    		if (filp->f_flags & O_NONBLOCK)//
    			return -EAGAIN;
    		PDEBUG("\"%s\" reading: going to sleep\n", current->comm);
    		if (wait_event_interruptible(dev->inq, (dev->rp != dev->wp)))//条件为真则继续执行，否则进入等待队列dev->inq。读写指针不等即说明缓冲区内有数据
    			return -ERESTARTSYS; /* signal: tell the fs layer to handle it */
    		/* otherwise loop, but first reacquire the lock *///运行到这里，说明条件为真，缓冲区内有数据，进程取得信号量后读缓冲区
    		if (down_interruptible(&dev->sem))
    			return -ERESTARTSYS;
    	}
    	/* ok, data is there, return something */
    	if (dev->wp > dev->rp)
    		count = min(count, (size_t)(dev->wp - dev->rp));
    	else /* the write pointer has wrapped, return data up to dev->end */
    		count = min(count, (size_t)(dev->end - dev->rp));//若读指针在高地址，则读到缓冲区尾即止。
    	if (copy_to_user(buf, dev->rp, count)) {
    		up (&dev->sem);
    		return -EFAULT;
    	}
    	dev->rp += count;
    	if (dev->rp == dev->end)//读指针回卷
    		dev->rp = dev->buffer; /* wrapped */
    	up (&dev->sem);
    
    	/* finally, awake any writers and return */
    	wake_up_interruptible(&dev->outq);//唤醒写队列中的所有进程
    	PDEBUG("\"%s\" did read %li bytes\n",current->comm, (long)count);
    	return count;
    }
    
    /* Wait for space for writing; caller must hold device semaphore.  On
     * error the semaphore will be released before returning. */
    static int scull_getwritespace(struct scull_pipe *dev, struct file *filp)
    {
    	while (spacefree(dev) == 0) { /* full */
    		DEFINE_WAIT(wait);		//定义一个等待队列入口wait
    		
    		up(&dev->sem);
    		if (filp->f_flags & O_NONBLOCK)
    			return -EAGAIN;
    		PDEBUG("\"%s\" writing: going to sleep\n",current->comm);
    		prepare_to_wait(&dev->outq, &wait, TASK_INTERRUPTIBLE);//准备休眠等待，可能会在下面的schedule()中才休眠
    		if (spacefree(dev) == 0)//再次查看确认空间是空的
    			schedule();	//调度进程，当进程被调度回来时接着向下运行
    		finish_wait(&dev->outq, &wait);//完成休眠等待，这时候可能缓冲区已经有写的空间了
    		if (signal_pending(current))
    			return -ERESTARTSYS; /* signal: tell the fs layer to handle it */
    		if (down_interruptible(&dev->sem))//获得锁之后如果缓冲区已经有写的空间则跳出while循环，函数返回。
    			return -ERESTARTSYS;				
    	}
    	return 0;
    }	
    
    /* How much space is free? */
    static int spacefree(struct scull_pipe *dev)//返回（真正可用空间-1）的大小
    {
    	if (dev->rp == dev->wp)
    		return dev->buffersize - 1;
    	return ((dev->rp + dev->buffersize - dev->wp) % dev->buffersize) - 1;
    }
    
    static ssize_t scull_p_write(struct file *filp, const char __user *buf, size_t count,
                    loff_t *f_pos)
    {
    	struct scull_pipe *dev = filp->private_data;
    	int result;
    
    	if (down_interruptible(&dev->sem))	//先得到设备的信号量，因为接下来要访问设备
    		return -ERESTARTSYS;
    
    	/* Make sure there's space to write *///有空间写则返回0
    	result = scull_getwritespace(dev, filp);//如果函数返回则说明已经有写的空间了，并且获得了锁
    	if (result)
    		return result; /* scull_getwritespace called up(&dev->sem) */
    
    	/* ok, space is there, accept something */
    	count = min(count, (size_t)spacefree(dev));
    	if (dev->wp >= dev->rp)
    		count = min(count, (size_t)(dev->end - dev->wp)); /* to end-of-buf */
    	else /* the write pointer has wrapped, fill up to rp-1 */
    		count = min(count, (size_t)(dev->rp - dev->wp - 1));
    	PDEBUG("Going to accept %li bytes to %p from %p\n", (long)count, dev->wp, buf);
    	if (copy_from_user(dev->wp, buf, count)) {
    		up (&dev->sem);
    		return -EFAULT;
    	}
    	dev->wp += count;
    	if (dev->wp == dev->end)
    		dev->wp = dev->buffer; /* wrapped */
    	up(&dev->sem);
    
    	/* finally, awake any reader */
    	wake_up_interruptible(&dev->inq);  /* blocked in read() and select() */
    
    	/* and signal asynchronous readers, explained late in chapter 5 */
    	if (dev->async_queue)//如果有异步的读进程
    		kill_fasync(&dev->async_queue, SIGIO, POLL_IN);//向异步队列发送SIGIO信号，POLL_IN标识设备可读
    	PDEBUG("\"%s\" did write %li bytes\n",current->comm, (long)count);
    	return count;
    }
    
    static unsigned int scull_p_poll(struct file *filp, poll_table *wait)//设备提供轮询功能，用户态程序可以使用select来轮询设备
    {
    	struct scull_pipe *dev = filp->private_data;
    	unsigned int mask = 0;
    
    	/*
    	 * The buffer is circular; it is considered full
    	 * if "wp" is right behind "rp" and empty if the
    	 * two are equal.
    	 */
    	down(&dev->sem);
    	poll_wait(filp, &dev->inq,  wait);//把读等待队列添加到poll_table wait中
    	poll_wait(filp, &dev->outq, wait);//把写等待队列添加到poll_table wait中
    
    	if (dev->rp != dev->wp)
    		mask |= POLLIN | POLLRDNORM;	/* readable */
    	if (spacefree(dev))
    		mask |= POLLOUT | POLLWRNORM;	/* writable */
    	up(&dev->sem);
    	return mask;
    }
    
    
    
    
    
    static int scull_p_fasync(int fd, struct file *filp, int mode)
    {
    	struct scull_pipe *dev = filp->private_data;
    
    	return fasync_helper(fd, filp, mode, &dev->async_queue);
    }
    
    
    
    /* FIXME this should use seq_file */
    #ifdef SCULL_DEBUG
    static void scullp_proc_offset(char *buf, char **start, off_t *offset, int *len)
    {
    	if (*offset == 0)
    		return;
    	if (*offset >= *len) {	/* Not there yet */
    		*offset -= *len;
    		*len = 0;
    	}
    	else {			/* We're into the interesting stuff now */
    		*start = buf + *offset;
    		*offset = 0;
    	}
    }
    
    
    static int scull_read_p_mem(char *buf, char **start, off_t offset, int count,
    		int *eof, void *data)
    {
    	int i, len;
    	struct scull_pipe *p;
    
    #define LIMIT (PAGE_SIZE-200)	/* don't print any more after this size */
    	*start = buf;
    	len = sprintf(buf, "Default buffersize is %i\n", scull_p_buffer);
    	for(i = 0; i<scull_p_nr_devs && len <= LIMIT; i++) {
    		p = &scull_p_devices[i];
    		if (down_interruptible(&p->sem))
    			return -ERESTARTSYS;
    		len += sprintf(buf+len, "\nDevice %i: %p\n", i, p);
    /*		len += sprintf(buf+len, "   Queues: %p %p\n", p->inq, p->outq);*/
    		len += sprintf(buf+len, "   Buffer: %p to %p (%i bytes)\n", p->buffer, p->end, p->buffersize);
    		len += sprintf(buf+len, "   rp %p   wp %p\n", p->rp, p->wp);
    		len += sprintf(buf+len, "   readers %i   writers %i\n", p->nreaders, p->nwriters);
    		up(&p->sem);
    		scullp_proc_offset(buf, start, &offset, &len);
    	}
    	*eof = (len <= LIMIT);
    	return len;
    }
    
    
    #endif
    
    
    
    /*
     * The file operations for the pipe device
     * (some are overlayed with bare scull)
     */
    struct file_operations scull_pipe_fops = {
    	.owner =	THIS_MODULE,
    	.llseek =	no_llseek,//特殊的辅助函数，定义在<linux/fs.h>
    	.read =		scull_p_read,
    	.write =	scull_p_write,
    	.poll =		scull_p_poll,
    	.ioctl =	scull_ioctl,
    	.open =		scull_p_open,
    	.release =	scull_p_release,
    	.fasync =	scull_p_fasync,
    };
    
    
    /*
     * Set up a cdev entry.
     */
    static void scull_p_setup_cdev(struct scull_pipe *dev, int index)
    {
    	int err, devno = scull_p_devno + index;
        
    	cdev_init(&dev->cdev, &scull_pipe_fops);
    	dev->cdev.owner = THIS_MODULE;
    	err = cdev_add (&dev->cdev, devno, 1);
    	/* Fail gracefully if need be */
    	if (err)
    		printk(KERN_NOTICE "Error %d adding scullpipe%d", err, index);
    }
    
     
    
    /*
     * Initialize the pipe devs; return how many we did.
     */
    int scull_p_init(dev_t firstdev)//以firstdev开始，为scullpipe设备分配scull_p_nr_devs个设备号，此函数在main.c中被调用。
    {
    	int i, result;
    				//这里连续分配了scull_p_nr_devs个设备号
    	result = register_chrdev_region(firstdev, scull_p_nr_devs, "scullp");
    	if (result < 0) {
    		printk(KERN_NOTICE "Unable to get scullp region, error %d\n", result);
    		return 0;
    	}
    	scull_p_devno = firstdev;
    	scull_p_devices = kmalloc(scull_p_nr_devs * sizeof(struct scull_pipe), GFP_KERNEL);
    	if (scull_p_devices == NULL) {
    		unregister_chrdev_region(firstdev, scull_p_nr_devs);
    		return 0;
    	}
    	memset(scull_p_devices, 0, scull_p_nr_devs * sizeof(struct scull_pipe));
    	for (i = 0; i < scull_p_nr_devs; i++) {//
    		init_waitqueue_head(&(scull_p_devices[i].inq));
    		init_waitqueue_head(&(scull_p_devices[i].outq));
    		init_MUTEX(&scull_p_devices[i].sem);
    		scull_p_setup_cdev(scull_p_devices + i, i);
    	}
    #ifdef SCULL_DEBUG
    	create_proc_read_entry("scullpipe", 0, NULL, scull_read_p_mem, NULL);
    #endif
    	return scull_p_nr_devs;
    }
    
    /*
     * This is called by cleanup_module or on failure.
     * It is required to never fail, even if nothing was initialized first
     */
    void scull_p_cleanup(void)
    {
    	int i;
    
    #ifdef SCULL_DEBUG
    	remove_proc_entry("scullpipe", NULL);
    #endif
    
    	if (!scull_p_devices)
    		return; /* nothing else to release */
    
    	for (i = 0; i < scull_p_nr_devs; i++) {
    		cdev_del(&scull_p_devices[i].cdev);
    		kfree(scull_p_devices[i].buffer);
    	}
    	kfree(scull_p_devices);
    	unregister_chrdev_region(scull_p_devno, scull_p_nr_devs);
    	scull_p_devices = NULL; /* pedantic */
    }

注意：open操作中假设分配了0x1~ox9共9个字节的buffer，那么open之后内存如图所示，注意此时dev->end所指的空间是无效内存。

![1333028097_6895.png][]

这里主要理解了read和write函数，read和write函数也是其阻塞实现的核心，其他函数会在后续的学习中来补充理解完整。

scullpipe设备提供了轮询的功能

测试代码如下

#include <sys/types.h>
    #include <sys/stat.h>
    #include <stdio.h>
    #include <fcntl.h>
    #include <unistd.h>
    #include <sys/time.h>
    
    #define CLEAR 0x1
    int main(int argc, char **argv)
    {
    	int fd;
    	char rd_ch[100];
    	fd_set rfds,wfds;//文件描述符集合
      
    	fd = open("/dev/my_scullpipe", O_RDONLY | O_NONBLOCK);
    	if (fd ==  - 1){
    	printf("ERROR:open\n");
    	}
    	else{
    		while(1)
    			{
    				FD_ZERO(&rfds);//先清空
    				FD_ZERO(&wfds);
    				FD_SET(fd, &rfds);//将fd加入到读文件描述符集合
    				FD_SET(fd, &wfds);//将fd加入到写文件描述符集合
    				//检查文件描述符fd，select监视文件描述符集合rfds和wfds
    				select(fd + 1, &rfds, &wfds, NULL, NULL);
    		
    				if(FD_ISSET(fd, &rfds)){//可读
    					printf("the device can be read\n");
    				}
    		
    				if(FD_ISSET(fd, &wfds)){//可写
    					printf("the device can be written\n");
    				}      
    			}
    	}
    	
    }

[1333028097_6895.png]: https://img-my.csdn.net/uploads/201203/29/1333028097_6895.png