linux device driver 3 简单的字符驱动

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ldd 第三章

鉴于书中附带代码是综合性的，虽然很简单，但是对于初学者来说还是有点复杂，

我把作者的例子进行了一些简化，更有利于看清最重要的东西。也有利于一步步的学习，循序渐进。

首先，了解一下涉及到数据结构。

 

 

我们简化一下ldd中的设计，如下，

下面，我简要的介绍一下，例子中的函数的作用：

function

mydev_init_module:

申请设备号 register_chrdev_region or alloc_chrdev_region

分配，初始化设备，并添加设备

分配：静态或者 cdev_alloc

初始化：cdev_init

添加：cdev_add

mydev_clearup_module :

释放分配的用于存放数据的内存

删除设备cdev_del

释放“设备结构体”所占内存

释放申请的设备号

 

mydev_llseek:

修改文件的当前的读写位置(f_pos值)

mydev_open:

保存”设备“指针到文件结构体的private_data域

mydev_release:

open 的逆操作

mydev_read:

将写入的数据按照一定规则读出

mydev_write

讲用户数据保存到某个位置（内存或者硬件设备）

下面是简化后的代码结构

macro

MYDEBUG

ELEMENT_SIZE

 

struct

dataelem

mydev

 

variable

mydev0

mydev_major

mydev_minor

element_size

mydev_ops

mydev_init_module

mydev_clearup_module

 

function

mydev_init_module

mydev_setup_cdev

mydev_clearup_module

mydev_free_mem

mydev_llseek

mydev_open

mydev_release

mydev_read

mydev_write

 

说明：本例子为了更突出重点，而很简陋，重复打开设备的结果是不可预料的

 

 

源码：

//mydev0.c#include <linux/init.h>#include <linux/module.h>#include <linux/config.h>#include <linux/moduleparam.h>#include <linux/kernel.h> /* printk() */#include <linux/slab.h> /* kmalloc() */#include <linux/fs.h> /* everything... */#include <linux/errno.h> /* error codes */#include <linux/types.h> /* size_t */#include <linux/proc_fs.h>#include <linux/fcntl.h> /* O_ACCMODE */#include <linux/seq_file.h>#include <linux/cdev.h>#include <asm/system.h> /* cli(), *_flags */#include <asm/uaccess.h> /* copy_*_user */#define MYDEBUG 1 // For debug 1:print more information#define ELEMENT_SIZE 1024 //dataqueue struct data's size//Data queue elementstruct dataelem {void* data;//Pointer to the mem used to store your datastruct dataelem *next;};//My charactor devicestruct mydev {struct dataelem * data;//pointed to the first data elementunsigned long size;//The total data number(byte)struct cdev cdev;//characor device};///global variablesstruct mydev* mydev0;//pointer to mydevint mydev_major = 0; // major device numberint mydev_minor = 0; // minor numberint element_size = ELEMENT_SIZE;// function declarestruct dataelem* mydev_get_elem(loff_t *f_pos, struct file *filep);int mydev_init_module(void);ssize_t mydev_write (struct file* filep, const char __user * buf,size_t count,loff_t* f_pos);ssize_t mydev_read (struct file * filep, char __user * buf, size_t count, loff_t* f_pos);int mydev_release (struct inode* inode, struct file *filep);int mydev_open (struct inode *inode, struct file * filep);loff_t mydev_llseek (struct file* filep, loff_t off, int whence);void mydev_free_mem (struct mydev* mydev);void mydev_clearup_module (void);void mydev_setup_cdev (struct mydev* mydev);struct file_operations mydev_ops = {.owner = THIS_MODULE,.llseek = mydev_llseek,.read = mydev_read,.write = mydev_write,.open = mydev_open,.release = mydev_release,};int mydev_init_module(void){int result = 0;dev_t dev;result = alloc_chrdev_region(&dev, 0/*minor*/, 1/*dev nums*/,"mydev");mydev_major = MAJOR(dev);//store the major and minor number of our devicemydev_minor = MINOR(dev);if (result < 0) {printk (KERN_WARNING "mydev: alloc major error!/n");return result;}#ifdef MYDEBUGelse {printk (KERN_WARNING "mydev: major is %d/n", mydev_major);}#endifmydev0 = kmalloc (sizeof(struct mydev), GFP_KERNEL);if (!mydev0) {result = -ENOMEM;goto fail;}memset(mydev0, 0, sizeof(struct mydev));/* Init our device */mydev0->data = NULL;mydev0->size = 0;mydev_setup_cdev (mydev0);//init and add cdevreturn 0;fail:mydev_clearup_module ();#ifdef MYDEBUG{printk (KERN_WARNING "mydev_init_module success./n");}#endifreturn result;}/* Initialize the mydev.cdev item */void mydev_setup_cdev (struct mydev* mydev){int err, devno = MKDEV(mydev_major, mydev_minor);cdev_init(&mydev->cdev, &mydev_ops);mydev->cdev.owner = THIS_MODULE;mydev->cdev.ops = &mydev_ops;err = cdev_add (&mydev->cdev, devno, 1);if (err < 0) {printk (KERN_NOTICE "mydev setup error! /n");}#ifdef MYDEBUG{printk (KERN_WARNING "mydev_setup_cdev success./n");}#endif}/* free the device mem and free the device struct */void mydev_clearup_module (void){dev_t dev = MKDEV(mydev_major, mydev_minor);//free device memif (mydev0) {mydev_free_mem (mydev0);cdev_del (&mydev0->cdev);}// unregister charactor number regionunregister_chrdev_region(dev, 1);//free device structkfree (mydev0);#ifdef MYDEBUG{printk (KERN_WARNING "mydev_clearup_module success./n");}#endif}/* Free the memery allocate for the device to store data */void mydev_free_mem (struct mydev* mydev){struct dataelem *p = mydev->data;if (p) kfree(p);mydev->data = NULL;#ifdef MYDEBUG{printk (KERN_WARNING "mydev_free_mem success./n");}#endif}loff_t mydev_llseek (struct file* filep, loff_t off, int whence){struct mydev *dev = filep->private_data;loff_t newpos;switch(whence) {case 0:/* SEEK_SET */newpos = off;break;case 1:/* SEEK_CUR */newpos = filep->f_pos + off;break;case 2:/* SEEK_END */newpos = dev->size + off;break;default:return -EINVAL;}if (newpos < 0) return -EINVAL;filep->f_pos = newpos;#ifdef MYDEBUG{printk (KERN_WARNING "mydev_free_mem success./n");}#endifreturn newpos;}int mydev_open (struct inode *inode, struct file * filep){struct mydev *dev;dev = container_of (inode->i_cdev, struct mydev, cdev);filep->private_data = dev;/* If open was write-only, trim to 0 */if ((filep->f_flags & O_ACCMODE) == O_WRONLY) {mydev_free_mem (dev);}#ifdef MYDEBUGprintk(KERN_WARNING "function %s is called success /n",__func__);#endifreturn 0;}int mydev_release (struct inode* inode, struct file *filep){#ifdef MYDEBUGprintk(KERN_WARNING "function %s is called success /n",__func__);#endifreturn 0;}ssize_t mydev_read (struct file * filep, char __user * buf, size_t count,loff_t *f_pos){struct mydev* dev = filep->private_data;int elem_offset = (long)*f_pos % element_size;//*f_pos = 0;//for testif (!dev->data || ((long)*f_pos > element_size)) return -EINVAL; // no data or//parament errorif ((*f_pos + count) > dev->size) {// read only up to the end of dev->data[element_size]count = element_size - *f_pos;}//count = 3; //for testprintk (KERN_WARNING "before read count is %d/nbefore read *f_pos is %ld/n", (int)count,(long)*f_pos);if (copy_to_user(buf, dev->data + elem_offset/*f_pos + elem_offset*/, count)) {printk (KERN_WARNING "read error./n");return -EFAULT;}//updat the f_pos*f_pos += count;#ifdef MYDEBUGprintk(KERN_WARNING "after read count is %d /nelem_offset is %d/n",(int) count, elem_offset);printk(KERN_WARNING "function %s is called success /n",__func__);#endifreturn count;}ssize_t mydev_write (struct file* filep, const char __user* buf, size_t count,loff_t* f_pos){struct mydev* dev = filep->private_data;int elem_offset = (long)*f_pos % element_size;//here elem_offset is eaqul to f_posif ((long)*f_pos > element_size) {printk (KERN_WARNING "mydev_wrtie parameter error! /n");return -EINVAL;}if (dev->data == NULL) {//allocate fixed size memery at the first timedev->data = kmalloc (element_size, GFP_KERNEL);if (!dev->data) return -ENOMEM;memset (dev->data, 0, element_size);//clear the fresh memery}printk (KERN_WARNING "count before write is %d/n", (int)count);if (*f_pos + count > element_size) {//make sure data writing willn't overflowcount = element_size - *f_pos;}/* store data to the memery allocated before */if (copy_from_user(dev->data + elem_offset, buf, count)){printk (KERN_WARNING "data write error!/n");return -EFAULT;}//update the f_pos*f_pos += count;//update data sizedev->size += *f_pos;#ifdef MYDEBUGprintk(KERN_WARNING "function %s is called success /n",__func__);printk(KERN_WARNING "write count is %d", (int)count);#endifreturn count;}MODULE_LICENSE("Dual BSD/GPL");module_init(mydev_init_module);module_exit(mydev_clearup_module);s// Makefileobj-m := mydev0.odefault:make -C /lib/modules/${shell uname -r}/build M=${shell pwd} modulesclean:rm -f modules.order Module.symvers mydev0.ko mydev0.mod.c mydev0.mod.o mydev0.o

 

操作过程：

 

# make //生成mydev0.ko

# insmod mydev0.ko //插入生成的模块

# cat /proc/devices //查看分配的主设备号

...

250 mydev <-----我们的模块

251 hidraw

252 usbmon

...

# mknod /dev/mydev0 c 250 0 //创建设备结点

# echo “” > /var/log/messages //清空记录方便查看

# echo “ I love you Linux” >/dev/mydev0 //向设备中写入数据

#cat /dev/mydev0 //从设备中读出数据

#cat /var/log/messages //查看打印出来的调试信息