linux驱动学习（四） linux字符设备驱动 cdev

下面开始学习linux字符设备驱动，也是linux驱动中最简单的驱动模块。

在内存中虚拟出一段空间作为字符设备，并为之编写些列的驱动程序。

字符设备驱动cdev中用到的两个重要的结构体如下，现补充下基本知识

一、cdev

/*
*内核源码位置
*linux2.6.38/include/linux/cdev.h
*/

struct cdev {
	struct kobject kobj;
	struct module *owner;   //一般初始化为：THIS_MODULE
	const struct file_operations *ops;   //字符设备用到的例外一个重要的结构体file_operations，cdev初始化时与之绑定
	struct list_head list;
	dev_t dev;  //主设备号24位 与次设备号8位，dev_t为32位整形
	unsigned int count;
};

二、file_operations

熟悉c语言文件编程的应该知道 read write等函数，这些函数都在file_operations中声明，在read等函数中实现与硬件相关的操作，这样就让具体的硬件设备与操作系统联系在了一起

/*
~/include/linux/fs.h
*/

struct file_operations {
	struct module *owner;
	loff_t (*llseek) (struct file *, loff_t, int);
	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
	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 (*readdir) (struct file *, void *, filldir_t);
	unsigned int (*poll) (struct file *, struct poll_table_struct *);
	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
	int (*mmap) (struct file *, struct vm_area_struct *);
	int (*open) (struct inode *, struct file *);
	int (*flush) (struct file *, fl_owner_t id);
	int (*release) (struct inode *, struct file *);
	int (*fsync) (struct file *, int datasync);
	int (*aio_fsync) (struct kiocb *, int datasync);
	int (*fasync) (int, struct file *, int);
	int (*lock) (struct file *, int, struct file_lock *);
	ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
	unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
	int (*check_flags)(int);
	int (*flock) (struct file *, int, struct file_lock *);
	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
	int (*setlease)(struct file *, long, struct file_lock **);
	long (*fallocate)(struct file *file, int mode, loff_t offset,
			  loff_t len);
};

光有这cdev与file_operations定义的结构体变量是不行的，显然要让他们做一些初始化工作，然后通过某个函数，让我们定义的这两个结构体变量与内核联系在一起，所以，调用内核的下列函数

文件名：char_dev.c

void cdev_init(struct cdev *cdev, const struct file_operations *fops)
{
	memset(cdev, 0, sizeof *cdev);
	INIT_LIST_HEAD(&cdev->list);
	kobject_init(&cdev->kobj, &ktype_cdev_default);
	cdev->ops = fops;
}

为cdev开辟内存空间，然后，将file_operations定义的变量fops赋值给cdev中的ops成员变量，这样，他们就紧密的连在一起了

以上才仅仅将cdev初始化，还未将其真正的添加到系统内核中，因此调用下列函数：

文件名：char_dev.c

int cdev_add(struct cdev *p, dev_t dev, unsigned count)
{
	p->dev = dev;
	p->count = count;
	return kobj_map(cdev_map, dev, count, NULL, exact_match, exact_lock, p);
}

有了以上的字符设备基础时候后，在开始看一下字符设备的基本结构，其实就是在hello word的基础之上添加了设备读、写、控制的函数。

头文件：一般包含下面几个

#include<linux/cdev.h>
#include<linux/module.h>
#include<linux/types.h>
#include<linux/fs.h>
#include<linux/errno.h>
#include<linux/mm.h>
#include<linux/sched.h>
#include<linux/init.h>
#include<asm/io.h>
#include<asm/system.h>
#include<asm/uaccess.h>

定义的cdev结构体与设备空间数据

/*习惯上将内部数据空间与cdev 绑定，与其封装*/
struct mychar_dev{
	struct cdev cdev;
	unsigned char mem[MYCHAR_MEM_SIZE];
};

/*一个实例*/
struct mychar_dev* mychar_devp;

然后是读、写、ioctl函数的实现

/*实现file_operations结构体体的open函数*/
int mychar_open(struct inode *inode,struct file * filp)

int mychar_release(struct inode *inode,struct file* filp);

/*read*/
ssize_t mychar_read(struct file *filp,char __user *buf,size_t size ,loff_t *ppos );

/*write*/
ssize_t mychar_write(struct file *filp ,const char __user *buf,size_t size,loff_t *ppos);

/*llseek*/
static loff_t mychar_llseek(struct file *filp,loff_t offset,int orig);

/*ioctl*/
int mychar_ioctl(struct inode * inodep ,struct file *filp ,unsigned int cmd ,unsigned long arg);

以上函数实现后将里赋值到file_operations中相应的函数成员变量

static const struct file_operations mychar_fops = {
	.owner = THIS_MODULE,
	.llseek = mychar_llseek,
	.read = mychar_read,
	.write = mychar_write,
	.ioctl = mychar_ioctl,
	.open = mychar_open,
	.release =mychar_release,
};

最后是init 与exit函数

/*init*/
static int __init mychar_init(void);

/*exit*/
static void __exit mychar_exit(void);

最后是

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("ghostyu");

module_param(mychar_major,int,S_IRUGO);

module_init(mychar_init);
module_exit(mychar_exit);

下面看一下完整的源码：

/*在内存中申请1k 大小的内存做为简单的一个设备来访问*/
/*一般包含的头文件*/
#include<linux/cdev.h>
#include<linux/module.h>
#include<linux/types.h>
#include<linux/fs.h>
#include<linux/errno.h>
#include<linux/mm.h>
#include<linux/sched.h>
#include<linux/init.h>
#include<asm/io.h>
#include<asm/system.h>
#include<asm/uaccess.h>

/*设备空间*/
#define MYCHAR_MEM_SIZE 0x0400
/*主设备号*/
#define MYCHAR_MAJOR	260
/*自定义的清除内存的命令*/
#define MYCHAR_MEN_CLR	0x01
/*主设备号变量*/
static int mychar_major = MYCHAR_MAJOR;

/*习惯上将内部数据空间与cdev 绑定，与其封装*/
struct mychar_dev{
	struct cdev cdev;
	unsigned char mem[MYCHAR_MEM_SIZE];
};

/*一个实例*/
struct mychar_dev* mychar_devp;

/*实现file_operations结构体体的open函数*/
int mychar_open(struct inode *inode,struct file * filp)
{
	filp->private_data = mychar_devp;
	return 0;
}
/*同上*/
int mychar_release(struct inode *inode,struct file* filp)
{
	return 0;
}

/*read*/
ssize_t mychar_read(struct file *filp,char __user *buf,size_t size ,loff_t *ppos )
{
	unsigned long p=*ppos;
	unsigned int count = size;
	int ret = 0;
	struct mychar_dev *dev = filp->private_data;
	if(p>MYCHAR_MEM_SIZE)
		return 0;
	if(count > MYCHAR_MEM_SIZE-p)
		count = MYCHAR_MEM_SIZE-p;
	if( copy_to_user(buf,(void*)(dev->mem+p),count)){
		ret= -EFAULT;
	}else{
		*ppos +=count;
		ret = count;
		printk(KERN_INFO "read %u bytes(s) from %1u\n",count,p);
	}
	return ret;
}
/*write*/
ssize_t mychar_write(struct file *filp ,const char __user *buf,size_t size,loff_t *ppos)
{
	unsigned long p=*ppos;
	unsigned int count=size;
	int ret = 0;
	struct mychar_dev *dev = filp->private_data;
	if(p > MYCHAR_MEM_SIZE)
		return 0;
	if(count > MYCHAR_MEM_SIZE-p)
		count = MYCHAR_MEM_SIZE-p;
	if(copy_from_user((void*)(dev->mem),buf,count)){
		ret = -EFAULT;
	}else{
		*ppos +=count;
		ret = count;
		printk(KERN_INFO "written %u byte(s) from %1u\n",count,p);
	}
	return ret;
}
/*llseek*/
static loff_t mychar_llseek(struct file *filp,loff_t offset,int orig)
{
	loff_t ret = 0;
	switch(orig){
	case 0:	/*相对于文件开始位置偏移*/
		if(offset < 0)
			ret = -EINVAL;
		break;
		if((unsigned int)offset > MYCHAR_MEM_SIZE){
			ret = -EINVAL;
			break;
		}
		filp->f_pos =(unsigned int )offset;
		ret = filp->f_pos;
		break;
	case 1:	/*相对于文件当前位置*/
		if((filp->f_pos+offset)>MYCHAR_MEM_SIZE){
			ret = -EINVAL;
			break;
		}
		if((filp->f_pos+offset)< 0){
			ret = -EINVAL;
			break;
		}
		filp->f_pos +=offset;
		ret = filp->f_pos;
		break;
	default:
		ret = - EINVAL;
		break;
	}
	return ret;
}
/*ioctl*/
int mychar_ioctl(struct inode * inodep ,struct file *filp ,unsigned int cmd ,unsigned long arg)
{
	struct mychar_dev *dev =filp->private_data;
	switch(cmd){
	case MYCHAR_MEM_CLR:
		memset(dev->mem,0,MYCHAR_MEM_SIZE);
		printk(KERN_INFO "mychar memery is set to zero\n");
		break;
	default:
		return -EINVAL;
	}
	return 0;
}
static const struct file_operations mychar_fops = {
	.owner = THIS_MODULE,
	.llseek = mychar_llseek,
	.read = mychar_read,
	.write = mychar_write,
	.ioctl = mychar_ioctl,
	.open = mychar_open,
	.release =mychar_release,
};

/*cdev结构初始化*/
static void mychar_setup_cdev(struct mychar_dev *dev,int index)
{
	int err;
	int devno = MKDEV(mychar_major,index);
	cdev_init(&dev->cdev,&mychar_fops);
	dev->cdev.owner = THIS_MODULE;
	err = cdev_add(&dev->cdev,devno,1);
	if(err)
		printk(KERN_NOTICE " Error %d adding mychar %d",err,index);

}


/*init*/
static int __init mychar_init(void)
{
	int result;
	dev_t devno = MKDEV(mychar_major,0);
	if(mychar_major)
		result = register_chrdev_region(devno,1,"mychar");
	else{
		result = alloc_chrdev_region(&devno,0,1,"mychar");
		mychar_major = MAJOR(devno);
	}
	if(result<0)
		return result;
	mychar_devp = kmalloc(sizeof(struct mychar_dev),GFP_KERNEL);
	if(!mychar_devp){
		result = -ENOMEM;
		goto fall_malloc;

	}
	memset(mychar_devp,0,sizeof(struct mychar_dev));
	mychar_setup_cdev(mychar_devp,0);
	return 0;

fall_malloc:
	unregister_chrdev_region(devno,1);
	return result;
}

/*exit*/
static void __exit mychar_exit(void)
{
	cdev_del(&mychar_devp->cdev);
	kfree(mychar_devp);
	unregister_chrdev_region(MKDEV(mychar_major,0),1);
}


MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("ghostyu");

module_param(mychar_major,int,S_IRUGO);

module_init(mychar_init);
module_exit(mychar_exit);

 

我个人觉得，如果不太会使用vim的ctag功能，可以在windows平台下source insight中编写linux驱动。

只要事先先建立一个linux内核源码的工程，在在这个工程中添加自己的linux驱动程序源码。

这样就实现了一个IDE，函数、变量、宏定义不全功能，语法着色等等非常方便，加快开发速度。