简单的字符设备驱动

下面是一个最简单的字符设备驱动。

#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/slab.h>
#include <linux/uaccess.h>


static unsigned int simple_dev_major_num = 222;

module_param(simple_dev_major_num, uint, S_IRUGO);

struct simple_cdev{
    struct cdev scdev;
    unsigned char data;
};

static struct simple_cdev *psimle_cdev = NULL;


int simple_cdev_open(struct inode *pinode, struct file *pfile)
{
    return 0;
}

int simple_cdev_close(struct inode *pinode, struct file *pfile)
{
    return 0;
}


static const struct file_operations simple_cdev_fopt = {
    .owner = THIS_MODULE,
    .open = simple_cdev_open,
    .release =simple_cdev_close,
};

static int __init simple_cdev_init(void)
{
    int ret;
    dev_t devno = MKDEV(simple_dev_major_num, 0);
    
    //申请/注册设备号
    if ( 0 == simple_dev_major_num )
    {
        ret = register_chrdev_region(devno, 1, "simple cdev");
    }
    else
    {
        ret = alloc_chrdev_region(&devno, 0, 1, "simple cdev");
        simple_dev_major_num = devno;
    }

    if ( ret < 0 )
    {
        printk(KERN_ERR "simple cdev get device num error
");
        return ret;
    }
    
    psimle_cdev = kzalloc(sizeof(struct simple_cdev), GFP_KERNEL);
    if ( !psimle_cdev )
    {
        printk(KERN_ERR "simple cdev get memory failed
");
        unregister_chrdev_region(devno, 1);
        return -ENOMEM;
    }
    
    //初始化字符设备
    cdev_init(&psimle_cdev->scdev, &simple_cdev_fopt);
    
    psimle_cdev->scdev.owner = THIS_MODULE;
    
    //添加设备
    ret = cdev_add(&psimle_cdev->scdev, devno,  1);
    if ( ret < 0 )
    {
        printk(KERN_ERR "add cdev failed
");
        unregister_chrdev_region(devno, 1);
        return ret;

    }
    
	printk(KERN_INFO "simple cdev enter
");
	return 0;
}
module_init(simple_cdev_init);

static void __exit simple_cdev_exit(void)
{
    printk(KERN_INFO "simple cdev exit
 ");
    if ( NULL != psimle_cdev )
    {
        cdev_del(&psimle_cdev->scdev);
        kfree(psimle_cdev);
    }
    unregister_chrdev_region(simple_dev_major_num, 1);
}
module_exit(simple_cdev_exit);

MODULE_AUTHOR("thammer@163.com");
MODULE_LICENSE("GPL v2");

其中包含了几个关键的结构体struct file,struct inode,struct cdev,struct file_operations。这几个结构体将应用层对设备文件的open,read,write等操作和实际的设备驱动关联起来。下图展示了字符设备驱动和文件系统的关系。

通常一个具体的字符设备会以下面的方式描述：

struct simple_cdev{
    struct cdev scdev;
    unsigned char data;
	...
	...
};

相当于struct cdev是struct simple_cdev的基类，如果用C++来描述：

class cdev{
};

class simple_cdev::cdev{
    unsigned char data;
	...
	...
};

struct cdev的几个关键操作函数包含在文件 linux/fs/char_dev.c文件中

int register_chrdev_region(dev_t from, unsigned count, const char *name);
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
			const char *name);
void unregister_chrdev_region(dev_t from, unsigned count);
void cdev_init(struct cdev *cdev, const struct file_operations *fops);
int cdev_add(struct cdev *p, dev_t dev, unsigned count);
void cdev_del(struct cdev *p);

他们通过SYMBOL_EXPORT导出符号链接，以供其它LKM使用：

EXPORT_SYMBOL(register_chrdev_region);
EXPORT_SYMBOL(unregister_chrdev_region);
EXPORT_SYMBOL(alloc_chrdev_region);
EXPORT_SYMBOL(cdev_init);
EXPORT_SYMBOL(cdev_alloc);
EXPORT_SYMBOL(cdev_del);
EXPORT_SYMBOL(cdev_add);

前面三个x__chrdev_region函数都是和设备号申请，释放相关的操作函数。cdev_init将struct cdev和设备驱动里面定义的struct file_operations相关联。其实就是将struct cdev的成员ops赋值为指定的fops。cdev_add， cdev_del将设备向/从系统添加/删除，具体应该是kobject组织起来的相关的数据结构，暂时对kobject还不了解，不做分析。

struct cdev {
	struct kobject kobj;
	struct module *owner;
	const struct file_operations *ops;
	struct list_head list;
	dev_t dev;
	unsigned int count;
};