linux驱动开发之字符设备--自动创建设备节点

mdev

在/dev下创建设备的方法有两种，一种是手动的使用 mknod 进行创建。另外一种是使用 mdev 进行自动的创建设备。

mdev集成在busybox中，mdev 会自动检测 /sys/class 和 /sys/block 的所有类设备目录；如果在目录中含有名为“dev”的文件，同时如果内容是设备号的话，mdev
就会利用这些信息，在/dev下创建这个设备节点。

内核API

创建类

#define class_create(owner, name)       \({                      \    static struct lock_class_key __key; \    __class_create(owner, name, &__key);    \})

参数：

• owner: THIS_MODULE
• name :在/sys/class/下显示的name

创建设备

struct device *device_create(struct class *class, struct device *parent,                 dev_t devt, void *drvdata, const char *fmt, ...)

参数：

• class : 创建的类，
• parent: 父类。通常为NULL
• devt : 设备号
• drvdata : 通常为NULL
• fmt： 可变参数，通常为 设备的名字

有类、设备的创建，就有类、设备的销毁

void class_destroy(struct class *cls)

void device_destroy(struct class *class, dev_t devt)

实例

#include <linux/init.h>#include <linux/module.h>#include <linux/kernel.h>#include <linux/cdev.h>#include <linux/fs.h>#include <linux/device.h>#include <linux/sysfs.h>#include <linux/kobject.h>#include <linux/uaccess.h>//幻数#define IOCTL_TYPE 'b'#define COUNT   32struct  ioctl_arg {    int val;    char buf[COUNT];};//定义的命令码#define CMDCTL  _IO(IOCTL_TYPE,0)#define CMDR    _IOR(IOCTL_TYPE,1,struct ioctl_arg)#define CMDW    _IOW(IOCTL_TYPE,2,struct ioctl_arg)#define DEVICE_CLASS_NAME   "cdev_demo_class"#define DEVICE_NAME "cdev_demo"static struct cdev *pdev = NULL;static int major = 0;static int minor = 0;static int count = 2;static struct  class  *cdev_demo_class;struct kobject *cdev_obj;#define BUF_SIZE        (1024)static char kbuf[BUF_SIZE];static int  buf_count = 0;static int cdev_demo_open(struct inode * inode, struct file * file){    printk("%s,%d\n",__func__,__LINE__);    return 0;}static int cdev_demo_release(struct inode *inode, struct file * file){    printk("%s,%d\n",__func__,__LINE__);    return 0;}static ssize_t cdev_demo_read(struct file * file, char __user * buffer, size_t size, loff_t * loff){    printk("%s,%d\n",__func__,__LINE__);    if(0 == buf_count){        return -EAGAIN;    }    if(buf_count < size){        size = buf_count;    }    if(size == copy_to_user(buffer,kbuf,size)){        return -EAGAIN;    }    buf_count  = 0;    return size;}static ssize_t cdev_demo_write(struct file * file, const char __user * buffer, size_t size, loff_t * loff){    printk("%s,%d\n",__func__,__LINE__);    printk("buffer=%s   size=%d\n",buffer,size);    if(size >BUF_SIZE){        return -ENOMEM;    }    if(size == copy_from_user(kbuf,buffer,size)){        return -EAGAIN;    }    buf_count = size;    return size;}//ioctlstatic long cdev_demo_ioctl (struct file *filep, unsigned int cmd, unsigned long arg){    static struct ioctl_arg buf;    printk("%s,%d\n",__func__,__LINE__);    //分辨不同命令码    switch(cmd){        case CMDCTL:                printk("do CMDCTL\n");                break;        case CMDR:            //使用 _IOC_SIZE()获得命令码中的数据长度            if(sizeof(buf) != _IOC_SIZE(cmd)){                return -EINVAL;            }            if(sizeof(buf) == copy_to_user((struct ioctl_arg*)arg,&buf,sizeof(struct ioctl_arg))){                return -EAGAIN;            }            printk("do CMDR\n");            break;        case CMDW:            if(sizeof(buf)!= _IOC_SIZE(cmd)){                 return -EINVAL;            }            if(sizeof(buf) == copy_from_user(&buf,(struct ioctl_arg*)arg,sizeof(buf))){                return -EAGAIN;            }            printk("do CMDW\n");            printk("%d,%s \n",buf.val,buf.buf);            break;        default:            break;    }    return 0;}static struct file_operations fops ={    .owner   = THIS_MODULE,    .open    = cdev_demo_open,    .release = cdev_demo_release,    .read    = cdev_demo_read,    .write   = cdev_demo_write,    .unlocked_ioctl = cdev_demo_ioctl,};char cdev_buf[2] = "a";static ssize_t cdev_demo_show(struct device *dev,struct device_attribute *attr, char *buf)  {    char *s = buf;    s = sprintf(s,"%s",cdev_buf);    return sizeof(s);}static ssize_t cdev_demo_store(struct device *dev, struct device_attribute *attr, const char *buf, size_t count){    memcpy(cdev_buf,buf,count);    return count;}static DEVICE_ATTR(cdev_demo, 0666, cdev_demo_show, cdev_demo_store);static struct attribute  *g[] = {    &dev_attr_cdev_demo.attr,    NULL,};static struct attribute_group attr_group = {    .attrs = g,};static int __init cdev_demo_init(void){    dev_t dev;    int ret;    int i;    struct device * cdemo_device;    printk("%s,%d\n",__func__,__LINE__);    pdev = cdev_alloc();    if(NULL == pdev){        printk("cdev_alloc failed.\n");        return -ENOMEM;    }    cdev_init(pdev,&fops);    ret = alloc_chrdev_region(&dev,minor,count,DEVICE_NAME);    if(ret){        printk("alloc_chrdev_region failed.\n");        goto ERROR_CDEV;    }    major = MAJOR(dev);    ret = cdev_add(pdev, dev,count);    if(ret) {        printk("cdev_add failed.\n");        goto  ERROR_ADD;    }    cdev_obj = kobject_create_and_add("cdev_demo",NULL);    if(!cdev_obj){        return -ENOMEM;    }    ret = sysfs_create_group(cdev_obj,&attr_group);    if(ret)        goto SYSFS_ERR;    cdev_demo_class = class_create(THIS_MODULE,DEVICE_CLASS_NAME);    if(IS_ERR(cdev_demo_class)){        ret = PTR_ERR(cdev_demo_class);         printk("Unable to create class, err = %d\n", ret);        goto  CLASS_ERROR;    }    for(i = minor; i < minor + count; i ++){        cdemo_device = device_create(cdev_demo_class,NULL,MKDEV(major,i),NULL,"%s%d",DEVICE_NAME,i);        if(NULL == cdemo_device){            ret = -EIO;             goto DEVICE_ERROR;        }    }    return 0;DEVICE_ERROR:    for(i-- ; i  >= minor ; i--){        device_destroy(cdev_demo_class, MKDEV(major,i));    }        class_destroy(cdev_demo_class);CLASS_ERROR:    sysfs_remove_group(cdev_obj,&attr_group);SYSFS_ERR:    kobject_del(cdev_obj);    kobject_put(cdev_obj);ERROR_ADD:    unregister_chrdev_region(dev,count);ERROR_CDEV:    cdev_del(pdev);    return ret;}static void __exit cdev_demo_exit(void){    int i;    printk("%s,%d\n",__func__,__LINE__);    for(i = minor;i <count + minor;i ++){    device_destroy(cdev_demo_class, MKDEV(major,i));    }    class_destroy(cdev_demo_class);    sysfs_remove_group(cdev_obj,&attr_group);    kobject_del(cdev_obj);    kobject_put(cdev_obj);    unregister_chrdev_region(MKDEV(major,minor),count);    cdev_del(pdev);}module_init(cdev_demo_init);module_exit(cdev_demo_exit);MODULE_LICENSE("GPL");

结果:

//创建的类shell@tiny4412:/sys/class/cdev_demo_class # ls -lalrwxrwxrwx root     root              2014-01-01 20:01 cdev_demo ->../../devices/virtual/cdev_demo_class/cdev_demo

// 类下/dev的内容shell@tiny4412:/sys/class/cdev_demo_class/cdev_demo # cat dev247:0

//在/dev/下创建的设备ls -la /dev/cdev_demo                                                          crw------- root     root     247,   0 2014-01-01 20:01 cdev_demo

在创建设备的时候，没有指定parent，则设备创建在一下目录

/sys/device/vitual/cdev_demo_class/

参考文献

浅析为什么device_create()生成文件添加到/sys/devices/virtual/目录