设备驱动中cdev(kernel-4.7)

linux系统将设备分为3类：字符设备、块设备、网络设备

1、字符设备：是指只能一个字节一个字节读写的设备，不能随机读取设备内存中的某一数据，读取数据需要按照先后数据。字符设备是面向流的设备，常见的字符设备有鼠标、键盘、串口、控制台和LED设备等。
2、块设备：是指可以从设备的任意位置读取一定长度数据的设备。块设备包括硬盘、磁盘、U盘和SD卡等。

每一个字符设备或块设备都在/dev目录下对应一个设备文件。
linux用户程序通过设备文件（或称设备节点）来使用驱动程序操作字符设备和块设备。

字符设备

字符设备的核心结构体是cdev，该结构体定义在include/linux/cdev.h中：

struct cdev {
    struct kobject kobj;    // 内嵌的kobject对象 
    struct module *owner;    // 所属模块
    const struct file_operations *ops;    // 文件操作结构体
    struct list_head list;   //linux内核所维护的链表指针
    dev_t dev;               //设备号
    unsigned int count;    //设备数目
};

其中dev_t是专为设备号定义的数据类型，dev为设备号，设备号中包含了主设备号和次设备号。一个字符设备或块设备都有一个主设备号和一个次设备号。主设备号用来标识与设备文件相连的驱动程序，用来反映设备类型。次设备号被驱动程序用来辨别操作的是哪个设备，用来区分同类型的设备。

在内核中，dev_t用来保存设备编号，包括主设备号和次设备号。dev_t是一个32位的数，其中12位用来表示主设备号，其余20位用来标识次设备号。
通过dev_t获取主设备号和次设备号使用下面的宏：

#define MINORBITS   20
#define MINORMASK   ((1U << MINORBITS) - 1)

#define MAJOR(dev)  ((unsigned int) ((dev) >> MINORBITS))//获得主设备号
#define MINOR(dev)  ((unsigned int) ((dev) & MINORMASK)) //获得次设备号
#define MKDEV(ma,mi)    (((ma) << MINORBITS) | (mi))   //由主次设备号得到设备号 

对于字符设备的访问是通过文件系统中的设备名称进行的。它们通常位于/dev目录下。如下：

xxx@ubuntu:~$ ls -l /dev/  
total 0  
brw-rw----  1 root disk        7,   0  12月 25 10:34 loop0  
brw-rw----  1 root disk        7,   1  12月 25 10:34 loop1  
brw-rw----  1 root disk        7,   2  12月 25 10:34 loop2  
crw-rw-rw-  1 root tty         5,   0  12月 25 12:48 tty  
crw--w----  1 root tty         4,   0  12月 25 10:34 tty0  
crw-rw----  1 root tty         4,   1  12月 25 10:34 tty1  
crw--w----  1 root tty         4,  10  12月 25 10:34 tty10  

其中b代表块设备，c代表字符设备。对于普通文件来说，使用命令ls -l会列出文件的长度，而对于设备文件来说，上面的7,5,4等代表的是对应设备的主设备号，而后面的0,1,2,10等则是对应设备的次设备号。主设备号标识设备对应的驱动程序，也就是说1个主设备号对应一个驱动程序。当然，现在也有多个驱动程序共享主设备号的情况。而次设备号由内核使用，用于确定/dev下的设备文件对应的具体设备。举一个例子，虚拟控制台和串口终端有驱动程序4管理，而不同的终端分别有不同的次设备号。

cdev初始化

静态初始化

/**
 * cdev_init() - initialize a cdev structure
 * @cdev: the structure to initialize
 * @fops: the file_operations for this device
 *
 * Initializes @cdev, remembering @fops, making it ready to add to the
 * system with cdev_add().
 */
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;
}

动态初始化：

/**
 * cdev_alloc() - allocate a cdev structure
 *
 * Allocates and returns a cdev structure, or NULL on failure.
 */
struct cdev *cdev_alloc(void)
{
    struct cdev *p = kzalloc(sizeof(struct cdev), GFP_KERNEL);
    if (p) {
        INIT_LIST_HEAD(&p->list);
        kobject_init(&p->kobj, &ktype_cdev_dynamic);
    }
    return p;
}

cdev设备注册

cdev设备注册过程中为设备提供了两种分配设备号的方式：

1、指定主设备号

/**
 * register_chrdev_region() - register a range of device numbers
 * @from: the first in the desired range of device numbers; must include
 *        the major number.
 * @count: the number of consecutive device numbers required
 * @name: the name of the device or driver.
 *
 * Return value is zero on success, a negative error code on failure.
 */
int register_chrdev_region(dev_t from, unsigned count, const char *name)
{
    struct char_device_struct *cd;
    dev_t to = from + count;
    dev_t n, next;

    for (n = from; n < to; n = next) {
        next = MKDEV(MAJOR(n)+1, 0);
        if (next > to)
            next = to;
        cd = __register_chrdev_region(MAJOR(n), MINOR(n),
                   next - n, name);
        if (IS_ERR(cd))
            goto fail;
    }
    return 0;
fail:
    to = n;
    for (n = from; n < to; n = next) {
        next = MKDEV(MAJOR(n)+1, 0);
        kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
    }
    return PTR_ERR(cd);
}

2、系统自动分配主设备号

/**
 * alloc_chrdev_region() - register a range of char device numbers
 * @dev: output parameter for first assigned number
 * @baseminor: first of the requested range of minor numbers
 * @count: the number of minor numbers required
 * @name: the name of the associated device or driver
 *
 * Allocates a range of char device numbers.  The major number will be
 * chosen dynamically, and returned (along with the first minor number)
 * in @dev.  Returns zero or a negative error code.
 */
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
            const char *name)
{
    struct char_device_struct *cd;
    cd = __register_chrdev_region(0, baseminor, count, name);
    if (IS_ERR(cd))
        return PTR_ERR(cd);
    *dev = MKDEV(cd->major, cd->baseminor);
    return 0;
}

cdev注销

与上述两个分配设备号对应的注销设备号的函数如下

/**
 * unregister_chrdev_region() - unregister a range of device numbers
 * @from: the first in the range of numbers to unregister
 * @count: the number of device numbers to unregister
 *
 * This function will unregister a range of @count device numbers,
 * starting with @from.  The caller should normally be the one who
 * allocated those numbers in the first place...
 */
void unregister_chrdev_region(dev_t from, unsigned count)
{
    dev_t to = from + count;
    dev_t n, next;

    for (n = from; n < to; n = next) {
        next = MKDEV(MAJOR(n)+1, 0);
        if (next > to)
            next = to;
        kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
    }
}

添加字符设备cdev

/**
 * cdev_add() - add a char device to the system
 * @p: the cdev structure for the device
 * @dev: the first device number for which this device is responsible
 * @count: the number of consecutive minor numbers corresponding to this
 *         device
 *
 * cdev_add() adds the device represented by @p to the system, making it
 * live immediately.  A negative error code is returned on failure.
 */
int cdev_add(struct cdev *p, dev_t dev, unsigned count)
{
    int error;

    p->dev = dev;
    p->count = count;

    error = kobj_map(cdev_map, dev, count, NULL,
             exact_match, exact_lock, p);
    if (error)
        return error;

    kobject_get(p->kobj.parent);

    return 0;
}

删除字符设备cdev

/**
 * cdev_del() - remove a cdev from the system
 * @p: the cdev structure to be removed
 *
 * cdev_del() removes @p from the system, possibly freeing the structure
 * itself.
 */
void cdev_del(struct cdev *p)
{
    cdev_unmap(p->dev, p->count);
    kobject_put(&p->kobj);
}

字符设备是3大类设备（字符设备、块设备、网络设备）中较简单的一类设备，其驱动程序中完成的主要工作是初始化、添加和删除cdev结构体，申请和释放设备号，以及填充file_operation结构体中操作函数，并实现file_operations结构体中的read()、write()、ioctl()等重要函数。
如图所示为cdev结构体、file_operations和用户空间调用驱动的关系。

字符设备驱动程序实例分析：
memdev.h

#ifndef _MEMDEV_H_
#define _MEMDEV_H_

#ifndef MEMDEV_MAJOR
#define MEMDEV_MAJOR 251   /*预设的mem的主设备号*/
#endif

#ifndef MEMDEV_NR_DEVS
#define MEMDEV_NR_DEVS 2    /*设备数*/
#endif

#ifndef MEMDEV_SIZE
#define MEMDEV_SIZE 4096
#endif

/*mem设备描述结构体*/
struct mem_dev                                     
{                                                        
  char *data;                      
  unsigned long size;       
};

#endif /* _MEMDEV_H_ */

memdev.c

#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 <linux/cdev.h>  
#include <asm/io.h>  
#include <asm/uaccess.h>  
#include <linux/timer.h>  
#include <asm/atomic.h>  
#include <linux/slab.h>  
#include <linux/device.h>  

#include "memdev.h"

static mem_major = MEMDEV_MAJOR;

module_param(mem_major, int, S_IRUGO);

struct mem_dev *mem_devp; /*设备结构体指针*/

struct cdev cdev; 

/*文件打开函数*/
int mem_open(struct inode *inode, struct file *filp)
{
    struct mem_dev *dev;

    /*获取次设备号*/
    int num = MINOR(inode->i_rdev);

    if (num >= MEMDEV_NR_DEVS) 
            return -ENODEV;
    dev = &mem_devp[num];

    /*将设备描述结构指针赋值给文件私有数据指针*/
    filp->private_data = dev;

    return 0; 
}

/*文件释放函数*/
int mem_release(struct inode *inode, struct file *filp)
{
  return 0;
}

/*读函数*/
static ssize_t mem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos)
{
  unsigned long p =  *ppos;        /*记录文件指针偏移位置*/  
  unsigned int count = size;    /*记录需要读取的字节数*/ 
  int ret = 0;    /*返回值*/  
  struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*/

  /*判断读位置是否有效*/
  if (p >= MEMDEV_SIZE)    /*要读取的偏移大于设备的内存空间*/  
    return 0;
  if (count > MEMDEV_SIZE - p)     /*要读取的字节大于设备的内存空间*/ 
    count = MEMDEV_SIZE - p;

  /*读数据到用户空间:内核空间->用户空间交换数据*/  
  if (copy_to_user(buf, (void*)(dev->data + p), count))
  {
    ret =  - EFAULT;
  }
  else
  {
    *ppos += count;
    ret = count;

    printk(KERN_INFO "read %d bytes(s) from %d\n", count, p);
  }

  return ret;
}

/*写函数*/
static ssize_t mem_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 mem_dev *dev = filp->private_data; /*获得设备结构体指针*/

  /*分析和获取有效的写长度*/
  if (p >= MEMDEV_SIZE)
    return 0;
  if (count > MEMDEV_SIZE - p)    /*要写入的字节大于设备的内存空间*/
    count = MEMDEV_SIZE - p;

  /*从用户空间写入数据*/
  if (copy_from_user(dev->data + p, buf, count))
    ret =  - EFAULT;
  else
  {
    *ppos += count;      /*增加偏移位置*/  
    ret = count;      /*返回实际的写入字节数*/ 

    printk(KERN_INFO "written %d bytes(s) from %d\n", count, p);
  }

  return ret;
}

/* seek文件定位函数 */
static loff_t mem_llseek(struct file *filp, loff_t offset, int whence)
{ 
    loff_t newpos;      

    switch(whence) {
      case 0: /* SEEK_SET */       /*相对文件开始位置偏移*/ 
        newpos = offset;           /*更新文件指针位置*/
        break;

      case 1: /* SEEK_CUR */
        newpos = filp->f_pos + offset;    
        break;

      case 2: /* SEEK_END */
        newpos = MEMDEV_SIZE -1 + offset;
        break;

      default: /* can't happen */
        return -EINVAL;
    }
    if ((newpos<0) || (newpos>MEMDEV_SIZE))
        return -EINVAL;

    filp->f_pos = newpos;
    return newpos;

}

/*文件操作结构体*/
static const struct file_operations mem_fops =
{
  .owner = THIS_MODULE,
  .llseek = mem_llseek,
  .read = mem_read,
  .write = mem_write,
  .open = mem_open,
  .release = mem_release,
};

/*设备驱动模块加载函数*/
static int memdev_init(void)
{
  int result;
  int i;

  dev_t devno = MKDEV(mem_major, 0);

   /* 申请设备号，当xxx_major不为0时，表示静态指定；当为0时，表示动态申请*/ 
  /* 静态申请设备号*/
  if (mem_major)
    result = register_chrdev_region(devno, 2, "memdev");
  else  /* 动态分配设备号 */
  {
    result = alloc_chrdev_region(&devno, 0, 2, "memdev");
    mem_major = MAJOR(devno);    /*获得申请的主设备号*/
  }  

  if (result < 0)
    return result;

 /*初始化cdev结构，并传递file_operations结构指针*/ 
  cdev_init(&cdev, &mem_fops);    
  cdev.owner = THIS_MODULE;    /*指定所属模块*/
  cdev.ops = &mem_fops;

  /* 注册字符设备 */
  cdev_add(&cdev, MKDEV(mem_major, 0), MEMDEV_NR_DEVS);

  /* 为设备描述结构分配内存*/
  mem_devp = kmalloc(MEMDEV_NR_DEVS * sizeof(struct mem_dev), GFP_KERNEL);
  if (!mem_devp)    /*申请失败*/
  {
    result =  - ENOMEM;
    goto fail_malloc;
  }
  memset(mem_devp, 0, sizeof(struct mem_dev));

  /*为设备分配内存*/
  for (i=0; i < MEMDEV_NR_DEVS; i++) 
  {
        mem_devp[i].size = MEMDEV_SIZE;
        mem_devp[i].data = kmalloc(MEMDEV_SIZE, GFP_KERNEL);
        memset(mem_devp[i].data, 0, MEMDEV_SIZE);
  }

  return 0;

  fail_malloc: 
  unregister_chrdev_region(devno, 1);

  return result;
}

/*模块卸载函数*/
static void memdev_exit(void)
{
  cdev_del(&cdev);   /*注销设备*/
  kfree(mem_devp);     /*释放设备结构体内存*/
  unregister_chrdev_region(MKDEV(mem_major, 0), 2); /*释放设备号*/
}

MODULE_AUTHOR("David");
MODULE_LICENSE("GPL");

module_init(memdev_init);
module_exit(memdev_exit);

应用程序测试：
app-mem.c

#include <stdio.h>

int main()
{
    FILE *fp0 = NULL;
    char Buf[4096];

    /*初始化Buf*/
    strcpy(Buf,"Mem is char dev!");
    printf("BUF: %s\n",Buf);

    /*打开设备文件*/
    fp0 = fopen("/dev/memdev0","r+");
    if (fp0 == NULL)
    {
        printf("Open Memdev0 Error!\n");
        return -1;
    }

    /*写入设备*/
    fwrite(Buf, sizeof(Buf), 1, fp0);    
    /*重新定位文件位置（思考没有该指令，会有何后果)*/
    fseek(fp0,0,SEEK_SET);    
    /*清除Buf*/
    strcpy(Buf,"Buf is NULL!");
    printf("BUF: %s\n",Buf);        
    /*读出设备*/
    fread(Buf, sizeof(Buf), 1, fp0);    
    /*检测结果*/
    printf("BUF: %s\n",Buf);   
    return 0;    

}

测试：
1)cat /proc/devices看看有哪些编号已经被使用，我们选一个没有使用的设备号xxx。
2)挂载驱动模块：insmod memdev.ko
3)通过”mknod /dev/memdev0 c xxx 0“命令创建”/dev/memdev0“设备节点。
4)交叉编译app-mem.c文件，下载并执行：

#./app-mem
Mem is char dev!