《linux设备驱动开发详解》笔记——6字符设备驱动

6.1 字符设备驱动结构

先看看字符设备驱动的架构：

6.1.1 cdev

　　cdev结构体是字符设备的核心数据结构，用于描述一个字符设备，cdev定义如下：

　　

#include <linux/cdev.h>

struct cdev {
    struct kobject kobj;
    struct module *owner;
    const struct file_operations *ops;　　// 文件操作结构体
    struct list_head list;
    dev_t dev;　　　　　　　　　　　　　　　　// 设备号，12bit主设备号+20bit次设备号
    unsigned int count;　　　　　　　　　　 // 设备个数
};

设备号相关宏定义:
MAJOR(dev);　　　　　　// 获取主设备号
MINOR(dev);　　　　　　// 获取从设备号
MKDEV(major,minor);　//  生产设备号　

cdev相关函数接口:

void cdev_init(struct cdev *, const struct file_operations *);　　// 将cdev与file_operations挂钩
struct cdev *cdev_alloc(void);
void cdev_put(struct cdev *p);
int cdev_add(struct cdev *, dev_t, unsigned);　　　　　　　　　　　　// 向系统中添加cdev，同时将cdev与设备号挂钩（设备号和设备名称是文件系统的组成部分，与用户交互的桥梁）
                                                                 // 注意判断返回值， A negative error code is returned on failure.返回1个负数
void cdev_del(struct cdev *);　　　　　　　　　　　　　　　　　　　　　//  从系统中删除cdev

6.1.2 设备号

注册设备号和释放设备号，设备号也是系统资源，需要注意在注销函数时释放：

#include <linux/fs.h>

/**
* 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);　　// 动态分配，并注册设备号

/**
* 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);　　　　// 将已知的设备号注册到系统中

/**
* unregister_chrdev_region() - return 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)；　　// 注销设备号

6.1.3 file_operations

　　file_operations是字符设备驱动设计的主要内容， 应用程序进行open、close、write、read等操作时，通过文件系统简介调用file_operations里驱动实现的函数接口。

#include <linux/fs.h>

struct file_operations {
    struct module *owner;　　// module是内核表示模块的单元，THIS_MODULE表示当前模块，insmod时内核创建一个module结构体
    loff_t (*llseek) (struct file *, loff_t, int);　　// 修改文件当前读写位置，返回新位置，出错时返回-1
    ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);　　// 读数据，与应用程序的read和fread对应
    ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);　　// 写数据，与应用程序的write和fwrite对应;若未实现，应用调用时返回-EINVAL
    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 (*iterate) (struct file *, struct dir_context *);
    unsigned int (*poll) (struct file *, struct poll_table_struct *);　　// 多路复用
    long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); // 对应应用程序的ioctl
    long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
    int (*mmap) (struct file *, struct vm_area_struct *);　　// 帧缓存是有用，应用映射后可直接访问内存空间，与应用程序的mmap对应。若驱动未实现时如果应用调用，返回-ENODEV
    int (*open) (struct inode *, struct file *);　　　　// 若未定义，应用调用时返回成功
    int (*flush) (struct file *, fl_owner_t id);
    int (*release) (struct inode *, struct file *);
    int (*fsync) (struct file *, loff_t, loff_t, 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);
    int (*show_fdinfo)(struct seq_file *m, struct file *f);
};

【注意】
各结构体成员的参数是struct file和struct inode，而不是应用程序里的文件描述符fd；

6.1.4 生成设备文件

　　原书中没有介绍如何在/dev目录下自动生成设备文件，可使用如下函数实现

#include <linux/device.h>

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

　 /**
　　* class_create - create a struct class structure
　　* @owner: pointer to the module that is to "own" this struct class
　　* @name: pointer to a string for the name of this class.
　　* @key: the lock_class_key for this class; used by mutex lock debugging
　　*
　　* This is used to create a struct class pointer that can then be used
　　* in calls to device_create().　　
　　*
　　* Returns &struct class pointer on success, or ERR_PTR() on error.
　　*
　　* Note, the pointer created here is to be destroyed when finished by
　　* making a call to class_destroy().
　　*/
　　struct class *__class_create(struct module *owner, const char *name,struct lock_class_key *key)

/**
 * device_create - creates a device and registers it with sysfs
 * @class: pointer to the struct class that this device should be registered to
 * @parent: pointer to the parent struct device of this new device, if any
 * @devt: the dev_t for the char device to be added
 * @drvdata: the data to be added to the device for callbacks
 * @fmt: string for the device's name
 *
 * This function can be used by char device classes.  A struct device
 * will be created in sysfs, registered to the specified class.
 *
 * A "dev" file will be created, showing the dev_t for the device, if
 * the dev_t is not 0,0.
 * If a pointer to a parent struct device is passed in, the newly created
 * struct device will be a child of that device in sysfs.
 * The pointer to the struct device will be returned from the call.
 * Any further sysfs files that might be required can be created using this
 * pointer.
 *
 * Returns &struct device pointer on success, or ERR_PTR() on error.
 *
 * Note: the struct class passed to this function must have previously
 * been created with a call to class_create().
 */
struct device *device_create(struct class *class, struct device *parent, dev_t devt, void *drvdata, const char *fmt, ...)

/**
* device_destroy - removes a device that was created with device_create()
* @class: pointer to the struct class that this device was registered with
* @devt: the dev_t of the device that was previously registered
*
* This call unregisters and cleans up a device that was created with a
* call to device_create().
*/
void device_destroy(struct class *class, dev_t devt);　　

！！！6.1.5 字符设备驱动结构

6.1.4.1 模块加载和卸载

#include <linux/cdev.h>　　// cdev
#include <linux/fs.h>　　　// file_operations
#include <linux/device.h> // class,device

/* 设备结构体 */
struct xxx_dev_t {
　　struct cdev cdev;　　　　// 一般cdev结构体不进行动态分配，直接定义在驱动程序里
　　struct class class;　　 // 定义class，为了生成/dev/设备文件
　　....
}xxx_dev;

/* 驱动模块加载函数 */
static int __init xxx_init( void )
{
　　...
　　cdev_init( &xxx_dev.cdev, &xxx_fops );　　// 建立cdev与file_operations的关联
　　xxx_dev.cdev.owner = THIS_MODULE;

　　/* 注册设备号 */
　　if( xxx_major ){　
　　　　register_chrdev_region( xxx_dev_no, 1, DEV_NAME );
　　}
　　else{
　　　　alloc_chrdev_region( &xxx_dev_no,0,1,DEV_NAME );
　　}
　　　
　　ret = cdev_add( &xxx_dev.cdev, xxx_dev_no,1 );  　 // 注册字符设备
　　if(ret){
　　　　// err proccess　　
　　}
　
　　/* 设备文件相关 */
　　xxx_dev.class = class_create( THIS_MODULE, DEV_NAME );　　　　　　　　// 创建class
   device_create( &xxx_dev.class, NULL, xxx_dev_no,NULL, DEV_NAME );　　// 创建device，会在/dev目录下生成名字为DEV_NAME的设备文件
　　...
}

/* 驱动模块卸载函数 */
static void __exit xxx_exit( void )
{
　　unregister_chrdev_region( xxx_dev_no, 1);    // 释放占用的设备号
　　cdev_del( &xxx_dev.cdev );　　　　　　　　　　　// 注销字符设备
}

module_init(xxx_init);
module_eixt(xxx_exit);

【注意】

• init和exit函数的格式，static int __init xxx_init( void )，否则

6.1.4.2 file_operations成员函数

　　大多数驱动都要实现read、write、ioctl等函数
　　【注意】：

　　1.参数中的*f_pos实际应该就是指向filp->f_pos的指针，由文件系统统一管理的偏移，驱动中read/write要直接使用这个偏移，并在完成操作后更新这个指针；

　　2.llseek的参数中没有*f_pos，直接操作filp->f_pos即可。

struct file_operations xxx_fops = {
　　.owner = THIS_MODULE,
　　.read = xxx_read,
　　.write = xxx_write,
　　.unlocked_ioctl = xxx_ioctl,
　　...
}

ssize_t xxx_read( struct file * filp, char __user *buf, size_t count, loff_t * f_pos )
{
　　...
　　copy_to_user( buf, ..., ...);
　　...
}

ssize_t xxx_write( struct file * filp, char __user *buf, size_t count, loff_t * f_pos )
{
　　...
　　copy_from_user( buf, ..., ...);
　　...
}

long xxx_ioctrl( struct file * filp, unsigned int cmd, unsigned long arg )
{
　　...
　　switch( cmd ){
　　case XXX_CMD1:
　　　　...
　　　　break;
　　case XXX_CMD2:　　
　　　　...
　　　　break;
　　default:
　　　　// 不支持的命令
　　　　return -ENOTTY;
　　}
　　
　　return 0;
}

　　【注意】:

　　1.读写函数的参数中，buf是用户空间的，处于内核态的驱动不应该直接读写，应该调用特定接口，如下：

　　2. 注意copy_to/from_user的参数位置，都是从第2个参数拷贝到第一个参数，有点类似memcpy，别搞反了！

　　3. 注意检查copy函数的返回值，返回值n是剩余的字节数，正常时应该为0

#include <linux/uaccess.h>　　　　// 内部包含 arch/arm/inculde/asm/uaccess.h,看来除了linux源码根目录的include目录，arch/arm/include也是驱动搜索的头文件目录

static inline unsigned long __must_check copy_from_user(void *to, const void __user *from, unsigned long n)
{
    if (access_ok(VERIFY_READ, from, n))　　　　// 先检查是否传入的缓冲区是否属于用户空间
        n = __copy_from_user(to, from, n);
    else /* security hole - plug it */
        memset(to, , n);
    return n;
}

static inline unsigned long __must_check copy_to_user(void __user *to, const void *from, unsigned long n)
{
    if (access_ok(VERIFY_WRITE, to, n))
        n = __copy_to_user(to, from, n);
    return n;
}

对于简单类型，如char，int，long可以使用简单的函数进行复制，如下：

　　#define get_user(x,p) __get_user(x,p)　　　　// x是变量，p是用户空间地址， x = *p
　　#define put_user(x,p) __put_user(x,p)　　　　// *p = x

　　#define __get_user(x,ptr) \
　　({ \
　　　　long __gu_err = 0; \
　　　　__get_user_err((x),(ptr),__gu_err); \
　　　　__gu_err; \
　　})

　　#define __put_user(x,ptr) \
　　({ \
　　　　long __pu_err = 0; \
　　　　__put_user_err((x),(ptr),__pu_err); \
　　　　__pu_err; \
　　})

6.1.4.3 ioctl()命令

　　命令最好能区分不同设备，如果所有驱动都采用1,2,3这类命令，会造成命令码污染。linux有一套统一的ioctl命令生成的方式，最好使用该机制。  强制使用土鳖的1/2/3，没不会有啥大的问题。

设备类型type，是1个幻数，应参考内核中的ioctl-number.txt，不要与已使用的冲突；

序列号nr，8bit；

方向dir，_IOC_NONE（ 无数据传输） 、_IOC_READ（ 读） 、 _IOC_WRITE（ 写） 和_IOC_READ|_IOC_WRITE（ 双向） 。 数据传送的方向是从应用程序的角度来看的；

数据尺寸size，13~14bit；

用下述宏来生成命令：

#include <linux/ioctl.h>　　//　实际是 include/uapi/linux/ioctl.h，看来uapi也是驱动搜索的目录之一

#define _IO(type,nr)        _IOC(_IOC_NONE,(type),(nr),0)
#define _IOR(type,nr,size)    _IOC(_IOC_READ,(type),(nr),(_IOC_TYPECHECK(size)))
#define _IOW(type,nr,size)    _IOC(_IOC_WRITE,(type),(nr),(_IOC_TYPECHECK(size)))
#define _IOWR(type,nr,size)    _IOC(_IOC_READ|_IOC_WRITE,(type),(nr),(_IOC_TYPECHECK(size)))

// 例如
#define GLOBALMEM_CMD_CLR　　_IO('g',0)

6.2 globalmem设备驱动示例

　　这本书使用globalmem模拟一种物理设备，所有的驱动都在这个设备上进行实例操作。

　　【注意】

　　1.模块编译的Makefile文件，首字母必须大写，否则make modules找不到Makefile文件，貌似是make modules时写死了，只认"Makefile"字样的文件；

6.2.1 单一设备

#include <linux/module.h>
#include <linux/cdev.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <asm/uaccess.h>

#define DEV_NAME    "globalmem"

#define GLOBALMEN_LEN    1024

struct globalmem_dev_t
{
    struct cdev cdev;
    struct class * class;
    dev_t  dev_no;

    char buf[GLOBALMEN_LEN];
}globalmem_dev;

int globalmem_open(struct inode * inode, struct file * filp)
{
    filp->private_data = &globalmem_dev;
    return ;
}

ssize_t globalmem_read(struct file *filp, char __user * buf, size_t len, loff_t * pos)
{
    struct globalmem_dev_t * globalmem_devp;
    size_t len_rd;

    globalmem_devp = filp->private_data;
    if( (*pos)>GLOBALMEN_LEN )
        return ;

    if( ( (*pos)+len) > GLOBALMEN_LEN  )
        len_rd = GLOBALMEN_LEN-(*pos);

    else
        len_rd = len;

    if( copy_to_user(buf,&globalmem_devp->buf[(*pos)],len_rd) )
        return -EFAULT;
    printk("read %d bytes from %d pos\r\n",(int)len_rd,(int)*pos);
    *pos += len_rd;
    return len_rd;
}

ssize_t globalmem_write (struct file *filp, const char __user *buf, size_t len, loff_t *pos)
{
    struct globalmem_dev_t * globalmem_devp;
    size_t len_wr;

    globalmem_devp = filp->private_data;
    if( (*pos)>GLOBALMEN_LEN )
        return ;

    if( ((*pos)+len) > GLOBALMEN_LEN  )
        len_wr = GLOBALMEN_LEN-(*pos);

    else
        len_wr = len;
    if( copy_from_user(&globalmem_devp->buf[(*pos)],buf,len_wr) )
        return -EFAULT;
    printk("write %d bytes from %d pos\r\n",(int)len_wr,(int)*pos);
    *pos += len_wr;
    return len_wr;
}

loff_t globalmem_llseek(struct file *filp, loff_t offset, int whence )
{
    loff_t ret;    // 注意要有返回值

    switch(whence){
    case SEEK_SET:
        if( offset <  )
            return -EINVAL;
        if( offset > GLOBALMEN_LEN )
            return -EINVAL;
        filp->f_pos = offset;
        ret = filp->f_pos;
        break;
    case SEEK_CUR:
        if((filp->f_pos+offset)<  )
            return -EINVAL;
        if((filp->f_pos+offset)> GLOBALMEN_LEN )
            return -EINVAL;
        filp->f_pos += offset;
        ret = filp->f_pos;
        break;
    case SEEK_END:
        if((filp->f_pos+offset)<  )
            return -EINVAL;
        if((filp->f_pos+offset) > GLOBALMEN_LEN )
            return -EINVAL;
        filp->f_pos += (offset+GLOBALMEN_LEN);
        ret = filp->f_pos;
        break;
    default:
        return -EINVAL;
        break;
    }

    return ret;
}

struct file_operations globalmem_fops = {
    .owner = THIS_MODULE,
    .open = globalmem_open,
    .read = globalmem_read,
    .write = globalmem_write,
    .llseek = globalmem_llseek,
};

static int __init globalmem_init( void )
{
    int ret;

    printk("enter globalmem_init()\r\n");

    cdev_init(&globalmem_dev.cdev,&globalmem_fops);
    globalmem_dev.cdev.owner=THIS_MODULE;

    if( (ret=alloc_chrdev_region(&globalmem_dev.dev_no,,,DEV_NAME))< )
    {
        printk("alloc_chrdev_region err.\r\n");
        return ret;
    }
    ret = cdev_add(&globalmem_dev.cdev,globalmem_dev.dev_no,);
    if( ret )
    {
        printk("cdev_add err.\r\n");
        return ret;
    }

    /*
         * $ sudo insmod globalmem.ko    如果使用class_create,insmod时会报错，dmesg查看内核log信息发现找不到class相关函数
     *   insmod: ERROR: could not insert module globalmem.ko: Unknown symbol in module
     *   $ dmesg
     *    [ 5495.606920] globalmem: Unknown symbol __class_create (err 0)
     *    [ 5495.606943] globalmem: Unknown symbol class_destroy (err 0)
     *    [ 5495.607027] globalmem: Unknown symbol device_create (err 0)
     */    

    globalmem_dev.class = class_create( THIS_MODULE, DEV_NAME );
    device_create(globalmem_dev.class,NULL,globalmem_dev.dev_no,NULL,DEV_NAME);

    /* init mem and pos */
    memset(globalmem_dev.buf,,GLOBALMEN_LEN);
    return ;
}

static void __exit globalmem_exit( void )
{
    unregister_chrdev_region(globalmem_dev.dev_no, );
    cdev_del(&globalmem_dev.cdev);
    class_destroy(globalmem_dev.class);
}

module_init(globalmem_init);
module_exit(globalmem_exit);

MODULE_LICENSE("GPL");    // 不加此声明，会报上述Unknown symbol问题

$ echo "hello world" > /dev/globalmem
$ cat /dev/globalmem
hello world

另一个终端监视printk的显示（用dmesg脚本）

[ 3158.660191] enter globalmem_init()
[ 3231.664661] open filp->private_data = 0xc0270480
[ 3231.664669] open filp->f_pos = 0
[ 3231.717074] write 4 bytes from 0 pos
[ 3598.704506] open filp->private_data = 0xc0270480
[ 3598.704519] open filp->f_pos = 0
[ 3598.704547] read 1024 bytes from 0 pos
[ 3598.707040] read 0 bytes from 1024 pos

6.2.2 支持N个设备

原则：cdev和file_operations本质上与驱动程序对应，理论上只需要1个就可以。同理，class定义设备的一种类型， 也不用定义多个。  具体设备（多个globalmem的buf）是多个，驱动程序要想办法把驱动文件file、inode与设备对应上就可以了。

上述原则与书中测试用例不太一致，书中定义了多个cdev。

同一套驱动需要支持多个同类设备，对上述驱动进行简单改造：

【注意】exit时要把init时建立的文件都销毁，漏掉了很容易引发问题。一开始忘记了device_destroy,导致rmmod后再insmod出问题，因为没有卸干净，

/*
    一套驱动对应多个设备的方法，试着采用驱动和设备分离的思想
 */

#include <linux/module.h>
#include <linux/cdev.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <asm/uaccess.h>

#define DEV_NAME    "globalmem"
#define DEV_NUM        3
#define GLOBALMEN_LEN    1024

// 具体设备
struct globalmem_dev_t
{
    char buf[GLOBALMEN_LEN];
};

// 驱动
struct globalmem_driver_t
{
    struct cdev cdev;
    struct class * class;
};

struct globalmem_dev_t globalmem_dev[DEV_NUM];
struct globalmem_driver_t globalmem_driver;
dev_t  globalmem_devno;

int globalmem_open(struct inode * inode, struct file * filp)
{
    unsigned int minor = iminor(inode);　　// 完成文件与设备的对应

    filp->private_data = &globalmem_dev[minor];
    return ;
}

ssize_t globalmem_read(struct file *filp, char __user * buf, size_t len, loff_t * pos)
{
    struct globalmem_dev_t * globalmem_devp;
    size_t len_rd;

    globalmem_devp = filp->private_data;
    if( (*pos)>GLOBALMEN_LEN )
        return ;

    if( ( (*pos)+len) > GLOBALMEN_LEN  )
        len_rd = GLOBALMEN_LEN-(*pos);

    else
        len_rd = len;

    if( copy_to_user(buf,&globalmem_devp->buf[(*pos)],len_rd) )
        return -EFAULT;
    printk("read %d bytes from %d pos\r\n",(int)len_rd,(int)*pos);
    *pos += len_rd;
    return len_rd;
}

ssize_t globalmem_write (struct file *filp, const char __user *buf, size_t len, loff_t *pos)
{
    struct globalmem_dev_t * globalmem_devp;
    size_t len_wr;

    globalmem_devp = filp->private_data;
    if( (*pos)>GLOBALMEN_LEN )
        return ;

    if( ((*pos)+len) > GLOBALMEN_LEN  )
        len_wr = GLOBALMEN_LEN-(*pos);

    else
        len_wr = len;
    if( copy_from_user(&globalmem_devp->buf[(*pos)],buf,len_wr) )
        return -EFAULT;
    printk("write %d bytes from %d pos\r\n",(int)len_wr,(int)*pos);
    *pos += len_wr;
    return len_wr;
}

loff_t globalmem_llseek(struct file *filp, loff_t offset, int whence )
{
    loff_t ret;    // 注意要有返回值

    switch(whence){
    case SEEK_SET:
        if( offset <  )
            return -EINVAL;
        if( offset > GLOBALMEN_LEN )
            return -EINVAL;
        filp->f_pos = offset;
        ret = filp->f_pos;
        break;
    case SEEK_CUR:
        if((filp->f_pos+offset)<  )
            return -EINVAL;
        if((filp->f_pos+offset)> GLOBALMEN_LEN )
            return -EINVAL;
        filp->f_pos += offset;
        ret = filp->f_pos;
        break;
    case SEEK_END:
        if((filp->f_pos+offset)<  )
            return -EINVAL;
        if((filp->f_pos+offset) > GLOBALMEN_LEN )
            return -EINVAL;
        filp->f_pos += (offset+GLOBALMEN_LEN);
        ret = filp->f_pos;
        break;
    default:
        return -EINVAL;
        break;
    }

    return ret;
}

struct file_operations globalmem_fops = {
    .owner = THIS_MODULE,
    .open = globalmem_open,
    .read = globalmem_read,
    .write = globalmem_write,
    .llseek = globalmem_llseek,
};

static int __init globalmem_init( void )
{
    int ret;
    int index;
    dev_t dev_no;
    int major;
    char name[];

    printk("enter globalmem_init()\r\n");
    if( (ret=alloc_chrdev_region(&globalmem_devno,,DEV_NUM,DEV_NAME))< )
    {
        printk("alloc_chrdev_region err.\r\n");
        return ret;
    }
    else
        printk("alloc %d mem,first dev_no is %d.\r\n",DEV_NUM,(int)globalmem_devno);
    major = MAJOR(globalmem_devno);

    cdev_init(&globalmem_driver.cdev,&globalmem_fops);
    globalmem_driver.cdev.owner=THIS_MODULE;

    ret = cdev_add(&globalmem_driver.cdev,globalmem_devno,DEV_NUM);
    if( ret )
    {
        printk("cdev_add err.\r\n");
        return ret;
    }
    globalmem_driver.class = class_create( THIS_MODULE, DEV_NAME );

    for( index=;index<DEV_NUM;index++ ){
        dev_no = MKDEV( major, index );
        snprintf(name,,DEV_NAME"%d",index);
        printk("name:%s,dev_no %d.\r\n",name,dev_no);
        device_create(globalmem_driver.class,NULL,dev_no,NULL,DEV_NAME"%d",index );
        /* init mem and pos */
        memset(globalmem_dev[index].buf,,GLOBALMEN_LEN);
    }
    return ;
}

static void __exit globalmem_exit( void )
{
    int index;
    dev_t dev_no;

    printk("enter globalmem_exit(),dev_no %d\r\n",globalmem_devno);
    unregister_chrdev_region(globalmem_devno, DEV_NUM);
    cdev_del(&globalmem_driver.cdev);
    for( index=;index<DEV_NUM;index++){
        dev_no = MKDEV(MAJOR(globalmem_devno),index);
        device_destroy(globalmem_driver.class,dev_no);　　　　// 注意注销时与init时建立的文件都要消除，否则会出问题
    }
    class_destroy(globalmem_driver.class);

}

module_init(globalmem_init);
module_exit(globalmem_exit);

MODULE_LICENSE("GPL");    

$ echo "mem0" > /dev/globalmem0
$ echo "mem1" > /dev/globalmem1
$ echo "mem2" > /dev/globalmem2
$ cat /dev/globalmem0
mem0
$ cat /dev/globalmem1
mem1
$ cat /dev/globalmem2
mem2

6.3 内核驱动的调试，printk

【问题1】 printk在ubuntu的shell终端里无法显示

　　printk打印的是控制台，也就是/dev/console。而图形界面中的终端，其实是把stdin,stdout,stderr三个文件重定向了一下。所以printk是无法再图形界面中的终端中显示的，当然可以再/var/log/syslog或者用dmesg查看。

　　在嵌入式设备中，其中初始化的时候把stdin，stdout,stderr均定向到了/dev/console中（main.c中的init_post函数里）。一般的情况下控制台就是串口。

【问题2】 ubuntu里怎么显示printk

　　ubuntu这类的发行版linux，有专门的守护进程负责记录log信息，有些log信息里就保存着printk的打印信息（内核信息），可以用dmesg命令或者查看/proc/kmsg文件

　　在一个单独的窗口里，可以输入如下命令，单独监测内核的日志文件，从而监控printk信息

// 方法1：
sudo cat /proc/kmsg　　　　// 阻塞显示内核信息，只显示增量信息,感觉没有dmesg好用，会漏一些信息

// 方法2：可运行如下脚本，定期用dmesg查看内核信息
while true
do
　　sudo dmesg -c　　// dmesg不是增量显示，会打印所有的存量信息，所以用-c，显示后删除
　　sleep 1　　　　　　
done

6.4 错误类型说明

驱动中注意使用这些标准的错误返回码

// base error
#define    EPERM         1    /* Operation not permitted */
#define    ENOENT         2    /* No such file or directory */
#define    ESRCH         3    /* No such process */
#define    EINTR         4    /* Interrupted system call */
#define    EIO         5    /* I/O error */
#define    ENXIO         6    /* No such device or address */
#define    E2BIG         7    /* Argument list too long */
#define    ENOEXEC         8    /* Exec format error */
#define    EBADF         9    /* Bad file number */
#define    ECHILD        10    /* No child processes */
#define    EAGAIN        11    /* Try again */
#define    ENOMEM        12    /* Out of memory */
#define    EACCES        13    /* Permission denied */
#define    EFAULT        14    /* Bad address */
#define    ENOTBLK        15    /* Block device required */
#define    EBUSY        16    /* Device or resource busy */
#define    EEXIST        17    /* File exists */
#define    EXDEV        18    /* Cross-device link */
#define    ENODEV        19    /* No such device */
#define    ENOTDIR        20    /* Not a directory */
#define    EISDIR        21    /* Is a directory */
#define    EINVAL        22    /* Invalid argument */
#define    ENFILE        23    /* File table overflow */
#define    EMFILE        24    /* Too many open files */
#define    ENOTTY        25    /* Not a typewriter */
#define    ETXTBSY        26    /* Text file busy */
#define    EFBIG        27    /* File too large */
#define    ENOSPC        28    /* No space left on device */
#define    ESPIPE        29    /* Illegal seek */
#define    EROFS        30    /* Read-only file system */
#define    EMLINK        31    /* Too many links */
#define    EPIPE        32    /* Broken pipe */
#define    EDOM        33    /* Math argument out of domain of func */
#define    ERANGE        34    /* Math result not representable */

// not base error
#define    EDEADLK        35    /* Resource deadlock would occur */
#define    ENAMETOOLONG    36    /* File name too long */
#define    ENOLCK        37    /* No record locks available */
#define    ENOSYS        38    /* Function not implemented */
#define    ENOTEMPTY    39    /* Directory not empty */
#define    ELOOP        40    /* Too many symbolic links encountered */
#define    EWOULDBLOCK    EAGAIN    /* Operation would block */
#define    ENOMSG        42    /* No message of desired type */
#define    EIDRM        43    /* Identifier removed */
#define    ECHRNG        44    /* Channel number out of range */
#define    EL2NSYNC    45    /* Level 2 not synchronized */
#define    EL3HLT        46    /* Level 3 halted */
#define    EL3RST        47    /* Level 3 reset */
#define    ELNRNG        48    /* Link number out of range */
#define    EUNATCH        49    /* Protocol driver not attached */
#define    ENOCSI        50    /* No CSI structure available */
#define    EL2HLT        51    /* Level 2 halted */
#define    EBADE        52    /* Invalid exchange */
#define    EBADR        53    /* Invalid request descriptor */
#define    EXFULL        54    /* Exchange full */
#define    ENOANO        55    /* No anode */
#define    EBADRQC        56    /* Invalid request code */
#define    EBADSLT        57    /* Invalid slot */

#define    EDEADLOCK    EDEADLK

#define    EBFONT        59    /* Bad font file format */
#define    ENOSTR        60    /* Device not a stream */
#define    ENODATA        61    /* No data available */
#define    ETIME        62    /* Timer expired */
#define    ENOSR        63    /* Out of streams resources */
#define    ENONET        64    /* Machine is not on the network */
#define    ENOPKG        65    /* Package not installed */
#define    EREMOTE        66    /* Object is remote */
#define    ENOLINK        67    /* Link has been severed */
#define    EADV        68    /* Advertise error */
#define    ESRMNT        69    /* Srmount error */
#define    ECOMM        70    /* Communication error on send */
#define    EPROTO        71    /* Protocol error */
#define    EMULTIHOP    72    /* Multihop attempted */
#define    EDOTDOT        73    /* RFS specific error */
#define    EBADMSG        74    /* Not a data message */
#define    EOVERFLOW    75    /* Value too large for defined data type */
#define    ENOTUNIQ    76    /* Name not unique on network */
#define    EBADFD        77    /* File descriptor in bad state */
#define    EREMCHG        78    /* Remote address changed */
#define    ELIBACC        79    /* Can not access a needed shared library */
#define    ELIBBAD        80    /* Accessing a corrupted shared library */
#define    ELIBSCN        81    /* .lib section in a.out corrupted */
#define    ELIBMAX        82    /* Attempting to link in too many shared libraries */
#define    ELIBEXEC    83    /* Cannot exec a shared library directly */
#define    EILSEQ        84    /* Illegal byte sequence */
#define    ERESTART    85    /* Interrupted system call should be restarted */
#define    ESTRPIPE    86    /* Streams pipe error */
#define    EUSERS        87    /* Too many users */
#define    ENOTSOCK    88    /* Socket operation on non-socket */
#define    EDESTADDRREQ    89    /* Destination address required */
#define    EMSGSIZE    90    /* Message too long */
#define    EPROTOTYPE    91    /* Protocol wrong type for socket */
#define    ENOPROTOOPT    92    /* Protocol not available */
#define    EPROTONOSUPPORT    93    /* Protocol not supported */
#define    ESOCKTNOSUPPORT    94    /* Socket type not supported */
#define    EOPNOTSUPP    95    /* Operation not supported on transport endpoint */
#define    EPFNOSUPPORT    96    /* Protocol family not supported */
#define    EAFNOSUPPORT    97    /* Address family not supported by protocol */
#define    EADDRINUSE    98    /* Address already in use */
#define    EADDRNOTAVAIL    99    /* Cannot assign requested address */
#define    ENETDOWN    100    /* Network is down */
#define    ENETUNREACH    101    /* Network is unreachable */
#define    ENETRESET    102    /* Network dropped connection because of reset */
#define    ECONNABORTED    103    /* Software caused connection abort */
#define    ECONNRESET    104    /* Connection reset by peer */
#define    ENOBUFS        105    /* No buffer space available */
#define    EISCONN        106    /* Transport endpoint is already connected */
#define    ENOTCONN    107    /* Transport endpoint is not connected */
#define    ESHUTDOWN    108    /* Cannot send after transport endpoint shutdown */
#define    ETOOMANYREFS    109    /* Too many references: cannot splice */
#define    ETIMEDOUT    110    /* Connection timed out */
#define    ECONNREFUSED    111    /* Connection refused */
#define    EHOSTDOWN    112    /* Host is down */
#define    EHOSTUNREACH    113    /* No route to host */
#define    EALREADY    114    /* Operation already in progress */
#define    EINPROGRESS    115    /* Operation now in progress */
#define    ESTALE        116    /* Stale file handle */
#define    EUCLEAN        117    /* Structure needs cleaning */
#define    ENOTNAM        118    /* Not a XENIX named type file */
#define    ENAVAIL        119    /* No XENIX semaphores available */
#define    EISNAM        120    /* Is a named type file */
#define    EREMOTEIO    121    /* Remote I/O error */
#define    EDQUOT        122    /* Quota exceeded */

#define    ENOMEDIUM    123    /* No medium found */
#define    EMEDIUMTYPE    124    /* Wrong medium type */
#define    ECANCELED    125    /* Operation Canceled */
#define    ENOKEY        126    /* Required key not available */
#define    EKEYEXPIRED    127    /* Key has expired */
#define    EKEYREVOKED    128    /* Key has been revoked */
#define    EKEYREJECTED    129    /* Key was rejected by service */

/* for robust mutexes */
#define    EOWNERDEAD    130    /* Owner died */
#define    ENOTRECOVERABLE    131    /* State not recoverable */

#define ERFKILL        132    /* Operation not possible due to RF-kill */

#define EHWPOISON    133    /* Memory page has hardware error */

6.5 总结

总结一下应注意的事项：

1.write和read的pos指针的含义，llseek的f_pos的处理
2.copy_to/from_user的参数顺序
3.printk的查看方法
4.没有注册license引发的问题，MODULE_LICENSE("GPL")
5.错误类型及返回
6.使用私有数据的方法
7.一套驱动对应多个设备的方法