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