【Linux驱动】字符设备驱动

,本文假定读者具备一定linux基础以及对linux驱动基础有所了解。

➜  ~  cat /proc/version
Linux version 3.13.0-43-generic (buildd@akateko) (gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1) )
#72-Ubuntu SMP Mon Dec 8 19:35:44 UTC 2014

globalmem.c

关键词：register_chrdev_region()、alloc_chrdev_region()、unregister_chrdev_region()、cdev_init()、cdev_add()、cdev_del()

#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 <linux/slab.h>

MODULE_LICENSE("Dual BSD/GPL");

#define GLOBALMEM_SIZE 0X1000
#define MEM_CLEAR 0x1
#define GLOBALMEM_MAJOR 240

static int globalmem_major = GLOBALMEM_MAJOR;

//globalmem设备结构体
struct globalmem_dev{
struct cdev cdev;
unsigned char mem[GLOBALMEM_SIZE];//全局内存
};

struct globalmem_dev *globalmem_devp;//设备结构指针

//文件打开函数
int globalmem_open(struct inode *inode, struct file *filp)
{
//将设备结构体指针赋值给文件私有数据指针
filp->private_data = globalmem_devp;
printk(KERN_ALERT "globalmem open\n");
return 0;
}

//文件释放函数
int globalmem_release(struct inode *inode, struct file *filp)
{
printk(KERN_ALERT "globalmem release\n");
return 0;
}

//ioctl设备控制函数,linux kernel高版本中ioctl已经发生了改变，用了其余两个替换
/*static int globalmem_ioctl(struct inode *inodep, struct file *filp,
unsigned int cmd, unsigned long arg)
{
struct globalmem_dev *dev = filp->private_data;//获得设备结构体指针
switch(cmd)
{
case MEM_CLEAR://清空全局内存
memset(dev->mem, 0, GLOBALMEM_SIZE);
printk(KERN_INFO "globalmem is set to zero\n");
break;
default:
return -EINVAL;
}
return 0;
}*/

//读函数
static ssize_t globalmem_read(struct file *filp, char __user *buf, size_t count, loff_t *ppos)
{
unsigned long p = *ppos;
int ret = 0;
struct globalmem_dev *dev = filp->private_data;

if(p >= GLOBALMEM_SIZE)
return 0;
if(count > GLOBALMEM_SIZE - p)
count = GLOBALMEM_SIZE -p;
if(copy_to_user(buf, (void*)(dev->mem +p), count))
ret = -EFAULT;
else
{
*ppos += count;
ret = count;
//      printk(KERN_INFO "read %d byte(s) from %d", count, p);
}
return ret;
}

//写函数
static ssize_t globalmem_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 globalmem_dev *dev = filp->private_data;

if(p >= GLOBALMEM_SIZE)
return 0;
if(count > GLOBALMEM_SIZE - p)
count = GLOBALMEM_SIZE - p;

if(copy_from_user(dev->mem+p,buf,count))
ret = -EFAULT;
else
{
*ppos += count;
ret = count;
//      printk(KERN_INFO "written %d byte(s) form %d\n", count, p);
}
return ret;
}

//seek文件定位函数
static loff_t globalmem_llseek(struct file *filp, loff_t offset, int orig)
{
loff_t ret = 0;
switch(orig)
{
case 0:
if(offset < 0)
{
ret = -EINVAL;
break;
}
if((unsigned int)offset > GLOBALMEM_SIZE)
{
ret = -EINVAL;
break;
}
filp->f_pos = (unsigned int)offset;
ret = filp->f_pos;
break;
case 1:
if((filp->f_pos + offset) > GLOBALMEM_SIZE)
{
ret = -EINVAL;;
break;
}
if((filp->f_pos + offset) < 0)
{
ret = -EINVAL;
break;
}
filp->f_pos += offset;
ret = filp->f_pos;
break;
default:
ret = -EINVAL;
break;
}
return ret;
}

//文件操作结构体
static const struct file_operations globalmem_fops =
{
.owner = THIS_MODULE,
.llseek = globalmem_llseek,
.read = globalmem_read,
.write = globalmem_write,
//  .ioctl = globalmem_ioctl,//Linux kernel高版本中ioctl已被两个XX_ioctl替代，参见下面注释
.open = globalmem_open,
.release = globalmem_release,
};

//初始化并注册cdev
static void globalmem_setup_cdev(struct globalmem_dev *dev, int index)
{
int err,devno = MKDEV(globalmem_major, index);
cdev_init(&dev->cdev, &globalmem_fops);
dev->cdev.owner = THIS_MODULE;
dev->cdev.ops = &globalmem_fops;
err = cdev_add(&dev->cdev, devno, 1);
if(err)
printk(KERN_NOTICE "Error %d adding globalmem %d", err, index);
}

//设备驱动模块加载函数
int globalmem_init(void)
{
int result;
dev_t devno = MKDEV(globalmem_major, 0);

//申请设备号
if(globalmem_major)
result = register_chrdev_region(devno,1,"globalmem");
else//动态申请设备号
{
result = alloc_chrdev_region(&devno,0,1,"globalmem");
globalmem_major = MAJOR(devno);
}
if(result < 0)
return result;
printk(KERN_ALERT "T1\n");
//动态申请设备结构体的内存
globalmem_devp = (struct globalmem_dev *)kmalloc(sizeof(struct globalmem_dev),GFP_KERNEL);
printk(KERN_ALERT "T0\n");

if(!globalmem_devp)
{
result = -ENOMEM;
goto fail_malloc;
}
memset(globalmem_devp, 0, sizeof(struct globalmem_dev));
globalmem_setup_cdev(globalmem_devp,0);
return 0;

fail_malloc:
unregister_chrdev_region(devno,1);

return result;
}

//模块卸载函数
void globalmem_exit(void)
{
cdev_del(&globalmem_devp->cdev);
kfree(globalmem_devp);
unregister_chrdev_region(MKDEV(globalmem_major,0),1);
}

module_param(globalmem_major, int,S_IRUGO);

module_init(globalmem_init);
module_exit(globalmem_exit);

下面是Linux kernel 3.14的源码中的file_operations结构体（include/linux/fs.h），由于有时候内核版本的升级会给接口带来一定的变化，所以在写驱动时，应该同时具有该版本的源码，以编写出对应内核接口的函数。所以对于上面的ioctl函数，自行参考修改一下即可编译通过。

struct file_operations {
struct module *owner;
loff_t (*llseek) (struct file *, loff_t, int);
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
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 *);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
......
}

Makefile

ifneq ($(KERNELRELEASE),)
obj-m := globalmem.o
else
KERNELDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
all:
$(MAKE) -C $(KERNELDIR) SUBDIRS=$(PWD) modules
endif
clean:
rm -f *.o *.ko *.mod.c .globalmem*

用户态程序test.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
int fd;
fd = open("/dev/globalmem", O_RDWR);

if(fd < 0)
{
perror("open");
}
return EXIT_SUCCESS;
}

运行测试程序

root@selfimpr-pc:/home/selfimpr/wenqian/lkp/globalmem#insmod globalmem.ko
root@selfimpr-pc:/home/selfimpr/wenqian/lkp/globalmem# cat /proc/devices  | grep "globalmem"
240 globalmem
root@selfimpr-pc:/home/selfimpr/wenqian/lkp/globalmem# mknod /dev/globalmem c 240 0
root@selfimpr-pc:/home/selfimpr/wenqian/lkp/globalmem# ll /dev/globalmem
crw-r--r-- 1 root root 240, 0 12月  2 19:47 /dev/globalmem
root@selfimpr-pc:/home/selfimpr/wenqian/lkp/globalmem#./test
root@selfimpr-pc:/home/selfimpr/wenqian/lkp/globalmem#dmesg
[210299.896657] T1
[210299.896665] T0
[210393.405124] globalmem open
[210393.405201] globalmem release

上面一个简单的case，只是大致的概述下linux字符设备驱动的编写流程，但这也是字符驱动的一个最初雏形，类似于单片机的最小系统之类。

在linux内核中，使用cdev结构体描述一个字符设备

struct cdev
{
struct kobject kobj;//内嵌的kobject对象
struct module *owner;//所属模块
struct file_operations *ops;//文件操作结构体
struct list_head list;
dev_t dev;//设备号,长度为32位，其中高12为主设备号，低20位为此设备号
unsigned int count;
};

关于设备号请具体参考任何一本linux字符驱动书籍。

cdev结构体中一个重要成员file_operations定义了字符设备驱动提供给虚拟文件系统的接口函数（file_operations请参考前面的博文文件系统系列）。而下层的具体实现，则根据用户及驱动编写。

linux内核提供了一组函数用于操作cdev结构体：

void cdev_init(struct cdev *, struct file_operations *);//初始化cdev成员
struct cdev *cdev_alloc(void);//动态申请一个cdev内存
int cdev_add(struct cdev *, dev_t, unsigned);//向系统添加一个cdev
void cdev_del(struct cdev *);//向系统删除一个cdev

分配和释放设备号

在调用cdev_add()函数向系统注册字符设备之前，应首先调用 register_chrdev_region() 或 alloc_chrdev_region()函数向系统申请设备号

int register_chrdev_region(dev_t first, unsigned count, const char *name);
int alloc_chrdev_region(dev_t *dev, unsigned firstminor, unsigned int count, const char *name);

其中register_chrdev_region函数用于已知起始设备的设备号的情况，而alloc_chrdev_region函数用于设备号未知，向系统动态申请未被占用的设备号的情况，函数调用成功以后，会把得到的设备放入第一个参数dev中。

相反的，在调用cdev_del函数从系统注销字符设备之后，unregiter_chrdev_region()应该用以释放原先申请的设备号：

void unregister_chrdev_region(dev_t first, unsigned count);

我们可以使用下列宏从dev_t获得主设备号和次设备号：

MAJOR(dev_t dev)
MINOR(dev_t dev)

使用下列宏（也是我们通常使用的）则可以通过主设备号和次设备号生成dev_t

MKDEV(int major, int minor)