Linux内核驱动学习（三）字符型设备驱动之初体验
Linux字符型设备驱动之初体验
文章目录
前言
驱动总共分为字符型设备驱动，块设备驱动，网络设备驱动。对于字符型设备驱动的资料，网上比较多，《Linux Kernel Driver》这本书可以了解一下，对于学习Linux驱动有很大的帮助，当然还有很多优秀的书籍，暂不一一列举，本文简单总结了在学习字符型设备驱动的过程中遇到的问题，以及对该类驱动的理解。
框架
字符型设备
什么是字符型设备？字符型以字符(Byte/Char)为单位进行数据传输的设备，如键盘，串口等等设备，所以Linux环境编程中文件I/O进行操作的系统接口如open，read，write，close等等，在字符型设备驱动中同样需要支持这些接口。这里会用到file_operations结构体，在后面会讲到。
程序实现
下面是一个简单字符型设备驱动程序，可以在系统注册一个字符型设备驱动，目前未实现open，read，write，close等接口。
#include <linux/init.h>
#include <linux/module.h>
#include <linux/cdev.h>
#include <linux/fs.h>
#include <linux/slab.h>
#define DRIVER_DATA_SIZE 4096
static int major_dev_index = 0;
struct cnc_character_st{
struct cdev device;
u8 data[DRIVER_DATA_SIZE];
};
static struct cnc_character_st *character_dev;
//TODO
static ssize_t cnc_character_read (struct file * fd, char __user * data, size_t len, loff_t * offset){
ssize_t ret = 0;
printk("%s call\n",__func__);
return ret;
}
//TODO
static ssize_t cnc_character_write (struct file * fd, const char __user * data, size_t len, loff_t * offset){
ssize_t ret = 0;
return ret;
}
//TODO
static long cnc_character_unlocked_ioctl (struct file * fd, unsigned int data, unsigned long cmd){
long ret = 0;
return ret;
}
//TODO
static int cnc_character_open (struct inode * node, struct file * fd){
int ret = 0;
return ret;
}
//TODO
static int cnc_character_release (struct inode * node, struct file * fd){
int ret = 0;
return ret;
}
static const struct file_operations cnc_character_ops = {
.owner = THIS_MODULE,
.read = cnc_character_read,
.write = cnc_character_write,
.open = cnc_character_open,
.unlocked_ioctl = cnc_character_unlocked_ioctl,
.release = cnc_character_release,
};
static int register_device(struct cnc_character_st *mdev,int major_dev_index,int minor_dev_index){
int ret = 0;
int dev_no = MKDEV(major_dev_index, minor_dev_index);
// 初始化dev
cdev_init(&mdev->device, &cnc_character_ops);
mdev->device.owner = THIS_MODULE;
ret = cdev_add(&mdev->device,dev_no,1);
if(ret){
printk(KERN_ERR "cdev add device failed\n");
}
return ret;
}
static int unregister_device(struct cnc_character_st *mdev){
int ret= 0;
kfree(character_dev);
return ret;
}
static int __init cnc_character_init(void){
int ret = 0;
dev_t devno = MKDEV(major_dev_index, 0);
if(major_dev_index){
ret = register_chrdev_region(devno, 1, "cnc_character");
}else{
ret = alloc_chrdev_region(&devno, 0, 1, "cnc_character");
major_dev_index = MAJOR(devno);
}
if(ret < 0){
return ret;
}
character_dev = kmalloc(sizeof(struct cnc_character_st),GFP_KERNEL);
if(!character_dev){
printk("%s failed malloc character_dev call\n",__func__);
ret = -ENOMEM;
goto failed;
}else{
printk("%s success malloc character_dev call\n",__func__);
}
register_device(character_dev,major_dev_index,0);
return 0;
failed:
unregister_chrdev_region(devno, 1);
return ret;
}
module_init(cnc_character_init);
static void __exit cnc_character_exit(void){
printk("%s call\n",__func__);
unregister_device(character_dev);
}
module_exit(cnc_character_exit);
MODULE_AUTHOR("zhaojunhui@cncgroup.top");
MODULE_VERSION("1.0");
MODULE_LICENSE("GPL");
cdev
在linux/cdev.h中可以阅读相关字符型设备驱动的信息，其中包括cdev结构体可以做一下分析，先定位到源码做一下分析
#ifndef _LINUX_CDEV_H
#define _LINUX_CDEV_H
#include <linux/kobject.h>
#include <linux/kdev_t.h>
#include <linux/list.h>
struct file_operations;
struct inode;
struct module;
struct cdev {
struct kobject kobj;
struct module *owner;
const struct file_operations *ops;
struct list_head list;
dev_t dev;
unsigned int count;
};
void cdev_init(struct cdev *, const struct file_operations *);
struct cdev *cdev_alloc(void);
void cdev_put(struct cdev *p);
int cdev_add(struct cdev *, dev_t, unsigned);
void cdev_del(struct cdev *);
void cd_forget(struct inode *);
#endif
其中包括结构体cdev和cdev的一系列函数接口cdev_init，cdev_alloc，cdev_put，cdev_add，cdev_del，cd_forget。
kobj
kobject是所有设备驱动模型的基类，而cdev可以理解为是它的派生类，这里使用了面向对象的思想，通过访问cdev中的kobj成员，就能使用kobject中所有功能。关于kobject的详细内容可以参考内核文档Documentation/kobject.txt。
owner
首先明确一点的是owner是struct module的指针变量，owner=THIS_MODULE;，这里将指针指向当前的模块，关于THIS_MODULE以及struct module的知识可以参考这篇博客。
file_operations
这个结构体位于/linux/include/fs.h，代码如下。
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 (*read_iter) (struct kiocb *, struct iov_iter *);
ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
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);
int (*mmap) (struct file *, struct vm_area_struct *);
int (*open) (struct inode *, struct file *);
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 **, void **);
long (*fallocate)(struct file *file, int mode, loff_t offset,
loff_t len);
void (*show_fdinfo)(struct seq_file *m, struct file *f);
#ifndef CONFIG_MMU
unsigned (*mmap_capabilities)(struct file *);
#endif
};
在file_operations定义了很多I/O操作接口，这里同样使用了面向对象编程的思想，每个接口可以在重新定义file_operations结构体变量的时候，重新赋于自定义功能的函数，如下，可以理解read，write，open，unlocked_ioctl，release是对抽象函数的实现。
static const struct file_operations cnc_character_ops = {
.owner = THIS_MODULE,
.read = cnc_character_read,
.write = cnc_character_write,
.open = cnc_character_open,
.unlocked_ioctl = cnc_character_unlocked_ioctl,
.release = cnc_character_release,
};
dev_t
设备注册过程
设备的初始化在函数cnc_character_init中完成具体的功能实现，主要分为两个部分，设备号的申请和设备的注册。其中设备注册单独封装到register_device函数中。
申请设备号
dev_t devno = MKDEV(major_dev_index, 0);
if(major_dev_index){
ret = register_chrdev_region(devno, 1, "cnc_character");
}else{
ret = alloc_chrdev_region(&devno, 0, 1, "cnc_character");
major_dev_index = MAJOR(devno);
}
注册设备
character_dev = kmalloc(sizeof(struct cnc_character_st),GFP_KERNEL);
if(!character_dev){
printk("%s failed malloc character_dev call\n",__func__);
ret = -ENOMEM;
goto failed;
}else{
printk("%s success malloc character_dev call\n",__func__);
}
register_device(character_dev,major_dev_index,0);
register_device
在register_device中，主要用到了cdev提供的函数接口。
cdev_init初始化一个字符型设备并传入自定义的file_operations类型变量cnc_character_ops。
cdev_add将初始化的字符型设备添加到内核，并分配已经申请好的设备号。
static int register_device(struct cnc_character_st *mdev,int major_dev_index,int minor_dev_index){
int ret = 0;
int dev_no = MKDEV(major_dev_index, minor_dev_index);
// 初始化dev
cdev_init(&mdev->device, &cnc_character_ops);
mdev->device.owner = THIS_MODULE;
ret = cdev_add(&mdev->device,dev_no,1);
if(ret){
printk(KERN_ERR "cdev add device failed\n");
}
return ret;
}
如何构建
模块编译
使用这个Makefile
KVERS = $(shell uname -r)
# Kernel modules
obj-m += demo_character.o
# Specify flags for the module compilation.
EXTRA_CFLAGS=-g -O0
build: kernel_modules
kernel_modules:
make -C /lib/modules/$(KVERS)/build M=$(CURDIR) modules
clean:
make -C /lib/modules/$(KVERS)/build M=$(CURDIR) clean
内核编译
Makefile
obj-$(CONFIG_DEMO_CHARACTER_DRIVER) +=demo_character.o
Kconfig
menuconfig DEMO_DRIVERS
tristate "demo drivers"
config DEMO_CHARACTER_DRIVER
tristate "the most simplest character driver"
help
character driver
endif
总结
总体上来说，字符型设备驱动框架还是相对简单的，通过这次学习加深了对cdev的认识和linux内核源码中面向对象的设计思想，但是这里还没有对devfs和sysfs做相应的介绍，后面继续学习这两者的区别以及总线驱动模型，总之，加油吧。
参考
https://blog.csdn.net/lucky_liuxiang/article/details/83413946
https://www.cnblogs.com/helloahui/p/3677192.html
https://blog.csdn.net/jk110333/article/details/8563647
