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

• Linux字符型设备驱动之初体验
• 前言
• 框架
• 字符型设备
• 程序实现
• cdev
• kobj
• owner
• file_operations
• dev_t
• 设备注册过程
• 申请设备号
• 注册设备
• register_device
• 如何构建
• 模块编译
• 内核编译
• Makefile
• Kconfig
• 总结
• 参考

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
",__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
");
	}
	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
",__func__);
		ret = -ENOMEM;
		goto failed;
	}else{
		printk("%s success malloc character_dev call
",__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
",__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
",__func__);
    ret = -ENOMEM;
    goto failed;
}else{
    printk("%s success malloc character_dev call
",__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
");
	}
	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