linux驱动：[2]字符设备驱动memdev(cdev结构解析)

linux驱动：[2]字符设备驱动memdev

Linux 内存模拟字符设备 驱动程序

测试平台： Xunlong Orange Pi Zero

代码一览（解析见下方）

驱动程序以及Makefile如下：

• memdev.c:

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

#ifndef MEMDEV_MAJOR
#define MEMDEV_MAJOR 254
#endif

#ifndef MEMDEV_NR_DEVS
#define MEMDEV_NR_DEVS 2
#endif

#ifndef MEMDEV_SIZE
#define MEMDEV_SIZE 4096
#endif

struct mem_dev {
    char *data;
    unsigned long size;
};

static int mem_major = MEMDEV_MAJOR;

module_param(mem_major, int, S_IRUGO);

struct mem_dev *mem_devp;

struct cdev cdev;

int mem_open(struct inode *inode, struct file *filp)
{
    struct mem_dev *dev;

    int num = MINOR(inode->i_rdev);

    if (num >= MEMDEV_NR_DEVS)
        return -ENODEV;
    dev = &mem_devp[num];

    filp->private_data = dev;

    return 0;
}

int mem_release(struct inode *inode, struct file *filp)
{
    return 0;
}

static ssize_t mem_read(struct file *filp, char __user *buf, size_t size, loff_t *poss)
{
    unsigned long p = *poss;
    unsigned int count = size;
    int ret = 0;
    struct mem_dev *dev = filp->private_data;

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

    if(copy_to_user(buf, (void*)(dev->data + p), count)) {
        ret = -EFAULT;
    } else {
        *poss += count;
        ret = count;
        printk(KERN_INFO "read %d bytes from %lu\n", count, p);
    }

    return ret;
}

static ssize_t mem_write(struct file *filp, const char __user *buf, size_t size, loff_t *poss)
{
    unsigned long p = *poss;
    unsigned int count = size;
    int ret = 0;
    struct mem_dev *dev = filp->private_data;

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

    if (copy_from_user(dev->data + p, buf, count)) {
        ret = -EFAULT;
    } else {
        *poss += count;
        ret = count;
        printk(KERN_INFO "write %d bytes from %lu\n", count, p);
    }

    return ret;
}

static loff_t mem_llseek(struct file *filp, loff_t offset, int whence)
{
    loff_t newpos;

    switch (whence) {
    case 0:
        newpos = offset;
        break;
    case 1:
        newpos = filp->f_pos + offset;
        break;
    case 2:
        newpos = MEMDEV_SIZE - 1 + offset;
        break;
    default:
        return -EINVAL;
    }
    if ((newpos < 0) || (newpos > MEMDEV_SIZE))
        return -EINVAL;

    filp->f_pos = newpos;
    return newpos;
}

static const struct file_operations mem_fops =
{
    .owner = THIS_MODULE,
    .llseek = mem_llseek,
    .read = mem_read,
    .write = mem_write,
    .open = mem_open,
    .release = mem_release,
};

static int memdev_init(void)
{
    int result;
    int i;
    dev_t devno = MKDEV(mem_major, 0);

    if (mem_major)
        result = register_chrdev_region(devno, 2, "memdev");
    else {
        result = alloc_chrdev_region(&devno, 0, 2, "memdev");
        mem_major = MAJOR(devno);
    }

    if (result < 0)
        return result;

    cdev_init(&cdev, &mem_fops);
    cdev.owner = THIS_MODULE;
    cdev.ops = &mem_fops;

    cdev_add(&cdev, MKDEV(mem_major, 0), MEMDEV_NR_DEVS);

    mem_devp = kmalloc(MEMDEV_NR_DEVS * sizeof(struct mem_dev), GFP_KERNEL);
    if (!mem_devp) {
        result = -ENOMEM;
        goto fail_malloc;
    }

    memset(mem_devp, 0, MEMDEV_NR_DEVS * sizeof(struct mem_dev));

    for (i = 0; i < MEMDEV_NR_DEVS; i++) {
        mem_devp[i].size = MEMDEV_SIZE;
        mem_devp[i].data = kmalloc(MEMDEV_SIZE, GFP_KERNEL);
        memset(mem_devp[i].data, 0, MEMDEV_SIZE);
    }

    printk("memdev init success\n");
    return 0;

fail_malloc:
    unregister_chrdev_region(devno, 2);
    return result;
}

static void memdev_exit(void)
{
    cdev_del(&cdev);
    kfree(mem_devp);
    unregister_chrdev_region(MKDEV(mem_major, 0), 2);
    printk("memdev exit success\n");
}

MODULE_AUTHOR("Ziping Chen ");
MODULE_LICENSE("GPL");

module_init(memdev_init);
module_exit(memdev_exit);

• Makefile:

obj-m := memdev.o #编译进模块
KERNELDIR := /lib/modules/4.11.0-rc4-00064-g89970a0-dirty/build #此处为linux内核库目录
PWD := $(shell pwd) #获取当前目录
OUTPUT := $(obj-m) $(obj-m:.o=.ko) $(obj-m:.o=.mod.o) $(obj-m:.o=.mod.c) modules.order Module.symvers
 
modules:
    $(MAKE) -C $(KERNELDIR) M=$(PWD) modules

clean:
    rm -rf $(OUTPUT)

在shell中使用以下命令装载驱动程序：
(这里以主设备号为181进行测试)

$ make
$ insmod memdev.ko mem_major=181
$ mknod /dev/memdev0 c 181 0

使用linux c进行测试：

• memapp.c:

#include 
#include 
#include 
#include 
#include 

int main()
{
    int fd;
    char buf[4096];

    strcpy(buf,"memory simulate char device test...\n");
    printf("original buf:%s\n",buf); 

    fd = open("/dev/memdev0",O_RDWR);
    if (fd == -1) {
        printf("open memdev failed!\n");
        return -1;
    }
    write(fd, buf, sizeof(buf));
    lseek(fd, 0, SEEK_SET);
    strcpy(buf, "nothing here");
    read(fd, buf, sizeof(buf));
    printf("read buf:%s\n", buf);

    return 0;
}

进行编译、测试：

$ gcc -o memapp memapp.c

实验成功！

---

代码解析：

一、分配设备号

if (mem_major)
        result = register_chrdev_region(devno, 2, "memdev");
else {
        result = alloc_chrdev_region(&devno, 0, 2, "memdev");
        mem_major = MAJOR(devno);
}

如果定义的参数mem_major不为0(上面测试用了181),则进行静态分配

register_chrdev_region(devno, 2, "memdev");//静态分配设备号为devno的设备

如果mem_major为0则进行动态分配

alloc_chrdev_region(&devno, 0, 2, "memdev");//动态分配主设备号为devno，次设备号为0的设备

二、初始化cdev结构

/linux/include/linux/cdev.h:

struct cdev {
    struct kobject kobj;//每个 cdev 都是一个 kobject
    struct module *owner;//指向实现驱动的模块
    const struct file_operations *ops;//操纵这个字符设备文件的方法
    struct list_head list;//与 cdev 对应的字符设备文件的 inode->i_devices 的链表头
    dev_t dev;//起始设备编号
    unsigned int count;//设备范围号大小
};

一个 cdev 一般它有两种定义初始化方式：静态的和动态的。

静态内存定义初始化：

struct cdev my_cdev;
cdev_init(&my_cdev, &fops);
my_cdev.owner = THIS_MODULE;

动态内存定义初始化：

struct cdev *my_cdev = cdev_alloc();
my_cdev->ops = &fops;
my_cdev->owner = THIS_MODULE;

两种使用方式的功能是一样的，只是使用的内存区不一样，一般视实际的数据结构需求而定。

源码分析：

struct cdev *cdev_alloc(void)
{
   struct cdev *p = kzalloc(sizeof(struct cdev), GFP_KERNEL);
   if (p) {
       INIT_LIST_HEAD(&p->list);
       kobject_init(&p->kobj, &ktype_cdev_dynamic);
   }
   return p;
}

void cdev_init(struct cdev *cdev, const struct file_operations *fops)
{
   memset(cdev, 0, sizeof *cdev);
   INIT_LIST_HEAD(&cdev->list);
   kobject_init(&cdev->kobj, &ktype_cdev_default);
   cdev->ops = fops;
}

可见，两个函数完成都功能基本一致。

三、添加cdev

初始化 cdev 后，需要把它添加到系统中去。为此可以调用 cdev_add() 函数。传入 cdev 结构的指针，起始设备编号，以及设备编号范围。

int cdev_add(struct cdev *p, dev_t dev, unsigned count)
{
   p->dev = dev;
   p->count = count;
   return kobj_map(cdev_map, dev, count, NULL, exact_match, exact_lock, p);
}

简单地说，设备驱动程序通过调用cdev_add把它所管理的设备对象的指针嵌入到一个类型为struct probe的节点之中，然后再把该节点加入到cdev_map所实现的哈希链表中。

对系统而言，当设备驱动程序成功调用了cdev_add之后，就意味着一个字符设备对象已经加入到了系统，在需要的时候，系统就可以找到它。对用户态的程序而言，cdev_add调用之后，就已经可以通过文件系统的接口调用到我们的驱动程序。

四、卸载cdev

当一个字符设备驱动不再需要的时候（比如模块卸载），就可以用 cdev_del() 函数来释放 cdev 占用的内存。

void cdev_del(struct cdev *p)
{
   cdev_unmap(p->dev, p->count);
   kobject_put(&p->kobj);
}

其中 cdev_unmap() 调用 kobj_unmap() 来释放 cdev_map 散列表中的对象。kobject_put() 释放 cdev 结构本身。

---

• 我的个人主页：http://www.techping.cn/
• 我的个人站点博客：http://www.techping.cn/blog/wordpress/
• 我的CSDN博客：http://blog.csdn.net/techping
• 我的：http://www.jianshu.com/users/b2a36e431d5e/timeline
• 我的GitHub：https://github.com/techping
  欢迎相互follow～