Linux字符设备驱动实例—globalmem驱动

1、globalmem虚拟设备实例

globalmem为“全局内存”的意思，在globalmem字符设备中会分配一片大小为GLOBALMEM_SIZE（4KB）的内存空间，并在驱动中提供对这片内存的读写、控制和定位函数，供用户空间的进程能通过Linux系统调用获取和设置这片内存。

（1）头文件、宏以及设备结构体

#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/slab.h>
#include <linux/uaccess.h>

#define GLOBALMEM_SIZE  0x1000
#define MEM_CLEAR  0x1
#define GLOBALMEM_MAJOR  230

static int globalmem_major = GLOBALMEM_MAJOR;
module_param(globalmem_major, int, S_IRUGO);

struct globalmem_dev {
    struct cdev cdev;
    unsigned char mem[GLOBALMEM_SIZE];
};

struct globalmem_dev *globalmem_devp;

定义的globalmem_dev结构体中，包含了对应于globalmem字符设备的cdev，使用的内存mem[GLOBALMEM_SIZE]。

（2）globalmem设备驱动模块的加载和卸载函数

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;
    err = cdev_add(&dev->cdev, devno, 1);
    if (err) {
        printk(KERN_NOTICE "Error %d adding globalmem %d", err, index);
    }
    
}

static int __init globalmem_init(void)
{
    int ret;
    dev_t devno = MKDEV(globalmem_major, 0);

    if (globalmem_major) {
        ret = register_chrdev_region(devno, 1, "globalmem");
    } else {
        ret = alloc_chrdev_region(&devno, 0, 1, "globalmem");
        globalmem_major = MAJOR(devno);
    }
    if (ret < 0)
        return ret;

    globalmem_devp = kzalloc(sizeof(struct globalmem_dev), GFP_KERNEL);
    if (!globalmem_devp) {
        ret = -ENOMEM;
        goto fail_malloc;
    }

    globalmem_setup_cdev(globalmem_devp, 0);
    return 0;

fail_malloc:
    unregister_chrdev_region(devno, 1);
    return ret;
}

static void __exit globalmem_exit(void)
{
    cdev_del(&globalmem_devp->cdev);
    kfree(globalmem_devp);
    unregister_chrdev_region(MKDEV(globalmem_major, 0), 1);
}

globalmem_setup_cdev()函数完成cdev的初始化化和添加，kzalloc()申请了一份globalmem_dev结构体的内存，并将其清0，在cdev_init()函数中，与globalmem的cdev关联的file_operations结构体如下所示：

static const struct file_operations globalmem_fops = {
    .owner = THIS_MODULE,
    .open = globalmem_open,
    .release = globalmem_release,
    .read = globalmem_read,
    .write = globalmem_write,
    .llseek = globalmem_llseek,
    .unlocked_ioctl = globalmem_ioctl,
};

（3）读写函数的实现

首先是读函数，函数的实现如下所示：

static ssize_t globalmem_read(struct file *filp, 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_to_user(buf, dev->mem + p, count)) {
        ret = -EFAULT;
    } else {
        *ppos += count;
        ret = count;

        printk(KERN_INFO "read %u bytes(s) from %lu\n", count, p);
    }

    return ret;
}

其中*ppos是读的位置相对于文件开头的漂移，如果该漂移大于或等于GLOBALMEM_SIZE，表示文件已经到了末尾，返回0（EOF）。

写函数的实现如下所示：

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)) {
        return -EFAULT;
    } else {
        *ppos += count;
        ret = count;

        printk(KERN_INFO "written %u bytes(s) from %lu\n", count, p);
    }

    return ret;
}

（4）seek函数的实现

seek()函数对文件定位的起始地址可以是文件开头（SEEK_SET,0）、当前位置（SEEK_CUR,1）和文件末尾（SEEK_END,2），在定位的时候，要检查用户请求的合法性，若不合法，函数返回错误号，若合法，更新文件的当前位置，并返回新的位置，实现如下所示：

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;
}

（5）ioctl函数实现

static long globalmem_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    struct globalmem_dev *dev = filp->private_data;

    switch (cmd) {
    case MEM_CLEAR:
        memset(dev->mem, 0, GLOBALMEM_SIZE);
        printk(KERN_INFO "globalmem is set to zero\n");
        break;
    default:
        return -EINVAL;
    }

    return 0;
}

（6）使用文件的私有数据

将文件的私有数据private_data指向设备的结构体，然后使用read()、write()、ioctl()、llseek()等函数通过private_data访问设备结构体，如下所示：

static int globalmem_open(struct inode *inode, struct file *filp)
{
    filp->private_data = globalmem_devp;
    return 0;
}

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

（7）完整的globalmem驱动代码

#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/slab.h>
#include <linux/uaccess.h>

#define GLOBALMEM_SIZE  0x1000
#define MEM_CLEAR  0x1
#define GLOBALMEM_MAJOR  230

static int globalmem_major = GLOBALMEM_MAJOR;
module_param(globalmem_major, int, S_IRUGO);

struct globalmem_dev {
    struct cdev cdev;
    unsigned char mem[GLOBALMEM_SIZE];
};

struct globalmem_dev *globalmem_devp;

static int globalmem_open(struct inode *inode, struct file *filp)
{
    filp->private_data = globalmem_devp;
    return 0;
}

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

static long globalmem_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    struct globalmem_dev *dev = filp->private_data;

    switch (cmd) {
    case MEM_CLEAR:
        memset(dev->mem, 0, GLOBALMEM_SIZE);
        printk(KERN_INFO "globalmem is set to zero\n");
        break;
    default:
        return -EINVAL;
    }

    return 0;
}

static ssize_t globalmem_read(struct file *filp, 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_to_user(buf, dev->mem + p, count)) {
        ret = -EFAULT;
    } else {
        *ppos += count;
        ret = count;

        printk(KERN_INFO "read %u bytes(s) from %lu\n", count, p);
    }

    return ret;
}

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)) {
        return -EFAULT;
    } else {
        *ppos += count;
        ret = count;

        printk(KERN_INFO "written %u bytes(s) from %lu\n", count, p);
    }

    return ret;
}

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;
}

static const struct file_operations globalmem_fops = {
    .owner = THIS_MODULE,
    .llseek = globalmem_llseek,
    .read = globalmem_read,
    .write = globalmem_write,
    .unlocked_ioctl = globalmem_ioctl,
    .open = globalmem_open,
    .release = globalmem_release,
};

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;
    err = cdev_add(&dev->cdev, devno, 1);
    if (err) {
        printk(KERN_NOTICE "Error %d adding globalmem %d", err, index);
    }
    
}

static int __init globalmem_init(void)
{
    int ret;
    dev_t devno = MKDEV(globalmem_major, 0);

    if (globalmem_major) {
        ret = register_chrdev_region(devno, 1, "globalmem");
    } else {
        ret = alloc_chrdev_region(&devno, 0, 1, "globalmem");
        globalmem_major = MAJOR(devno);
    }
    if (ret < 0)
        return ret;

    globalmem_devp = kzalloc(sizeof(struct globalmem_dev), GFP_KERNEL);
    if (!globalmem_devp) {
        ret = -ENOMEM;
        goto fail_malloc;
    }

    globalmem_setup_cdev(globalmem_devp, 0);
    return 0;

fail_malloc:
    unregister_chrdev_region(devno, 1);
    return ret;
}

static void __exit globalmem_exit(void)
{
    cdev_del(&globalmem_devp->cdev);
    kfree(globalmem_devp);
    unregister_chrdev_region(MKDEV(globalmem_major, 0), 1);
}

module_init(globalmem_init);
module_exit(globalmem_exit);

MODULE_AUTHOR("HLY");
MODULE_LICENSE("GPL");

（8）globalmem驱动验证

使用make命令将源文件编译出驱动模块globalmem.ko文件，编译需要的Makefile如下所示：

# Makefile for globalmem driver

obj-m += globalmem.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

然后使用驱动模块命令加载模块，如下：

$ sudo insmod globalmem.ko
$ lsmod

然后使用下面的命令查看globalmem虚拟设备的设备号：

$ cat /proc/devices

然后，使用mknod创建设备节点：

# mknod /dev/globalmem c 230 0
# ls -al /dev/globalmem

接下来使用命令对该文件进行读写以测试：

# echo “Hello World” > /dev/globalmem
# cat /dev/globalmem

也可以使用系统调用函数open、write和read进行该虚拟设备的测试，测试的app.c文件如下：

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

#define LENGTH  100

int main(int argc, char *argv[])
{
    int fd,len;
    char str[LENGTH];

    fd = open("/dev/globalmem", O_RDWR);
    if (fd) {
        write(fd, "Hello World", strlen("Hello World"));
        close(fd);
    }

    fd = open("/dev/globalmem", O_RDWR);
    len = read(fd, str, LENGTH);
    str[len] = '\0';
    printf("str:%s\n", str);
    close(fd);

    return 0;
}

编写该app.c的Makefile文件，如下：

# Makefile by HLY

all: myapp

# Which compiler
CC = gcc

# Where are include files
INCLUDE = .

# Options for development
CFLAGS = -g -Wall -ansi

myapp: app.o
    $(CC) -o myapp app.o

clean:
    rm -rf *.o myapp

使用make命令将app.c编译成可执行文件myapp，然后执行程序，即可完成文件的读写。