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HaveFunWithEmbeddedSystem/Chapter8_SOC_与_Linux/8.6_字符设备驱动.md

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8.6 字符设备驱动

Linux 系统中，字符设备、块设备、网络设备是从使用角度来划分的。

字符设备：是指只能一个字节一个字节读写的设备，不能随机读取设备内存中的某一数据，读取数据需要按照先后数据。字符设备是面向流的设备，常见的字符设备有鼠标、键盘、串口、控制台和 LED 设备等。

块设备：是指可以从设备的任意位置读取一定长度数据的设备。块设备包括硬盘、磁盘、U 盘和 SD 卡等。

字符设备驱动示例

/**
 * @file    demo_char.c
 * @author  Rick Chan (cy187lion@sina.com)
 * @brief   Linux char driver demo.
 * @version 0.1.0
 * @date    2020-04-27
 *
 * @copyright   Copyright (c) 2020
 *
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <linux/uaccess.h>
#include <linux/fs.h>

#include <linux/slab.h>
#include <linux/device.h>

MODULE_AUTHOR("Rick Chan");
MODULE_LICENSE("GPL");

#define DEMO_MAGIC          'R'
#define DEMO_IOC_SIZE       8
#define DEMO_CTL_IOC        _IOC(_IOC_READ|_IOC_WRITE, DEMO_MAGIC, 0, DEMO_IOC_SIZE)
#define DEMO_CTL_IO         _IO(DEMO_MAGIC, 1)
#define DEMO_CTL_IOR        _IOR(DEMO_MAGIC, 2, uint16_t)
#define DEMO_CTL_IOW        _IOW(DEMO_MAGIC, 3, int32_t)
#define DEMO_CTL_IOWR       _IOWR(DEMO_MAGIC, 4, uint32_t)
#define DEMO_CTL_MAX        5
#define DEMO_MODULE_NAME    "demo_char"

#define DEMO_DEV_CNT        2

// This is a test.
#define DEMO_DATA_SIZE      5

static int demo_major =     0;

struct demo_dev
{
    struct cdev cdev;
    struct device *dev;
    // This is a test.
    char demo_text[DEMO_DATA_SIZE];
};

struct class *class;
struct demo_dev* demo_devp;

static int demo_open(struct inode *inode, struct file *filp)
{
    struct demo_dev *demo;
    demo = container_of(inode->i_cdev, struct demo_dev, cdev);
    filp->private_data = demo;

    return 0;
}

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

static loff_t demo_llseek(struct file *filp, loff_t offset, int origin)
{
    struct demo_dev *devp = filp->private_data;
    loff_t ret;

    (void)devp;
    // This is a test.
    switch(origin)
    {
    case 0:     // 从文件开头开始偏移
        if(offset<0)
        {
            ret = -EINVAL;
            break;
        }
        if((unsigned int)offset>DEMO_DATA_SIZE)
        {
            ret = -EINVAL;
            break;
        }
        filp->f_pos = (unsigned int)offset;
        ret = filp->f_pos;
        break;
    case 1:     // 从当前位置开始偏移
        if((filp->f_pos+offset)>DEMO_DATA_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;
    }

    return ret;
    return filp->f_pos;
}

static ssize_t demo_read(struct file *filp, char __user *buffer, size_t count, loff_t *position)
{
    struct demo_dev *devp = filp->private_data;
    loff_t p = *position;
    ssize_t ret = 0;

    // This is a test.
    // 分析和获取有效的读长度
    if(DEMO_DATA_SIZE<=p)   // 要读的偏移位置越界
        return 0;           // End of a file
    if(DEMO_DATA_SIZE<(count+p))    // 要读的字节数太大
        count = DEMO_DATA_SIZE-p;

    if(copy_to_user((void*)buffer, &devp->demo_text[p], count))
        ret = -EFAULT;
    else
    {
        *position += count;
        ret = count;
    }
    return ret;
}

static ssize_t demo_write(struct file *filp, const char __user *buffer, size_t count, loff_t *position)
{
    struct demo_dev *devp = filp->private_data;
    const char __user *p = buffer;

    (void)devp;
    return p-buffer;
}

static long demo_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    struct demo_dev *devp = filp->private_data;
    (void)devp;

    // 检测 cmd 合法性
    if (DEMO_MAGIC!=_IOC_TYPE(cmd))
        return -EINVAL;
    if (DEMO_CTL_MAX<_IOC_NR(cmd))
        return -EINVAL;

    switch(cmd)
    {
    case DEMO_CTL_IOC:
    {
        uint8_t tmp[DEMO_IOC_SIZE];
        copy_from_user(&tmp, (void*)arg, _IOC_SIZE(cmd));
        copy_to_user((void*)arg, &tmp, _IOC_SIZE(cmd));
    }
        break;
    case DEMO_CTL_IO:
        break;
    case DEMO_CTL_IOR:
    {
        uint16_t tmp = 0x55AA;
        copy_to_user((void*)arg, &tmp, _IOC_SIZE(cmd));
    }
        break;
    case DEMO_CTL_IOW:
    {
        int32_t tmp = 0;
        copy_from_user(&tmp, (void*)arg, _IOC_SIZE(cmd));
    }
        break;
    case DEMO_CTL_IOWR:
    {
        uint32_t tmp = 0;
        copy_from_user(&tmp, (void*)arg, _IOC_SIZE(cmd));
        copy_to_user((void*)arg, &tmp, _IOC_SIZE(cmd));
    }
        break;
    default:
        return -ENOIOCTLCMD;
    }
    return 0;
}

static int demo_mmap(struct file *filp, struct vm_area_struct *vma)
{
    struct demo_dev *devp = filp->private_data;
    (void)devp;

    return 0;
}

static struct file_operations demo_fops = {
    .owner          = THIS_MODULE,
    .open           = demo_open,
    .release        = demo_release,
    .llseek         = demo_llseek,
    .read           = demo_read,
    .write          = demo_write,
    .unlocked_ioctl = demo_ioctl,
    .mmap           = demo_mmap
};

static int demo_setup_cdev(struct demo_dev *devp, int index)
{
    char name[16];
    int err, devno = MKDEV(demo_major, index);

    cdev_init(&devp->cdev, &demo_fops);
    devp->cdev.owner = THIS_MODULE;
    err = cdev_add(&devp->cdev, devno, 1);
    if(err)
    {
        printk(KERN_ERR "demo add cdev:%d error:%d.\r\n", index, err);
        goto out_cdev;
    }

    // 创建设备节点
    memset(name, 0, 16);
    sprintf(name, DEMO_MODULE_NAME"%d", index);
    printk(KERN_INFO "demo new dev name:%s", name);
    devp->dev = device_create(class, NULL, devp->cdev.dev, NULL, name);
    // This is a test.
    devp->demo_text[DEMO_DATA_SIZE-2] = '\n';
    devp->demo_text[DEMO_DATA_SIZE-1] = 0;
    sprintf(devp->demo_text, "%d", index);
    return 0;
out_cdev:
    cdev_del(&devp->cdev);
    kfree(devp);
    return err;
}

static int __init demo_init(void)
{
    int err, i;
    dev_t devno = MKDEV(demo_major, 0);

    printk(KERN_ALERT "demo_init.demo_major: %d\n", demo_major);
    if(demo_major)  // 使用固定主设备号
        err = register_chrdev_region(devno, DEMO_DEV_CNT, DEMO_MODULE_NAME);
    else            // 动态分配主设备号
    {
        err = alloc_chrdev_region(&devno, 0, DEMO_DEV_CNT, DEMO_MODULE_NAME);
        demo_major = MAJOR(devno);
    }

    if(err<0)
        return err;

    demo_devp = kzalloc(DEMO_DEV_CNT*sizeof(struct demo_dev), GFP_KERNEL);
    if(!demo_devp)
    {
        err = -ENOMEM;
        goto out;
    }

    // 创建设备类, 子设备属于同一个设备类
    class = class_create(THIS_MODULE, DEMO_MODULE_NAME);
    for(i=0; i<DEMO_DEV_CNT; i++)
    {
        err = demo_setup_cdev(&demo_devp[i], i);
        if(err)
            goto out_class;
    }

    return 0;

out_class:
    class_destroy(class);
out:
    unregister_chrdev_region(devno, DEMO_DEV_CNT);
    return err;
}

static void demo_clean_cdev(struct demo_dev *devp)
{
    device_destroy(class, devp->cdev.dev);
    cdev_del(&devp->cdev);
}

static void __exit demo_exit(void)
{
    int i;

    for(i=0; i<DEMO_DEV_CNT; i++)
        demo_clean_cdev(&demo_devp[i]);

    class_destroy(class);
    kfree(demo_devp);
    unregister_chrdev_region(MKDEV(demo_major, 0), DEMO_DEV_CNT);
}

// insmod 时可传入参数
module_param(demo_major, int, S_IRUGO);

module_init(demo_init);
module_exit(demo_exit);

MODULE_AUTHOR("Rick Chan <cy187lion@sina.com>");
MODULE_DESCRIPTION("Char driver demo");
MODULE_LICENSE("GPL");
MODULE_VERSION("1.0.0");

Makefile 文件如下：

obj-m:= \
    demochar.o

demochar-objs:= \
    demo_char.o

EXTRA_CFLAGS += \
    -I$(PWD)

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

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

编译和验证方法：

make
sudo insmod demochar.ko demo_major=200
ls /dev/demo*
ll /sys/class/demo_char/
sudo rmmod demochar

字符设备驱动示例说明

设备分配设备号

MKDEV 是将主设备号和次设备号转换成 dev_t 类型的一个内核函数。dev_t 定义了设备号，为 32 位，其中高 12 位为主设备号，低 20 位为次设备号。使用下列宏可以从 dev_t 获得主设备号和次设备号。

MAJOR(dev_t dev)
MINOR(dev_t dev)

以下两段代码：

err = register_chrdev_region(devno, DEMO_DEV_CNT, DEMO_MODULE_NAME);
err = alloc_chrdev_region(&devno, 0, DEMO_DEV_CNT, DEMO_MODULE_NAME);

用于向系统静态/动态申请设备号。其原型如下：

int register_chrdev_region(dev_t from, unsigned count, const char *name);
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count, const char *name);

是否为动态注册是在驱动加载时决定的，module_param 用于指定该模块参数：

module_param(demo_major, int, S_IRUGO);

如果在加载驱动时没有指定设备号，则使用动态分配，否则使用静态分配。

分配内存资源

kzalloc 可实现内核内存空间的分配：

demo_devp = kzalloc(DEMO_DEV_CNT*sizeof(struct demo_dev), GFP_KERNEL);

用于分配 demo_dev 对象空间。

注册字符设备

cdev_add() 函数和 cdev_del() 函数分别向系统添加和删除一个 cdev 对象，完成字符设备的注册和注销。对应代码如下：

err = cdev_add(&devp->cdev, devno, 1);
cdev_del(&devp->cdev);

udev 文件系统

示例代码通过：

class = class_create(THIS_MODULE, DEMO_MODULE_NAME);
devp->dev = device_create(class, NULL, devp->cdev.dev, NULL, name);

两个接口来与 udev 文件系统交互，产生用户态设备加载消息，生成设备节点。

file_operations

示例通过以下代码注册了字符设备的操作接口函数：

static struct file_operations demo_fops = {
    .owner          = THIS_MODULE,
    .open           = demo_open,
    .release        = demo_release,
    .llseek         = demo_llseek,
    .read           = demo_read,
    .write          = demo_write,
    .unlocked_ioctl = demo_ioctl,
    .mmap           = demo_mmap
};

cdev_init(&devp->cdev, &demo_fops);

访问用户态数据

内核中不能直接使用用户态数据，需要通过 copy_from_user() 和 copy_to_user() 将数据拷贝到内核，或将新数据拷贝回用户态。如 demo_read() 中的处理：

static ssize_t demo_read(struct file *filp, char __user *buffer, size_t count, loff_t *position)
{
    struct demo_dev *devp = filp->private_data;
    loff_t p = *position;
    ssize_t ret = 0;

    // This is a test.
    // 分析和获取有效的读长度
    if(DEMO_DATA_SIZE<=p)   // 要读的偏移位置越界
        return 0;           // End of a file
    if(DEMO_DATA_SIZE<(count+p))    // 要读的字节数太大
        count = DEMO_DATA_SIZE-p;

    if(copy_to_user((void*)buffer, &devp->demo_text[p], count))
        ret = -EFAULT;
    else
    {
        *position += count;
        ret = count;
    }
    return ret;
}

IOCTL 操作

示例定义了以下 IOCTL 类型：

#define DEMO_MAGIC          'R'
#define DEMO_IOC_SIZE       8
#define DEMO_CTL_IOC        _IOC(_IOC_READ|_IOC_WRITE, DEMO_MAGIC, 0, DEMO_IOC_SIZE)
#define DEMO_CTL_IO         _IO(DEMO_MAGIC, 1)
#define DEMO_CTL_IOR        _IOR(DEMO_MAGIC, 2, uint16_t)
#define DEMO_CTL_IOW        _IOW(DEMO_MAGIC, 3, int32_t)
#define DEMO_CTL_IOWR       _IOWR(DEMO_MAGIC, 4, uint32_t)

用户态调用 ioctl 接口时，必须使用相同的宏，最后才会在 demo_ioctl() 中进行对应的分派处理。

  • _IOC：用于定义 IOCTL 命令。
  • _IOR：用于创建只读命令。
  • _IOW：用于创建只写命令。
  • _IOWR：用于创建读写命令。

练习

  1. 为字符设备驱动编写应用程序，用于验证各个接口。
  2. 在字符设备驱动的 open、read、write、llseek、unlocked_ioctl 和 release 接口中增加 printk 函数，用于验证来自于用户态的操作。
  3. 完善 demo_write() 函数，实现将用户数据写入内核空间，并最后可再通过 demo_read() 读出。