Linux 字符設備驅動開發全流程：file_operations、cdev 註冊與用戶空間交互

字符設備是 Linux 設備驅動中最常見的一類，支援順序數據流訪問。本文基於一個完整的驅動實例，系統講解字符設備驅動的模塊架構、file_operations 實現、cdev 註冊流程、設備號管理與用戶空間交互接口。

字符設備 vs 塊設備

驅動模塊架構

// 必須的模塊聲明
MODULE_LICENSE("GPL");
module_init(dvr_tdm_init);
module_exit(dvr_tdm_exit);

// __init/__exit 作用：標記代碼段，init 代碼執行後釋放內存，exit 代碼僅模塊卸載時運行

file_operations 核心接口

static struct file_operations dvr_tdm_fops = {
    .read    = dvr_tdm_read,
    .write   = dvr_tdm_write,
    .open    = dvr_tdm_open,
    .release = dvr_tdm_release,
    // 進階接口（註釋部分）
    // .poll    = dvr_tdm_poll,        // 支援 select/poll
    // .mmap    = dvr_tdm_mmap,        // 支援內存映射
    // .unlocked_ioctl = dvr_tdm_ioctl, // 非 BKL ioctl
    // .fasync  = dvr_tdm_fasync,      // 異步通知
    // .lock    = dvr_tdm_lock,        // 文件鎖
};

字符設備註冊流程

static struct cdev *dvr_tdm_cdev;
static dev_t tdm_dev_t;  // 設備號（主設備號 + 次設備號）

static int __init dvr_tdm_init(void)
{
    int ret;

    // 1. 動態分配設備號範圍
    ret = alloc_chrdev_region(&tdm_dev_t, 0, DVR_TDM_COUNT, DRV_NAME);
    if (ret != 0) { printk(KERN_ERR "alloc_chrdev_region failed"); goto out; }

    // 2. 分配 cdev 結構
    dvr_tdm_cdev = cdev_alloc();
    if (!dvr_tdm_cdev) { ret = -ENOMEM; goto out_unreg; }

    // 3. 綁定 file_operations
    dvr_tdm_cdev->ops = &dvr_tdm_fops;
    dvr_tdm_cdev->owner = THIS_MODULE;

    // 4. 初始化並添加到系統
    cdev_init(dvr_tdm_cdev, &dvr_tdm_fops);
    ret = cdev_add(dvr_tdm_cdev, tdm_dev_t, DVR_TDM_COUNT);
    if (ret) { printk("cdev_add failed"); goto out_cdev; }

    printk("chr device added");
    return 0;

out_cdev:
    cdev_del(dvr_tdm_cdev);
out_unreg:
    unregister_chrdev_region(tdm_dev_t, DVR_TDM_COUNT);
out:
    return ret;
}

static void __exit dvr_tdm_exit(void)
{
    unregister_chrdev_region(tdm_dev_t, DVR_TDM_COUNT);
    cdev_del(dvr_tdm_cdev);
}

關鍵函數語義

核心接口實現範例

open：模塊引用計數管理

static int dvr_tdm_open(struct inode *inode, struct file *filp)
{
    if (!try_module_get(THIS_MODULE)) {  // 增加模塊引用計數
        printk("try_module_get error");
        return -EINVAL;
    }
    printk("driver run ok");
    return 0;
}

release：釋放模塊引用

static int dvr_tdm_release(struct inode *inode, struct file *filp)
{
    module_put(THIS_MODULE);  // 遞減模塊引用計數
    printk("driver release ok");
    return 0;
}

read：內核 → 用戶空間數據拷貝

static ssize_t dvr_tdm_read(struct file *file, char __user *buf,
                            size_t count, loff_t *pos)
{
    short signed int pcm_buffer[TDM_FRAME_LENGTH];
    int ret;

    // ... 從硬件獲取數據到 pcm_buffer ...

    ret = copy_to_user(buf, pcm_buffer, TDM_FRAME_LENGTH);
    if (ret)
        printk("dvr_tdm_read: %x bytes can't read for user", ret);
    return TDM_FRAME_LENGTH;  // 返回實際讀取字節數
}

write：用戶 → 內核空間數據拷貝

static ssize_t dvr_tdm_write(struct file *file, const char __user *buf,
                             size_t count, loff_t *offset)
{
    short signed int pcm_buffer[TDM_FRAME_LENGTH];
    int ret;

    ret = copy_from_user(pcm_buffer, buf, count);
    if (ret)
        printk("dvr_tdm_write: %x bytes can't read from user", ret);

    // ... 將 pcm_buffer 寫入硬件 ...
    return count;  // 返回實際寫入字節數
}

進階功能擴展指引

Makefile 與設備節點創建

# Makefile
ifneq ($(KERNELRELEASE),)
    obj-m += mpc8315_tdm.o
else
    KERNELDIR ?= /path/to/kernel/build
    obj-m += dvr_tdm.o
    dvr_tdm-objs := tdm_driver.o
    PWD := $(shell pwd)
    all:
        $(MAKE) ARCH=powerpc CROSS_COMPILE=powerpc-e300c3-linux-gnu- -C $(KERNELDIR) M=$(PWD) modules
    clean:
        rm -f *.o *.ko
endif

#!/bin/sh  # 創建設備節點腳本
module="dvr_tdm"
device="dvr_tdm"
mode="664"

insmod ./$module.ko || exit 1
major=$(awk "\$2==\"$device\" {print \$1}" /proc/devices)
if [ -z "$major" ]; then echo "Error: Cannot find device"; exit 1; fi

rm -f /dev/${device}
mknod /dev/${device} c $major 0
chmod $mode /dev/${device}

總結

字符設備驅動開發的標準化流程：定義 file_operations → 分配設備號 alloc_chrdev_region → 分配並初始化 cdev → cdev_add 註冊 → 實現 read/write/open/release 等接口 → 模塊加載腳本創建 /dev 節點。掌握 copy_to_user/copy_from_user、模塊引用計數、cdev 生命週期管理，是編寫穩健字符設備驅動的基石。