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@(从Linux内核LED驱动来理解字符设备驱动开发流程)

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开发环境

1. Linux字符设备驱动的组成

引自宋宝华《Linux设备驱动开发详解--基于最新的Linux 4.0内核》P138内容：

在Linux中，字符设备驱动由如下几个部分组成。
1. 字符设备驱动模块加载与卸载函数
2. 字符设备驱动的file_operations 结构体中的成员函数

这里先介绍一下字符设备的开发流程：字符设备驱动是通过设备号 与上位机程序连接。而上位机程序对驱动的控制则是通过文件操作，即read、write、ioctl等完成。

• ps.（对于Linux系统而言，一切皆文件，驱动加载成功后，会在/proc/devices里面添加驱动节点号信息）

因此一个字符设备驱动应包含1. 设备号的注册、卸载和2. 文件操作两个功能，注册的设备号用于提供接口，而文件操作用于对驱动的操作。

字符设备驱动的结构如下图所示：

对于cdev_init函数中，建立file_operations之间的连接的疑问，看一下cdev_init的实现

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;
}

可以看出，最后的一个语句cdev->ops = fops;完成了在cdev中的file_operations的绑定
下面从程序语言角度感性的认识一下设备号的注册、卸载函数原型，和文件操作函数原型。

1.1 字符设备驱动模块加载与卸载函数

//加载函数
static int __init xxx_init(void)
{
        ... ...
}
//卸载函数
static int __exit xxx_exit(void)
{
        ... ...
}

1.2 字符设备驱动的file_operations 结构体中的成员函数

static const struct file_operations xxx_fileops = {
    .owner  = THIS_MODULE,
    .write  = xxx_write,
    .read   = xxx_read,
    .open   = xxx_open,
    .unlocked_ioctl  = xxx_ioctl,
    ... ...
};

static int xxx_open( struct inode *inodes, struct file *filp )
{
        ... ...
}
static long xxx_ioctl( struct file  *file, unsigned int cmd, unsigned long arg )
{
        ... ...
}
static ssize_t xxx_write( struct file *filp, const char __user *buffer, size_t size, loff_t *f_pos )
{
        ... ...
}
static ssize_t xxx_read( struct file *filp, const char __user *buffer, size_t size, loff_t *f_pos )
{
        ... ...
}

2. 字符设备驱动——设备号注册卸载

以我写的字符设备驱动源代码为例，路径为linux-3.4.2\drivers\char\s3c2440_leds.c，文章后附有完整的代码
设备号的注册由static int __init s3c2440_leds_init(void)完成
设备号的卸载由static int __init s3c2440_leds_exit(void)完成
首先分析设备号的注册，然后分析卸载

2.1 设备号注册

设备号分为主设备号和次设备号，若源代码中定义了主设备号(次设备号一般为0)，那么可以直接完成设备号的注册，其流程为

注册成功后，可通过cat /proc/devices命令查看设备号

2.2 设备号注销

相比设备号的注册，注销流程就十分简单：

3. 字符设备驱动——文件操作

上位机程序首先要调用open函数打开此驱动，具体方法就是，打开该设备号对应的文件，一般而言，该设备号文件在/dev/文件夹下，驱动在内核中注册成功后会在/proc/devices中包含设备号信息，但/dev/文件夹内并没有创建该设备号对应的文件，因此需要手动创建该设备号文件，命令为：

mknod /dev/leds c 230 0

表示在/dev文件夹下创建名为leds的字符设备文件，其主设备号为230，次设备号为0。
字符设备文件名可以另取，但设备号一定要对应/proc/devices里面的设备号。

然后通过fd = open("/dev/leds",0);完成设备驱动的打开

当上位机程序通过调用open函数打开(链接上)相应的驱动程序后，open函数会返回一个==文件描述符==暂且记为fd，然后对该驱动的read、write、ioctl等操作都可以通过使用fd完成。简单的字符设备驱动程序大多采用ioctl函数控制驱动程序，而这个ioctl函数本身也不难，其实现为：

static long s3c2440_leds_ioctl( struct file  *file, unsigned int cmd, unsigned long arg )

函数中
第一个参数：表示要操作的文件描述符
第二个参数：表示传递的命令字
第三个参数：表示传递的变量字
第二个参数和第三个参数的含义没有硬性规定，传递的参数符合对应的关键字限定类型即可

下面的给出示例参考

static long s3c2440_leds_ioctl( struct file  *file, unsigned int cmd, unsigned long arg )
{
    printk(DRV_NAME "\tRecv cmd: %u\n", cmd);
    printk(DRV_NAME "\tRecv arg: %lu\n", arg);
    //IO operations function.
    if(arg > 4) {
        return -EINVAL;
    }

    switch (cmd) {
        case IOCTL_LED_ON:          //#define IOCTL_LED_ON 1
            s3c2410_gpio_setpin(S3C2410_GPF(arg+3), 0);//Set pin
            printk("Open LED %lu ",arg);
        return 0;

        case IOCTL_LED_OFF:         //#define IOCTL_LED_OFF 0
            s3c2410_gpio_setpin(S3C2410_GPF(arg+3), 1);
            printk("Close LED %lu ",arg);
        return 0;

        default:
            return -EINVAL;
    }
}

参考资料

1. 宋宝华《Linux设备驱动开发详解–基于最新的Linux 4.0内核》 第6章 字符设备驱动
2. Jonathan Corbet《linux设备驱动程序第三版》 P50-P51

示例代码

/*
 *  Driver for S3C2440 base board.
 *
 *  Copyright (C) 2019  Alan NWPU <alantian.at@gmail.com>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *  MULTIBEANS, NPU Youyi West Ave, Beilin District, Xi'an, China.
 */

#include <linux/module.h>   /* Every Linux kernel module must include this head */
#include <linux/init.h>     /* Every Linux kernel module must include this head */
#include <linux/kernel.h>   /* printk() */
#include <linux/fs.h>       /* struct fops */
#include <linux/errno.h>    /* error codes */
#include <linux/cdev.h>     /* cdev_alloc()  */
#include <linux/ioport.h>   /* request_mem_region() */
#include <linux/delay.h>
#include <linux/moduleparam.h>
#include <linux/types.h>
#include <linux/gpio.h>
#include <linux/device.h>
#include <linux/types.h>
#include <linux/miscdevice.h>
#include <linux/slab.h>
#include <linux/uaccess.h>

#include <asm/irq.h>
#include <mach/gpio-nrs.h>
#include <mach/gpio.h>
#include <mach/hardware.h>
#include <plat/gpio-cfg.h>

#define DRV_NAME "s3c2440_leds"
#define DRV_AUTHOR "Alan Tian <alantian.at@gmail.com>"
#define DRV_DESC "S3C2440 LED Pin Driver"

#define S3C2440_LED_SIZE 0x1000

#define S3C2440_LED_MAJOR 230
#define S3C2440_LED_MINOR 0

static int major = S3C2440_LED_MAJOR;
static int minor = S3C2440_LED_MINOR;

/* 应用程序执行ioctl(fd, cmd, arg)时的第2个参数 */
#define IOCTL_LED_ON    0
#define IOCTL_LED_OFF   1

static int s3c2440_leds_open( struct inode *inodes, struct file *filp );
static long s3c2440_leds_ioctl( struct file  *file, unsigned int cmd, unsigned long arg );
static ssize_t s3c2440_leds_write( struct file *filp, const char __user *buffer,                                      size_t size, loff_t *f_pos );
static ssize_t s3c2440_leds_read( struct file *filp, const char __user *buffer,                                      size_t size, loff_t *f_pos );

struct s3c2440_leds_dev_t
{
    struct cdev cdev;
    unsigned char mem[S3C2440_LED_SIZE];
} *s3c2440_leds_dev;

//Step 2: Add file operations
static const struct file_operations s3c2440_leds_fileops = {
    .owner  = THIS_MODULE,
    .write  = s3c2440_leds_write,
    .read   = s3c2440_leds_read,
    .open   = s3c2440_leds_open,
    .unlocked_ioctl  = s3c2440_leds_ioctl,
};

static int s3c2440_leds_open( struct inode *inodes, struct file *filp )
{
    //int ret;

    filp->private_data = s3c2440_leds_dev;
    printk(DRV_NAME"\tS3C2440 open function...\n");

    return 0;
}

static long s3c2440_leds_ioctl( struct file  *file, unsigned int cmd, unsigned long arg )
{
    printk(DRV_NAME "\tRecv cmd: %u\n", cmd);
    printk(DRV_NAME "\tRecv arg: %lu\n", arg);
    //IO operations function.
    if(arg > 4) {
        return -EINVAL;
    }

    switch (cmd) {
        case IOCTL_LED_ON:
            s3c2410_gpio_setpin(S3C2410_GPF(arg+3), 0);//Set pin
            printk("Open LED %lu ",arg);
        return 0;

        case IOCTL_LED_OFF:
            s3c2410_gpio_setpin(S3C2410_GPF(arg+3), 1);
            printk("Close LED %lu ",arg);
        return 0;

        default:
            return -EINVAL;
    }
}


static ssize_t s3c2440_leds_write( struct file *filp, const char __user *buffer,                                      size_t size, loff_t *f_pos )
{
    unsigned long p = *f_pos;
    unsigned int count = size;
    int ret = 0;
    struct s3c2440_leds_dev_t *dev = filp->private_data;

    if(p >= S3C2440_LED_SIZE)
        return 0;
    if(count > S3C2440_LED_SIZE - p)
        count = S3C2440_LED_SIZE - p;

    memset(dev->mem, 0, S3C2440_LED_SIZE);

    if(copy_from_user(dev->mem + p, buffer, count)) {
        ret = -EFAULT;
    } else {
        *f_pos += count;
        ret = count;
        printk(KERN_INFO "writter %u bytes(s) from %lu\n", count, p);
    }

    return ret;
    
}

static ssize_t s3c2440_leds_read( struct file *filp, const char __user *buffer,                                      size_t size, loff_t *f_pos )
{
    unsigned long p = *f_pos;
    unsigned int count = size;
    int ret = 0;
    struct s3c2440_leds_dev_t *dev = filp->private_data;
    
    if(p >= S3C2440_LED_SIZE)
        return 0;
    if(count > S3C2440_LED_SIZE - p)
        count = S3C2440_LED_SIZE - p;
    if(copy_to_user(buffer, dev->mem + p, count)) {
        ret = -EFAULT;
    } else {
        *f_pos += count;
        ret = count;
        printk(KERN_INFO "read %u bytes(s) from %lu\n", count, p);
    }

    return ret;
}

static int __init s3c2440_leds_init(void)
{
    int ret,err;
    dev_t devid;
    
    if(major) {
        devid = MKDEV(major, 0);
        ret = register_chrdev_region(devid, 1, DRV_NAME);
        printk("Origin Creat node %d\n",major);
    } else {
        ret = alloc_chrdev_region(&devid, 0, 1, DRV_NAME);
        major = MAJOR(devid);
        printk("Arrage1 Creat node %d\n",major);
    }
    if(ret < 0) {
        printk(DRV_NAME "\ts3c2440 new device failed\n");
        //goto fail_malloc;
        return ret;
    }
    s3c2440_leds_dev = kzalloc(sizeof(struct s3c2440_leds_dev_t), GFP_KERNEL);
    if(!s3c2440_leds_dev) {
        ret = -ENOMEM;
        goto fail_malloc;
    }
    printk("success init leds\n");
    
    cdev_init(&s3c2440_leds_dev->cdev, &s3c2440_leds_fileops);
    err = cdev_add(&s3c2440_leds_dev->cdev, devid, 1);
    if(err)
        printk(KERN_NOTICE "Error %d adding s2c2440_leds %d",err, 1);
    return 0;
    
fail_malloc:
    unregister_chrdev_region(devid, 1);
    return ret;
}

static void __exit s3c2440_leds_exit(void)
{
    printk("Starting delet node %d\n",major);
    cdev_del(&s3c2440_leds_dev->cdev);
    kfree(s3c2440_leds_dev);
    unregister_chrdev_region(MKDEV(major, minor), 1);
    printk("Delete node %d\n",major);
}

module_init(s3c2440_leds_init);
module_exit(s3c2440_leds_exit);

MODULE_AUTHOR(DRV_AUTHOR);
MODULE_DESCRIPTION(DRV_DESC);
MODULE_LICENSE("GPL");

从Linux内核LED驱动来理解字符设备驱动开发流程

标签：图片   描述   应用程序   sof   general   开发流程   div   成员   ext   

原文地址：https://www.cnblogs.com/multimicro/p/11625821.html