Linux驱动程序

驱动程序包含以下类型的功能函数：drv_open（打开设备），drv_read（读），drv_write（写），drv_ioctl（读/写）等。
驱动程序包括字符设备驱动和块设备驱动。

如何编写驱动程序：

1. 确定主设备号，或者让系统分配
2. 定义file_operations结构体
3. 实现对应的drv_open/drv_read/drv_write等函数，填入file_operations结构体
4. 在内核注册驱动（register_chrdev）
5. 入口函数，在安装驱动（insmod xxx.ko）时调用
6. 出口函数，在卸载驱动（rmmod xxx.ko）程序时调用
7. 在内核解除注册
8. 其他内容，比如设备信息，自动创建设备节点：class_create，device_create

驱动程序架构

1. 用户层（APP层）：在APP里面使用驱动程序进行相关的驱动操作，使用open打开设备，使用write写值，比如LED驱动，我们需要在APP里面调用LED的on（亮）和off（灭）方法
2. 驱动库层：编写某一个驱动，有时候需要适配很多机型或者芯片主板，我们需要建立一个公共的驱动库，上层提供给APP调用，底层基于不同单板的代码进行自动适配
3. 硬件层：基于不同的单板编写针对性的代码，通过芯片手册找到需要操作的寄存器并编写相关操作功能

做驱动开发的准备工作

初识linux驱动，可以从linux内核源代码入手，源代码中提供可以参考的众多驱动程序代码。
前往下载linux源代码 https://www.kernel.org/
下载之后在windows上解压
下载源码阅读工具Source Insight https://www.sourceinsight.com/
安装Source Insight并且新建项目，添加linux内核源代码。具体步骤参考：https://blog.csdn.net/ceasadan/article/details/52120825
可参考的驱动源码misc.c内容如下：

/*
 * linux/drivers/char/misc.c
 *
 * Generic misc open routine by Johan Myreen
 *
 * Based on code from Linus
 *
 * Teemu Rantanen's Microsoft Busmouse support and Derrick Cole's
 *   changes incorporated into 0.97pl4
 *   by Peter Cervasio (pete%q106fm.uucp@wupost.wustl.edu) (08SEP92)
 *   See busmouse.c for particulars.
 *
 * Made things a lot mode modular - easy to compile in just one or two
 * of the misc drivers, as they are now completely independent. Linus.
 *
 * Support for loadable modules. 8-Sep-95 Philip Blundell <pjb27@cam.ac.uk>
 *
 * Fixed a failing symbol register to free the device registration
 *      Alan Cox <alan@lxorguk.ukuu.org.uk> 21-Jan-96
 *
 * Dynamic minors and /proc/mice by Alessandro Rubini. 26-Mar-96
 *
 * Renamed to misc and miscdevice to be more accurate. Alan Cox 26-Mar-96
 *
 * Handling of mouse minor numbers for kerneld:
 *  Idea by Jacques Gelinas <jack@solucorp.qc.ca>,
 *  adapted by Bjorn Ekwall <bj0rn@blox.se>
 *  corrected by Alan Cox <alan@lxorguk.ukuu.org.uk>
 *
 * Changes for kmod (from kerneld):
 *  Cyrus Durgin <cider@speakeasy.org>
 *
 * Added devfs support. Richard Gooch <rgooch@atnf.csiro.au>  10-Jan-1998
 */

#include <linux/module.h>

#include <linux/fs.h>
#include <linux/errno.h>
#include <linux/miscdevice.h>
#include <linux/kernel.h>
#include <linux/major.h>
#include <linux/mutex.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/stat.h>
#include <linux/init.h>
#include <linux/device.h>
#include <linux/tty.h>
#include <linux/kmod.h>
#include <linux/gfp.h>

/*
 * Head entry for the doubly linked miscdevice list
 */
static LIST_HEAD(misc_list);
static DEFINE_MUTEX(misc_mtx);

/*
 * Assigned numbers, used for dynamic minors
 */
#define DYNAMIC_MINORS 64 /* like dynamic majors */
static DECLARE_BITMAP(misc_minors, DYNAMIC_MINORS);

#ifdef CONFIG_PROC_FS
static void *misc_seq_start(struct seq_file *seq, loff_t *pos)
{
    mutex_lock(&misc_mtx);
    return seq_list_start(&misc_list, *pos);
}

static void *misc_seq_next(struct seq_file *seq, void *v, loff_t *pos)
{
    return seq_list_next(v, &misc_list, pos);
}

static void misc_seq_stop(struct seq_file *seq, void *v)
{
    mutex_unlock(&misc_mtx);
}

static int misc_seq_show(struct seq_file *seq, void *v)
{
    const struct miscdevice *p = list_entry(v, struct miscdevice, list);

    seq_printf(seq, "%3i %s\n", p->minor, p->name ? p->name : "");
    return 0;
}


static const struct seq_operations misc_seq_ops = {
    .start = misc_seq_start,
    .next  = misc_seq_next,
    .stop  = misc_seq_stop,
    .show  = misc_seq_show,
};

static int misc_seq_open(struct inode *inode, struct file *file)
{
    return seq_open(file, &misc_seq_ops);
}

static const struct file_operations misc_proc_fops = {
    .owner   = THIS_MODULE,
    .open    = misc_seq_open,
    .read    = seq_read,
    .llseek  = seq_lseek,
    .release = seq_release,
};
#endif

static int misc_open(struct inode * inode, struct file * file)
{
    int minor = iminor(inode);
    struct miscdevice *c;
    int err = -ENODEV;
    const struct file_operations *new_fops = NULL;

    mutex_lock(&misc_mtx);

    list_for_each_entry(c, &misc_list, list) {
        if (c->minor == minor) {
            new_fops = fops_get(c->fops);
            break;
        }
    }

    if (!new_fops) {
        mutex_unlock(&misc_mtx);
        request_module("char-major-%d-%d", MISC_MAJOR, minor);
        mutex_lock(&misc_mtx);

        list_for_each_entry(c, &misc_list, list) {
            if (c->minor == minor) {
                new_fops = fops_get(c->fops);
                break;
            }
        }
        if (!new_fops)
            goto fail;
    }

    /*
     * Place the miscdevice in the file's
     * private_data so it can be used by the
     * file operations, including f_op->open below
     */
    file->private_data = c;

    err = 0;
    replace_fops(file, new_fops);
    if (file->f_op->open)
        err = file->f_op->open(inode,file);
fail:
    mutex_unlock(&misc_mtx);
    return err;
}

static struct class *misc_class;

static const struct file_operations misc_fops = {
    .owner      = THIS_MODULE,
    .open       = misc_open,
    .llseek     = noop_llseek,
};

/**
 *  misc_register   -   register a miscellaneous device
 *  @misc: device structure
 *
 *  Register a miscellaneous device with the kernel. If the minor
 *  number is set to %MISC_DYNAMIC_MINOR a minor number is assigned
 *  and placed in the minor field of the structure. For other cases
 *  the minor number requested is used.
 *
 *  The structure passed is linked into the kernel and may not be
 *  destroyed until it has been unregistered. By default, an open()
 *  syscall to the device sets file->private_data to point to the
 *  structure. Drivers don't need open in fops for this.
 *
 *  A zero is returned on success and a negative errno code for
 *  failure.
 */

int misc_register(struct miscdevice * misc)
{
    dev_t dev;
    int err = 0;
    bool is_dynamic = (misc->minor == MISC_DYNAMIC_MINOR);

    INIT_LIST_HEAD(&misc->list);

    mutex_lock(&misc_mtx);

    if (is_dynamic) {
        int i = find_first_zero_bit(misc_minors, DYNAMIC_MINORS);
        if (i >= DYNAMIC_MINORS) {
            err = -EBUSY;
            goto out;
        }
        misc->minor = DYNAMIC_MINORS - i - 1;
        set_bit(i, misc_minors);
    } else {
        struct miscdevice *c;

        list_for_each_entry(c, &misc_list, list) {
            if (c->minor == misc->minor) {
                err = -EBUSY;
                goto out;
            }
        }
    }

    dev = MKDEV(MISC_MAJOR, misc->minor);

    misc->this_device =
        device_create_with_groups(misc_class, misc->parent, dev,
                      misc, misc->groups, "%s", misc->name);
    if (IS_ERR(misc->this_device)) {
        if (is_dynamic) {
            int i = DYNAMIC_MINORS - misc->minor - 1;

            if (i < DYNAMIC_MINORS && i >= 0)
                clear_bit(i, misc_minors);
            misc->minor = MISC_DYNAMIC_MINOR;
        }
        err = PTR_ERR(misc->this_device);
        goto out;
    }

    /*
     * Add it to the front, so that later devices can "override"
     * earlier defaults
     */
    list_add(&misc->list, &misc_list);
 out:
    mutex_unlock(&misc_mtx);
    return err;
}

/**
 *  misc_deregister - unregister a miscellaneous device
 *  @misc: device to unregister
 *
 *  Unregister a miscellaneous device that was previously
 *  successfully registered with misc_register().
 */

void misc_deregister(struct miscdevice *misc)
{
    int i = DYNAMIC_MINORS - misc->minor - 1;

    if (WARN_ON(list_empty(&misc->list)))
        return;

    mutex_lock(&misc_mtx);
    list_del(&misc->list);
    device_destroy(misc_class, MKDEV(MISC_MAJOR, misc->minor));
    if (i < DYNAMIC_MINORS && i >= 0)
        clear_bit(i, misc_minors);
    mutex_unlock(&misc_mtx);
}

EXPORT_SYMBOL(misc_register);
EXPORT_SYMBOL(misc_deregister);

static char *misc_devnode(struct device *dev, umode_t *mode)
{
    struct miscdevice *c = dev_get_drvdata(dev);

    if (mode && c->mode)
        *mode = c->mode;
    if (c->nodename)
        return kstrdup(c->nodename, GFP_KERNEL);
    return NULL;
}

static int __init misc_init(void)
{
    int err;
    struct proc_dir_entry *ret;

    ret = proc_create("misc", 0, NULL, &misc_proc_fops);
    misc_class = class_create(THIS_MODULE, "misc");
    err = PTR_ERR(misc_class);
    if (IS_ERR(misc_class))
        goto fail_remove;

    err = -EIO;
    if (register_chrdev(MISC_MAJOR,"misc",&misc_fops))
        goto fail_printk;
    misc_class->devnode = misc_devnode;
    return 0;

fail_printk:
    printk("unable to get major %d for misc devices\n", MISC_MAJOR);
    class_destroy(misc_class);
fail_remove:
    if (ret)
        remove_proc_entry("misc", NULL);
    return err;
}
subsys_initcall(misc_init);

下面创建一个字符设备驱动hello_drv为例

hello_drv.c:

#include <linux/module.h>

#include <linux/fs.h>
#include <linux/errno.h>
#include <linux/miscdevice.h>
#include <linux/kernel.h>
#include <linux/major.h>
#include <linux/mutex.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/stat.h>
#include <linux/init.h>
#include <linux/device.h>
#include <linux/tty.h>
#include <linux/kmod.h>
#include <linux/gfp.h>





/*1. 确定主设备号，或者让系统分配*/
static int major = 0;
static char kernel_buf[1024];
static struct class *hello_class;

#define MIN(a,b) (a < b ? a : b)


/*3. 实现对应的drv_open/drv_read/drv_write等函数，填入file_operations结构体*/

static ssize_t hello_drv_read(struct file *file, char __user *buf, size_t size, loff_t *offset)
{
    int err;
    printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
    err = copy_to_user(buf, kernel_buf, MIN(1024, size));
    return MIN(1024, size);

}
static ssize_t hello_drv_write(struct file *file, const char __user *buf, size_t size, loff_t *offset)
{
    int err;
    printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
    err = copy_from_user(kernel_buf, buf, MIN(1024, size));
    return MIN(1024, size);

}
static int hello_drv_open(struct inode *node, struct file *file)
{
    printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
    return 0;

}
static int hello_drv_close(struct inode *node, struct file *file)
{
    printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
    return 0;

}


/*2. 定义file_operations结构体*/
static struct file_operations hello_drv = 
{
    .owner = THIS_MODULE,
        .open = hello_drv_open,
        .write = hello_drv_write,
        .read = hello_drv_read,
        .release = hello_drv_close,
};


/*4. 在内核注册驱动（register_chrdev）*/
/*5. 入口函数，在安装驱动（insmod xxx.ko）时调用*/
static int __init hello_init(void)
{
    
    /*6. 出口函数，在卸载驱动（rmmod xxx.ko）程序时调用*/
    int err;
    printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
    major = register_chrdev(0, "huanghai_hello", &hello_drv);
    hello_class = class_create(THIS_MODULE, "hello_class");
    err = PTR_ERR(hello_class);
    if (IS_ERR(hello_class))
    {
        printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
        unregister_chrdev(0, "huanghai_hello");
        return -1;

    }

    device_create(hello_class, NULL, MKDEV(major, 0), NULL, "huanghai_hello");
        
    return 0;
}




static void __exit hello_exit(void)
{
    /*7. 在内核解除注册*/
    printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
    device_destroy(hello_class, MKDEV(major, 0));
    class_destroy(hello_class);
    unregister_chrdev(0, "huanghai_hello");
}


/*8. 其他内容，比如设备信息，自动创建设备节点：class_create，device_create*/
module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");

hello_drv_test.c:

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

/*

./hello_drv_test -w abc
./hello_drv_test -r

*/

int main(int argc, char **argv)
{
    int fd;
    char buf[1024];
    int len;

    /*1.判断参数*/
    if(argc < 2)
    {
        printf("Usage: %s -w <string>\n",argv[0]);
        printf("       %s -r\n",argv[0]);
        return -1;

    }

    /*2.打开文件*/
    fd = open("/dev/huanghai_hello", O_RDWR);
    if(fd == -1)
    {
        printf("can not open file /dev/huanghai_hello\n");
    }

    /*3.写文件或读文件*/
    if((0 == strcmp(argv[1], "-w")) && (argc == 3))
    {
        len = strlen(argv[2]) + 1;
        len = len < 1024 ? len : 1024;
        write(fd, argv[2], len);

    }
    else
    {
        len = read(fd,buf,1024);
        buf[1023] = '\0';
        printf("APP READ : %s\n", buf);

    }
    close(fd);
}

Makefile:

# 1. 使用不同的开发板内核时，一定要修改KERN_DIR
# 2. KERN_DIR中的内核要事先配置、编译，为了能编译内核，要先设置下列环境变量
# 2.1 ARCH， 比如：export ARCH=arm64
# 2.2 CROSS_COMPILE, 比如： export CROSS_COMPILE=aarch64-linux-gnu-
# 2.3 PATH,比如：export PATH=$PATH:/home/book/100ask_rocr-k3399-pc/ToolChain-6.3.1/gcc-linaro-6.3.1-2017.05-x86_64_aarch64-linux-gnu/bin
# 注意：不同的开发板不同的编译器上述3个环境变量不一定相同，请参考各开发板的高级用户使用手册

#KERN_DIR = /home/book/work/linux-4.4.210
KERN_DIR = /usr/src/linux-headers-4.18.0-15

all:
    make -C $(KERN_DIR) M=`pwd` modules
    $(CROSS_COMPILE)gcc -o hello_drv_test hello_drv_test.c

clean:
    make -C $(KERN_DIR) M=`pwd` modules clean
    rm -rf modules.order
    rm -f hello_drv_test

obj-m   += hello_drv.o

配置Makefile环境变量，确认linux内核目录并且编译过内核。
将以上3个文件拷贝到ubuntu中进行编译
编译完成使用insmod hello_drv.ko进行安装驱动
使用./hello_drv_test -w "hello my first driver" 进行写入测试
使用./hello_drv_test -r 进行读测试

参考链接

Firefly论坛-韦东山老师嵌入式Linux教学视频汇总：http://dev.t-firefly.com/thread-100207-1-1.html
百问网-嵌入式Linux应用开发完全手册第 2 版：https://book.100ask.net/
百问网-嵌入式初学者学习路线：http://wiki.100ask.org/BeginnerLearningRoute