为安卓编写Linux内核驱动程序

参考博文：https://blog.csdn.net/Luoshengyang/article/details/6568411
参考博文：
https://www.jianshu.com/p/4ea84d1ce4e5

为安卓编写一个简单的设备驱动

环境：Andriod12 + kernel 5.*内核 + 一部手机

我们在配置硬件较多的地方创建freg目录及其相关文件

---Android源码
------kernel5.*
---------drivers/
------------freg/
---------------freg.c
---------------freg.h
---------------Makefile
---------------Kconfig
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freg.c

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

#include "freg.h"

static int freg_major = 0;
static int freg_minor = 0;

static struct class* freg_class = NULL;
static struct fake_reg_dev* freg_dev = NULL;

static int freg_open(struct inode* inode, struct file* filp);
static int freg_release(struct inode* inode, struct file* filp);
static ssize_t freg_read(struct file* filp, char __user *buf, size_t count, loff_t* f_pos);
static ssize_t freg_write(struct file* filp, const char __user *buf, size_t count, loff_t* f_pos);

static struct file_operations freg_fops = {
    .owner = THIS_MODULE,
    .open = freg_open,
    .release = freg_release,
    .read = freg_read,
    .write = freg_write,
};

#define SEQ_printf(m, x...)     \
    do {                \
        if (m)          \
            seq_printf(m, x);   \
        else            \
            pr_err(x);      \
    } while (0)

static int freg_proc_show(struct seq_file *m, void *v)
{
    SEQ_printf(m, "%d\n", freg_dev->val);
    return 0;
}

static int freg_proc_open(struct inode *inode, struct file *file)
{
    return single_open(file, freg_proc_show, inode->i_private);
}

static ssize_t __freg_set_val(struct fake_reg_dev* dev, const char* buf, size_t count){
    int val = 0;

    val = simple_strtol(buf, NULL, 10);

    if(down_interruptible(&(dev->sem))){
        return -ERESTARTSYS;
    }

    dev->val = val;
    up(&(dev->sem));

    return count;
}

/*
static ssize_t freg_proc_read(char* page, char** start, off_t off, int count, int* eof, void* data){
    if(off >0 ){
        *eof = 1;
        return 0;
    }

    return __freg_get_val(freg_dev, page);
}*/

static ssize_t freg_proc_write(struct file *filp, const char *ubuf, size_t cnt, loff_t *data){
    int err = 0;
    char* page = NULL;

    if(cnt > PAGE_SIZE){
        printk(KERN_ALERT"The buff is too large: %lu.\n", cnt);
        return -EFAULT;
    }

    page = (char*) __get_free_page(GFP_KERNEL);
    if(!page){
        printk(KERN_ALERT"Failed to alloc page.\n");
        return -ENOMEM;
    }

    if(raw_copy_from_user(page, ubuf, cnt)){
        printk(KERN_ALERT"Failed to copy buff from user.\n");
        err = -EFAULT;
        goto out;
    }

    err = __freg_set_val(freg_dev, page, cnt);

out:
    free_page((unsigned long)page);
    return err;
}

static const struct file_operations freg_proc_fops = {
    .open = freg_proc_open,
    .write = freg_proc_write,
    .read = seq_read,
    .llseek = seq_lseek,
    .release = single_release,
};


static ssize_t freg_val_show(struct device* dev, struct device_attribute* attr, char* buf);
static ssize_t freg_val_store(struct device* dev, struct device_attribute* attr, const char* buf, size_t count);

static DEVICE_ATTR(val, S_IRUGO | S_IWUSR, freg_val_show, freg_val_store);

static int freg_open(struct inode * inode, struct file * filp){
    struct fake_reg_dev * dev;

    dev = container_of(inode->i_cdev, struct fake_reg_dev, dev);
    filp->private_data = dev;

    return 0;
}

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

static ssize_t freg_read(struct file* filp, char __user *buf, size_t count, loff_t* f_pos){
    ssize_t err = 0;
    struct fake_reg_dev* dev = filp->private_data;

    if(down_interruptible(&(dev->sem))){
        return -ERESTARTSYS;
    }

    if(count < sizeof(dev->val)){
        goto out;
    }

    if(raw_copy_to_user(buf, &(dev->val), sizeof(dev->val))){
        err = -EFAULT;
        goto out;
    }

    err = sizeof(dev->val);

out:
    up(&(dev->sem));
    return err;
}

static ssize_t freg_write(struct file * filp, const char __user * buf, size_t count, loff_t * f_pos){
    struct fake_reg_dev* dev = filp->private_data;
    ssize_t err = 0;

    if(down_interruptible(&(dev->sem))){
        return -ERESTARTSYS;
    }

    if(count != sizeof(dev->val)){
        goto out;
    }

    if(raw_copy_from_user(&(dev->val), buf, count)){
        err = -EFAULT;
        goto out;
    }

    err = sizeof(dev->val);

out:
    up(&(dev->sem));
    return err;
}

static ssize_t __freg_get_val(struct fake_reg_dev* dev, char* buf){
    int val = 0;

    if(down_interruptible(&dev->sem)){
        return -ERESTARTSYS;
    }

    val = dev->val;
    up(&(dev->sem));

    return snprintf(buf, PAGE_SIZE, "%d\n", val);
}


static ssize_t freg_val_show(struct device* dev, struct device_attribute* attr, char* buf){
    struct fake_reg_dev* hdev = (struct fake_reg_dev*)dev_get_drvdata(dev);

    return __freg_get_val(hdev, buf);
}

static ssize_t freg_val_store(struct device*dev, struct device_attribute* attr, const char* buf, size_t count){
    struct fake_reg_dev* hdev = (struct fake_reg_dev*)dev_get_drvdata(dev);

    return __freg_set_val(hdev, buf, count);
}


static void freg_create_proc(void){
    proc_create(FREG_DEVICE_PROC_NAME, 0644, 0,  &freg_proc_fops);
}

static void freg_remove_proc(void){
    remove_proc_entry(FREG_DEVICE_PROC_NAME, NULL);
}

static int __freg_setup_dev(struct fake_reg_dev* dev){
    int err;
    dev_t devno = MKDEV(freg_major, freg_minor);

    memset(dev, 0, sizeof(struct fake_reg_dev));

    cdev_init(&(dev->dev), &freg_fops);
    dev->dev.owner = THIS_MODULE;
    dev->dev.ops = &freg_fops;

    err = cdev_add(&(dev->dev), devno, 1);
    if(err){
        return err;
    }

    //init_MUTEX(&(dev->sem));
    sema_init(&(dev->sem), 1);
    dev->val = 0;

    return 0;
}

static int __init freg_init(void){
    int err = -1;
    dev_t dev = 0;
    struct device* temp = NULL;

    printk(KERN_ALERT"Initializing freg device.\n");

    err = alloc_chrdev_region(&dev, 0, 1, FREG_DEVICE_NODE_NAME);
    if(err < 0){
        printk(KERN_ALERT"Failed to alloc char dev region.\n");
        goto fail;
    }

    freg_major = MAJOR(dev);
    freg_minor = MINOR(dev);

    freg_dev = kmalloc(sizeof(struct fake_reg_dev), GFP_KERNEL);
    if(!freg_dev){
        err = -ENOMEM;
        printk(KERN_ALERT"Failed to alloc freg device.\n");
        goto unregister;
    }

    err = __freg_setup_dev(freg_dev);
    if(err){
        printk(KERN_ALERT"Failed to setup freg device: %d.\n", err);
        goto cleanup;
    }

    freg_class = class_create(THIS_MODULE, FREG_DEVICE_CLASS_NAME);
    if(IS_ERR(freg_class)){
        err = PTR_ERR(freg_class);
        printk(KERN_ALERT"Failed to create freg device class.\n");
        goto destroy_cdev;
    }

    temp = device_create(freg_class, NULL, dev, NULL, "%s", FREG_DEVICE_FILE_NAME);
    if(IS_ERR(temp)){
        err = PTR_ERR(temp);
        printk(KERN_ALERT"Failed to create freg device.\n");
        goto destroy_class;
    }

    err = device_create_file(temp, &dev_attr_val);
    if(err < 0){
        printk(KERN_ALERT"Failed to create attribute val of freg device.\n");
        goto destroy_device;
    }

    dev_set_drvdata(temp, freg_dev);

    freg_create_proc();

    printk(KERN_ALERT"Succedded to initialize freg device.\n");

    return 0;

destroy_device:
    device_destroy(freg_class, dev);
destroy_class:
    class_destroy(freg_class);
destroy_cdev:
    cdev_del(&(freg_dev->dev));
cleanup:
    kfree(freg_dev);
unregister:
    unregister_chrdev_region(MKDEV(freg_major, freg_minor), 1);
fail:
    return err;
}

static void __exit freg_exit(void){
    dev_t devno = MKDEV(freg_major, freg_minor);

    printk(KERN_ALERT"Destory freg device.\n");

    freg_remove_proc();

    if(freg_class){
        device_destroy(freg_class, MKDEV(freg_major, freg_minor));
        class_destroy(freg_class);
    }

    if(freg_dev){
        cdev_del(&(freg_dev->dev));
        kfree(freg_dev);
    }

    unregister_chrdev_region(devno, 1);
}

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Fake Register Driver");

module_init(freg_init);
module_exit(freg_exit);
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此处代码参考第二篇博文，新内核弃用copy_from_user与copy_to_user 需要改成raw_copy_from_user与raw_copy_to_user

freg.h

#ifndef _FAKE_REG_H_
#define _FAKE_REG_H_

#include 
#include 

#define FREG_DEVICE_NODE_NAME  "freg"
#define FREG_DEVICE_FILE_NAME  "freg"
#define FREG_DEVICE_PROC_NAME  "freg"
#define FREG_DEVICE_CLASS_NAME "freg"

struct fake_reg_dev {
    int val;
    struct semaphore sem;
    struct cdev dev;
};

#endif

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Kconfig

config FREG
    tristate "Fake Register Driver"
    default y
    help
    This is the freg driver for android system.
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Makefile

obj-$(CONFIG_FREG) += freg.o
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然后我们要去修改上级目录的Kconfig文件与Makefile文件也就是/drivers下的2个文件

在drivers中的Kconfig文件中我们需要在

menu "Device Drivers"
source "drivers/lizi/Kconfig"
...
...
endmenu
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menu 与 endmenu之间加下面这句话

/drivers Kconfig

source "drivers/freg/Kconfig"
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/drivers Makefile

obj-$(CONFIG_FREG) += freg/
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到此我们就完成了代码部分,然后我们回到安卓源码目录下编译，不是kernel目录

make bootimage -j4
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然后我们在编译后生成的out目录下找到将boot.img刷到手机里通过adb shell命令进入可一看到proc和dev目录下有freg文件

adb shell
/proc/freg
/dev/freg
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我们在proc目录下输入

cat freg
0
echo '5' > freg
5
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此时我们可看到/sys/class/freg/freg/下的val值

cat /sys/class/freg/freg/val
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