按键读取我们需要实现read函数，

read 函数的 第二个参数被__user 修饰，原则上在内核层我们不能直接访问，需要调用

copy_to_user()这个函数，从内核获取数据到上层。

copy_to_user(void __user volatile * to, const void * from, unsigned long n)

参数1：目标地址，

参数2：数据从哪里来

参数3：多少个字节

 当我们想实现write时，就需要使用，copy_from_user() 从上层获取数据到内核，

copy_from_user(void * to, const void __user volatile * from, unsigned long n)

参数1：目标地址

参数2：数据从哪里来

参数3：多少字节

下面来实现驱动代码

#include "linux/cdev.h"
#include "linux/device.h"
#include "linux/device/class.h"
#include "linux/export.h"
#include "linux/fs.h"
#include "linux/gpio.h"
#include "linux/module.h"
#include "linux/of_device.h"
#include "linux/of_gpio.h"
#include "linux/platform_device.h"
#include "linux/types.h"
#include "linux/uaccess.h"
#include "linux/zconf.h"


uint32_t pin;
dev_t dev_num;
struct cdev *cdev;
struct class *cls;
ssize_t read(struct file *f, char __user *b, size_t s, loff_t *offt)
{
    uint8_t val=gpio_get_value(pin);
    int a=copy_to_user(b,&val,1);
	copy_from_user(void * to, const void __user volatile * from, unsigned long n)
    if(a)
    {
        
    }
    return 0;
}
struct file_operations fops={
   .owner=THIS_MODULE,
   .read=read,
};
int probe(struct platform_device *d)
{
    pin=of_get_named_gpio(d->dev.of_node,"key_pin",0);
    printk("%d\r\n",pin);
    gpio_request(pin,"key");
    gpio_direction_input(pin);
    alloc_chrdev_region(&dev_num, 0, 1,"key");
    cdev=cdev_alloc();
    cdev->ops=&fops;
    cdev_add(cdev,dev_num,1);
    cls=class_create(THIS_MODULE, "key");
    device_create(cls, NULL,dev_num,NULL,"key");
    return 0;
}
int remove(struct platform_device *d)
{
    return 0;
}
static struct of_device_id match={
    .compatible="key",
};

static struct platform_driver driver={
    .probe=probe,
    .remove=remove,
    .driver={
        .name="key",
        .of_match_table=&match,
    },
};
static int __init start(void)
{
    platform_driver_register(&driver);
    printk(KERN_INFO "Hello, world!\n");
    return 0;
}
static void __exit stop(void)
{
    platform_driver_unregister(&driver);
    printk(KERN_INFO "Goodbye, world!\n");
}
module_init(start);
module_exit(stop);
MODULE_LICENSE("GPL");