学习笔记 --- LINUX I2C设备驱动的实现

上一篇文章中介绍了框架，但是没有实现具体的I2C设备的操作，这里完善这个步骤，修改上篇文章中的驱动端：

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/platform_device.h>
#include <linux/i2c.h>
#include <linux/err.h>
#include <linux/regmap.h>
#include <linux/slab.h>
#include <linux/fs.h>
#include <asm/uaccess.h>

static int major;
static struct class *class;
static struct i2c_client *at24cxx_client;

/* 传入: buf[0] : addr
 * 输出: buf[0] : data
 */
static ssize_t at24cxx_read(struct file * file, char __user *buf, size_t count, loff_t *off)
{
	unsigned char addr, data;
	
	copy_from_user(&addr, buf, 1);
	data = i2c_smbus_read_byte_data(at24cxx_client, addr);
	copy_to_user(buf, &data, 1);
	return 1;
}

/* buf[0] : addr
 * buf[1] : data
 */
static ssize_t at24cxx_write(struct file *file, const char __user *buf, size_t count, loff_t *off)
{
	unsigned char ker_buf[2];
	unsigned char addr, data;

	copy_from_user(ker_buf, buf, 2);
	addr = ker_buf[0];
	data = ker_buf[1];

	printk("addr = 0x%02x, data = 0x%02x\n", addr, data);

	if (!i2c_smbus_write_byte_data(at24cxx_client, addr, data))
		return 2;
	else
		return -EIO;	
}

static struct file_operations at24cxx_fops = {
	.owner = THIS_MODULE,
	.read  = at24cxx_read,
	.write = at24cxx_write,
};

static int __devinit at24cxx_probe(struct i2c_client *client,
				  const struct i2c_device_id *id)
{
	at24cxx_client = client;
		
	//printk("%s %s %d\n", __FILE__, __FUNCTION__, __LINE__);
	major = register_chrdev(0, "at24cxx", &at24cxx_fops);
	class = class_create(THIS_MODULE, "at24cxx");
	device_create(class, NULL, MKDEV(major, 0), NULL, "at24cxx"); /* /dev/at24cxx */
	
	return 0;
}

static int __devexit at24cxx_remove(struct i2c_client *client)
{
	//printk("%s %s %d\n", __FILE__, __FUNCTION__, __LINE__);
	device_destroy(class, MKDEV(major, 0));
	class_destroy(class);
	unregister_chrdev(major, "at24cxx");
		
	return 0;
}

static const struct i2c_device_id at24cxx_id_table[] = {
	{ "at24c08", 0 },
	{}
};


/* 1. 分配/设置i2c_driver */
static struct i2c_driver at24cxx_driver = {
	.driver	= {
		.name	= "100ask",
		.owner	= THIS_MODULE,
	},
	.probe		= at24cxx_probe,
	.remove		= __devexit_p(at24cxx_remove),
	.id_table	= at24cxx_id_table,
};

static int at24cxx_drv_init(void)
{
	/* 2. 注册i2c_driver */
	i2c_add_driver(&at24cxx_driver);
	
	return 0;
}

static void at24cxx_drv_exit(void)
{
	i2c_del_driver(&at24cxx_driver);
}

module_init(at24cxx_drv_init);
module_exit(at24cxx_drv_exit);
MODULE_LICENSE("GPL");

这样就完成了一个完整的at24c08的驱动。

上面使用了i2c_smbus_read_byte_data和i2c_smbus_write_byte_data来实现对底层设备的读写：

i2c_smbus_read_byte_data --- i2c_smbus_xfer(client->adapter, client->addr, client->flags,
I2C_SMBUS_READ, command,
I2C_SMBUS_BYTE_DATA, &data);

i2c_smbus_write_byte_data --- i2c_smbus_xfer(client->adapter,client->addr,client->flags,
                     I2C_SMBUS_WRITE,command,
                     I2C_SMBUS_BYTE_DATA,&data);

最终会调用适配器里面的smbus_xfer操作函数来实现数据的读写，这个适配器提供的函数由适配器驱动（总线驱动）来完成，下篇文章再讲：

res = adapter->algo->smbus_xfer(adapter, addr, flags,
read_write, command,
protocol, data);

这里顺便说下这个smbus：

smbus是I2C总线的一个子集，一般的I2C设备都支持这个子集的通讯协议。