内核中编写i2c设备驱动
二、内核中编写i2c设备驱动
内核编写i2c设备驱动支持两种方式：Adapter方式（LEGACY）和Probe方式（new style）。
1.legacy方式
此方法驱动需要自己创建i2c_client，并且要知道芯片的地址，在内核目录documentation/i2c/upgrading-clients中有一个例程。
i2c_driver构建
static struct i2c_driver at24c02_driver = {
.driver = {
.name = "at24c02",
},
.id = I2C_DRIVERID_EEPROM,
.attach_adapter = at24c02_attach_adapter,
.detach_client = at24c02_detach_client,
};
I2C设备驱动的加载与卸载函数模板
static int __init at24c02_init(void)
{
return i2c_add_driver(&at24c02_driver);
}
i2c_add_driver的执行回引发i2c_driver结构体at24c02_attach_adapter的执行，若内核中注册了i2c适配器，就顺序调用这些适配器来连接i2c设备，at24c02_attach_adapter调用i2c核心i2c_probe探测设备
static int at24c02_attach_adapter(struct i2c_adapter *adapter)
{
return i2c_probe(adapter, &addr_data, at24c02_detect);
}
static unsigned short normal_i2c[] = { 0x50, 0x51, 0x52, 0x53, 0x54,
0x55, 0x56, 0x57, I2C_CLIENT_END };
/* Insmod parameters */
I2C_CLIENT_INSMOD_1(at24c02);
其中第二个参数add_data是i2c_client_address_data类型的变量，由normal_i2c[]通过宏I2C_CLIENT_INSMOD_1构建而成，具体方法参考i2c.h文件。normal_i2c是i2c芯片的地址，如果地址与芯片对应不上，无法探测到设备，当探测到目标设备后，调用at24c02_detect，把探测到得地址address作为参数传入。
#define AT24C02_MAJOR 250
static int at24c02_major = AT24C02_MAJOR;
struct at24c02_data {
struct cdev cdev;
struct i2c_client client;
struct mutex update_lock;
u8 data[EEPROM_SIZE]; /* Register values */
};
static int at24c02_detect(struct i2c_adapter *adapter, int address, int kind)
{
struct i2c_client *new_client;
struct at24c02_data *data;
int err = 0,result;
dev_t at24c02_dev=MKDEV(at24c02_major,0);
if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_READ_BYTE_DATA
| I2C_FUNC_SMBUS_BYTE)) //判断适配器功能
goto exit;
if (!(data = kzalloc(sizeof(struct at24c02_data), GFP_KERNEL))) {
err = -ENOMEM;
goto exit;
}
new_client = &data->client;
memset(data->data, 0xff, EEPROM_SIZE);
i2c_set_clientdata(new_client, data);
new_client->addr = address;
new_client->adapter = adapter;
new_client->driver = &at24c02_driver;
new_client->flags = 0;
/* Fill in the remaining client fields */
strlcpy(new_client->name, "at24c02", I2C_NAME_SIZE);
mutex_init(&data->update_lock);
/* Tell the I2C layer a new client has arrived */
if ((err = i2c_attach_client(new_client)))
goto exit_kfree;
if(at24c02_major)
{
result=register_chrdev_region(at24c02_dev,1,”at24c02”);
}
else
{
result=alloc_chrdev_region(&at24c02_dev,0,1,”at24c02”);
at24c02_major=MAJOR(at24c02_dev);
}
if(result<0)
{
printk(KERN_NOTICE “Unable to get region %d/n”,result);
err=result;
goto exit_detach;
}
at24c02_setup_cdev(data,0); //注册字符设备at24c02_fops
return 0;
exit_detach:
i2c_detach_client(new_client);
exit_kfree:
kfree(data);
exit:
return err;
}
static void at24c02_setup_cdev(struct at24c02_data *dev,int index)
{
int err,at24c02_dev=MKDEV(at24c02_major,index);
cdev_init(&dev->cdev,&at24c02_fops);
dev_cdev.owner=THIS_MODULE;
err=cdev_add(&dev->cdev,at24c02_dev,1);
if(err)
printk(KERN_NOTICE "error %d adding at24c02 %d/n",err,index);
}
i2c设备驱动卸载函数进行i2c_del_driver调用后，顺序调用内核中注册的适配器断开注册过的i2c设备，此过程是调用at24c02_detach_client实现的。
static void __exit at24c02_exit(void)
{
i2c_del_driver(&at24c02_driver);
}
static int at24c02_detach_client(struct i2c_client *client)
{
int err;
struct at24c02_data *data;
data=i2c_get_client(client);
cdev_del(&(data->cdev));
unregister_chrdev_region(MKDEV(at24c02_major, 0), 1);
err = i2c_detach_client(client);
if (err)
return err;
kfree(data);
return 0;
}
字符驱动的实现
struct file_operations at24c02_fops = {
.owner = THIS_MODULE,
.read= at24c02 _read,
.write= at24c02 _write,
.open= at24c02 _open,
.release = at24c02_release,
};
字符设备驱动不在详述，主要说明如何调用适配器完成数据传输。首先构造消息，通过i2c_transfer来传递，i2c_transfer找到对应适配器algorithm通信方法master_xfer最终完成i2c消息处理。下面是一个关于设备驱动的读函数，其他类似。
static int at24c02_open(struct inode *inode, struct file *file)
{
struct at24c02_data *data;
data=container_of(inode->i_cdev,struct at24c02_data,cdev);
file->private_data = data;
return 0;
}
static int at24c02_read(struct file *filp, char __user *buff,size_t count, loff_t *offp)
{
int ret;
struct i2c_msg msg[2];
char addr = 0;
struct at24c02_data *data=file->private_data;
if (count > 1024)
{
return -EINVAL;
}
msg[0].addr = data->client->addr;
msg[0].flags = 0; /* write */
msg[0].len = 1; /* 1个地址 */
msg[0].buf = &addr;
msg[1].addr = data->client->addr;
msg[1].flags = I2C_M_RD; /* read */
msg[1].len = count; /* 要读的数据个数 */
msg[1].buf = data->data;
ret = i2c_transfer(at24c02_client->adapter, msg, 2);
if (ret == 2)
{
copy_to_user(buff, data->data, count);
return count;
}
else
return -EIO;
}
目前适配器主要支持的传输方法有两种：master_xfer和smbus_xfer，从i2c_algorithm结构体可看出。一般来说，若适配器支持master_xfer那么它可以模拟支持smbus，但只实现smbus_xfer，则不支持master_xfer传输。当然，上面的驱动也可以采用smbus方式完成传输。采用LEGACY方式在2.6.32.2内核下编译不过去，主要是i2c核心中有些函数不存在，所以，设备驱动发展方向是new style方式。
2.new style方式
构建i2c_driver
static struct i2c_driver at24c02_driver = {
.driver = {
.name = "at24c02",
.owner = THIS_MODULE,
},
.probe = at24c02_probe,
.remove = __devexit_p(at24c02_remove),
.id_table = at24c02_id,
};
加载与注销
static int __init at24c02_init(void)
{
return i2c_add_driver(&at24c02_driver);
}
module_init(at24c02_init);
i2c_add_driver会将驱动注册到总线上，探测到i2c设备就会调用at24c02_probe，探测主要是用i2c_match_id函数比较client的名字和id_table中名字，如果相等，则探测到i2c设备，本驱动中id_table如下：
static const struct i2c_device_id at24c02_id[] = {
{ "at24c02", 0 },
{ }
};
MODULE_DEVICE_TABLE(i2c, at24c02_id);
MODULE_DEVICE_TABLE宏是用来生成i2c_device_id。在legacy方式中i2c_client是自己创建的，而此处的i2c_client如何得到？实际上是在i2c_register_board_info函数注册i2c_board_info过程中构建的，所以arch/arm/mach-s3c2440/mach-smdk2440.c中添加注册信息。
static struct i2c_board_info i2c_devices[] __initdata = {
{ I2C_BOARD_INFO("at24c02", 0x50), },
};
static void __init smdk2440_machine_init(void)
{
i2c_register_board_info(0,i2c_devices,ARRAY_SIZE(i2c_devices));
s3c24xx_fb_set_platdata(&smdk2440_fb_info);
s3c_i2c0_set_platdata(NULL);
platform_add_devices(smdk2440_devices, ARRAY_SIZE(smdk2440_devices));
smdk_machine_init();
}
如果没有注册i2c信息，就探测不到i2c设备。探测到at24c02设备后就会调用at24c02_probe函数。
static int __devinit at24c08b_probe(struct i2c_client *client,const struct i2c_device_id *id)
{
struct at24c02_data *data;
int err = 0,result;
dev_t at24c02_dev=MKDEV(at24c02_major,0);
if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_READ_BYTE_DATA
| I2C_FUNC_SMBUS_BYTE)) //判断适配器功能
goto exit;
if (!(data = kzalloc(sizeof(struct at24c02_data), GFP_KERNEL))) {
err = -ENOMEM;
goto exit;
}
memset(data, 0, sizeof(struct at24c02_data));
data->client=client;
i2c_set_clientdata(client, data);
if(at24c02_major)
{
result=register_chrdev_region(at24c02_dev,1,”at24c02”);
}
else
{
result=alloc_chrdev_region(&at24c02_dev,0,1,”at24c02”);
at24c02_major=MAJOR(at24c02_dev);
}
if(result<0)
{
printk(KERN_NOTICE “Unable to get region %d/n”,result);
err=result;
goto exit_kfree;
}
at24c02_setup_cdev(data,0); //注册字符设备at24c02_fops
return 0;
exit_kfree:
kfree(data);
exit:
return err;
}
probe函数主要注册了字符设备，通过data->client=client获得的相关信息。关于at24c02_fops结构体的完善，由于与前面一种方法类似，这里就不详述。最后就是驱动的注销。
static void __exit at24c02_exit(void)
{
i2c_del_driver(&at24c02_driver);
}
module_exit(at24c02_exit);
static int __devexit at24c02_remove(struct i2c_client *client)
{
int err;
struct at24c02_data *data;
data=i2c_get_client(client);
cdev_del(&(data->cdev));
unregister_chrdev_region(MKDEV(at24c02_major, 0), 1);
kfree(data);
return 0;
}