08、通用 I2C 驱动讲解i2c-dev.c
Linux系统中管理I2C设备的核心驱动是drivers/i2c/i2c-dev.c文件。这个驱动程序主要做了两件事：
- 提供统一框架 通过"I2C设备"和"I2C驱动"两个模块，统一管理所有I2C外设。设备负责硬件识别，驱动负责功能实现。
- 提供用户接口 在系统/dev目录下自动生成类似/dev/i2c-1这样的设备文件（数字代表总线编号）。应用程序只需通过标准文件操作：
- 用open()打开设备节点
- 用ioctl()发送控制指令
- 用read()/write()收发数据 就能直接与I2C从设备通信。
如果发现/dev目录缺少i2c-X设备文件，说明需要开启内核配置。进入系统后检查是否存在对应的设备节点，没有的话需要重新配置编译内核。
一、i2c字符设备的创建
首先，通过i2c_dev_init接口在内核中注册一个字符设备，主设备号设为I2C_MAJOR，次设备号设为0。但此时不会立即在文件系统中生成对应的设备文件。
当某个I2C适配器被添加到系统总线时：
- 内核会触发一个"添加设备"的通知事件
- 这个事件会自动调用i2cdev_attach_adapter函数
- 该函数向系统发送一个设备添加信号（kobject的uevent事件）
用户空间的设备管理工具（如udevd或mdev）会捕获到这个信号：
- 根据信号中的设备信息
- 在/dev目录下自动生成对应的设备文件节点
整个过程：先预注册设备号，实际设备文件的创建要等到具体I2C适配器被识别时，由内核与用户空间工具配合完成。
drivers/i2c/i2c-dev.c
C
static int __init i2c_dev_init(void)
{
int res;
printk(KERN_INFO "i2c /dev entries driver\n");
//注册字符设备驱动,主设备号为 I2C_MAJOR,次设备号范围为 0 到 I2C_MINORS-1,设备名为"i2c"
res = register_chrdev_region(MKDEV(I2C_MAJOR, 0), I2C_MINORS, "i2c");
if (res)
goto out;
// 创建一个 class 对象,名称为 "i2c-dev",用于在用
i2c_dev_class = class_create(THIS_MODULE, "i2c-dev");
if (IS_ERR(i2c_dev_class)) {
res = PTR_ERR(i2c_dev_class);
goto out_unreg_chrdev;
}
// 将 i2c_groups 数组设置为该 class 的 dev_grou
i2c_dev_class->dev_groups = i2c_groups;
// 注册一个总线通知函数 i2cdev_notifier,用于追踪 i2c 总线上新添加或删除的适配器
res = bus_register_notifier(&i2c_bus_type, &i2cdev_notifier);
if (res)
goto out_unreg_class;
// 立即绑定已经存在的 i2c 适配器到 i2c设备
i2c_for_each_dev(NULL, i2cdev_attach_adapter);
return 0;
out_unreg_class:
class_destroy(i2c_dev_class);
out_unreg_chrdev:
unregister_chrdev_region(MKDEV(I2C_MAJOR, 0), I2C_MINORS);
out:
printk(KERN_ERR "%s: Driver Initialisation failed\n", __FILE__);
return res;
}二、i2c-dev字符设备文件操作接口
i2c_dev 结构体中的 cdev 字段指定的文件操作集结构体为 i2cdev_fops，具体内容如下所示：
C
static const struct file_operations i2cdev_fops = {
.owner = THIS_MODULE,
.llseek = no_llseek,
.read = i2cdev_read,
.write = i2cdev_write,
.unlocked_ioctl = i2cdev_ioctl,
.compat_ioctl = compat_i2cdev_ioctl,
.open = i2cdev_open,
.release = i2cdev_release,
};分别实现了常用的 open、read、write 和 ioctl，接下来对上述函数的实现进行讲解，首先 来看 i2cdev_open 函数，函数的具体内容如下所示：
2.1、i2cdev_read
i2cdev 的read接口的内核实现间接调用了 i2c_master_recv() 接口， 在打开 i2cdev 后， 使用ioctl设定要访问的i2c设备的地址， 然后调用read()即可完成读操作
C
static ssize_t i2cdev_read(struct file *file, char __user *buf, size_t count,
loff_t *offset)
{
char *tmp;
int ret;
struct i2c_client *client = file->private_data;
if (count > 8192)
count = 8192;
tmp = kzalloc(count, GFP_KERNEL);
if (tmp == NULL)
return -ENOMEM;
pr_debug("i2c-dev: i2c-%d reading %zu bytes.\n",
iminor(file_inode(file)), count);
ret = i2c_master_recv(client, tmp, count);
if (ret >= 0)
if (copy_to_user(buf, tmp, ret))
ret = -EFAULT;
kfree(tmp);
return ret;
}2.2、i2cdev_write
i2cdev 的write接口的内核实现间接调用了 i2c_master_send() 接口， 在打开i2cdev后， 使用ioctl设定要访问的i2c设备的地址， 然后调用write()即可完成读操作
C
static ssize_t i2cdev_write(struct file *file, const char __user *buf,
size_t count, loff_t *offset)
{
int ret;
char *tmp;
struct i2c_client *client = file->private_data;
if (count > 8192)
count = 8192;
tmp = memdup_user(buf, count);
if (IS_ERR(tmp))
return PTR_ERR(tmp);
pr_debug("i2c-dev: i2c-%d writing %zu bytes.\n",
iminor(file_inode(file)), count);
ret = i2c_master_send(client, tmp, count);
kfree(tmp);
return ret;
}2.3、i2cdev_ioctl
C
static long i2cdev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
struct i2c_client *client = file->private_data;
unsigned long funcs;
dev_dbg(&client->adapter->dev, "ioctl, cmd=0x%02x, arg=0x%02lx\n",
cmd, arg);
switch (cmd) {
case I2C_SLAVE:
case I2C_SLAVE_FORCE:
if ((arg > 0x3ff) ||
(((client->flags & I2C_M_TEN) == 0) && arg > 0x7f))
return -EINVAL;
if (cmd == I2C_SLAVE && i2cdev_check_addr(client->adapter, arg))
return -EBUSY;
/* REVISIT: address could become busy later */
client->addr = arg;
return 0;
case I2C_TENBIT:
if (arg)
client->flags |= I2C_M_TEN;
else
client->flags &= ~I2C_M_TEN;
return 0;
case I2C_PEC:
/*
* Setting the PEC flag here won't affect kernel drivers,
* which will be using the i2c_client node registered with
* the driver model core. Likewise, when that client has
* the PEC flag already set, the i2c-dev driver won't see
* (or use) this setting.
*/
if (arg)
client->flags |= I2C_CLIENT_PEC;
else
client->flags &= ~I2C_CLIENT_PEC;
return 0;
case I2C_FUNCS:
funcs = i2c_get_functionality(client->adapter);
return put_user(funcs, (unsigned long __user *)arg);
case I2C_RDWR: {
struct i2c_rdwr_ioctl_data rdwr_arg;
struct i2c_msg *rdwr_pa;
if (copy_from_user(&rdwr_arg,
(struct i2c_rdwr_ioctl_data __user *)arg,
sizeof(rdwr_arg)))
return -EFAULT;
if (!rdwr_arg.msgs || rdwr_arg.nmsgs == 0)
return -EINVAL;
/*
* Put an arbitrary limit on the number of messages that can
* be sent at once
*/
if (rdwr_arg.nmsgs > I2C_RDWR_IOCTL_MAX_MSGS)
return -EINVAL;
rdwr_pa = memdup_user(rdwr_arg.msgs,
rdwr_arg.nmsgs * sizeof(struct i2c_msg));
if (IS_ERR(rdwr_pa))
return PTR_ERR(rdwr_pa);
return i2cdev_ioctl_rdwr(client, rdwr_arg.nmsgs, rdwr_pa);
}
case I2C_SMBUS: {
struct i2c_smbus_ioctl_data data_arg;
if (copy_from_user(&data_arg,
(struct i2c_smbus_ioctl_data __user *) arg,
sizeof(struct i2c_smbus_ioctl_data)))
return -EFAULT;
return i2cdev_ioctl_smbus(client, data_arg.read_write,
data_arg.command,
data_arg.size,
data_arg.data);
}
case I2C_RETRIES:
if (arg > INT_MAX)
return -EINVAL;
client->adapter->retries = arg;
break;
case I2C_TIMEOUT:
if (arg > INT_MAX)
return -EINVAL;
/* For historical reasons, user-space sets the timeout
* value in units of 10 ms.
*/
client->adapter->timeout = msecs_to_jiffies(arg * 10);
break;
default:
/* NOTE: returning a fault code here could cause trouble
* in buggy userspace code. Some old kernel bugs returned
* zero in this case, and userspace code might accidentally
* have depended on that bug.
*/
return -ENOTTY;
}
return 0;
}