Linux内核学习：I2C_SLAVE_FORCE
在Linux内核源代码include/linux/i2c-dev.h文件内，有如下定义：
#define I2C_SLAVE 0x0703 /* Use this slave address */
#define I2C_SLAVE_FORCE 0x0706 /* Use this slave address, even if it
is already in use by a driver! */看注释，意思就是当某个i2c设备地址已经关联了某个内核driver时，再用I2C_SLAVE作为ioctl的flag就无法取得该设备的控制权了。这时，应该使用I2C_SLAVE_FORCE。
再次搜索，找到drivers/i2c/i2c-dev.c文件：
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:
/* NOTE: devices set up to work with "new style" drivers
* can't use I2C_SLAVE, even when the device node is not
* bound to a driver. Only I2C_SLAVE_FORCE will work.
*
* 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 > 0x3ff) || // 如果地址不只10位
(((client->flags & I2C_M_TEN) == 0) && arg > 0x7f)) // 如果地址设置为7位但实际却超过7位
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:
这里case I2C_SLAVE后面是空的，实际上应该是转移到下一条分支case I2C_SLAVE_FORCE去了，也就是说这两种情况的后续处理放在了一起。
如果cmd == I2C_SLAVE并且总线上挂的设备中已经有地址为arg的设备在用了，则给出-EBUSY，这其实就是说你往相同的i2c总线上挂两个i2c地址一样的设备，肯定有潜在的问题，Linux不建议你这样干。
这个也是我之前遇到过的情况。解决办法就是设置flag为I2C_SLAVE_FORCE，这样，就可以跳过这个检测条件，将我想要使用的设备地址arg强行传递给该总线，而不理可能导致的问题，这就叫FORCE，后果自负！
client->addr = arg;另外一个问题：client为什么是通过file->private_data传递的？这就要看下面的代码了。
它描述的大概流程是这样的：inode -> minor -> i2c_dev -> adap -> client，即通过inode一步步生成了client，最后把client传递进file->private_data，和i2cdev_ioctl()中的传递方向正好相反。而且这个i2cdev_open()发生在上面的i2cdev_ioctl()之前。
static int i2cdev_open(struct inode *inode, struct file *file)
{
unsigned int minor = iminor(inode);
struct i2c_client *client;
struct i2c_adapter *adap;
struct i2c_dev *i2c_dev;
i2c_dev = i2c_dev_get_by_minor(minor);
if (!i2c_dev)
return -ENODEV;
adap = i2c_get_adapter(i2c_dev->adap->nr);
if (!adap)
return -ENODEV;
/* This creates an anonymous i2c_client, which may later be
* pointed to some address using I2C_SLAVE or I2C_SLAVE_FORCE.
*
* This client is ** NEVER REGISTERED ** with the driver model
* or I2C core code!! It just holds private copies of addressing
* information and maybe a PEC flag.
*/
client = kzalloc(sizeof(*client), GFP_KERNEL);
if (!client) {
i2c_put_adapter(adap);
return -ENOMEM;
}
snprintf(client->name, I2C_NAME_SIZE, "i2c-dev %d", adap->nr);
client->adapter = adap;
file->private_data = client;
return 0;
}注意到上面的注释中的一段话，意思是它只创建一个匿名的i2c_client，这个i2c_client以后可以通过I2C_SLAVE或I2C_SLAVE_FORCE flag指向某些i2c设备地址。而且这个i2c_client绝对不会注册到driver model或者I2C core代码中。它只是用于暂时的hold住地址信息的私有拷贝(and maybe a PEC flag，这句话暂时不理吧).