在Linux內核源代碼include/linux/i2c-dev.h文件內，有如下定義：

#define I2C_SLAVE0x0703/* Use this slave address */

#define I2C_SLAVE_FORCE0x0706/* Use this slave address, even if it

is already in use by a driver! */

看注釋，意思就是當某個i2c設備地址已經關聯了某個內核driver時，再用I2C_SLAVE作為ioctl的flag就無法取得該設備的控制權了。這時，應該使用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)) // 如果地址設置為7位但實際卻超過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:上面的第一個參數file，舉個例子，它可以是/dev/i2c-2，那么由此得到的client應該是掛在這個i2c-2總線上的所有i2c設備吧，這是我的理解。

這里case I2C_SLAVE后面是空的，實際上應該是轉移到下一條分支case I2C_SLAVE_FORCE去了，也就是說這兩種情況的后續處理放在了一起。

如果cmd == I2C_SLAVE并且總線上掛的設備中已經有地址為arg的設備在用了，則給出-EBUSY，這其實就是說你往相同的i2c總線上掛兩個i2c地址一樣的設備，肯定有潛在的問題，Linux不建議你這樣干。

這個也是我之前遇到過的情況。解決辦法就是設置flag為I2C_SLAVE_FORCE，這樣，就可以跳過這個檢測條件，將我想要使用的設備地址arg強行傳遞給該總線，而不理可能導致的問題，這就叫FORCE，后果自負！

client->addr = arg;

另外一個問題：client為什么是通過file->private_data傳遞的？這就要看下面的代碼了。

它描述的大概流程是這樣的：inode -> minor -> i2c_dev -> adap -> client，即通過inode一步步生成了client，最后把client傳遞進file->private_data，和i2cdev_ioctl()中的傳遞方向正好相反。而且這個i2cdev_open()發生在上面的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;

}

注意到上面的注釋中的一段話，意思是它只創建一個匿名的i2c_client，這個i2c_client以后可以通過I2C_SLAVE或I2C_SLAVE_FORCE flag指向某些i2c設備地址。而且這個i2c_client絕對不會注冊到driver model或者I2C core代碼中。它只是用于暫時的hold住地址信息的私有拷貝(and maybe a PEC flag，這句話暫時不理吧).

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