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_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:

上面的第一个参数file，举个例子，它可以是/dev/i2c-2，那么由此得到的client应该是挂在这个i2c-2总线上的所有i2c设备吧，这是我的理解。

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).