15.2 Linux I2C核心
I2C核心（drivers/i2c/i2c-core.c）中提供一组不依赖于硬件平台的接口函数，这个文件一般不需要被修改，但是理解其中的主要函数非常关键，因为I2C总线驱动和设备驱动之间以I2C核心作为纽带。
I2C核心中的主要函数如下：
include/linux/i2c.h
（1）增加/删除i2c_adapter
int i2c_add_adapter(struct i2c_adapter *adap);
void i2c_del_adapter(struct i2c_adapter *adap);
（2）增加/删除i2c_driver
int i2c_register_driver(struct module *owner, struct i2c_driver *driver);
void i2c_del_driver(struct i2c_driver *driver);
/* use a define to avoid include chaining to get THIS_MODULE */
#define i2c_add_driver(driver) \
i2c_register_driver(THIS_MODULE, driver)
（3）I2C传输、发送和接收
int i2c_transfer(struct i2c_adapter * adap, struct i2c_msg *msgs, int num);
int i2c_master_send(struct i2c_client *client,const char *buf ,int count); // 写
int i2c_master_recv(struct i2c_client *client, char *buf ,int count);// 读
备注：
i2c_transfer（）函数用于进行I2C适配器和I2C设备之间的一组消息交互，其中第2个参数是一个指向i2c_msg数组的指针，所以i2c_transfer（）一次可以传输多个i2c_msg（考虑到很多外设的读写波形比较复杂，比如读寄存器可能要先写，所以需要两个以上的消息）。对于时序比较简单的外设，i2c_master_send（）函数和i2c_master_recv（）函数内部会调用i2c_transfer（）函数分别完成一条写消息和一条读消息，如代码清单15.6、15.7所示。
代码清单15.6 I2C核心的i2c_master_send（）函数
/**
* i2c_master_send - issue a single I2C message in master transmit mode
* @client: Handle to slave device
* @buf: Data that will be written to the slave
* @count: How many bytes to write, must be less than 64k since msg.len is u16
*
* Returns negative errno, or else the number of bytes written.
*/
int i2c_master_send(const struct i2c_client *client, const char *buf, int count)
{
int ret;
struct i2c_adapter *adap = client->adapter;
struct i2c_msg msg;
msg.addr = client->addr;
msg.flags = client->flags & I2C_M_TEN;
msg.len = count;
msg.buf = (char *)buf;
ret = i2c_transfer(adap, &msg, 1);// 完成一条写消息
/*
* If everything went ok (i.e. 1 msg transmitted), return #bytes
* transmitted, else error code.
*/
return (ret == 1) ? count : ret;
}
EXPORT_SYMBOL(i2c_master_send);
代码清单15.7 I2C核心的i2c_master_recv（）函数
/**
* i2c_master_recv - issue a single I2C message in master receive mode
* @client: Handle to slave device
* @buf: Where to store data read from slave
* @count: How many bytes to read, must be less than 64k since msg.len is u16
*
* Returns negative errno, or else the number of bytes read.
*/
int i2c_master_recv(const struct i2c_client *client, char *buf, int count)
{
struct i2c_adapter *adap = client->adapter;
struct i2c_msg msg;
int ret;
msg.addr = client->addr;
msg.flags = client->flags & I2C_M_TEN;
msg.flags |= I2C_M_RD;
msg.len = count;
msg.buf = buf;
ret = i2c_transfer(adap, &msg, 1);// 完成一条读消息
/*
* If everything went ok (i.e. 1 msg received), return #bytes received,
* else error code.
*/
return (ret == 1) ? count : ret;
}
EXPORT_SYMBOL(i2c_master_recv);
备注：
i2c_transfer（）函数本身不具备驱动适配器物理硬件以完成消息交互的能力，i2c_transfer（）只是寻找到与
i2c_adapter对应的i2c_algorithm，并使用i2c_algorithm的master_xfer（）函数真正驱动硬件流程，如代码清
单15.8所示。
代码清单15.8 I2C核心的i2c_transfer（）函数
/**
* i2c_transfer - execute a single or combined（组合的） I2C message
* @adap: Handle to I2C bus
* @msgs: One or more messages to execute before STOP is issued to
* terminate the operation; each message begins with a START.
* @num: Number of messages to be executed.
*
* Returns negative errno, else the number of messages executed.
*
* Note that there is no requirement that each message be sent to
* the same slave address, although that is the most common model.
*/
int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
int ret;
if (adap->algo->master_xfer) {
#ifdef DEBUG
for (ret = 0; ret < num; ret++) {
dev_dbg(&adap->dev, "master_xfer[%d] %c, addr=0x%02x, "
"len=%d%s\n", ret, (msgs[ret].flags & I2C_M_RD)
? 'R' : 'W', msgs[ret].addr, msgs[ret].len,
(msgs[ret].flags & I2C_M_RECV_LEN) ? "+" : "");
}
#endif
if (in_atomic() || irqs_disabled()) {
ret = i2c_trylock_adapter(adap);
if (!ret)
/* I2C activity is ongoing. */
return -EAGAIN;
} else {
i2c_lock_adapter(adap);
}
ret = __i2c_transfer(adap, msgs, num);
i2c_unlock_adapter(adap);
return ret;
} else {
dev_dbg(&adap->dev, "I2C level transfers not supported\n");
return -EOPNOTSUPP;
}
}
EXPORT_SYMBOL(i2c_transfer);
/**
* __i2c_transfer - unlocked flavor of i2c_transfer
* @adap: Handle to I2C bus
* @msgs: One or more messages to execute before STOP is issued to
* terminate the operation; each message begins with a START.
* @num: Number of messages to be executed.
*
* Returns negative errno, else the number of messages executed.
*
* Adapter lock must be held when calling this function. No debug logging
* takes place. adap->algo->master_xfer existence isn't checked.
*/
int __i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
unsigned long orig_jiffies;
int ret, try;
/* i2c_trace_msg gets enabled when tracepoint i2c_transfer gets
* enabled. This is an efficient way of keeping the for-loop from
* being executed when not needed.
*/
if (static_key_false(&i2c_trace_msg)) {
int i;
for (i = 0; i < num; i++)
if (msgs[i].flags & I2C_M_RD)
trace_i2c_read(adap, &msgs[i], i);
else
trace_i2c_write(adap, &msgs[i], i);
}
/* Retry automatically on arbitration loss */
orig_jiffies = jiffies;
for (ret = 0, try = 0; try <= adap->retries; try++) {
ret = adap->algo->master_xfer(adap, msgs, num);
if (ret != -EAGAIN)
break;
if (time_after(jiffies, orig_jiffies + adap->timeout))
break;
}
if (static_key_false(&i2c_trace_msg)) {
int i;
for (i = 0; i < ret; i++)
if (msgs[i].flags & I2C_M_RD)
trace_i2c_reply(adap, &msgs[i], i);
trace_i2c_result(adap, i, ret);
}
return ret;
}
EXPORT_SYMBOL(__i2c_transfer);