在前面我们介绍了I2C host的数据结构描述struct i2c_adapter，其中提到结构中 struct i2c_algorithm *algo; 这个数据结构用于描述或者说用于控制I2C总线上数据格式，其体现了host对传输数据要求，一般我们 在i2c_add_adapter()时需要实现这样一个对象。struct i2c_algorithm定义如下：
/**
* struct i2c_algorithm - represent I2C transfer method
* @master_xfer: Issue a set of i2c transactions to the given I2C adapter
* defined by the msgs array, with num messages available to transfer via
* the adapter specified by adap.
* @smbus_xfer: Issue smbus transactions to the given I2C adapter. If this
* is not present, then the bus layer will try and convert the SMBus calls
* into I2C transfers instead.
* @functionality: Return the flags that this algorithm/adapter pair supports
* from the I2C_FUNC_* flags.
* @reg_slave: Register given client to I2C slave mode of this adapter
* @unreg_slave: Unregister given client from I2C slave mode of this adapter
*
* The following structs are for those who like to implement new bus drivers:
* i2c_algorithm is the interface to a class of hardware solutions which can
* be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
* to name two of the most common.
*
* The return codes from the @master_xfer field should indicate the type of
* error code that occurred during the transfer, as documented in the kernel
* Documentation file Documentation/i2c/fault-codes.
*/
struct i2c_algorithm {
/* If an adapter algorithm can't do I2C-level access, set master_xfer
to NULL. If an adapter algorithm can do SMBus access, set
smbus_xfer. If set to NULL, the SMBus protocol is simulated
using common I2C messages */
/* master_xfer should return the number of messages successfully
processed, or a negative value on error */
int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
int num);
int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
unsigned short flags, char read_write,
u8 command, int size, union i2c_smbus_data *data); /* To determine what the adapter supports */
u32 (*functionality) (struct i2c_adapter *); #if IS_ENABLED(CONFIG_I2C_SLAVE)
int (*reg_slave)(struct i2c_client *client);
int (*unreg_slave)(struct i2c_client *client);
#endif
};
由于这些跟具体的host相关，所以，在理解host ic之上实现这些函数。 其实根据上面两个函数master_xfer和smbus_xfer就可以明白，I2C支持两个类别的传输。对此 内核封装两个函数：i2c_smbus_xfer和__i2c_transfer()，对于后者，其提供封装接口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; if (adap->quirks && i2c_check_for_quirks(adap, msgs, num))
return -EOPNOTSUPP; /* 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;
}
/**
* i2c_smbus_xfer - execute SMBus protocol operations
* @adapter: Handle to I2C bus
* @addr: Address of SMBus slave on that bus
* @flags: I2C_CLIENT_* flags (usually zero or I2C_CLIENT_PEC)
* @read_write: I2C_SMBUS_READ or I2C_SMBUS_WRITE
* @command: Byte interpreted by slave, for protocols which use such bytes
* @protocol: SMBus protocol operation to execute, such as I2C_SMBUS_PROC_CALL
* @data: Data to be read or written
*
* This executes an SMBus protocol operation, and returns a negative
* errno code else zero on success.
*/
s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr, unsigned short flags,
char read_write, u8 command, int protocol,
union i2c_smbus_data *data)
{
unsigned long orig_jiffies;
int try;
s32 res; /* If enabled, the following two tracepoints are conditional on
* read_write and protocol.
*/
trace_smbus_write(adapter, addr, flags, read_write,
command, protocol, data);
trace_smbus_read(adapter, addr, flags, read_write,
command, protocol); flags &= I2C_M_TEN | I2C_CLIENT_PEC | I2C_CLIENT_SCCB; if (adapter->algo->smbus_xfer) {
i2c_lock_bus(adapter, I2C_LOCK_SEGMENT); /* Retry automatically on arbitration loss */
orig_jiffies = jiffies;
for (res = 0, try = 0; try <= adapter->retries; try++) {
res = adapter->algo->smbus_xfer(adapter, addr, flags,
read_write, command,
protocol, data);
if (res != -EAGAIN)
break;
if (time_after(jiffies,
orig_jiffies + adapter->timeout))
break;
}
i2c_unlock_bus(adapter, I2C_LOCK_SEGMENT); if (res != -EOPNOTSUPP || !adapter->algo->master_xfer)
goto trace;
/*
* Fall back to i2c_smbus_xfer_emulated if the adapter doesn't
* implement native support for the SMBus operation.
*/
} res = i2c_smbus_xfer_emulated(adapter, addr, flags, read_write,
command, protocol, data); trace:
/* If enabled, the reply tracepoint is conditional on read_write. */
trace_smbus_reply(adapter, addr, flags, read_write,
command, protocol, data);
trace_smbus_result(adapter, addr, flags, read_write,
command, protocol, res); return res;
}
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