i2c_smbus系列函数有：
s32 i2c_smbus_read_byte(const struct i2c_client *client);
s32 i2c_smbus_write_byte(const struct i2c_client *client, u8 value);
s32 i2c_smbus_read_byte_data(const struct i2c_client *client, u8 command);
s32 i2c_smbus_write_byte_data(const struct i2c_client *client,
u8 command, u8 value);
s32 i2c_smbus_read_word_data(const struct i2c_client *client, u8 command);
s32 i2c_smbus_write_word_data(const struct i2c_client *client,
u8 command, u16 value);
s32 i2c_smbus_read_block_data(const struct i2c_client *client,
u8 command, u8 *values);
s32 i2c_smbus_write_block_data(const struct i2c_client *client,
u8 command, u8 length, const u8 *values);
s32 i2c_smbus_read_i2c_block_data(const struct i2c_client *client,
u8 command, u8 length, u8 *values);
s32 i2c_smbus_write_i2c_block_data(const struct i2c_client *client,
u8 command, u8 length, const u8 *values);
smbs是基于i2c总线的，但同i2c总线还是稍微有点区别的，这里也不再关注这点。
上面的一系列函数最终都是调用的i2c_smbus_xfer()函数，i2c_smbus_xfer()函数代码如下：
/**首先在判断主控制器是否支持smbus_xfer传输，但是通常i2c主控制器都是不支持的，所以直接调用i2c_smbus_xfer_emulated()函数。
* 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_adapter(adapter);
/* 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_adapter(adapter);
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;
}
EXPORT_SYMBOL(i2c_smbus_xfer);
/* Simulate a SMBus command using the i2c protocol No checking of parameters is done! */static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr, unsigned short flags, char read_write, u8 command, int size, union i2c_smbus_data *data){ /* So we need to generate a series of msgs. In the case of writing, we need to use only one message; when reading, we need two. We initialize most things with sane defaults, to keep the code below somewhat simpler. */ unsigned char msgbuf0[I2C_SMBUS_BLOCK_MAX+3]; unsigned char msgbuf1[I2C_SMBUS_BLOCK_MAX+2]; int num = read_write == I2C_SMBUS_READ ? 2 : 1; int i; u8 partial_pec = 0; int status; struct i2c_msg msg[2] = { { .addr = addr, .flags = flags, .len = 1, .buf = msgbuf0, }, { .addr = addr, .flags = flags | I2C_M_RD, .len = 0, .buf = msgbuf1, }, }; msgbuf0[0] = command; switch (size) { case I2C_SMBUS_QUICK: msg[0].len = 0; /* Special case: The read/write field is used as data */ msg[0].flags = flags | (read_write == I2C_SMBUS_READ ? I2C_M_RD : 0); num = 1; break; case I2C_SMBUS_BYTE: if (read_write == I2C_SMBUS_READ) { /* Special case: only a read! */ msg[0].flags = I2C_M_RD | flags; num = 1; } break; case I2C_SMBUS_BYTE_DATA: if (read_write == I2C_SMBUS_READ) msg[1].len = 1; else { msg[0].len = 2; msgbuf0[1] = data->byte; } break; case I2C_SMBUS_WORD_DATA: if (read_write == I2C_SMBUS_READ) msg[1].len = 2; else { msg[0].len = 3; msgbuf0[1] = data->word & 0xff; msgbuf0[2] = data->word >> 8; } break; case I2C_SMBUS_PROC_CALL: num = 2; /* Special case */ read_write = I2C_SMBUS_READ; msg[0].len = 3; msg[1].len = 2; msgbuf0[1] = data->word & 0xff; msgbuf0[2] = data->word >> 8; break; case I2C_SMBUS_BLOCK_DATA: if (read_write == I2C_SMBUS_READ) { msg[1].flags |= I2C_M_RECV_LEN; msg[1].len = 1; /* block length will be added by the underlying bus driver */ } else { msg[0].len = data->block[0] + 2; if (msg[0].len > I2C_SMBUS_BLOCK_MAX + 2) { dev_err(&adapter->dev, "Invalid block write size %d\n", data->block[0]); return -EINVAL; } for (i = 1; i < msg[0].len; i++) msgbuf0[i] = data->block[i-1]; } break; case I2C_SMBUS_BLOCK_PROC_CALL: num = 2; /* Another special case */ read_write = I2C_SMBUS_READ; if (data->block[0] > I2C_SMBUS_BLOCK_MAX) { dev_err(&adapter->dev, "Invalid block write size %d\n", data->block[0]); return -EINVAL; } msg[0].len = data->block[0] + 2; for (i = 1; i < msg[0].len; i++) msgbuf0[i] = data->block[i-1]; msg[1].flags |= I2C_M_RECV_LEN; msg[1].len = 1; /* block length will be added by the underlying bus driver */ break; case I2C_SMBUS_I2C_BLOCK_DATA: if (read_write == I2C_SMBUS_READ) { msg[1].len = data->block[0]; } else { msg[0].len = data->block[0] + 1; if (msg[0].len > I2C_SMBUS_BLOCK_MAX + 1) { dev_err(&adapter->dev, "Invalid block write size %d\n", data->block[0]); return -EINVAL; } for (i = 1; i <= data->block[0]; i++) msgbuf0[i] = data->block[i]; } break; default: dev_err(&adapter->dev, "Unsupported transaction %d\n", size); return -EOPNOTSUPP; } i = ((flags & I2C_CLIENT_PEC) && size != I2C_SMBUS_QUICK && size != I2C_SMBUS_I2C_BLOCK_DATA); if (i) { /* Compute PEC if first message is a write */ if (!(msg[0].flags & I2C_M_RD)) { if (num == 1) /* Write only */ i2c_smbus_add_pec(&msg[0]); else /* Write followed by read */ partial_pec = i2c_smbus_msg_pec(0, &msg[0]); } /* Ask for PEC if last message is a read */ if (msg[num-1].flags & I2C_M_RD) msg[num-1].len++; } status = i2c_transfer(adapter, msg, num); if (status < 0) return status; /* Check PEC if last message is a read */ if (i && (msg[num-1].flags & I2C_M_RD)) { status = i2c_smbus_check_pec(partial_pec, &msg[num-1]); if (status < 0) return status; } if (read_write == I2C_SMBUS_READ) switch (size) { case I2C_SMBUS_BYTE: data->byte = msgbuf0[0]; break; case I2C_SMBUS_BYTE_DATA: data->byte = msgbuf1[0]; break; case I2C_SMBUS_WORD_DATA: case I2C_SMBUS_PROC_CALL: data->word = msgbuf1[0] | (msgbuf1[1] << 8); break; case I2C_SMBUS_I2C_BLOCK_DATA: for (i = 0; i < data->block[0]; i++) data->block[i+1] = msgbuf1[i]; break; case I2C_SMBUS_BLOCK_DATA: case I2C_SMBUS_BLOCK_PROC_CALL: for (i = 0; i < msgbuf1[0] + 1; i++) data->block[i] = msgbuf1[i]; break; } return 0;}
首先定义了两个i2c_msg，还记得i2c_transfer()函数吗，这两个msg的buf分别为msgbuf0、msgbuf1。
然后来到switch处这里，这里主要就是针对的上面的i2c_smbus一系列函数。
1. i2c_smbus_read_byte()、i2c_smbus_write_byte()
这两个函数是没有太大意义的，如果是read，则连读的基地址都没有，如果是write，则是简单将value发送过去了，设备或许都不知道这是发送写的基地址还是写的数据，这两个函数有点类似于i2c_master_recv()、i2c_master_send()。
2. i2c_smbus_read_byte_data()、i2c_smbus_write_byte_data()
首先是read，如果是read将传送两个msg，第一个msg用来传送读的基地址，第二个msg用来读取数据。
然后是write，write只传送一个msg，只发送一个msg不好的地方是只能单方向传输，比如写就只能写，读就只能读（注意读的基地址是写方向）。但是这里为什么可以只传送一个msg呢，因为传输的方向并没有改变，包括写基地址，写数据都是写方向，但是写数据的长度为2，写基地址还是command，写的数据为value。
3. i2c_smbus_read_word_data()、i2c_smbus_write_word_data()
同上面类似，只是一次读写两个字节。
4. i2c_smbus_read_block_data()、i2c_smbus_write_block_data()
读注意，这里并没有指定读数据的长度。
5. i2c_smbus_read_i2c_block_data()、i2c_smbus_write_i2c_block_data()
同上面不一样的是，read时有长度指定。
而write同上面不同的是，i2c_smbus_write_block_data()函数会将发送数据的长度一起发送给设备，而这里则不会。
最终调用i2c_transfer函数来发送msg。
所以在我们的i2c设备驱动那边，可以简单的调用上面的i2c_smbs接口函数，也可以自己构造i2c_msg，然后调用i2c_transfer来发送数据。