数据收发流程

驱动层收发接口

首先了解，驱动层向协议（算法）层注册的接口。

注释写的清楚：

1）如果不支持I2C 层级的访问，则设置i2c收发接口master_xfer为空，否则设置I2C控制器的接口；

2）如果控制器支持smbus协议，则对应的驱动设置收发接口smbus_xfer.

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 (*master_xfer_atomic)(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);int (*smbus_xfer_atomic)(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 *adap);#if IS_ENABLED(CONFIG_I2C_SLAVE)int (*reg_slave)(struct i2c_client *client);int (*unreg_slave)(struct i2c_client *client);
#endif
};

驱动层注册

对于此数据结构的使用，内核i801(intel smbus控制器)和 imx i2c 驱动，给出了示例：

smbus控制器

static const struct i2c_algorithm smbus_algorithm = {//由于控制器芯片支持smbus，不支持i2c级别。所以这里采用smbus_xfer接口.smbus_xfer	= i801_access,   .functionality	= i801_func,
};

i2c控制器

static const struct i2c_algorithm i2c_imx_algo = {//i2c 控制器，实现i2c级别数据传输，不支持smbus接口。.master_xfer = i2c_imx_xfer,.master_xfer_atomic = i2c_imx_xfer_atomic,.functionality = i2c_imx_func,.reg_slave	= i2c_imx_reg_slave,.unreg_slave	= i2c_imx_unreg_slave,
};

协议接口使用

对于注册接口的不同，在 drivers\i2c \ i2c-core-smbus.c __i2c_smbus_xfer 接口可以看出。


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)
{int (*xfer_func)(struct i2c_adapter *adap, u16 addr,unsigned short flags, char read_write,u8 command, int size, union i2c_smbus_data *data);unsigned long orig_jiffies;int try;s32 res;res = __i2c_check_suspended(adapter);if (res)return 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;//查看控制器向协议层注册的smbus_xfer接口xfer_func = adapter->algo->smbus_xfer; if (i2c_in_atomic_xfer_mode()) {if (adapter->algo->smbus_xfer_atomic)xfer_func = adapter->algo->smbus_xfer_atomic;else if (adapter->algo->master_xfer_atomic)xfer_func = NULL; /* fallback to I2C emulation */}if (xfer_func) { //对于smbus(i801)，此接口不为空，则调用i801驱动接口收发数据/* Retry automatically on arbitration loss */orig_jiffies = jiffies;for (res = 0, try = 0; try <= adapter->retries; try++) {res = xfer_func(adapter, addr, flags, read_write,command, protocol, data);if (res != -EAGAIN)break;if (time_after(jiffies,orig_jiffies + adapter->timeout))break;}//如果不是i2c控制器驱动，即master_xfer为空，则直接跳转到trace,// 不会调用i2c_smbus_xfer_emulated接口进行smbus协议生成。// 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, res);trace_smbus_result(adapter, addr, flags, read_write,command, protocol, res);return res;
}

总体流程

图 1 i2c /smbus 数据传输路径

数据收发关键数据结构

以上描述了i2c smbus从应用层到驱动控制器的数据流转路径。

本章描述在此路径上流转所采用的数据载体形式。在整个i2c 驱动中采用了如下数据结构，此数据结构表示一帧 i2c/smbus数据。

struct i2c_msg {__u16 addr;     //i2c slave device address__u16 flags;
#define I2C_M_RD		0x0001	/* guaranteed to be 0x0001! */
#define I2C_M_TEN		0x0010	/* use only if I2C_FUNC_10BIT_ADDR */
#define I2C_M_DMA_SAFE		0x0200	/* use only in kernel space */
#define I2C_M_RECV_LEN		0x0400	/* use only if I2C_FUNC_SMBUS_READ_BLOCK_DATA */
#define I2C_M_NO_RD_ACK		0x0800	/* use only if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_IGNORE_NAK	0x1000	/* use only if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_REV_DIR_ADDR	0x2000	/* use only if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_NOSTART		0x4000	/* use only if I2C_FUNC_NOSTART */
#define I2C_M_STOP		0x8000	/* use only if I2C_FUNC_PROTOCOL_MANGLING */__u16 len;   //传输的数据长度，即i2c一帧数据中，data的个数。__u8 *buf;   //存放数据的缓存。
};

使用示例--eeprom的读写

假设我们采用的eeprom 地址是16位的，eeprom期望的时序（报文协议）如下：

可以看到此处有两个START，则需要两个i2c_msg数据结构分别携带此两段数据。其中：

第一个start，为写，i2c_msg的字段填写为：

  i2c_msg.addr = slave_addr;  i2c_msg .flags = I2C_M_WR;        i2c_msg.len = 2;  //在第一个START中，数据data即为两个字节的地址，因而这里填写2              i2c_msg.buf[0] = (eep_byte_addr >> 8) & 0x0ff; i2c_msg.buf[1] = eep_byte_addr  & 0x0ff;

第二个start，为读， i2cmsg的字段信息填写为

  i2c_msg.addr = slave_addr;  i2c_msg .flags = I2C_M_RD;        i2c_msg.len = 需要读取的数据长度;  //i2c_msg.buf = 存放读取回来的数据

使用示例--smbus的读写接口

i2cdetect程序里面提供了一套读写接口，最终调用驱动的代码。此处采用两个i2c_msg，即可以支持读，又可以支持写。通常读采用两个i2c_msg完成；写采用一个，可以看到默认msg[1].len=0。

i2c_smbus_xfer_emulatedstruct i2c_msg msg[2] = {{.addr = addr,.flags = flags,.len = 1,.buf = msgbuf0,}, {.addr = addr,.flags = flags | I2C_M_RD,.len = 0,        .buf = msgbuf1,},};

在smbus的协议中，不论是读，还是写，都存在一个command字段，例如写字节的协议报文