struct_i2c_msg(9) an I2C transaction segment beginning with START


struct i2c_msg {
__u16 addr;
__u16 flags;
#define I2C_M_TEN 0x0010
#define I2C_M_RD 0x0001
#define I2C_M_NOSTART 0x4000
#define I2C_M_REV_DIR_ADDR 0x2000
#define I2C_M_IGNORE_NAK 0x1000
#define I2C_M_NO_RD_ACK 0x0800
#define I2C_M_RECV_LEN 0x0400
__u16 len;
__u8 * buf;



Slave address, either seven or ten bits. When this is a ten bit address, I2C_M_TEN must be set in flags and the adapter must support I2C_FUNC_10BIT_ADDR.


I2C_M_RD is handled by all adapters. No other flags may be provided unless the adapter exported the relevant I2C_FUNC_* flags through i2c_check_functionality.


Number of data bytes in buf being read from or written to the I2C slave address. For read transactions where I2C_M_RECV_LEN is set, the caller guarantees that this buffer can hold up to 32 bytes in addition to the initial length byte sent by the slave (plus, if used, the SMBus PEC); and this value will be incremented by the number of block data bytes received.


The buffer into which data is read, or from which it's written.


An i2c_msg is the low level representation of one segment of an I2C transaction. It is visible to drivers in the i2c_transfer() procedure, to userspace from i2c-dev, and to I2C adapter drivers through the i2c_adapter.master_xfer() method.

Except when I2C "protocol mangling" is used, all I2C adapters implement the standard rules for I2C transactions. Each transaction begins with a START. That is followed by the slave address, and a bit encoding read versus write. Then follow all the data bytes, possibly including a byte with SMBus PEC. The transfer terminates with a NAK, or when all those bytes have been transferred and ACKed. If this is the last message in a group, it is followed by a STOP. Otherwise it is followed by the next i2c_msg transaction segment, beginning with a (repeated) START.

Alternatively, when the adapter supports I2C_FUNC_PROTOCOL_MANGLING then passing certain flags may have changed those standard protocol behaviors. Those flags are only for use with broken/nonconforming slaves, and with adapters which are known to support the specific mangling options they need (one or more of IGNORE_NAK, NO_RD_ACK, NOSTART, and REV_DIR_ADDR).