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User space structures used by I2C ioctl().

From include/uapi/linux/i2c.h:

/**
 * struct i2c_msg - an I2C transaction segment beginning with START
 * @addr: 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.
 * @flags: 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().
 * @len: 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.
 * @buf: 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).
 */
struct i2c_msg {
        __u16 addr;     /* slave address                        */
        __u16 flags;
#define I2C_M_RD                0x0001  /* read data, from slave to master */
                                        /* I2C_M_RD is guaranteed to be 0x0001! */
#define I2C_M_TEN               0x0010  /* this is a ten bit chip address */
#define I2C_M_DMA_SAFE          0x0200  /* the buffer of this message is DMA safe */
                                        /* makes only sense in kernelspace */
                                        /* userspace buffers are copied anyway */
#define I2C_M_RECV_LEN          0x0400  /* length will be first received byte */
#define I2C_M_NO_RD_ACK         0x0800  /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_IGNORE_NAK        0x1000  /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_REV_DIR_ADDR      0x2000  /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_NOSTART           0x4000  /* if I2C_FUNC_NOSTART */
#define I2C_M_STOP              0x8000  /* if I2C_FUNC_PROTOCOL_MANGLING */
        __u16 len;              /* msg length                           */
        __u8 *buf;              /* pointer to msg data                  */
};