i2c_txn_alloc
txn = i2c_txn_alloc(ctrl, I2C_LOCK_TAG_BUS_CONFIG, pdip);
txn = i2c_txn_alloc(ctrl, I2C_LOCK_TAG_BUS_UNCONFIG, pdip);
i2c_txn_t *txn = i2c_txn_alloc(ctrl, I2C_LOCK_TAG_DIP_DETACH, dip);
extern i2c_txn_t *i2c_txn_alloc(i2c_ctrl_t *, i2c_txn_tag_t, const void *);
txn = i2c_txn_alloc(ctrl, tag, arg);
i2c_txn_t *txn = i2c_txn_alloc(ctrl, I2C_LOCK_TAG_MUX_UNREG, mux);
i2c_txn_t *txn = i2c_txn_alloc(ctrl, I2C_LOCK_TAG_MUX_REG, reg->mr_dip);
i2c_txn_t *txn = i2c_txn_alloc(ctrl, tag, user);