__fbnic_mbx_rd_desc
u64 desc = __fbnic_mbx_rd_desc(fbd, mbx_idx, i);
desc = __fbnic_mbx_rd_desc(fbd, FBNIC_IPC_MBX_RX_IDX, head);
desc = __fbnic_mbx_rd_desc(fbd, FBNIC_IPC_MBX_TX_IDX, head);
u64 __fbnic_mbx_rd_desc(struct fbnic_dev *fbd, int mbx_idx, int desc_idx);