net: ena: optimise calculations for CQ doorbell

BugLink: https://bugs.launchpad.net/bugs/1850175

This patch initially checks if CQ doorbell
is needed before proceeding with the calculations.
Signed-off-by: Igor Chauskin <igorch@amazon.com>
Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit d9186098)
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Khaled Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

drivers/net/ethernet/amazon/ena/ena_eth_com.h

@@ -195,16 +195,18 @@ static inline int ena_com_update_dev_comp_head(struct ena_com_io_cq *io_cq)
 	u16 unreported_comp, head;
 	bool need_update;
 
+	if (unlikely(io_cq->cq_head_db_reg)) {
 		head = io_cq->head;
 		unreported_comp = head - io_cq->last_head_update;
 		need_update = unreported_comp > (io_cq->q_depth / ENA_COMP_HEAD_THRESH);
 
-	if (io_cq->cq_head_db_reg && need_update) {
+		if (unlikely(need_update)) {
 			pr_debug("Write completion queue doorbell for queue %d: head: %d\n",
 				 io_cq->qid, head);
 			writel(head, io_cq->cq_head_db_reg);
 			io_cq->last_head_update = head;
 		}
+	}
 
 	return 0;
 }