[RFC,v2,5/7] net/mlx5: add descriptive comment for a barrier
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Message ID 20200410164127.54229-6-gavin.hu@arm.com
State New
Delegated to: Thomas Monjalon
Headers show
  • introduce new barrier class and use it for mlx5 PMD
Related show


Context Check Description
ci/Intel-compilation success Compilation OK
ci/checkpatch success coding style OK

Commit Message

Gavin Hu April 10, 2020, 4:41 p.m. UTC
The barrier is not required or can be moved down if HW waits for the
doorbell ring to execute the WQE.

This is not the case as HW can start executing the WQE until it gets the
ownership(passed by SW writing the doorbell record).

Add a decriptive comment for this HW specific behavior.

Signed-off-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
 drivers/net/mlx5/mlx5_rxtx.h | 5 +++++
 1 file changed, 5 insertions(+)

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diff --git a/drivers/net/mlx5/mlx5_rxtx.h b/drivers/net/mlx5/mlx5_rxtx.h
index 228e37de5..737d5716d 100644
--- a/drivers/net/mlx5/mlx5_rxtx.h
+++ b/drivers/net/mlx5/mlx5_rxtx.h
@@ -655,6 +655,11 @@  mlx5_tx_dbrec_cond_wmb(struct mlx5_txq_data *txq, volatile struct mlx5_wqe *wqe,
 	uint64_t *dst = MLX5_TX_BFREG(txq);
 	volatile uint64_t *src = ((volatile uint64_t *)wqe);
+	/* The ownership of WQE is passed to HW by updating the doorbell
+	 * record. Once WQE ownership has been passed to the HCA, HW can
+	 * execute the WQE. The barrier is to ensure the WQE are visible
+	 * to HW before HW execute it.
+	 */
 	*txq->qp_db = rte_cpu_to_be_32(txq->wqe_ci);
 	/* Ensure ordering between DB record and BF copy. */