[PATCH net] net/mlx5e: fix BQL reset on SQ re-activation

[Bobby Eshleman <bobbyeshleman@gmail.com>]

From: Bobby Eshleman <bobbyeshleman@meta.com>

mlx5e_queue_start() deactivates and re-activates all channels but closes
only the queue being restarted. mlx5e_activate_txqsq() then
unconditionally calls netdev_tx_reset_queue(), zeroing the BQL counters
of channels that kept their in-flight TX WQEs. The next completion then
over-charges and trips the BUG_ON() in dql_completed():

  kernel BUG at lib/dynamic_queue_limits.c:99!
  RIP: 0010:dql_completed+0x23d/0x280
  Call Trace:
   <IRQ>
   mlx5e_poll_tx_cq+0x668/0xa60
   mlx5e_napi_poll+0x5b/0x7b0
   net_rx_action+0x15a/0x580

Reset BQL only when the SQ has no bytes in flight (sq->cc == sq->pc).

In the case that reset is skipped, the outstanding WQEs will eventually
complete and rebalance the dql. The dql->limit is carried across the
reset.

Fixes: b2588ea40ec9 ("net/mlx5e: Implement queue mgmt ops and single channel swap")
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
 drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 4 +++-
 drivers/net/ethernet/mellanox/mlx5/core/en_tx.c   | 3 ++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
index 4a8351f95b27..44834890a409 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
@@ -1940,7 +1940,9 @@ void mlx5e_activate_txqsq(struct mlx5e_txqsq *sq)
 {
 	sq->txq = netdev_get_tx_queue(sq->netdev, sq->txq_ix);
 	set_bit(MLX5E_SQ_STATE_ENABLED, &sq->state);
-	netdev_tx_reset_queue(sq->txq);
+	/* Reset BQL only when the SQ has no bytes in flight. */
+	if (sq->cc == sq->pc)
+		netdev_tx_reset_queue(sq->txq);
 	netif_tx_start_queue(sq->txq);
 	netif_queue_set_napi(sq->netdev, sq->txq_ix, NETDEV_QUEUE_TYPE_TX, sq->cq.napi);
 }
diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
index 0b5e600e4a6a..d0ad1b8d6988 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
@@ -909,7 +909,8 @@ void mlx5e_free_txqsq_descs(struct mlx5e_txqsq *sq)
 	sq->cc = sqcc;
 
 	/* Do not update BQL for TXQs that got replaced by new active ones, as
-	 * netdev_tx_reset_queue() is called for them in mlx5e_activate_txqsq().
+	 * netdev_tx_reset_queue() is called for them in mlx5e_activate_txqsq()
+	 * if the queue has zero in-flight WQEs.
 	 */
 	if (sq == sq->priv->txq2sq[sq->txq_ix])
 		netdev_tx_completed_queue(sq->txq, npkts, nbytes);

---
base-commit: b515dc54795ef370be3cb396e7c12ad91686b6d1
change-id: 20260728-mlx5-bql-84e02a5a46c8

Best regards,
-- 
Bobby Eshleman <bobbyeshleman@meta.com>