[PATCH net v3] net/mlx5e: fix BQL reset on SQ re-activation

From: Bobby Eshleman <[email protected]>

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 <[email protected]>
---
Changes in v3:
- Sample sq->cc/sq->pc before setting MLX5E_SQ_STATE_ENABLED, so a
  concurrent mlx5e_poll_tx_cq() cannot un-charge BQL between the check
  and netdev_tx_reset_queue() (Sashiko:
  https://netdev-ai.bots.linux.dev/sashiko/#/patchset/[email protected])
- Link to v2: https://lore.kernel.org/r/[email protected]

Changes in v2:
- remove misleading comment (Tariq)
- Link to v1: https://lore.kernel.org/r/[email protected]
---
 drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
index 7d47a1da8b6b..f0407a850ea8 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
@@ -1939,8 +1939,10 @@ int mlx5e_open_txqsq(struct mlx5e_channel *c, u32 tisn, int txq_ix,
 void mlx5e_activate_txqsq(struct mlx5e_txqsq *sq)
 {
 	sq->txq = netdev_get_tx_queue(sq->netdev, sq->txq_ix);
+	/* Reset BQL only when the SQ has no bytes in flight. */
+	if (sq->cc == sq->pc)
+		netdev_tx_reset_queue(sq->txq);
 	set_bit(MLX5E_SQ_STATE_ENABLED, &sq->state);
-	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);
 }

---
base-commit: d1000fd7995e51deec872d154e0a40d82f7a539f
change-id: 20260728-mlx5-bql-84e02a5a46c8

Best regards,
-- 
Bobby Eshleman <[email protected]>