net/mlx5e: Calculate SQ stop room in a robust way

Currently, different formulas are used to estimate the space that may be
taken by WQEs in the SQ during a single packet transmit. This space is
called stop room, and it's checked in the end of packet transmit to find
out if the next packet could overflow the SQ. If it could, the driver
tells the kernel to stop sending next packets.

Many factors affect the stop room:

1. Padding with NOPs to avoid WQEs spanning over page boundaries.

2. Enabled and disabled offloads (TLS, upcoming MPWQE).

3. The maximum size of a WQE.

The padding is performed before every WQE if it doesn't fit the current
page.

The current formula assumes that only one padding will be required per
packet, and it doesn't take into account that the WQEs posted during the
transmission of a single packet might exceed the page size in very rare
circumstances. For example, to hit this condition with 4096-byte pages,
TLS offload will have to interrupt an almost-full MPWQE session, be in
the resync flow and try to transmit a near to maximum amount of data.

To avoid SQ overflows in such rare cases after MPWQE is added, this
patch introduces a more robust formula to estimate the stop room. The
new formula uses the fact that a WQE of size X will not require more
than X-1 WQEBBs of padding. More exact estimations are possible, but
they result in much more complex and error-prone code for little gain.

Before this patch, the TLS stop room included space for both INNOVA and
ConnectX TLS offloads that couldn't run at the same time anyway, so this
patch accounts only for the active one.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

1 files changed, 6 insertions, 6 deletions

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls.h b/drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls.h
index dabbc5f226ce..c6180892cfcb 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls.h
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_accel/ktls.h
@@ -102,15 +102,16 @@ bool mlx5e_ktls_handle_tx_skb(struct tls_context *tls_ctx, struct mlx5e_txqsq *s
 void mlx5e_ktls_tx_handle_resync_dump_comp(struct mlx5e_txqsq *sq,
 					   struct mlx5e_tx_wqe_info *wi,
 					   u32 *dma_fifo_cc);
+u16 mlx5e_ktls_get_stop_room(struct mlx5e_txqsq *sq);
+
 static inline u8
-mlx5e_ktls_dumps_num_wqebbs(struct mlx5e_txqsq *sq, unsigned int nfrags,
-			    unsigned int sync_len)
+mlx5e_ktls_dumps_num_wqes(struct mlx5e_txqsq *sq, unsigned int nfrags,
+			  unsigned int sync_len)
 {
 	/* Given the MTU and sync_len, calculates an upper bound for the
-	 * number of WQEBBs needed for the TX resync DUMP WQEs of a record.
+	 * number of DUMP WQEs needed for the TX resync of a record.
 	 */
-	return MLX5E_KTLS_DUMP_WQEBBS *
-		(nfrags + DIV_ROUND_UP(sync_len, sq->hw_mtu));
+	return nfrags + DIV_ROUND_UP(sync_len, sq->hw_mtu);
 }
 #else
 
@@ -122,7 +123,6 @@ static inline void
 mlx5e_ktls_tx_handle_resync_dump_comp(struct mlx5e_txqsq *sq,
 				      struct mlx5e_tx_wqe_info *wi,
 				      u32 *dma_fifo_cc) {}
-
 #endif
 
 #endif /* __MLX5E_TLS_H__ */