net/mlx5e: Add an underflow warning comment

mlx5e_mpwqe_get_log_rq_size calculates the number of WQEs (N) based on
the requested number of frames in the RQ (F) and the number of packets
per WQE (P). It ensures that N is not less than the minimum number of
WQEs in an RQ (N_min). Arithmetically, it means that F / P >= N_min
should be true. This function deals with logarithms, so it should check
that log(F) - log(P) >= log(N_min). However, if F < P, this expression
will cause an unsigned underflow. Check log(F) >= log(P) + log(N_min)
instead.
Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

drivers/net/ethernet/mellanox/mlx5/core/en/params.c

@@ -62,11 +62,14 @@ bool mlx5e_rx_mpwqe_is_linear_skb(struct mlx5_core_dev *mdev,
 
 u8 mlx5e_mpwqe_get_log_rq_size(struct mlx5e_params *params)
 {
+	u8 log_pkts_per_wqe = mlx5e_mpwqe_log_pkts_per_wqe(params);
+
+	/* Numbers are unsigned, don't subtract to avoid underflow. */
 	if (params->log_rq_mtu_frames <
-	    mlx5e_mpwqe_log_pkts_per_wqe(params) + MLX5E_PARAMS_MINIMUM_LOG_RQ_SIZE_MPW)
+	    log_pkts_per_wqe + MLX5E_PARAMS_MINIMUM_LOG_RQ_SIZE_MPW)
 		return MLX5E_PARAMS_MINIMUM_LOG_RQ_SIZE_MPW;
 
-	return params->log_rq_mtu_frames - mlx5e_mpwqe_log_pkts_per_wqe(params);
+	return params->log_rq_mtu_frames - log_pkts_per_wqe;
 }
 
 u8 mlx5e_mpwqe_get_log_stride_size(struct mlx5_core_dev *mdev,