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net/mlx5e: Add an underflow warning comment

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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>
This commit is contained in:
Maxim Mikityanskiy 2019-03-27 13:39:21 +02:00 committed by Saeed Mahameed
parent 9a22d5d839
commit b1b187e102
1 changed files with 5 additions and 2 deletions
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7
drivers/net/ethernet/mellanox/mlx5/core/en/params.c
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@ -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,