mlx4: fix potential divide by 0 in mlx4_en_auto_moderation()

1) In the case where rate == priv->pkt_rate_low == priv->pkt_rate_high,
mlx4_en_auto_moderation() does a divide by zero.

2) We want to properly change the moderation parameters if rx_frames
was changed (like in ethtool -C eth0 rx-frames 16)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_netdev.c

@@ -1382,6 +1382,7 @@ static void mlx4_en_set_default_moderation(struct mlx4_en_priv *priv)
 static void mlx4_en_auto_moderation(struct mlx4_en_priv *priv)
 {
 	unsigned long period = (unsigned long) (jiffies - priv->last_moder_jiffies);
+	u32 pkt_rate_high, pkt_rate_low;
 	struct mlx4_en_cq *cq;
 	unsigned long packets;
 	unsigned long rate;
@@ -1395,37 +1396,40 @@ static void mlx4_en_auto_moderation(struct mlx4_en_priv *priv)
 	if (!priv->adaptive_rx_coal || period < priv->sample_interval * HZ)
 		return;
 
+	pkt_rate_low = READ_ONCE(priv->pkt_rate_low);
+	pkt_rate_high = READ_ONCE(priv->pkt_rate_high);
+
 	for (ring = 0; ring < priv->rx_ring_num; ring++) {
 		rx_packets = READ_ONCE(priv->rx_ring[ring]->packets);
 		rx_bytes = READ_ONCE(priv->rx_ring[ring]->bytes);
 
-		rx_pkt_diff = ((unsigned long) (rx_packets -
-				priv->last_moder_packets[ring]));
+		rx_pkt_diff = rx_packets - priv->last_moder_packets[ring];
 		packets = rx_pkt_diff;
 		rate = packets * HZ / period;
-		avg_pkt_size = packets ? ((unsigned long) (rx_bytes -
-				priv->last_moder_bytes[ring])) / packets : 0;
+		avg_pkt_size = packets ? (rx_bytes -
+				priv->last_moder_bytes[ring]) / packets : 0;
 
 		/* Apply auto-moderation only when packet rate
 		 * exceeds a rate that it matters */
 		if (rate > (MLX4_EN_RX_RATE_THRESH / priv->rx_ring_num) &&
 		    avg_pkt_size > MLX4_EN_AVG_PKT_SMALL) {
-			if (rate < priv->pkt_rate_low)
+			if (rate <= pkt_rate_low)
 				moder_time = priv->rx_usecs_low;
-			else if (rate > priv->pkt_rate_high)
+			else if (rate >= pkt_rate_high)
 				moder_time = priv->rx_usecs_high;
 			else
-				moder_time = (rate - priv->pkt_rate_low) *
+				moder_time = (rate - pkt_rate_low) *
 					(priv->rx_usecs_high - priv->rx_usecs_low) /
-					(priv->pkt_rate_high - priv->pkt_rate_low) +
+					(pkt_rate_high - pkt_rate_low) +
 					priv->rx_usecs_low;
 		} else {
 			moder_time = priv->rx_usecs_low;
 		}
 
-		if (moder_time != priv->last_moder_time[ring]) {
+		cq = priv->rx_cq[ring];
+		if (moder_time != priv->last_moder_time[ring] ||
+		    cq->moder_cnt != priv->rx_frames) {
 			priv->last_moder_time[ring] = moder_time;
-			cq = priv->rx_cq[ring];
 			cq->moder_time = moder_time;
 			cq->moder_cnt = priv->rx_frames;
 			err = mlx4_en_set_cq_moder(priv, cq);