net: stmmac: Do not keep rearming the coalesce timer in stmmac_xmit

This is cutting down performance. Once the timer is armed it should run
after the time expires for the first packet sent and not the last one.

After this change, running iperf, the performance gain is +/- 24%.
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Vitor Soares <soares@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac.h

@@ -105,6 +105,7 @@ struct stmmac_priv {
 	u32 tx_count_frames;
 	u32 tx_coal_frames;
 	u32 tx_coal_timer;
+	bool tx_timer_armed;
 
 	int tx_coalesce;
 	int hwts_tx_en;

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -3158,13 +3158,16 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 	 * element in case of no SG.
 	 */
 	priv->tx_count_frames += nfrags + 1;
-	if (likely(priv->tx_coal_frames > priv->tx_count_frames)) {
+	if (likely(priv->tx_coal_frames > priv->tx_count_frames) &&
+	    !priv->tx_timer_armed) {
 		mod_timer(&priv->txtimer,
 			  STMMAC_COAL_TIMER(priv->tx_coal_timer));
+		priv->tx_timer_armed = true;
 	} else {
 		priv->tx_count_frames = 0;
 		stmmac_set_tx_ic(priv, desc);
 		priv->xstats.tx_set_ic_bit++;
+		priv->tx_timer_armed = false;
 	}
 
 	skb_tx_timestamp(skb);