net: stmmac: fix handling of zero coalescing tx-usecs

Setting ethtool -C eth0 tx-usecs 0 is supposed to disable the use of the
coalescing timer but currently it gets programmed with zero delay
instead.

Disable the use of the coalescing timer if tx-usecs is zero by
preventing it from being restarted.  Note that to keep things simple we
don't start/stop the timer when the coalescing settings are changed, but
just let that happen on the next transmit or timer expiry.

Fixes: 8fce3331 ("net: stmmac: Rework coalesce timer and fix multi-queue races")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -2704,9 +2704,7 @@ static int stmmac_tx_clean(struct stmmac_priv *priv, int budget, u32 queue)
 
 	/* We still have pending packets, let's call for a new scheduling */
 	if (tx_q->dirty_tx != tx_q->cur_tx)
-		hrtimer_start(&tx_q->txtimer,
-			      STMMAC_COAL_TIMER(priv->tx_coal_timer[queue]),
-			      HRTIMER_MODE_REL);
+		stmmac_tx_timer_arm(priv, queue);
 
 	flags = u64_stats_update_begin_irqsave(&tx_q->txq_stats.syncp);
 	tx_q->txq_stats.tx_packets += tx_packets;
@@ -2995,9 +2993,13 @@ static int stmmac_init_dma_engine(struct stmmac_priv *priv)
 static void stmmac_tx_timer_arm(struct stmmac_priv *priv, u32 queue)
 {
 	struct stmmac_tx_queue *tx_q = &priv->dma_conf.tx_queue[queue];
+	u32 tx_coal_timer = priv->tx_coal_timer[queue];
+
+	if (!tx_coal_timer)
+		return;
 
 	hrtimer_start(&tx_q->txtimer,
-		      STMMAC_COAL_TIMER(priv->tx_coal_timer[queue]),
+		      STMMAC_COAL_TIMER(tx_coal_timer),
 		      HRTIMER_MODE_REL);
 }