net: stmmac: Fixup the tail addr setting in xmit path

Currently we are always setting the tail address of descriptor list to
the end of the pre-allocated list.

According to databook this is not correct. Tail address should point to
the last available descriptor + 1, which means we have to update the
tail address everytime we call the xmit function.

This should make no impact in older versions of MAC but in newer
versions there are some DMA features which allows the IP to fetch
descriptors in advance and in a non sequential order so its critical
that we set the tail address correctly.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Fixes: f748be5 ("stmmac: support new GMAC4")
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@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_main.c

@@ -2213,8 +2213,7 @@ static int stmmac_init_dma_engine(struct stmmac_priv *priv)
 		stmmac_init_tx_chan(priv, priv->ioaddr, priv->plat->dma_cfg,
 				    tx_q->dma_tx_phy, chan);
 
-		tx_q->tx_tail_addr = tx_q->dma_tx_phy +
-			    (DMA_TX_SIZE * sizeof(struct dma_desc));
+		tx_q->tx_tail_addr = tx_q->dma_tx_phy;
 		stmmac_set_tx_tail_ptr(priv, priv->ioaddr,
 				       tx_q->tx_tail_addr, chan);
 	}
@@ -3003,6 +3002,7 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	netdev_tx_sent_queue(netdev_get_tx_queue(dev, queue), skb->len);
 
+	tx_q->tx_tail_addr = tx_q->dma_tx_phy + (tx_q->cur_tx * sizeof(*desc));
 	stmmac_set_tx_tail_ptr(priv, priv->ioaddr, tx_q->tx_tail_addr, queue);
 
 	return NETDEV_TX_OK;
@@ -3210,6 +3210,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	stmmac_enable_dma_transmission(priv, priv->ioaddr);
 
+	tx_q->tx_tail_addr = tx_q->dma_tx_phy + (tx_q->cur_tx * sizeof(*desc));
 	stmmac_set_tx_tail_ptr(priv, priv->ioaddr, tx_q->tx_tail_addr, queue);
 
 	return NETDEV_TX_OK;