r8169: add workaround for RTL8168evl TSO hw issues

Add workaround for hw issues with TSO on RTL8168evl. This workaround is
based on information I got from Realtek, and *should* allow to safely
enable TSO on this chip version.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/r8169_main.c

@@ -4306,6 +4306,37 @@ static netdev_tx_t rtl8169_start_xmit(struct sk_buff *skb,
 	return NETDEV_TX_BUSY;
 }
 
+static unsigned int rtl_last_frag_len(struct sk_buff *skb)
+{
+	struct skb_shared_info *info = skb_shinfo(skb);
+	unsigned int nr_frags = info->nr_frags;
+
+	if (!nr_frags)
+		return UINT_MAX;
+
+	return skb_frag_size(info->frags + nr_frags - 1);
+}
+
+/* Workaround for hw issues with TSO on RTL8168evl */
+static netdev_features_t rtl8168evl_fix_tso(struct sk_buff *skb,
+					    netdev_features_t features)
+{
+	/* IPv4 header has options field */
+	if (vlan_get_protocol(skb) == htons(ETH_P_IP) &&
+	    ip_hdrlen(skb) > sizeof(struct iphdr))
+		features &= ~NETIF_F_ALL_TSO;
+
+	/* IPv4 TCP header has options field */
+	else if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4 &&
+		 tcp_hdrlen(skb) > sizeof(struct tcphdr))
+		features &= ~NETIF_F_ALL_TSO;
+
+	else if (rtl_last_frag_len(skb) <= 6)
+		features &= ~NETIF_F_ALL_TSO;
+
+	return features;
+}
+
 static netdev_features_t rtl8169_features_check(struct sk_buff *skb,
 						struct net_device *dev,
 						netdev_features_t features)
@@ -4314,6 +4345,9 @@ static netdev_features_t rtl8169_features_check(struct sk_buff *skb,
 	struct rtl8169_private *tp = netdev_priv(dev);
 
 	if (skb_is_gso(skb)) {
+		if (tp->mac_version == RTL_GIGA_MAC_VER_34)
+			features = rtl8168evl_fix_tso(skb, features);
+
 		if (transport_offset > GTTCPHO_MAX &&
 		    rtl_chip_supports_csum_v2(tp))
 			features &= ~NETIF_F_ALL_TSO;