@@ -52,6 +52,7 @@ struct phylink_link_state;
 #define DSA_TAG_PROTO_BRCM_LEGACY_VALUE		22
 #define DSA_TAG_PROTO_SJA1110_VALUE		23
 #define DSA_TAG_PROTO_RTL8_4_VALUE		24
+#define DSA_TAG_PROTO_RTL8_4T_VALUE		25
 
 enum dsa_tag_protocol {
 	DSA_TAG_PROTO_NONE		= DSA_TAG_PROTO_NONE_VALUE,

@@ -79,6 +80,7 @@ enum dsa_tag_protocol {
 	DSA_TAG_PROTO_SEVILLE		= DSA_TAG_PROTO_SEVILLE_VALUE,
 	DSA_TAG_PROTO_SJA1110		= DSA_TAG_PROTO_SJA1110_VALUE,
 	DSA_TAG_PROTO_RTL8_4		= DSA_TAG_PROTO_RTL8_4_VALUE,
+	DSA_TAG_PROTO_RTL8_4T		= DSA_TAG_PROTO_RTL8_4T_VALUE,
 };
 
 struct dsa_switch;

@@ -9,11 +9,6 @@
  *
  * This tag header has the following format:
  *
- *  -------------------------------------------
- *  | MAC DA | MAC SA | 8 byte tag | Type | ...
- *  -------------------------------------------
- *     _______________/            \______________________________________
- *    /                                                                   \
  *  0                                  7|8                                 15
  *  |-----------------------------------+-----------------------------------|---
  *  |                               (16-bit)                                | ^

@@ -58,6 +53,28 @@
  *    TX/RX      | TX (switch->CPU): port number the packet was received on
  *               | RX (CPU->switch): forwarding port mask (if ALLOW=0)
  *               |                   allowance port mask (if ALLOW=1)
+ *
+ * The tag can be positioned before Ethertype, using tag "rtl8_4":
+ *
+ *  +--------+--------+------------+------+-----
+ *  | MAC DA | MAC SA | 8 byte tag | Type | ...
+ *  +--------+--------+------------+------+-----
+ *
+ * If checksum offload is enabled for CPU port device, it might break if the
+ * driver does not use csum_start/csum_offset.
+ *
+ * The tag can also appear between the end of the payload and before the CRC,
+ * using tag "rtl8_4t":
+ *
+ * +--------+--------+------+-----+---------+------------+-----+
+ * | MAC DA | MAC SA | TYPE | ... | payload | 8-byte tag | CRC |
+ * +--------+--------+------+-----+---------+------------+-----+
+ *
+ * The added bytes after the payload will break most checksums, either in
+ * software or hardware. To avoid this issue, if the checksum is still pending,
+ * this tagger checksum the packet before adding the tag, rendering any
+ * checksum offload useless.
+ *
  */
 
 #include <linux/bitfield.h>

@@ -84,87 +101,133 @@
 #define RTL8_4_TX			GENMASK(3, 0)
 #define RTL8_4_RX			GENMASK(10, 0)
 
-static struct sk_buff *rtl8_4_tag_xmit(struct sk_buff *skb,
-				       struct net_device *dev)
+static void rtl8_4_write_tag(struct sk_buff *skb, struct net_device *dev,
+			     void *tag)
 {
 	struct dsa_port *dp = dsa_slave_to_port(dev);
-	__be16 *tag;
-
-	skb_push(skb, RTL8_4_TAG_LEN);
-
-	dsa_alloc_etype_header(skb, RTL8_4_TAG_LEN);
-	tag = dsa_etype_header_pos_tx(skb);
+	__be16 tag16[RTL8_4_TAG_LEN / 2];
 
 	/* Set Realtek EtherType */
-	tag[0] = htons(ETH_P_REALTEK);
+	tag16[0] = htons(ETH_P_REALTEK);
 
 	/* Set Protocol; zero REASON */
-	tag[1] = htons(FIELD_PREP(RTL8_4_PROTOCOL, RTL8_4_PROTOCOL_RTL8365MB));
+	tag16[1] = htons(FIELD_PREP(RTL8_4_PROTOCOL, RTL8_4_PROTOCOL_RTL8365MB));
 
 	/* Zero FID_EN, FID, PRI_EN, PRI, KEEP; set LEARN_DIS */
-	tag[2] = htons(FIELD_PREP(RTL8_4_LEARN_DIS, 1));
+	tag16[2] = htons(FIELD_PREP(RTL8_4_LEARN_DIS, 1));
 
 	/* Zero ALLOW; set RX (CPU->switch) forwarding port mask */
-	tag[3] = htons(FIELD_PREP(RTL8_4_RX, BIT(dp->index)));
+	tag16[3] = htons(FIELD_PREP(RTL8_4_RX, BIT(dp->index)));
+
+	memcpy(tag, tag16, RTL8_4_TAG_LEN);
+}
+
+static struct sk_buff *rtl8_4_tag_xmit(struct sk_buff *skb,
+				       struct net_device *dev)
+{
+	skb_push(skb, RTL8_4_TAG_LEN);
+
+	dsa_alloc_etype_header(skb, RTL8_4_TAG_LEN);
+
+	rtl8_4_write_tag(skb, dev, dsa_etype_header_pos_tx(skb));
 
 	return skb;
 }
 
-static struct sk_buff *rtl8_4_tag_rcv(struct sk_buff *skb,
-				      struct net_device *dev)
+static struct sk_buff *rtl8_4t_tag_xmit(struct sk_buff *skb,
+					struct net_device *dev)
 {
-	__be16 *tag;
+	/* Calculate the checksum here if not done yet as trailing tags will
+	 * break either software and hardware based checksum
+	 */
+	if (skb->ip_summed == CHECKSUM_PARTIAL && skb_checksum_help(skb))
+		return NULL;
+
+	rtl8_4_write_tag(skb, dev, skb_put(skb, RTL8_4_TAG_LEN));
+
+	return skb;
+}
+
+static int rtl8_4_read_tag(struct sk_buff *skb, struct net_device *dev,
+			   void *tag)
+{
+	__be16 tag16[RTL8_4_TAG_LEN / 2];
 	u16 etype;
 	u8 reason;
 	u8 proto;
 	u8 port;
 
-	if (unlikely(!pskb_may_pull(skb, RTL8_4_TAG_LEN)))
-		return NULL;
-
-	tag = dsa_etype_header_pos_rx(skb);
+	memcpy(tag16, tag, RTL8_4_TAG_LEN);
 
 	/* Parse Realtek EtherType */
-	etype = ntohs(tag[0]);
+	etype = ntohs(tag16[0]);
 	if (unlikely(etype != ETH_P_REALTEK)) {
 		dev_warn_ratelimited(&dev->dev,
 				     "non-realtek ethertype 0x%04x\n", etype);
-		return NULL;
+		return -EPROTO;
 	}
 
 	/* Parse Protocol */
-	proto = FIELD_GET(RTL8_4_PROTOCOL, ntohs(tag[1]));
+	proto = FIELD_GET(RTL8_4_PROTOCOL, ntohs(tag16[1]));
 	if (unlikely(proto != RTL8_4_PROTOCOL_RTL8365MB)) {
 		dev_warn_ratelimited(&dev->dev,
 				     "unknown realtek protocol 0x%02x\n",
 				     proto);
-		return NULL;
+		return -EPROTO;
 	}
 
 	/* Parse REASON */
-	reason = FIELD_GET(RTL8_4_REASON, ntohs(tag[1]));
+	reason = FIELD_GET(RTL8_4_REASON, ntohs(tag16[1]));
 
 	/* Parse TX (switch->CPU) */
-	port = FIELD_GET(RTL8_4_TX, ntohs(tag[3]));
+	port = FIELD_GET(RTL8_4_TX, ntohs(tag16[3]));
 	skb->dev = dsa_master_find_slave(dev, 0, port);
 	if (!skb->dev) {
 		dev_warn_ratelimited(&dev->dev,
 				     "could not find slave for port %d\n",
 				     port);
-		return NULL;
+		return -ENOENT;
 	}
 
+	if (reason != RTL8_4_REASON_TRAP)
+		dsa_default_offload_fwd_mark(skb);
+
+	return 0;
+}
+
+static struct sk_buff *rtl8_4_tag_rcv(struct sk_buff *skb,
+				      struct net_device *dev)
+{
+	if (unlikely(!pskb_may_pull(skb, RTL8_4_TAG_LEN)))
+		return NULL;
+
+	if (unlikely(rtl8_4_read_tag(skb, dev, dsa_etype_header_pos_rx(skb))))
+		return NULL;
+
 	/* Remove tag and recalculate checksum */
 	skb_pull_rcsum(skb, RTL8_4_TAG_LEN);
 
 	dsa_strip_etype_header(skb, RTL8_4_TAG_LEN);
 
-	if (reason != RTL8_4_REASON_TRAP)
-		dsa_default_offload_fwd_mark(skb);
+	return skb;
+}
+
+static struct sk_buff *rtl8_4t_tag_rcv(struct sk_buff *skb,
+				       struct net_device *dev)
+{
+	if (skb_linearize(skb))
+		return NULL;
+
+	if (unlikely(rtl8_4_read_tag(skb, dev, skb_tail_pointer(skb) - RTL8_4_TAG_LEN)))
+		return NULL;
+
+	if (pskb_trim_rcsum(skb, skb->len - RTL8_4_TAG_LEN))
+		return NULL;
 
 	return skb;
 }
 
+/* Ethertype version */
 static const struct dsa_device_ops rtl8_4_netdev_ops = {
 	.name = "rtl8_4",
 	.proto = DSA_TAG_PROTO_RTL8_4,

@@ -172,7 +235,28 @@ static const struct dsa_device_ops rtl8_4_netdev_ops = {
 	.rcv = rtl8_4_tag_rcv,
 	.needed_headroom = RTL8_4_TAG_LEN,
 };
-module_dsa_tag_driver(rtl8_4_netdev_ops);
 
-MODULE_LICENSE("GPL");
+DSA_TAG_DRIVER(rtl8_4_netdev_ops);
+
 MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_RTL8_4);
+
+/* Tail version */
+static const struct dsa_device_ops rtl8_4t_netdev_ops = {
+	.name = "rtl8_4t",
+	.proto = DSA_TAG_PROTO_RTL8_4T,
+	.xmit = rtl8_4t_tag_xmit,
+	.rcv = rtl8_4t_tag_rcv,
+	.needed_tailroom = RTL8_4_TAG_LEN,
+};
+
+DSA_TAG_DRIVER(rtl8_4t_netdev_ops);
+
+MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_RTL8_4L);
+
+static struct dsa_tag_driver *dsa_tag_drivers[] = {
+	&DSA_TAG_DRIVER_NAME(rtl8_4_netdev_ops),
+	&DSA_TAG_DRIVER_NAME(rtl8_4t_netdev_ops),
+};
+module_dsa_tag_drivers(dsa_tag_drivers);
+
+MODULE_LICENSE("GPL");