net: dsa: tag_rtl4_a: Support also egress tags

Support also transmitting frames using the custom "8899 A"
4 byte tag.

Qingfang came up with the solution: we need to pad the
ethernet frame to 60 bytes using eth_skb_pad(), then the
switch will happily accept frames with custom tags.

Cc: Mauri Sandberg <sandberg@mailfence.com>
Reported-by: DENG Qingfang <dqfext@gmail.com>
Fixes: efd7fe68 ("net: dsa: tag_rtl4_a: Implement Realtek 4 byte A tag")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_rtl4_a.c

@@ -12,9 +12,7 @@
  *
  * The 2 bytes tag form a 16 bit big endian word. The exact
  * meaning has been guessed from packet dumps from ingress
- * frames, as no working egress traffic has been available
- * we do not know the format of the egress tags or if they
- * are even supported.
+ * frames.
  */
 
 #include <linux/etherdevice.h>
@@ -36,17 +34,34 @@
 static struct sk_buff *rtl4a_tag_xmit(struct sk_buff *skb,
 				      struct net_device *dev)
 {
-	/*
-	 * Just let it pass thru, we don't know if it is possible
-	 * to tag a frame with the 0x8899 ethertype and direct it
-	 * to a specific port, all attempts at reverse-engineering have
-	 * ended up with the frames getting dropped.
-	 *
-	 * The VLAN set-up needs to restrict the frames to the right port.
-	 *
-	 * If you have documentation on the tagging format for RTL8366RB
-	 * (tag type A) then please contribute.
-	 */
+	struct dsa_port *dp = dsa_slave_to_port(dev);
+	u8 *tag;
+	u16 *p;
+	u16 out;
+
+	/* Pad out to at least 60 bytes */
+	if (unlikely(eth_skb_pad(skb)))
+		return NULL;
+	if (skb_cow_head(skb, RTL4_A_HDR_LEN) < 0)
+		return NULL;
+
+	netdev_dbg(dev, "add realtek tag to package to port %d\n",
+		   dp->index);
+	skb_push(skb, RTL4_A_HDR_LEN);
+
+	memmove(skb->data, skb->data + RTL4_A_HDR_LEN, 2 * ETH_ALEN);
+	tag = skb->data + 2 * ETH_ALEN;
+
+	/* Set Ethertype */
+	p = (u16 *)tag;
+	*p = htons(RTL4_A_ETHERTYPE);
+
+	out = (RTL4_A_PROTOCOL_RTL8366RB << 12) | (2 << 8);
+	/* The lower bits is the port numer */
+	out |= (u8)dp->index;
+	p = (u16 *)(tag + 2);
+	*p = htons(out);
+
 	return skb;
 }