kernel: backport flow_dissect support for tag_brcm

This change helps BCM4908 SoC. It was capable of 940 Mb/s masq NAT while
using iperf but users reported lower & unstable speeds for HTTP based
tests.

For LibreSpeed tests this bumps NAT speed from ~930 Mb/s to ~990 Mb/s
(not that this value seems real but speed bump definitely is).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/generic/backport-5.4/701-v5.7-net-dsa-Implement-flow-dissection-for-tag_brcm.c.patch

@@ -0,0 +1,62 @@
+From 52015366e361a88c569550a285c71f72bb095661 Mon Sep 17 00:00:00 2001
+From: Florian Fainelli <f.fainelli@gmail.com>
+Date: Sun, 22 Mar 2020 14:09:57 -0700
+Subject: [PATCH] net: dsa: Implement flow dissection for tag_brcm.c
+
+Provide a flow_dissect callback which returns the network offset and
+where to find the skb protocol, given the tags structure a common
+function works for both tagging formats that are supported.
+
+Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
+Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
+Signed-off-by: David S. Miller <davem@davemloft.net>
+---
+ net/dsa/tag_brcm.c | 23 +++++++++++++++++++++++
+ 1 file changed, 23 insertions(+)
+
+--- a/net/dsa/tag_brcm.c
++++ b/net/dsa/tag_brcm.c
+@@ -144,6 +144,27 @@ static struct sk_buff *brcm_tag_rcv_ll(s
+ 
+ 	return skb;
+ }
++
++static int brcm_tag_flow_dissect(const struct sk_buff *skb, __be16 *proto,
++				 int *offset)
++{
++	/* We have been called on the DSA master network device after
++	 * eth_type_trans() which pulled the Ethernet header already.
++	 * Frames have one of these two layouts:
++	 * -----------------------------------
++	 * | MAC DA | MAC SA | 4b tag | Type | DSA_TAG_PROTO_BRCM
++	 * -----------------------------------
++	 * -----------------------------------
++	 * | 4b tag | MAC DA | MAC SA | Type | DSA_TAG_PROTO_BRCM_PREPEND
++	 * -----------------------------------
++	 * skb->data points 2 bytes before the actual Ethernet type field and
++	 * we have an offset of 4bytes between where skb->data and where the
++	 * payload starts.
++	 */
++	*offset = BRCM_TAG_LEN;
++	*proto = ((__be16 *)skb->data)[1];
++	return 0;
++}
+ #endif
+ 
+ #if IS_ENABLED(CONFIG_NET_DSA_TAG_BRCM)
+@@ -179,6 +200,7 @@ static const struct dsa_device_ops brcm_
+ 	.xmit	= brcm_tag_xmit,
+ 	.rcv	= brcm_tag_rcv,
+ 	.overhead = BRCM_TAG_LEN,
++	.flow_dissect = brcm_tag_flow_dissect,
+ };
+ 
+ DSA_TAG_DRIVER(brcm_netdev_ops);
+@@ -207,6 +229,7 @@ static const struct dsa_device_ops brcm_
+ 	.xmit	= brcm_tag_xmit_prepend,
+ 	.rcv	= brcm_tag_rcv_prepend,
+ 	.overhead = BRCM_TAG_LEN,
++	.flow_dissect = brcm_tag_flow_dissect,
+ };
+ 
+ DSA_TAG_DRIVER(brcm_prepend_netdev_ops);