Patchwork [3/6,NET] dsa: add support for original DSA tagging format

login
register
mail settings
Submitter Lennert Buytenhek
Date Sept. 29, 2008, 2:38 a.m.
Message ID <20080929023829.GI21560@xi.wantstofly.org>
Download mbox | patch
Permalink /patch/1853/
State Superseded
Delegated to: David Miller
Headers show

Comments

Lennert Buytenhek - Sept. 29, 2008, 2:38 a.m.
Most of the DSA switches currently in the field do not support the
Ethertype DSA tagging format that one of the previous patches added
support for, but only the original DSA tagging format.

The original DSA tagging format carries the same information as the
Ethertype DSA tagging format, but with the difference that it does not
have an ethertype field.  In other words, when receiving a packet that
is tagged with an original DSA tag, there is no way of telling in
eth_type_trans() that this packet is in fact a DSA-tagged packet.

This patch adds a hook into eth_type_trans() which is only compiled in
if support for a switch chip that doesn't support Ethertype DSA is
selected, and which checks whether there is a DSA switch driver
instance attached to this network device which uses the old tag format.
If so, it sets the protocol field to ETH_P_DSA without looking at the
packet, so that the packet ends up in the right place.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Tested-by: Nicolas Pitre <nico@marvell.com>
---
 include/linux/if_ether.h  |    1 +
 include/linux/netdevice.h |   11 +++
 include/net/dsa.h         |    2 +
 net/dsa/Kconfig           |    4 +
 net/dsa/Makefile          |    1 +
 net/dsa/dsa.c             |   16 ++++
 net/dsa/dsa_priv.h        |    4 +
 net/dsa/mv88e6123_61_65.c |   17 +++--
 net/dsa/slave.c           |    5 +
 net/dsa/tag_dsa.c         |  194 +++++++++++++++++++++++++++++++++++++++++++++
 net/ethernet/eth.c        |   10 +++
 11 files changed, 258 insertions(+), 7 deletions(-)
 create mode 100644 net/dsa/tag_dsa.c
Ben Hutchings - Oct. 3, 2008, 5:25 p.m.
On Mon, 2008-09-29 at 04:38 +0200, Lennert Buytenhek wrote:
> Most of the DSA switches currently in the field do not support the
> Ethertype DSA tagging format that one of the previous patches added
> support for, but only the original DSA tagging format.
> 
> The original DSA tagging format carries the same information as the
> Ethertype DSA tagging format, but with the difference that it does not
> have an ethertype field.  In other words, when receiving a packet that
> is tagged with an original DSA tag, there is no way of telling in
> eth_type_trans() that this packet is in fact a DSA-tagged packet.
> 
> This patch adds a hook into eth_type_trans() which is only compiled in
> if support for a switch chip that doesn't support Ethertype DSA is
> selected, and which checks whether there is a DSA switch driver
> instance attached to this network device which uses the old tag format.
> If so, it sets the protocol field to ETH_P_DSA without looking at the
> packet, so that the packet ends up in the right place.
[...]

Why should this go in eth_type_trans()?  Why don't you put the hook into
the specific network driver(s) that need it?

For that matter, why should dsa_ptr go in struct net_device and not in
the private state for the specific network drivers that need it?

Ben.
stephen hemminger - Oct. 3, 2008, 5:36 p.m.
Please use stronger types..

> +static inline int netdev_uses_dsa_tags(struct net_device *dev)

I would use:
static inline bool netdev_uses_dsa_tags(const struct net_device *dev)


> +int dsa_uses_dsa_tags(void *dsa_ptr);
extern bool dsa_uses_dsa_tags(const struct dsa_switch *dsa);

--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Lennert Buytenhek - Oct. 3, 2008, 7:32 p.m.
On Fri, Oct 03, 2008 at 06:25:59PM +0100, Ben Hutchings wrote:

> > Most of the DSA switches currently in the field do not support the
> > Ethertype DSA tagging format that one of the previous patches added
> > support for, but only the original DSA tagging format.
> > 
> > The original DSA tagging format carries the same information as the
> > Ethertype DSA tagging format, but with the difference that it does not
> > have an ethertype field.  In other words, when receiving a packet that
> > is tagged with an original DSA tag, there is no way of telling in
> > eth_type_trans() that this packet is in fact a DSA-tagged packet.
> > 
> > This patch adds a hook into eth_type_trans() which is only compiled in
> > if support for a switch chip that doesn't support Ethertype DSA is
> > selected, and which checks whether there is a DSA switch driver
> > instance attached to this network device which uses the old tag format.
> > If so, it sets the protocol field to ETH_P_DSA without looking at the
> > packet, so that the packet ends up in the right place.
> [...]
> 
> Why should this go in eth_type_trans()?  Why don't you put the hook into
> the specific network driver(s) that need it?
> 
> For that matter, why should dsa_ptr go in struct net_device and not in
> the private state for the specific network drivers that need it?

DSA is just another protocol.  Putting hooks in specific network driver
to handle a certain protocol doesn't seem like the right thing to do to
me.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Lennert Buytenhek - Oct. 3, 2008, 7:47 p.m.
On Fri, Oct 03, 2008 at 07:36:01PM +0200, Stephen Hemminger wrote:

> Please use stronger types..
> 
> > +static inline int netdev_uses_dsa_tags(struct net_device *dev)
> 
> I would use:
> static inline bool netdev_uses_dsa_tags(const struct net_device *dev)
> 
> 
> > +int dsa_uses_dsa_tags(void *dsa_ptr);
> extern bool dsa_uses_dsa_tags(const struct dsa_switch *dsa);

OK.  Did the same thing for the trailer tag support.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Ben Hutchings - Oct. 4, 2008, 11:22 a.m.
Lennert Buytenhek wrote:
> On Fri, Oct 03, 2008 at 06:25:59PM +0100, Ben Hutchings wrote:
> 
> > > Most of the DSA switches currently in the field do not support the
> > > Ethertype DSA tagging format that one of the previous patches added
> > > support for, but only the original DSA tagging format.
> > > 
> > > The original DSA tagging format carries the same information as the
> > > Ethertype DSA tagging format, but with the difference that it does not
> > > have an ethertype field.  In other words, when receiving a packet that
> > > is tagged with an original DSA tag, there is no way of telling in
> > > eth_type_trans() that this packet is in fact a DSA-tagged packet.
> > > 
> > > This patch adds a hook into eth_type_trans() which is only compiled in
> > > if support for a switch chip that doesn't support Ethertype DSA is
> > > selected, and which checks whether there is a DSA switch driver
> > > instance attached to this network device which uses the old tag format.
> > > If so, it sets the protocol field to ETH_P_DSA without looking at the
> > > packet, so that the packet ends up in the right place.
> > [...]
> > 
> > Why should this go in eth_type_trans()?  Why don't you put the hook into
> > the specific network driver(s) that need it?
> >
> > For that matter, why should dsa_ptr go in struct net_device and not in
> > the private state for the specific network drivers that need it?
> 
> DSA is just another protocol.  Putting hooks in specific network driver
> to handle a certain protocol doesn't seem like the right thing to do to
> me.

If it's just another protocol then put dsa_ptr along with the other
protocol pointers (ip_ptr etc.).

For those drivers that work with the original DSA tagging format, it's
apparently not just another protocol.

Ben.
Lennert Buytenhek - Oct. 5, 2008, 11:56 p.m.
On Sat, Oct 04, 2008 at 12:22:37PM +0100, Ben Hutchings wrote:

> > > > Most of the DSA switches currently in the field do not support the
> > > > Ethertype DSA tagging format that one of the previous patches added
> > > > support for, but only the original DSA tagging format.
> > > > 
> > > > The original DSA tagging format carries the same information as the
> > > > Ethertype DSA tagging format, but with the difference that it does not
> > > > have an ethertype field.  In other words, when receiving a packet that
> > > > is tagged with an original DSA tag, there is no way of telling in
> > > > eth_type_trans() that this packet is in fact a DSA-tagged packet.
> > > > 
> > > > This patch adds a hook into eth_type_trans() which is only compiled in
> > > > if support for a switch chip that doesn't support Ethertype DSA is
> > > > selected, and which checks whether there is a DSA switch driver
> > > > instance attached to this network device which uses the old tag format.
> > > > If so, it sets the protocol field to ETH_P_DSA without looking at the
> > > > packet, so that the packet ends up in the right place.
> > > [...]
> > > 
> > > Why should this go in eth_type_trans()?  Why don't you put the hook into
> > > the specific network driver(s) that need it?
> > >
> > > For that matter, why should dsa_ptr go in struct net_device and not in
> > > the private state for the specific network drivers that need it?
> > 
> > DSA is just another protocol.  Putting hooks in specific network driver
> > to handle a certain protocol doesn't seem like the right thing to do to
> > me.
> 
> If it's just another protocol then put dsa_ptr along with the other
> protocol pointers (ip_ptr etc.).

OK, I'll do that.


> For those drivers that work with the original DSA tagging format, it's
> apparently not just another protocol.

The original DSA tagging format can work with any NIC, as long as you
hook up that NIC directly to a DSA switch chip instead of to a PHY.  In
embedded designs you mostly see platform bus drivers like mv643xx_eth
and gianfar and such, but there's no reason you can't do it with any
other MAC that exposes a (R)(G)MII interface.

The only reason that eth_type_trans() needs a hack is that the original
DSA tagging format has information starting right after the ethernet
source address, so there's no way to tell that original DSA tagged
frames are in fact original DSA tagged frames.  (The way we tell is by
having the DSA switch driver decide -- it configures the directly
attached switch chip to always send DSA tagged packets to the cpu's
switch port, and then forces all packets received on the underlying
ethernet device to get ETH_P_DSA assigned to skb->protocol.)
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Patch

diff --git a/include/linux/if_ether.h b/include/linux/if_ether.h
index a85f5a2..864982b 100644
--- a/include/linux/if_ether.h
+++ b/include/linux/if_ether.h
@@ -100,6 +100,7 @@ 
 #define ETH_P_ECONET	0x0018		/* Acorn Econet			*/
 #define ETH_P_HDLC	0x0019		/* HDLC frames			*/
 #define ETH_P_ARCNET	0x001A		/* 1A for ArcNet :-)            */
+#define ETH_P_DSA	0x001B		/* Distributed Switch Arch.	*/
 
 /*
  *	This is an Ethernet frame header.
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index f67435d..5226308 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -42,6 +42,7 @@ 
 #include <linux/workqueue.h>
 
 #include <net/net_namespace.h>
+#include <net/dsa.h>
 
 struct vlan_group;
 struct ethtool_ops;
@@ -801,6 +802,16 @@  void dev_net_set(struct net_device *dev, struct net *net)
 #endif
 }
 
+static inline int netdev_uses_dsa_tags(struct net_device *dev)
+{
+#ifdef CONFIG_NET_DSA_TAG_DSA
+	if (dev->dsa_ptr != NULL)
+		return dsa_uses_dsa_tags(dev->dsa_ptr);
+#endif
+
+	return 0;
+}
+
 /**
  *	netdev_priv - access network device private data
  *	@dev: network device
diff --git a/include/net/dsa.h b/include/net/dsa.h
index 96eb2f3..a331f64 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -20,5 +20,7 @@  struct dsa_platform_data {
 	char		*port_names[DSA_MAX_PORTS];
 };
 
+int dsa_uses_dsa_tags(void *dsa_ptr);
+
 
 #endif
diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
index 7cf55e5..6b68016 100644
--- a/net/dsa/Kconfig
+++ b/net/dsa/Kconfig
@@ -10,6 +10,10 @@  menuconfig NET_DSA
 if NET_DSA
 
 # tagging formats
+config NET_DSA_TAG_DSA
+	bool
+	default n
+
 config NET_DSA_TAG_EDSA
 	bool
 	default n
diff --git a/net/dsa/Makefile b/net/dsa/Makefile
index b59a6f6..8b92123 100644
--- a/net/dsa/Makefile
+++ b/net/dsa/Makefile
@@ -1,4 +1,5 @@ 
 # tagging formats
+obj-$(CONFIG_NET_DSA_TAG_DSA) += tag_dsa.o
 obj-$(CONFIG_NET_DSA_TAG_EDSA) += tag_edsa.o
 
 # switch drivers
diff --git a/net/dsa/dsa.c b/net/dsa/dsa.c
index a010e41..d16a66e 100644
--- a/net/dsa/dsa.c
+++ b/net/dsa/dsa.c
@@ -203,6 +203,22 @@  static void dsa_switch_destroy(struct dsa_switch *ds)
 }
 
 
+/* hooks for ethertype-less tagging formats *********************************/
+/*
+ * The original DSA tag format and some other tag formats have no
+ * ethertype, which means that we need to add a little hack to the
+ * networking receive path to make sure that received frames get
+ * the right ->protocol assigned to them when one of those tag
+ * formats is in use.
+ */
+int dsa_uses_dsa_tags(void *dsa_ptr)
+{
+	struct dsa_switch *ds = dsa_ptr;
+
+	return !!(ds->tag_type == TAG_TYPE_DSA);
+}
+
+
 /* link polling *************************************************************/
 static void dsa_link_poll_work(struct work_struct *ugly)
 {
diff --git a/net/dsa/dsa_priv.h b/net/dsa/dsa_priv.h
index 5233aba..385a380 100644
--- a/net/dsa/dsa_priv.h
+++ b/net/dsa/dsa_priv.h
@@ -18,6 +18,7 @@ 
 #include <net/dsa.h>
 
 enum dsa_tag_type {
+	TAG_TYPE_DSA,
 	TAG_TYPE_EDSA,
 };
 
@@ -101,6 +102,9 @@  void unregister_switch_driver(struct dsa_switch_driver *type);
 void dsa_slave_mii_bus_init(struct dsa_switch *ds);
 struct net_device *dsa_slave_create(struct dsa_switch *ds, int port);
 
+/* tag_dsa.c */
+int dsa_xmit(struct sk_buff *skb, struct net_device *dev);
+
 /* tag_edsa.c */
 int edsa_xmit(struct sk_buff *skb, struct net_device *dev);
 
diff --git a/net/dsa/mv88e6123_61_65.c b/net/dsa/mv88e6123_61_65.c
index b53233d..e8cc553 100644
--- a/net/dsa/mv88e6123_61_65.c
+++ b/net/dsa/mv88e6123_61_65.c
@@ -192,15 +192,18 @@  static int mv88e6123_61_65_setup_port(struct dsa_switch *ds, int p)
 
 	/*
 	 * Port Control: disable Drop-on-Unlock, disable Drop-on-Lock,
-	 * configure the EDSA tagging mode if this is the CPU port,
-	 * disable Header mode, enable IGMP/MLD snooping, disable VLAN
-	 * tunneling, determine priority by looking at 802.1p and IP
-	 * priority fields (IP prio has precedence), and set STP state
-	 * to Forwarding.  Finally, if this is the CPU port, additionally
-	 * enable forwarding of unknown unicast and multicast addresses.
+	 * configure the requested (DSA/EDSA) tagging mode if this is
+	 * the CPU port, disable Header mode, enable IGMP/MLD snooping,
+	 * disable VLAN tunneling, determine priority by looking at
+	 * 802.1p and IP priority fields (IP prio has precedence), and
+	 * set STP state to Forwarding.  Finally, if this is the CPU
+	 * port, additionally enable forwarding of unknown unicast and
+	 * multicast addresses.
 	 */
 	REG_WRITE(addr, 0x04,
-			(p == ds->cpu_port) ? 0x373f : 0x0433);
+			(p == ds->cpu_port) ?
+			 (ds->tag_type == TAG_TYPE_DSA) ? 0x053f : 0x373f :
+			 0x0433);
 
 	/*
 	 * Port Control 1: disable trunking.  Also, if this is the
diff --git a/net/dsa/slave.c b/net/dsa/slave.c
index afc8ef0..ac8ed09 100644
--- a/net/dsa/slave.c
+++ b/net/dsa/slave.c
@@ -238,6 +238,11 @@  struct net_device *dsa_slave_create(struct dsa_switch *ds, int port)
 	slave_dev->open = dsa_slave_open;
 	slave_dev->stop = dsa_slave_close;
 	switch (ds->tag_type) {
+#ifdef CONFIG_NET_DSA_TAG_DSA
+	case TAG_TYPE_DSA:
+		slave_dev->hard_start_xmit = dsa_xmit;
+		break;
+#endif
 #ifdef CONFIG_NET_DSA_TAG_EDSA
 	case TAG_TYPE_EDSA:
 		slave_dev->hard_start_xmit = edsa_xmit;
diff --git a/net/dsa/tag_dsa.c b/net/dsa/tag_dsa.c
new file mode 100644
index 0000000..63a79fa
--- /dev/null
+++ b/net/dsa/tag_dsa.c
@@ -0,0 +1,194 @@ 
+/*
+ * net/dsa/tag_dsa.c - (Non-ethertype) DSA tagging
+ * Copyright (c) 2008 Marvell Semiconductor
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+#include <linux/etherdevice.h>
+#include <linux/list.h>
+#include <linux/netdevice.h>
+#include "dsa_priv.h"
+
+#define DSA_HLEN	4
+
+int dsa_xmit(struct sk_buff *skb, struct net_device *dev)
+{
+	struct dsa_slave_priv *p = netdev_priv(dev);
+	char *dsa_header;
+
+	dev->stats.tx_packets++;
+	dev->stats.tx_bytes += skb->len;
+
+	/*
+	 * Convert the outermost 802.1q tag to a DSA tag for tagged
+	 * packets, or insert a DSA tag between the addresses and
+	 * the ethertype field for untagged packets.
+	 */
+	if (skb->protocol == htons(ETH_P_8021Q)) {
+		if (skb_cow_head(skb, 0) < 0)
+			goto out_free;
+
+		/*
+		 * Construct tagged FROM_CPU DSA tag from 802.1q tag.
+		 */
+		dsa_header = skb->data + 2 * ETH_ALEN;
+		dsa_header[0] = 0x60;
+		dsa_header[1] = p->port << 3;
+
+		/*
+		 * Move CFI field from byte 2 to byte 1.
+		 */
+		if (dsa_header[2] & 0x10) {
+			dsa_header[1] |= 0x01;
+			dsa_header[2] &= ~0x10;
+		}
+	} else {
+		if (skb_cow_head(skb, DSA_HLEN) < 0)
+			goto out_free;
+		skb_push(skb, DSA_HLEN);
+
+		memmove(skb->data, skb->data + DSA_HLEN, 2 * ETH_ALEN);
+
+		/*
+		 * Construct untagged FROM_CPU DSA tag.
+		 */
+		dsa_header = skb->data + 2 * ETH_ALEN;
+		dsa_header[0] = 0x40;
+		dsa_header[1] = p->port << 3;
+		dsa_header[2] = 0x00;
+		dsa_header[3] = 0x00;
+	}
+
+	skb->protocol = htons(ETH_P_DSA);
+
+	skb->dev = p->parent->master_dev;
+	dev_queue_xmit(skb);
+
+	return NETDEV_TX_OK;
+
+out_free:
+	kfree_skb(skb);
+	return NETDEV_TX_OK;
+}
+
+static int dsa_rcv(struct sk_buff *skb, struct net_device *dev,
+		   struct packet_type *pt, struct net_device *orig_dev)
+{
+	struct dsa_switch *ds = dev->dsa_ptr;
+	u8 *dsa_header;
+	int source_port;
+
+	if (unlikely(ds == NULL))
+		goto out_drop;
+
+	skb = skb_unshare(skb, GFP_ATOMIC);
+	if (skb == NULL)
+		goto out;
+
+	if (unlikely(!pskb_may_pull(skb, DSA_HLEN)))
+		goto out_drop;
+
+	/*
+	 * The ethertype field is part of the DSA header.
+	 */
+	dsa_header = skb->data - 2;
+
+	/*
+	 * Check that frame type is either TO_CPU or FORWARD, and
+	 * that the source device is zero.
+	 */
+	if ((dsa_header[0] & 0xdf) != 0x00 && (dsa_header[0] & 0xdf) != 0xc0)
+		goto out_drop;
+
+	/*
+	 * Check that the source port is a registered DSA port.
+	 */
+	source_port = (dsa_header[1] >> 3) & 0x1f;
+	if (source_port >= DSA_MAX_PORTS || ds->ports[source_port] == NULL)
+		goto out_drop;
+
+	/*
+	 * Convert the DSA header to an 802.1q header if the 'tagged'
+	 * bit in the DSA header is set.  If the 'tagged' bit is clear,
+	 * delete the DSA header entirely.
+	 */
+	if (dsa_header[0] & 0x20) {
+		u8 new_header[4];
+
+		/*
+		 * Insert 802.1q ethertype and copy the VLAN-related
+		 * fields, but clear the bit that will hold CFI (since
+		 * DSA uses that bit location for another purpose).
+		 */
+		new_header[0] = (ETH_P_8021Q >> 8) & 0xff;
+		new_header[1] = ETH_P_8021Q & 0xff;
+		new_header[2] = dsa_header[2] & ~0x10;
+		new_header[3] = dsa_header[3];
+
+		/*
+		 * Move CFI bit from its place in the DSA header to
+		 * its 802.1q-designated place.
+		 */
+		if (dsa_header[1] & 0x01)
+			new_header[2] |= 0x10;
+
+		/*
+		 * Update packet checksum if skb is CHECKSUM_COMPLETE.
+		 */
+		if (skb->ip_summed == CHECKSUM_COMPLETE) {
+			__wsum c = skb->csum;
+			c = csum_add(c, csum_partial(new_header + 2, 2, 0));
+			c = csum_sub(c, csum_partial(dsa_header + 2, 2, 0));
+			skb->csum = c;
+		}
+
+		memcpy(dsa_header, new_header, DSA_HLEN);
+	} else {
+		/*
+		 * Remove DSA tag and update checksum.
+		 */
+		skb_pull_rcsum(skb, DSA_HLEN);
+		memmove(skb->data - ETH_HLEN,
+			skb->data - ETH_HLEN - DSA_HLEN,
+			2 * ETH_ALEN);
+	}
+
+	skb->dev = ds->ports[source_port];
+	skb_push(skb, ETH_HLEN);
+	skb->protocol = eth_type_trans(skb, skb->dev);
+
+	skb->dev->last_rx = jiffies;
+	skb->dev->stats.rx_packets++;
+	skb->dev->stats.rx_bytes += skb->len;
+
+	netif_receive_skb(skb);
+
+	return 0;
+
+out_drop:
+	kfree_skb(skb);
+out:
+	return 0;
+}
+
+static struct packet_type dsa_packet_type = {
+	.type	= __constant_htons(ETH_P_DSA),
+	.func	= dsa_rcv,
+};
+
+static int __init dsa_init_module(void)
+{
+	dev_add_pack(&dsa_packet_type);
+	return 0;
+}
+module_init(dsa_init_module);
+
+static void __exit dsa_cleanup_module(void)
+{
+	dev_remove_pack(&dsa_packet_type);
+}
+module_exit(dsa_cleanup_module);
diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
index a80839b..e0aca95 100644
--- a/net/ethernet/eth.c
+++ b/net/ethernet/eth.c
@@ -57,6 +57,7 @@ 
 #include <net/sock.h>
 #include <net/ipv6.h>
 #include <net/ip.h>
+#include <net/dsa.h>
 #include <asm/uaccess.h>
 #include <asm/system.h>
 
@@ -184,6 +185,15 @@  __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
 			skb->pkt_type = PACKET_OTHERHOST;
 	}
 
+	/*
+	 * Some variants of DSA tagging don't have an ethertype field
+	 * at all, so we check here whether one of those tagging
+	 * variants has been configured on the receiving interface,
+	 * and if so, set skb->protocol without looking at the packet.
+	 */
+	if (netdev_uses_dsa_tags(dev))
+		return htons(ETH_P_DSA);
+
 	if (ntohs(eth->h_proto) >= 1536)
 		return eth->h_proto;