[PATCH net-next v3 3/4] net: dsa: initial MT7628 tagging driver

Joris Vaisvila
2026-04-28T18:55:48Z

Add support for the MT7628 embedded switch's tag.

The MT7628 tag is merged with the VLAN TPID field when a VLAN is
appended by the switch hardware. It is not installed if the VLAN tag is
already there on ingress. Due to this hardware quirk the tag cannot be
trusted for port 0 if we don't know that the VLAN was added by the
hardware. As a workaround for this the switch is configured to always
append the port PVID tag even if the incoming packet is already tagged.
The tagging driver can then trust that the tag is always accurate and
the whole VLAN tag can be removed on ingress as it's only metadata for
the tagger.

On egress the MT7628 tag allows precise TX, but the correct VLAN tag
from tag_8021q is still appended or the switch will not forward the
packet.

Signed-off-by: Joris Vaisvila <joey@tinyisr.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
---
include/net/dsa.h | 2 +
net/dsa/Kconfig | 6 +++
net/dsa/Makefile | 1 +
net/dsa/tag_mt7628.c | 89 ++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 98 insertions(+)
create mode 100644 net/dsa/tag_mt7628.c

diff --git a/include/net/dsa.h b/include/net/dsa.h
index 8b6d34e8a6f0..0a314f62023d 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -58,6 +58,7 @@ struct tc_action;
#define DSA_TAG_PROTO_YT921X_VALUE 30
#define DSA_TAG_PROTO_MXL_GSW1XX_VALUE 31
#define DSA_TAG_PROTO_MXL862_VALUE 32
+#define DSA_TAG_PROTO_MT7628_VALUE 33

enum dsa_tag_protocol {
DSA_TAG_PROTO_NONE = DSA_TAG_PROTO_NONE_VALUE,
@@ -93,6 +94,7 @@ enum dsa_tag_protocol {
DSA_TAG_PROTO_YT921X = DSA_TAG_PROTO_YT921X_VALUE,
DSA_TAG_PROTO_MXL_GSW1XX = DSA_TAG_PROTO_MXL_GSW1XX_VALUE,
DSA_TAG_PROTO_MXL862 = DSA_TAG_PROTO_MXL862_VALUE,
+ DSA_TAG_PROTO_MT7628 = DSA_TAG_PROTO_MT7628_VALUE,
};

struct dsa_switch;
diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
index 5ed8c704636d..946f44f0b843 100644
--- a/net/dsa/Kconfig
+++ b/net/dsa/Kconfig
@@ -98,6 +98,12 @@ config NET_DSA_TAG_EDSA
Say Y or M if you want to enable support for tagging frames for the
Marvell switches which use EtherType DSA headers.

+config NET_DSA_TAG_MT7628
+ tristate "Tag driver for the MT7628 embedded switch"
+ help
+ Say Y or M if you want to enable support for tagging frames for the
+ switch embedded in the MT7628 SoC.
+
config NET_DSA_TAG_MTK
tristate "Tag driver for Mediatek switches"
help
diff --git a/net/dsa/Makefile b/net/dsa/Makefile
index bf7247759a64..a84f33f0963d 100644
--- a/net/dsa/Makefile
+++ b/net/dsa/Makefile
@@ -27,6 +27,7 @@ obj-$(CONFIG_NET_DSA_TAG_GSWIP) += tag_gswip.o
obj-$(CONFIG_NET_DSA_TAG_HELLCREEK) += tag_hellcreek.o
obj-$(CONFIG_NET_DSA_TAG_KSZ) += tag_ksz.o
obj-$(CONFIG_NET_DSA_TAG_LAN9303) += tag_lan9303.o
+obj-$(CONFIG_NET_DSA_TAG_MT7628) += tag_mt7628.o
obj-$(CONFIG_NET_DSA_TAG_MTK) += tag_mtk.o
obj-$(CONFIG_NET_DSA_TAG_MXL_862XX) += tag_mxl862xx.o
obj-$(CONFIG_NET_DSA_TAG_MXL_GSW1XX) += tag_mxl-gsw1xx.o
diff --git a/net/dsa/tag_mt7628.c b/net/dsa/tag_mt7628.c
new file mode 100644
index 000000000000..f0e346595f30
--- /dev/null
+++ b/net/dsa/tag_mt7628.c
@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026, Joris Vaisvila <joey@tinyisr.com>
+ * MT7628 switch tag support
+ */
+
+#include <linux/etherdevice.h>
+#include <linux/dsa/8021q.h>
+#include <net/dsa.h>
+
+#include "tag.h"
+
+/*
+ * The MT7628 tag is encoded in the VLAN TPID field.
+ * On TX the lower 6 bits encode the destination port bitmask.
+ * On RX the lower 3 bits encode the source port number.
+ *
+ * The switch hardware will not modify the TPID of an incoming packet if it is
+ * already VLAN tagged. To work around this the switch is configured to always
+ * append a tag_8021q standalone VLAN tag for each port. That means we can
+ * safely strip the outer VLAN tag after parsing it.
+ *
+ * A VLAN tag is constructed on egress to target the standalone VLAN and
+ * destination port.
+ */
+
+#define MT7628_TAG_NAME "mt7628"
+
+#define MT7628_TAG_TX_PORT GENMASK(5, 0)
+#define MT7628_TAG_RX_PORT GENMASK(2, 0)
+#define MT7628_TAG_LEN 4
+
+static struct sk_buff *mt7628_tag_xmit(struct sk_buff *skb,
+ struct net_device *dev)
+{
+ struct dsa_port *dp;
+ u16 xmit_vlan;
+ __be16 *tag;
+
+ dp = dsa_user_to_port(dev);
+ xmit_vlan = dsa_tag_8021q_standalone_vid(dp);
+
+ skb_push(skb, MT7628_TAG_LEN);
+ dsa_alloc_etype_header(skb, MT7628_TAG_LEN);
+
+ tag = dsa_etype_header_pos_tx(skb);
+
+ tag[0] = htons(ETH_P_8021Q |
+ FIELD_PREP(MT7628_TAG_TX_PORT,
+ dsa_xmit_port_mask(skb, dev)));
+ tag[1] = htons(xmit_vlan);
+
+ return skb;
+}
+
+static struct sk_buff *mt7628_tag_rcv(struct sk_buff *skb,
+ struct net_device *dev)
+{
+ __be16 *phdr;
+
+ if (unlikely(!pskb_may_pull(skb, MT7628_TAG_LEN)))
+ return NULL;
+
+ phdr = dsa_etype_header_pos_rx(skb);
+ skb->dev =
+ dsa_conduit_find_user(dev, 0,
+ FIELD_GET(MT7628_TAG_RX_PORT, ntohs(*phdr)));
+ if (!skb->dev)
+ return NULL;
+
+ skb_pull_rcsum(skb, MT7628_TAG_LEN);
+ dsa_strip_etype_header(skb, MT7628_TAG_LEN);
+ dsa_default_offload_fwd_mark(skb);
+ return skb;
+}
+
+static const struct dsa_device_ops mt7628_tag_ops = {
+ .name = MT7628_TAG_NAME,
+ .proto = DSA_TAG_PROTO_MT7628,
+ .xmit = mt7628_tag_xmit,
+ .rcv = mt7628_tag_rcv,
+ .needed_headroom = MT7628_TAG_LEN,
+};
+
+module_dsa_tag_driver(mt7628_tag_ops);
+
+MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_MT7628, MT7628_TAG_NAME);
+MODULE_DESCRIPTION("DSA tag driver for MT7628 switch");
+MODULE_LICENSE("GPL");
--
2.54.0