[PATCH net-next v4 5/8] net: dsa: Add tag handling for SoC-e switches
From: Vasilij Strassheim
Date: Thu Nov 05 2026 - 09:30:00 EST
SoC-e switches exchange frames with the conduit using an 8-byte SDSA
header inserted between the source MAC address and the original
EtherType. The header identifies itself with EtherType 0xdcdc and
carries the frame direction and a 10-bit source or destination port.
Add transmit support for FROM_CPU frames and receive support for TO_CPU
frames. Validate the EtherType, frame type, header length, and source
port before accepting received frames.
The SDSA header can also carry an 802.1Q TCI. Encode
hardware-accelerated 802.1Q VLAN metadata into this field on transmit
and clear the skb VLAN metadata to prevent subsequent VLAN insertion by
the conduit.
Leave existing in-band VLAN headers in the encapsulated frame, after the
SDSA header. Whether an 802.1Q tag reaches the tagger as accelerated
metadata or an in-band header depends on the VLAN TX features inherited
from the conduit.
Since SDSA carries no TPID, move accelerated tags with other VLAN
protocols into the frame before inserting the SDSA header. On receive,
the switch keeps an ingress 802.1Q header in the frame and duplicates
its TCI in the SDSA header. Drop the in-band copy and restore the TCI as
accelerated 802.1Q metadata.
Signed-off-by: Vasilij Strassheim <v.strassheim@xxxxxxxxxxxxx>
---
include/net/dsa.h | 2 +
net/dsa/Kconfig | 6 ++
net/dsa/Makefile | 1 +
net/dsa/tag_sdsa.c | 168 +++++++++++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 177 insertions(+)
diff --git a/include/net/dsa.h b/include/net/dsa.h
index 5d12191b6f6f..8287fba8b107 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -62,6 +62,7 @@ struct tc_action;
#define DSA_TAG_PROTO_KSZ8463_VALUE 34
#define DSA_TAG_PROTO_MT7628_VALUE 35
#define DSA_TAG_PROTO_KS8995_VALUE 36
+#define DSA_TAG_PROTO_SDSA_VALUE 37
enum dsa_tag_protocol {
DSA_TAG_PROTO_NONE = DSA_TAG_PROTO_NONE_VALUE,
@@ -101,6 +102,7 @@ enum dsa_tag_protocol {
DSA_TAG_PROTO_KSZ8463 = DSA_TAG_PROTO_KSZ8463_VALUE,
DSA_TAG_PROTO_MT7628 = DSA_TAG_PROTO_MT7628_VALUE,
DSA_TAG_PROTO_KS8995 = DSA_TAG_PROTO_KS8995_VALUE,
+ DSA_TAG_PROTO_SDSA = DSA_TAG_PROTO_SDSA_VALUE,
};
struct dsa_switch;
diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
index 4f44bf3ede23..9b299cada316 100644
--- a/net/dsa/Kconfig
+++ b/net/dsa/Kconfig
@@ -194,6 +194,12 @@ config NET_DSA_TAG_RZN1_A5PSW
Renesas RZ/N1 embedded switch that uses an 8 byte tag located after
destination MAC address.
+config NET_DSA_TAG_SDSA
+ tristate "Tag driver for SoC-e switches using EtherType SDSA headers"
+ help
+ Say Y or M if you want to enable support for tagging frames for the
+ SoC-e switches.
+
config NET_DSA_TAG_LAN9303
tristate "Tag driver for SMSC/Microchip LAN9303 family of switches"
help
diff --git a/net/dsa/Makefile b/net/dsa/Makefile
index 1f9cc30e9988..a4c8d0e6bc0f 100644
--- a/net/dsa/Makefile
+++ b/net/dsa/Makefile
@@ -40,6 +40,7 @@ obj-$(CONFIG_NET_DSA_TAG_QCA) += tag_qca.o
obj-$(CONFIG_NET_DSA_TAG_RTL4_A) += tag_rtl4_a.o
obj-$(CONFIG_NET_DSA_TAG_RTL8_4) += tag_rtl8_4.o
obj-$(CONFIG_NET_DSA_TAG_RZN1_A5PSW) += tag_rzn1_a5psw.o
+obj-$(CONFIG_NET_DSA_TAG_SDSA) += tag_sdsa.o
obj-$(CONFIG_NET_DSA_TAG_SJA1105) += tag_sja1105.o
obj-$(CONFIG_NET_DSA_TAG_TRAILER) += tag_trailer.o
obj-$(CONFIG_NET_DSA_TAG_VSC73XX_8021Q) += tag_vsc73xx_8021q.o
diff --git a/net/dsa/tag_sdsa.c b/net/dsa/tag_sdsa.c
new file mode 100644
index 000000000000..dcfa5de0ac6b
--- /dev/null
+++ b/net/dsa/tag_sdsa.c
@@ -0,0 +1,168 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2020-2026 System on Chip engineering, S.L.
+ * Copyright (c) 2026 Linutronix GmbH
+ * Author: Vasilij Strassheim <v.strassheim@xxxxxxxxxxxxx>
+ */
+
+#include <linux/bitfield.h>
+#include <linux/byteorder/generic.h>
+#include <linux/etherdevice.h>
+#include <linux/if_vlan.h>
+#include <linux/unaligned.h>
+
+#include "tag.h"
+
+#define SDSA_HLEN 8
+
+#define SDSA_NAME "sdsa"
+#define ETH_P_SDSA 0xDCDC
+
+/* SDSA tag byte layout (after the 12-byte MAC header):
+ * Bytes 0-1: SDSA EtherType (0xDCDC)
+ * Bytes 2-3: Reserved
+ * Byte 4: Frame type (bits 7-6), VLAN-info bit (bit 5), port[9:5] (bits 4-0)
+ * Byte 5: Port[4:0] (bits 7-3)
+ * Bytes 6-7: PCP (bits 7-5) / CFI (bit 4) / VID (bits 3-0 + byte 7), only
+ * meaningful when the VLAN-info bit is set.
+ */
+#define SDSA_TAG_FRAME_TYPE_MASK GENMASK(7, 6)
+#define SDSA_TAG_VLAN_BIT BIT(5)
+#define SDSA_TAG_PORT_HI_MASK GENMASK(4, 0)
+#define SDSA_TAG_PORT_LO_MASK GENMASK(7, 3)
+#define SDSA_TAG_PORT_HI_SHIFT 5
+#define SDSA_FRAME_TYPE_TO_CPU 0
+#define SDSA_FRAME_TYPE_FROM_CPU 1
+
+struct sdsa_tag {
+ __be16 ethertype;
+ __be16 reserved;
+ u8 frame_type_port_hi;
+ u8 port_lo;
+ __be16 vlan;
+};
+
+static struct sk_buff *sdsa_xmit(struct sk_buff *skb, struct net_device *dev)
+{
+ struct dsa_port *dp = dsa_user_to_port(dev);
+ struct sdsa_tag *tag;
+ bool encode_vlan;
+ bool tag_present;
+ u16 vlan_tci;
+
+ BUILD_BUG_ON(sizeof(*tag) != SDSA_HLEN);
+
+ /* SDSA carries no TPID, so only encode 802.1Q C-tags. */
+ tag_present = skb_vlan_tag_present(skb);
+ encode_vlan = tag_present && skb->vlan_proto == htons(ETH_P_8021Q);
+ /* Prevent the conduit from inserting a VLAN header ahead of SDSA. */
+ if (tag_present && !encode_vlan) {
+ skb = __vlan_hwaccel_push_inside(skb);
+ if (!skb)
+ return NULL;
+ }
+
+ skb_push(skb, SDSA_HLEN);
+ dsa_alloc_etype_header(skb, SDSA_HLEN);
+
+ tag = dsa_etype_header_pos_tx(skb);
+ tag->ethertype = cpu_to_be16(ETH_P_SDSA);
+ tag->reserved = 0;
+ tag->frame_type_port_hi =
+ FIELD_PREP(SDSA_TAG_FRAME_TYPE_MASK, SDSA_FRAME_TYPE_FROM_CPU) |
+ FIELD_PREP(SDSA_TAG_PORT_HI_MASK,
+ dp->index >> SDSA_TAG_PORT_HI_SHIFT);
+ tag->port_lo = FIELD_PREP(SDSA_TAG_PORT_LO_MASK, dp->index);
+ if (encode_vlan) {
+ vlan_tci = skb_vlan_tag_get(skb);
+ __vlan_hwaccel_clear_tag(skb);
+ tag->frame_type_port_hi |= SDSA_TAG_VLAN_BIT;
+ tag->vlan = cpu_to_be16(vlan_tci);
+ } else {
+ tag->vlan = 0;
+ }
+
+ return skb;
+}
+
+static struct sk_buff *sdsa_rcv(struct sk_buff *skb, struct net_device *dev)
+{
+ enum skb_drop_reason reason;
+ struct sdsa_tag *tag;
+ u16 dummy_vlan_tci;
+ bool vlan_present;
+ int source_port;
+ u16 encap_proto;
+ u8 frame_type;
+ u16 vlan_tci;
+
+ reason = pskb_may_pull_reason(skb, SDSA_HLEN);
+ if (unlikely(reason != SKB_NOT_DROPPED_YET))
+ goto out_drop;
+
+ tag = dsa_etype_header_pos_rx(skb);
+ if (unlikely(be16_to_cpu(tag->ethertype) != ETH_P_SDSA)) {
+ reason = SKB_DROP_REASON_DEV_HDR;
+ goto out_drop;
+ }
+
+ frame_type = FIELD_GET(SDSA_TAG_FRAME_TYPE_MASK,
+ tag->frame_type_port_hi);
+ if (frame_type != SDSA_FRAME_TYPE_TO_CPU) {
+ reason = SKB_DROP_REASON_DEV_HDR;
+ goto out_drop;
+ }
+
+ vlan_present = tag->frame_type_port_hi & SDSA_TAG_VLAN_BIT;
+ vlan_tci = be16_to_cpu(tag->vlan);
+ encap_proto = get_unaligned_be16((u8 *)tag + SDSA_HLEN);
+
+ source_port = FIELD_GET(SDSA_TAG_PORT_HI_MASK,
+ tag->frame_type_port_hi) <<
+ SDSA_TAG_PORT_HI_SHIFT;
+ source_port |= FIELD_GET(SDSA_TAG_PORT_LO_MASK, tag->port_lo);
+
+ skb->dev = dsa_conduit_find_user(dev, 0, source_port);
+ if (!skb->dev) {
+ reason = SKB_DROP_REASON_DEV_HDR;
+ goto out_drop;
+ }
+
+ skb_pull_rcsum(skb, SDSA_HLEN);
+ dsa_strip_etype_header(skb, SDSA_HLEN);
+ if (vlan_present) {
+ /* The in-band tag duplicates the TCI carried in SDSA. */
+ if (encap_proto == ETH_P_8021Q) {
+ skb_push_rcsum(skb, ETH_HLEN);
+ skb_reset_mac_header(skb);
+ if (__skb_vlan_pop(skb, &dummy_vlan_tci)) {
+ reason = SKB_DROP_REASON_NOMEM;
+ goto out_drop;
+ }
+ skb_pull_rcsum(skb, ETH_HLEN);
+ }
+
+ __vlan_hwaccel_put_tag(skb, htons(ETH_P_8021Q), vlan_tci);
+ }
+
+ dsa_default_offload_fwd_mark(skb);
+ return skb;
+
+out_drop:
+ kfree_skb_reason(skb, reason);
+ return NULL;
+}
+
+static const struct dsa_device_ops sdsa_netdev_ops = {
+ .name = SDSA_NAME,
+ .proto = DSA_TAG_PROTO_SDSA,
+ .xmit = sdsa_xmit,
+ .rcv = sdsa_rcv,
+ .needed_headroom = SDSA_HLEN,
+};
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("DSA tag driver for SoC-e SDSA protocol");
+MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_SDSA, SDSA_NAME);
+
+module_dsa_tag_driver(sdsa_netdev_ops);
--
2.39.5