net: dsa: tag_ks8995: Add the KS8995 tag handling

The KS8995 100Mbit switch can do proper DSA per-port tagging
with the proper set-up. This adds the code to handle ingress
and egress KS8995 tags.

The tagger is used by the KSZ8995XA support added later in this
series.

The tag is a modified 0x8100 ethertype tag where a bit in the
last nibble is set for each target port.

Always insert a distinct outer switch tag. User ports remove this
field on egress, so reusing an existing 802.1Q header would consume
the sender VLAN tag instead of preserving it.

Push any hardware-accelerated VLAN tag into the packet first. This
keeps the switch tag at offset 12 while retaining 802.1Q, 802.1ad and
stacked VLAN headers as inner payload, and avoids relying on a valid
SKB MAC header in direct transmit paths.

Rate-limit receive untagging errors so repeated failures cannot flood
the kernel log.

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Signed-off-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260916-ks8995-to-ksz8-v15-2-8825ff5fd4be@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

MAINTAINERS

@@ -17772,6 +17772,7 @@ F:	Documentation/devicetree/bindings/net/dsa/microchip,lan937x.yaml
 F:	drivers/net/dsa/microchip/*
 F:	include/linux/dsa/ksz_common.h
 F:	include/linux/platform_data/microchip-ksz.h
+F:	net/dsa/tag_ks8995.c
 F:	net/dsa/tag_ksz.c
 
 MICROCHIP LAN743X ETHERNET DRIVER

include/net/dsa.h

@@ -61,6 +61,7 @@ struct tc_action;
 #define DSA_TAG_PROTO_NETC_VALUE		33
 #define DSA_TAG_PROTO_KSZ8463_VALUE		34
 #define DSA_TAG_PROTO_MT7628_VALUE		35
+#define DSA_TAG_PROTO_KS8995_VALUE		36
 
 enum dsa_tag_protocol {
 	DSA_TAG_PROTO_NONE		= DSA_TAG_PROTO_NONE_VALUE,
@@ -99,6 +100,7 @@ enum dsa_tag_protocol {
 	DSA_TAG_PROTO_NETC		= DSA_TAG_PROTO_NETC_VALUE,
 	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,
 };
 
 struct dsa_switch;

net/dsa/Kconfig

@@ -125,6 +125,12 @@ config NET_DSA_TAG_MXL_GSW1XX
 	  Say Y or M if you want to enable support for tagging frames for
 	  MaxLinear GSW1xx switches.
 
+config NET_DSA_TAG_KS8995
+	tristate "Tag driver for Micrel KS8995 switch"
+	help
+	  Say Y if you want to enable support for tagging frames for the
+	  Micrel KS8995 switch.
+
 config NET_DSA_TAG_KSZ
 	tristate "Tag driver for Microchip 8795/937x/9477/9893 families of switches"
 	help

net/dsa/Makefile

@@ -25,6 +25,7 @@ obj-$(CONFIG_NET_DSA_TAG_BRCM_COMMON) += tag_brcm.o
 obj-$(CONFIG_NET_DSA_TAG_DSA_COMMON) += tag_dsa.o
 obj-$(CONFIG_NET_DSA_TAG_GSWIP) += tag_gswip.o
 obj-$(CONFIG_NET_DSA_TAG_HELLCREEK) += tag_hellcreek.o
+obj-$(CONFIG_NET_DSA_TAG_KS8995) += tag_ks8995.o
 obj-$(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

net/dsa/tag_ks8995.c

@@ -0,0 +1,180 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2026 Linus Walleij <linusw@kernel.org>
+ */
+#include <linux/bitfield.h>
+#include <linux/etherdevice.h>
+#include <linux/log2.h>
+#include <linux/list.h>
+#include <linux/net.h>
+#include <linux/slab.h>
+
+#include "tag.h"
+
+/* The Micrel KS8995XA / Microchip KSZ8995XA Special Tag Packet ID (STPID)
+ * pushes its tag in a modified VLAN (802.1Q) tag.
+ * -----------------------------------------------------------
+ * | MAC DA | MAC SA | 2 bytes tag | 2 bytes TCI | EtherType |
+ * -----------------------------------------------------------
+ * The tag is: 0x8100 |= BIT(port), ports 0,1,2,3
+ */
+
+#define KS8995_NAME "ks8995"
+
+#define KS8995_TAG_LEN		VLAN_HLEN
+/* Reserve room for the switch tag and a hardware-accelerated VLAN tag. */
+#define KS8995_NEEDED_HEADROOM	(KS8995_TAG_LEN + VLAN_HLEN)
+
+#define KS8995M_STPID_STD	GENMASK(15, 4)
+#define KS8995M_STPID_PORTMASK	GENMASK(3, 0)
+#define KS8995M_STPID(portmask)	htons(ETH_P_8021Q | FIELD_PREP(KS8995M_STPID_PORTMASK, portmask))
+
+static struct sk_buff *ks8995_xmit(struct sk_buff *skb, struct net_device *dev)
+{
+	struct vlan_ethhdr *hdr;
+	u16 portmask;
+
+	/* VLAN insertion requires a complete, linear Ethernet header. */
+	if (unlikely(!pskb_may_pull(skb, ETH_HLEN))) {
+		kfree_skb(skb);
+		return NULL;
+	}
+
+	/* Prepare the special KS8995 tags */
+	portmask = dsa_xmit_port_mask(skb, dev);
+
+	/* The switch expects the special tag at offset 12. Move any hardware
+	 * accelerated VLAN tag into the payload so the conduit cannot insert
+	 * it outside the special tag.
+	 */
+	if (unlikely(skb_vlan_tag_present(skb))) {
+		skb = __vlan_hwaccel_push_inside(skb);
+		if (!skb)
+			return NULL;
+	}
+
+	/* Always add a distinct outer tag. The user port removes this field on
+	 * egress, so reusing an existing 802.1Q tag would consume that VLAN tag.
+	 */
+	skb = vlan_insert_tag(skb, KS8995M_STPID(portmask), 0);
+	/* vlan_insert_tag() drops the skb on failure */
+	if (!skb)
+		return NULL;
+	hdr = skb_vlan_eth_hdr(skb);
+	netdev_dbg(dev, "%s: inserted VLAN TAG %04x TCI %04x\n",
+		   __func__, ntohs(hdr->h_vlan_proto),
+		   ntohs(hdr->h_vlan_TCI));
+
+	return skb;
+}
+
+static struct sk_buff *ks8995_rcv(struct sk_buff *skb, struct net_device *dev)
+{
+	int portmask;
+	int port;
+	u16 etype;
+
+	/* The special tag may be in the packet or VLAN metadata. In either
+	 * case, its TPID must retain the source-port bits.
+	 *
+	 * Read the in-band TPID directly because skb->protocol contains
+	 * ETH_P_XDSA.
+	 */
+	if (skb_vlan_tag_present(skb))
+		etype = ntohs(skb->vlan_proto);
+	else
+		etype = ntohs(*(__be16 *)dsa_etype_header_pos_rx(skb));
+	if ((etype & KS8995M_STPID_STD) != ETH_P_8021Q) {
+		netdev_dbg(dev, "%s: dropped ethertype 0x%04x\n",
+			   __func__, etype);
+		kfree_skb(skb);
+		return NULL;
+	}
+
+	portmask = FIELD_GET(KS8995M_STPID_PORTMASK, etype);
+	if (unlikely(!is_power_of_2(portmask))) {
+		netdev_dbg(dev, "%s: dropped invalid port mask 0x%04x\n",
+			   __func__, portmask);
+		kfree_skb(skb);
+		return NULL;
+	}
+	port = ilog2(portmask);
+
+	netdev_dbg(dev, "%s: received ethertype %04x\n",
+		   __func__, etype);
+
+	/* Move an in-band special tag into VLAN metadata. If already
+	 * offloaded, its TPID was validated above.
+	 */
+	if (!skb_vlan_tag_present(skb)) {
+		skb = skb_vlan_untag(skb);
+		if (!skb) {
+			/* skb_vlan_untag drops the skb on failure */
+			if (net_ratelimit())
+				netdev_err(dev, "%s: unable to untag skb\n", __func__);
+			return NULL;
+		}
+	}
+
+	netdev_dbg(dev, "%s: etype %04x portmask %04x (%d)\n",
+		   __func__, etype, portmask, port);
+	skb->dev = dsa_conduit_find_user(dev, 0, port);
+	if (!skb->dev) {
+		kfree_skb(skb);
+		return NULL;
+	}
+
+	/* The special tag's TCI is now in the hardware-accelerated VLAN
+	 * metadata. The switch preserves the TCI of tagged
+	 * frames, but inserts the ingress port's default tag for untagged frames.
+	 * Since the driver programs the default tag to 0, a zero TCI identifies
+	 * an originally untagged frame. Preserve every non-zero TCI as an
+	 * 802.1Q tag, including VID 0 frames with PCP or DEI set.
+	 */
+	if (!skb->vlan_tci) {
+		netdev_dbg(dev, "%s: clear VLAN tag from frame\n", __func__);
+		__vlan_hwaccel_clear_tag(skb);
+	} else {
+		skb->vlan_proto = htons(ETH_P_8021Q);
+		netdev_dbg(dev, "%s: vlan_tci = 0x%04x VLAN frame\n",
+			   __func__, skb->vlan_tci);
+	}
+
+	dsa_default_offload_fwd_mark(skb);
+
+	return skb;
+}
+
+static void ks8995_flow_dissect(const struct sk_buff *skb, __be16 *proto,
+				int *offset)
+{
+	const __be16 *encap_proto;
+	__be16 buffer;
+
+	*proto = 0;
+	*offset = 0;
+
+	/* Extra TX headroom does not increase the in-band RX tag length. */
+	encap_proto = skb_header_pointer(skb, KS8995_TAG_LEN - sizeof(buffer),
+					 sizeof(buffer), &buffer);
+	if (!encap_proto)
+		return;
+
+	*proto = *encap_proto;
+	*offset = KS8995_TAG_LEN;
+}
+
+static const struct dsa_device_ops ks8995_netdev_ops = {
+	.name = KS8995_NAME,
+	.proto	= DSA_TAG_PROTO_KS8995,
+	.xmit = ks8995_xmit,
+	.rcv = ks8995_rcv,
+	.flow_dissect = ks8995_flow_dissect,
+	.needed_headroom = KS8995_NEEDED_HEADROOM,
+};
+
+MODULE_DESCRIPTION("DSA tag driver for Micrel KS8995 family of switches");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_KS8995, KS8995_NAME);
+
+module_dsa_tag_driver(ks8995_netdev_ops);