net: dsa: add tagging driver for MaxLinear GSW1xx switch family

Add support for a new DSA tagging protocol driver for the MaxLinear
GSW1xx switch family. The GSW1xx switches use a proprietary 8-byte
special tag inserted between the source MAC address and the EtherType
field to indicate the source and destination ports for frames
traversing the CPU port.

Implement the tag handling logic to insert the special tag on transmit
and parse it on receive.

Signed-off-by: Daniel Golle <[e-mail]>
Reviewed-by: Alexander Sverdlin <[e-mail]>
Tested-by: Alexander Sverdlin <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

MAINTAINERS

···
 K:	landlock
 K:	LANDLOCK
 
-LANTIQ / INTEL Ethernet drivers
+LANTIQ / MAXLINEAR / INTEL Ethernet DSA drivers
 M:	Hauke Mehrtens <[e-mail]>
 L:	[e-mail]
 S:	Maintained
···
 F:	drivers/net/dsa/lantiq/*
 F:	drivers/net/ethernet/lantiq_xrx200.c
 F:	net/dsa/tag_gswip.c
+F:	net/dsa/tag_mxl-gsw1xx.c
 
 LANTIQ MIPS ARCHITECTURE
 M:	John Crispin <[e-mail]>

include/net/dsa.h

···
 #define DSA_TAG_PROTO_VSC73XX_8021Q_VALUE	28
 #define DSA_TAG_PROTO_BRCM_LEGACY_FCS_VALUE	29
 #define DSA_TAG_PROTO_YT921X_VALUE		30
+#define DSA_TAG_PROTO_MXL_GSW1XX_VALUE		31
 
 enum dsa_tag_protocol {
 	DSA_TAG_PROTO_NONE		= DSA_TAG_PROTO_NONE_VALUE,
···
 	DSA_TAG_PROTO_LAN937X		= DSA_TAG_PROTO_LAN937X_VALUE,
 	DSA_TAG_PROTO_VSC73XX_8021Q	= DSA_TAG_PROTO_VSC73XX_8021Q_VALUE,
 	DSA_TAG_PROTO_YT921X		= DSA_TAG_PROTO_YT921X_VALUE,
+	DSA_TAG_PROTO_MXL_GSW1XX	= DSA_TAG_PROTO_MXL_GSW1XX_VALUE,
 };
 
 struct dsa_switch;

include/uapi/linux/if_ether.h

···
 #define ETH_P_ETHERCAT	0x88A4		/* EtherCAT			*/
 #define ETH_P_8021AD	0x88A8          /* 802.1ad Service VLAN		*/
 #define ETH_P_802_EX1	0x88B5		/* 802.1 Local Experimental 1.  */
+#define ETH_P_MXLGSW	0x88C3		/* MaxLinear GSW DSA [ NOT AN OFFICIALLY REGISTERED ID ] */
 #define ETH_P_PREAUTH	0x88C7		/* 802.11 Preauthentication */
 #define ETH_P_TIPC	0x88CA		/* TIPC 			*/
 #define ETH_P_LLDP	0x88CC		/* Link Layer Discovery Protocol */

net/dsa/Kconfig

···
 	  Say Y or M if you want to enable support for tagging frames for
 	  Mediatek switches.
 
+config NET_DSA_TAG_MXL_GSW1XX
+	tristate "Tag driver for MaxLinear GSW1xx switches"
+	help
+	  The GSW1xx family of switches supports an 8-byte special tag which
+	  can be used on the CPU port of the switch.
+	  Say Y or M if you want to enable support for tagging frames for
+	  MaxLinear GSW1xx switches.
+
 config NET_DSA_TAG_KSZ
 	tristate "Tag driver for Microchip 8795/937x/9477/9893 families of switches"
 	help

net/dsa/Makefile

···
 obj-$(CONFIG_NET_DSA_TAG_KSZ) += tag_ksz.o
 obj-$(CONFIG_NET_DSA_TAG_LAN9303) += tag_lan9303.o
 obj-$(CONFIG_NET_DSA_TAG_MTK) += tag_mtk.o
+obj-$(CONFIG_NET_DSA_TAG_MXL_GSW1XX) += tag_mxl-gsw1xx.o
 obj-$(CONFIG_NET_DSA_TAG_NONE) += tag_none.o
 obj-$(CONFIG_NET_DSA_TAG_OCELOT) += tag_ocelot.o
 obj-$(CONFIG_NET_DSA_TAG_OCELOT_8021Q) += tag_ocelot_8021q.o

net/dsa/tag_mxl-gsw1xx.c

···
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * DSA driver Special Tag support for MaxLinear GSW1xx switch chips
+ *
+ * Copyright (C) 2025 Daniel Golle <[e-mail]>
+ * Copyright (C) 2023 - 2024 MaxLinear Inc.
+ */
+
+#include <linux/bitops.h>
+#include <linux/etherdevice.h>
+#include <linux/skbuff.h>
+#include <net/dsa.h>
+
+#include "tag.h"
+
+/* To define the outgoing port and to discover the incoming port a special
+ * tag is used by the GSW1xx.
+ *
+ *       Dest MAC       Src MAC    special TAG        EtherType
+ * ...| 1 2 3 4 5 6 | 1 2 3 4 5 6 | 1 2 3 4 5 6 7 8 | 1 2 |...
+ *                                |<--------------->|
+ */
+
+#define GSW1XX_TAG_NAME		"gsw1xx"
+
+/* special tag header length (RX and TX) */
+#define GSW1XX_HEADER_LEN		8
+
+/* Word 0 = Ethertype -> 0x88C3 */
+
+/* Word 1 */
+#define GSW1XX_TX_PORT_MAP		GENMASK(7, 0)
+#define GSW1XX_TX_PORT_MAP_EN		BIT(15)
+#define GSW1XX_TX_CLASS_EN		BIT(14)
+#define GSW1XX_TX_TIME_STAMP_EN		BIT(13)
+#define GSW1XX_TX_LRN_DIS		BIT(12)
+#define GSW1XX_TX_CLASS			GENMASK(11, 8)
+
+/* special tag in RX path header */
+/* Word 2 */
+#define GSW1XX_RX_PORT_MAP		GENMASK(15, 8)
+
+static struct sk_buff *gsw1xx_tag_xmit(struct sk_buff *skb,
+				       struct net_device *dev)
+{
+	struct dsa_port *dp = dsa_user_to_port(dev);
+	__be16 *gsw1xx_tag;
+
+	/* provide additional space 'GSW1XX_HEADER_LEN' bytes */
+	skb_push(skb, GSW1XX_HEADER_LEN);
+
+	/* add space between MAC address and Ethertype */
+	dsa_alloc_etype_header(skb, GSW1XX_HEADER_LEN);
+
+	/* special tag ingress */
+	gsw1xx_tag = dsa_etype_header_pos_tx(skb);
+	gsw1xx_tag[0] = htons(ETH_P_MXLGSW);
+	gsw1xx_tag[1] = htons(GSW1XX_TX_PORT_MAP_EN | GSW1XX_TX_LRN_DIS |
+			FIELD_PREP(GSW1XX_TX_PORT_MAP, BIT(dp->index)));
+
+	gsw1xx_tag[2] = 0;
+	gsw1xx_tag[3] = 0;
+
+	return skb;
+}
+
+static struct sk_buff *gsw1xx_tag_rcv(struct sk_buff *skb,
+				      struct net_device *dev)
+{
+	int port;
+	__be16 *gsw1xx_tag;
+
+	if (unlikely(!pskb_may_pull(skb, GSW1XX_HEADER_LEN))) {
+		dev_warn_ratelimited(&dev->dev, "Dropping packet, cannot pull SKB\n");
+		return NULL;
+	}
+
+	gsw1xx_tag = dsa_etype_header_pos_rx(skb);
+
+	if (unlikely(ntohs(gsw1xx_tag[0]) != ETH_P_MXLGSW)) {
+		dev_warn_ratelimited(&dev->dev, "Dropping packet due to invalid special tag\n");
+		dev_warn_ratelimited(&dev->dev, "Tag: %8ph\n", gsw1xx_tag);
+		return NULL;
+	}
+
+	/* Get source port information */
+	port = FIELD_GET(GSW1XX_RX_PORT_MAP, ntohs(gsw1xx_tag[1]));
+	skb->dev = dsa_conduit_find_user(dev, 0, port);
+	if (!skb->dev) {
+		dev_warn_ratelimited(&dev->dev, "Dropping packet due to invalid source port\n");
+		dev_warn_ratelimited(&dev->dev, "Tag: %8ph\n", gsw1xx_tag);
+		return NULL;
+	}
+
+	/* remove the GSW1xx special tag between MAC addresses and the current
+	 * ethertype field.
+	 */
+	skb_pull_rcsum(skb, GSW1XX_HEADER_LEN);
+	dsa_strip_etype_header(skb, GSW1XX_HEADER_LEN);
+
+	return skb;
+}
+
+static const struct dsa_device_ops gsw1xx_netdev_ops = {
+	.name			= GSW1XX_TAG_NAME,
+	.proto			= DSA_TAG_PROTO_MXL_GSW1XX,
+	.xmit			= gsw1xx_tag_xmit,
+	.rcv			= gsw1xx_tag_rcv,
+	.needed_headroom	= GSW1XX_HEADER_LEN,
+};
+
+MODULE_DESCRIPTION("DSA tag driver for MaxLinear GSW1xx 8 byte protocol");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_MXL_GSW1XX, GSW1XX_TAG_NAME);
+
+module_dsa_tag_driver(gsw1xx_netdev_ops);