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 <daniel@makrotopia.org>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Tested-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Link: https://patch.msgid.link/0e973ebfd9433c30c96f50670da9e9449a0d98f2.1762170107.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 125 insertions, 0 deletions

diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
index 6b94028b1fcc..f86b30742122 100644
--- a/net/dsa/Kconfig
+++ b/net/dsa/Kconfig
@@ -104,6 +104,14 @@ config NET_DSA_TAG_MTK
 	  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
diff --git a/net/dsa/Makefile b/net/dsa/Makefile
index 4b011a1d5c87..42d173f5a701 100644
--- a/net/dsa/Makefile
+++ b/net/dsa/Makefile
@@ -28,6 +28,7 @@ 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_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
diff --git a/net/dsa/tag_mxl-gsw1xx.c b/net/dsa/tag_mxl-gsw1xx.c
new file mode 100644
index 000000000000..701a079955f2
--- /dev/null
+++ b/net/dsa/tag_mxl-gsw1xx.c
@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * DSA driver Special Tag support for MaxLinear GSW1xx switch chips
+ *
+ * Copyright (C) 2025 Daniel Golle <daniel@makrotopia.org>
+ * 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);