net: dsa: add tag format for MxL862xx switches

Add proprietary special tag format for the MaxLinear MXL862xx family of
switches. While using the same Ethertype as MaxLinear's GSW1xx switches,
the actual tag format differs significantly, hence we need a dedicated
tag driver for that.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/c64e6ddb6c93a4fac39f9ab9b2d8bf551a2b118d.1770433307.git.daniel@makrotopia.org
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

3 files changed, 118 insertions, 0 deletions

diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
index f86b30742122..5ed8c704636d 100644
--- a/net/dsa/Kconfig
+++ b/net/dsa/Kconfig
@@ -104,6 +104,13 @@ 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_862XX
+	tristate "Tag driver for MaxLinear MxL862xx switches"
+	help
+	  Say Y or M if you want to enable support for tagging frames for the
+	  MaxLinear MxL86252 and MxL86282 switches using their native 8-byte
+	  tagging protocol.
+
 config NET_DSA_TAG_MXL_GSW1XX
 	tristate "Tag driver for MaxLinear GSW1xx switches"
 	help
diff --git a/net/dsa/Makefile b/net/dsa/Makefile
index 42d173f5a701..bf7247759a64 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-$(CONFIG_NET_DSA_TAG_MXL_862XX) += tag_mxl862xx.o
 obj-$(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
diff --git a/net/dsa/tag_mxl862xx.c b/net/dsa/tag_mxl862xx.c
new file mode 100644
index 000000000000..01f215868271
--- /dev/null
+++ b/net/dsa/tag_mxl862xx.c
@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * DSA Special Tag for MaxLinear 862xx switch chips
+ *
+ * Copyright (C) 2025 Daniel Golle <daniel@makrotopia.org>
+ * Copyright (C) 2024 MaxLinear Inc.
+ */
+
+#include <linux/bitops.h>
+#include <linux/etherdevice.h>
+#include <linux/skbuff.h>
+#include <net/dsa.h>
+#include "tag.h"
+
+#define MXL862_NAME	"mxl862xx"
+
+#define MXL862_HEADER_LEN	8
+
+/* Word 0 -> EtherType */
+
+/* Word 2 */
+#define MXL862_SUBIF_ID		GENMASK(4, 0)
+
+/* Word 3 */
+#define MXL862_IGP_EGP		GENMASK(3, 0)
+
+static struct sk_buff *mxl862_tag_xmit(struct sk_buff *skb,
+				       struct net_device *dev)
+{
+	struct dsa_port *dp = dsa_user_to_port(dev);
+	struct dsa_port *cpu_dp = dp->cpu_dp;
+	unsigned int cpu_port, sub_interface;
+	__be16 *mxl862_tag;
+
+	cpu_port = cpu_dp->index;
+
+	/* target port sub-interface ID relative to the CPU port */
+	sub_interface = dp->index + 16 - cpu_port;
+
+	/* provide additional space 'MXL862_HEADER_LEN' bytes */
+	skb_push(skb, MXL862_HEADER_LEN);
+
+	/* shift MAC address to the beginning of the enlarged buffer,
+	 * releasing the space required for DSA tag (between MAC address and
+	 * Ethertype)
+	 */
+	dsa_alloc_etype_header(skb, MXL862_HEADER_LEN);
+
+	/* special tag ingress (from the perspective of the switch) */
+	mxl862_tag = dsa_etype_header_pos_tx(skb);
+	mxl862_tag[0] = htons(ETH_P_MXLGSW);
+	mxl862_tag[1] = 0;
+	mxl862_tag[2] = htons(FIELD_PREP(MXL862_SUBIF_ID, sub_interface));
+	mxl862_tag[3] = htons(FIELD_PREP(MXL862_IGP_EGP, cpu_port));
+
+	return skb;
+}
+
+static struct sk_buff *mxl862_tag_rcv(struct sk_buff *skb,
+				      struct net_device *dev)
+{
+	__be16 *mxl862_tag;
+	int port;
+
+	if (unlikely(!pskb_may_pull(skb, MXL862_HEADER_LEN))) {
+		dev_warn_ratelimited(&dev->dev, "Cannot pull SKB, packet dropped\n");
+		return NULL;
+	}
+
+	mxl862_tag = dsa_etype_header_pos_rx(skb);
+
+	if (unlikely(mxl862_tag[0] != htons(ETH_P_MXLGSW))) {
+		dev_warn_ratelimited(&dev->dev,
+				     "Invalid special tag marker, packet dropped, tag: %8ph\n",
+				     mxl862_tag);
+		return NULL;
+	}
+
+	/* Get source port information */
+	port = FIELD_GET(MXL862_IGP_EGP, ntohs(mxl862_tag[3]));
+	skb->dev = dsa_conduit_find_user(dev, 0, port);
+	if (unlikely(!skb->dev)) {
+		dev_warn_ratelimited(&dev->dev,
+				     "Invalid source port, packet dropped, tag: %8ph\n",
+				     mxl862_tag);
+		return NULL;
+	}
+
+	/* remove the MxL862xx special tag between the MAC addresses and the
+	 * current ethertype field.
+	 */
+	skb_pull_rcsum(skb, MXL862_HEADER_LEN);
+	dsa_strip_etype_header(skb, MXL862_HEADER_LEN);
+
+	return skb;
+}
+
+static const struct dsa_device_ops mxl862_netdev_ops = {
+	.name = MXL862_NAME,
+	.proto = DSA_TAG_PROTO_MXL862,
+	.xmit = mxl862_tag_xmit,
+	.rcv = mxl862_tag_rcv,
+	.needed_headroom = MXL862_HEADER_LEN,
+};
+
+MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_MXL862, MXL862_NAME);
+MODULE_DESCRIPTION("DSA tag driver for MaxLinear MxL862xx switches");
+MODULE_LICENSE("GPL");
+
+module_dsa_tag_driver(mxl862_netdev_ops);