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>
---
v5:
 * remove unused macro definitions

RFC v4:
 * describe fields and variables with comments
 * sub-interface is only 5 bits
 * harmonize Kconfig symbol name
 * maintain alphabetic order in Kconfig
 * fix typo s/beginnig/beginning/
 * fix typo s/swtiches/switches/
 * arrange local variables in reverse xmas tree order

RFC v3: no changes
RFC v2: make sure all tag fields are initialized

 MAINTAINERS            |   1 +
 include/net/dsa.h      |   2 +
 net/dsa/Kconfig        |   7 +++
 net/dsa/Makefile       |   1 +
 net/dsa/tag_mxl862xx.c | 116 +++++++++++++++++++++++++++++++++++++++++
 5 files changed, 127 insertions(+)
 create mode 100644 net/dsa/tag_mxl862xx.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 162859534a77c..686591074b34d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -15619,6 +15619,7 @@ M:	Daniel Golle <daniel@makrotopia.org>
 L:	netdev@vger.kernel.org
 S:	Maintained
 F:	Documentation/devicetree/bindings/net/dsa/maxlinear,mxl862xx.yaml
+F:	net/dsa/tag_mxl862xx.c
 
 MCAN DEVICE DRIVER
 M:	Markus Schneider-Pargmann <msp@baylibre.com>
diff --git a/include/net/dsa.h b/include/net/dsa.h
index 6b2b5ed64ea4c..1e33242b6d94d 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -57,6 +57,7 @@ struct tc_action;
 #define DSA_TAG_PROTO_BRCM_LEGACY_FCS_VALUE	29
 #define DSA_TAG_PROTO_YT921X_VALUE		30
 #define DSA_TAG_PROTO_MXL_GSW1XX_VALUE		31
+#define DSA_TAG_PROTO_MXL862_VALUE		32
 
 enum dsa_tag_protocol {
 	DSA_TAG_PROTO_NONE		= DSA_TAG_PROTO_NONE_VALUE,
@@ -91,6 +92,7 @@ enum dsa_tag_protocol {
 	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,
+	DSA_TAG_PROTO_MXL862		= DSA_TAG_PROTO_MXL862_VALUE,
 };
 
 struct dsa_switch;
diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
index f86b30742122f..efc95759a10e1 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 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 42d173f5a7013..bf7247759a64a 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 0000000000000..c4d0ea2eb48f9
--- /dev/null
+++ b/net/dsa/tag_mxl862xx.c
@@ -0,0 +1,116 @@
+// 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)
+{
+	unsigned int cpu_port, usr_port, sub_interface;
+	struct dsa_port *dp = dsa_user_to_port(dev);
+	struct dsa_port *cpu_dp = dp->cpu_dp;
+	__be16 *mxl862_tag;
+
+	/* switch firmware expects ports to be counted starting from 1 */
+	cpu_port = cpu_dp->index + 1;
+	usr_port = dp->index + 1;
+
+	/* target port sub-interface ID relative to the CPU port */
+	sub_interface = usr_port + 16 - cpu_port;
+
+	if (!skb)
+		return skb;
+
+	/* 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 */
+	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\n");
+		dev_warn_ratelimited(&dev->dev, "Rx Packet Tag: %8ph\n",
+				     mxl862_tag);
+		return NULL;
+	}
+
+	/* Get source port information */
+	port = FIELD_GET(MXL862_IGP_EGP, ntohs(mxl862_tag[3])) - 1;
+
+	skb->dev = dsa_conduit_find_user(dev, 0, port);
+	if (!skb->dev) {
+		dev_warn_ratelimited(&dev->dev, "Invalid source port, packet dropped\n");
+		dev_warn_ratelimited(&dev->dev, "Rx Packet 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",
+	.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);
-- 
2.52.0

On 1/12/26 2:51 PM, Daniel Golle wrote:
> 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>
> ---
> v5:
>  * remove unused macro definitions
> 
> RFC v4:
>  * describe fields and variables with comments
>  * sub-interface is only 5 bits
>  * harmonize Kconfig symbol name
>  * maintain alphabetic order in Kconfig
>  * fix typo s/beginnig/beginning/
>  * fix typo s/swtiches/switches/
>  * arrange local variables in reverse xmas tree order
> 
> RFC v3: no changes
> RFC v2: make sure all tag fields are initialized
> 
>  MAINTAINERS            |   1 +
>  include/net/dsa.h      |   2 +
>  net/dsa/Kconfig        |   7 +++
>  net/dsa/Makefile       |   1 +
>  net/dsa/tag_mxl862xx.c | 116 +++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 127 insertions(+)
>  create mode 100644 net/dsa/tag_mxl862xx.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 162859534a77c..686591074b34d 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -15619,6 +15619,7 @@ M:	Daniel Golle <daniel@makrotopia.org>
>  L:	netdev@vger.kernel.org
>  S:	Maintained
>  F:	Documentation/devicetree/bindings/net/dsa/maxlinear,mxl862xx.yaml
> +F:	net/dsa/tag_mxl862xx.c
>  
>  MCAN DEVICE DRIVER
>  M:	Markus Schneider-Pargmann <msp@baylibre.com>
> diff --git a/include/net/dsa.h b/include/net/dsa.h
> index 6b2b5ed64ea4c..1e33242b6d94d 100644
> --- a/include/net/dsa.h
> +++ b/include/net/dsa.h
> @@ -57,6 +57,7 @@ struct tc_action;
>  #define DSA_TAG_PROTO_BRCM_LEGACY_FCS_VALUE	29
>  #define DSA_TAG_PROTO_YT921X_VALUE		30
>  #define DSA_TAG_PROTO_MXL_GSW1XX_VALUE		31
> +#define DSA_TAG_PROTO_MXL862_VALUE		32
>  
>  enum dsa_tag_protocol {
>  	DSA_TAG_PROTO_NONE		= DSA_TAG_PROTO_NONE_VALUE,
> @@ -91,6 +92,7 @@ enum dsa_tag_protocol {
>  	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,
> +	DSA_TAG_PROTO_MXL862		= DSA_TAG_PROTO_MXL862_VALUE,
>  };
>  
>  struct dsa_switch;
> diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
> index f86b30742122f..efc95759a10e1 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 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 42d173f5a7013..bf7247759a64a 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 0000000000000..c4d0ea2eb48f9
> --- /dev/null
> +++ b/net/dsa/tag_mxl862xx.c
> @@ -0,0 +1,116 @@
> +// 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)
> +{
> +	unsigned int cpu_port, usr_port, sub_interface;
> +	struct dsa_port *dp = dsa_user_to_port(dev);
> +	struct dsa_port *cpu_dp = dp->cpu_dp;
> +	__be16 *mxl862_tag;
> +
> +	/* switch firmware expects ports to be counted starting from 1 */
> +	cpu_port = cpu_dp->index + 1;
> +	usr_port = dp->index + 1;
> +
> +	/* target port sub-interface ID relative to the CPU port */
> +	sub_interface = usr_port + 16 - cpu_port;
> +
> +	if (!skb)
> +		return skb;

I think skb can't be NULL here?!? Otherwise it would be better to move
the check at the beginning of the function and return NULL directly.

> +
> +	/* 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 */
> +	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\n");
> +		dev_warn_ratelimited(&dev->dev, "Rx Packet Tag: %8ph\n",
> +				     mxl862_tag);

Why breaking the message in 2 strings? a single dev_warn_ratelimited()
would be more readable and will generate smaller code. Similar thing below.

/P