[net-next v5 2/3] net: dsa: tag_yt921x: add support for Motorcomm YT921x tags

[David Yang <mmyangfl@gmail.com>]

Add support for Motorcomm YT921x tags, which includes a proper
configurable ethertype field (default to 0x9988).

Signed-off-by: David Yang <mmyangfl@gmail.com>
---
 include/net/dsa.h    |   2 +
 net/dsa/Kconfig      |   6 +++
 net/dsa/Makefile     |   1 +
 net/dsa/tag_yt921x.c | 126 +++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 135 insertions(+)
 create mode 100644 net/dsa/tag_yt921x.c

diff --git a/include/net/dsa.h b/include/net/dsa.h
index d73ea0880066..67762fdaf3c7 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -55,6 +55,7 @@ struct tc_action;
 #define DSA_TAG_PROTO_LAN937X_VALUE		27
 #define DSA_TAG_PROTO_VSC73XX_8021Q_VALUE	28
 #define DSA_TAG_PROTO_BRCM_LEGACY_FCS_VALUE	29
+#define DSA_TAG_PROTO_YT921X_VALUE		30
 
 enum dsa_tag_protocol {
 	DSA_TAG_PROTO_NONE		= DSA_TAG_PROTO_NONE_VALUE,
@@ -87,6 +88,7 @@ enum dsa_tag_protocol {
 	DSA_TAG_PROTO_RZN1_A5PSW	= DSA_TAG_PROTO_RZN1_A5PSW_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,
 };
 
 struct dsa_switch;
diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
index 869cbe57162f..6b94028b1fcc 100644
--- a/net/dsa/Kconfig
+++ b/net/dsa/Kconfig
@@ -190,4 +190,10 @@ config NET_DSA_TAG_XRS700X
 	  Say Y or M if you want to enable support for tagging frames for
 	  Arrow SpeedChips XRS700x switches that use a single byte tag trailer.
 
+config NET_DSA_TAG_YT921X
+	tristate "Tag driver for Motorcomm YT921x switches"
+	help
+	  Say Y or M if you want to enable support for tagging frames for
+	  Motorcomm YT921x switches.
+
 endif
diff --git a/net/dsa/Makefile b/net/dsa/Makefile
index 555c07cfeb71..4b011a1d5c87 100644
--- a/net/dsa/Makefile
+++ b/net/dsa/Makefile
@@ -39,6 +39,7 @@ obj-$(CONFIG_NET_DSA_TAG_SJA1105) += tag_sja1105.o
 obj-$(CONFIG_NET_DSA_TAG_TRAILER) += tag_trailer.o
 obj-$(CONFIG_NET_DSA_TAG_VSC73XX_8021Q) += tag_vsc73xx_8021q.o
 obj-$(CONFIG_NET_DSA_TAG_XRS700X) += tag_xrs700x.o
+obj-$(CONFIG_NET_DSA_TAG_YT921X) += tag_yt921x.o
 
 # for tracing framework to find trace.h
 CFLAGS_trace.o := -I$(src)
diff --git a/net/dsa/tag_yt921x.c b/net/dsa/tag_yt921x.c
new file mode 100644
index 000000000000..94a4c1cb61ff
--- /dev/null
+++ b/net/dsa/tag_yt921x.c
@@ -0,0 +1,126 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Motorcomm YT921x Switch Extend CPU Port
+ *
+ * Copyright (c) 2025 David Yang <mmyangfl@gmail.com>
+ *
+ * +----+----+-------+-----+----+---------
+ * | DA | SA | TagET | Tag | ET | Payload ...
+ * +----+----+-------+-----+----+---------
+ *   6    6      2      6    2       N
+ *
+ * Tag Ethertype: CPU_TAG_TPID_TPID (default: 0x9988)
+ * Tag:
+ *   2: Service VLAN Tag
+ *   2: Rx Port
+ *     15b: Rx Port Valid
+ *     14b-11b: Rx Port
+ *     10b-0b: Unknown value 0x80
+ *   2: Tx Port(s)
+ *     15b: Tx Port(s) Valid
+ *     10b-0b: Tx Port(s) Mask
+ */
+
+#include <linux/etherdevice.h>
+#include <linux/list.h>
+#include <linux/slab.h>
+
+#include "tag.h"
+
+#define YT921X_NAME	"yt921x"
+
+#define YT921X_TAG_LEN	8
+
+#define ETH_P_YT921X	0x9988
+
+#define YT921X_TAG_PORT_EN		BIT(15)
+#define YT921X_TAG_RX_PORT_M		GENMASK(14, 11)
+#define YT921X_TAG_RX_CMD_M		GENMASK(10, 0)
+#define  YT921X_TAG_RX_CMD(x)			FIELD_PREP(YT921X_TAG_RX_CMD_M, (x))
+#define   YT921X_TAG_RX_CMD_UNK_NORMAL			0x80
+#define YT921X_TAG_TX_PORTS_M		GENMASK(10, 0)
+#define  YT921X_TAG_TX_PORTn(port)		BIT(port)
+
+static struct sk_buff *
+yt921x_tag_xmit(struct sk_buff *skb, struct net_device *netdev)
+{
+	struct dsa_port *dp = dsa_user_to_port(netdev);
+	unsigned int port = dp->index;
+	__be16 *tag;
+	u16 tx;
+
+	skb_push(skb, YT921X_TAG_LEN);
+	dsa_alloc_etype_header(skb, YT921X_TAG_LEN);
+
+	tag = dsa_etype_header_pos_tx(skb);
+
+	/* We might use yt921x_priv::tag_eth_p, but
+	 * 1. CPU_TAG_TPID could be configured anyway;
+	 * 2. Are you using the right chip?
+	 */
+	tag[0] = htons(ETH_P_YT921X);
+	/* Service VLAN tag not used */
+	tag[1] = 0;
+	tag[2] = 0;
+	tx = YT921X_TAG_PORT_EN | YT921X_TAG_TX_PORTn(port);
+	tag[3] = htons(tx);
+
+	/* Now tell the conduit network device about the desired output queue
+	 * as well
+	 */
+	skb_set_queue_mapping(skb, port);
+
+	return skb;
+}
+
+static struct sk_buff *
+yt921x_tag_rcv(struct sk_buff *skb, struct net_device *netdev)
+{
+	unsigned int port;
+	__be16 *tag;
+	u16 rx;
+
+	if (unlikely(!pskb_may_pull(skb, YT921X_TAG_LEN)))
+		return NULL;
+
+	tag = (__be16 *)skb->data;
+
+	/* Locate which port this is coming from */
+	rx = ntohs(tag[1]);
+	if (unlikely((rx & YT921X_TAG_PORT_EN) == 0)) {
+		dev_warn_ratelimited(&netdev->dev,
+				     "Unexpected rx tag 0x%04x\n", rx);
+		return NULL;
+	}
+
+	port = FIELD_GET(YT921X_TAG_RX_PORT_M, rx);
+	skb->dev = dsa_conduit_find_user(netdev, 0, port);
+	if (unlikely(!skb->dev)) {
+		dev_warn_ratelimited(&netdev->dev,
+				     "Couldn't decode source port %u\n", port);
+		return NULL;
+	}
+
+	/* Remove YT921x tag and update checksum */
+	skb_pull_rcsum(skb, YT921X_TAG_LEN);
+
+	dsa_default_offload_fwd_mark(skb);
+
+	dsa_strip_etype_header(skb, YT921X_TAG_LEN);
+
+	return skb;
+}
+
+static const struct dsa_device_ops yt921x_netdev_ops = {
+	.name	= YT921X_NAME,
+	.proto	= DSA_TAG_PROTO_YT921X,
+	.xmit	= yt921x_tag_xmit,
+	.rcv	= yt921x_tag_rcv,
+	.needed_headroom = YT921X_TAG_LEN,
+};
+
+MODULE_DESCRIPTION("DSA tag driver for Motorcomm YT921x switches");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_YT921X, YT921X_NAME);
+
+module_dsa_tag_driver(yt921x_netdev_ops);
-- 
2.50.1