net: dsa: add out-of-band tagging protocol

This tagging protocol is designed for the situation where the link
between the MAC and the Switch is designed such that the Destination
Port, which is usually embedded in some part of the Ethernet Header, is
sent out-of-band, and isn't present at all in the Ethernet frame.

This can happen when the MAC and Switch are tightly integrated on an
SoC, as is the case with the Qualcomm IPQ4019 for example, where the DSA
tag is inserted directly into the DMA descriptors. In that case,
the MAC driver is responsible for sending the tag to the switch using
the out-of-band medium. To do so, the MAC driver needs to have the
information of the destination port for that skb.

Add a new tagging protocol based on SKB extensions to convey the
information about the destination port to the MAC driver

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

Documentation/networking/dsa/dsa.rst

@@ -66,7 +66,8 @@ Switch tagging protocols
 ------------------------
 
 DSA supports many vendor-specific tagging protocols, one software-defined
-tagging protocol, and a tag-less mode as well (``DSA_TAG_PROTO_NONE``).
+tagging protocol, a tag-less mode as well (``DSA_TAG_PROTO_NONE``) and an
+out-of-band tagging protocol (``DSA_TAG_PROTO_OOB``).
 
 The exact format of the tag protocol is vendor specific, but in general, they
 all contain something which:
@@ -217,6 +218,16 @@ receive all frames regardless of the value of the MAC DA. This can be done by
 setting the ``promisc_on_master`` property of the ``struct dsa_device_ops``.
 Note that this assumes a DSA-unaware master driver, which is the norm.
 
+Some SoCs have a tight integration between the conduit network interface and the
+embedded switch, such that the DSA tag isn't transmitted in the packet data,
+but through another media, using so-called out-of-band tagging. In that case,
+the host MAC driver is in charge of transmitting the tag to the switch.
+An example is the IPQ4019 SoC, that transmits the tag between the ipqess
+ethernet controller and the qca8k switch using the DMA descriptor. In that
+configuration, tag-chaining is permitted, but the OOB tag will always be the
+top-most switch in the tree. The tagger (``DSA_TAG_PROTO_OOB``) uses skb
+extensions to transmit the tag to and from the MAC driver.
+
 Master network devices
 ----------------------
 

MAINTAINERS

@@ -17049,6 +17049,7 @@ L:	netdev@vger.kernel.org
 S:	Maintained
 F:	Documentation/devicetree/bindings/net/qcom,ipq4019-ess-edma.yaml
 F:	drivers/net/ethernet/qualcomm/ipqess/
+F:	net/dsa/tag_oob.c
 
 QUALCOMM ETHQOS ETHERNET DRIVER
 M:	Vinod Koul <vkoul@kernel.org>

include/linux/dsa/oob.h

@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0-only
+ * Copyright (C) 2022 Maxime Chevallier <maxime.chevallier@bootlin.com>
+ */
+
+#ifndef _NET_DSA_OOB_H
+#define _NET_DSA_OOB_H
+
+#include <linux/skbuff.h>
+
+struct dsa_oob_tag_info {
+	u16 port;
+};
+
+int dsa_oob_tag_push(struct sk_buff *skb, struct dsa_oob_tag_info *ti);
+int dsa_oob_tag_pop(struct sk_buff *skb, struct dsa_oob_tag_info *ti);
+#endif

include/linux/skbuff.h

@@ -4573,6 +4573,9 @@ enum skb_ext_id {
 #endif
 #if IS_ENABLED(CONFIG_MCTP_FLOWS)
 	SKB_EXT_MCTP,
+#endif
+#if IS_ENABLED(CONFIG_NET_DSA_TAG_OOB)
+	SKB_EXT_DSA_OOB,
 #endif
 	SKB_EXT_NUM, /* must be last */
 };

include/net/dsa.h

@@ -55,6 +55,7 @@ struct phylink_link_state;
 #define DSA_TAG_PROTO_RTL8_4T_VALUE		25
 #define DSA_TAG_PROTO_RZN1_A5PSW_VALUE		26
 #define DSA_TAG_PROTO_LAN937X_VALUE		27
+#define DSA_TAG_PROTO_OOB_VALUE			28
 
 enum dsa_tag_protocol {
 	DSA_TAG_PROTO_NONE		= DSA_TAG_PROTO_NONE_VALUE,
@@ -85,6 +86,7 @@ enum dsa_tag_protocol {
 	DSA_TAG_PROTO_RTL8_4T		= DSA_TAG_PROTO_RTL8_4T_VALUE,
 	DSA_TAG_PROTO_RZN1_A5PSW	= DSA_TAG_PROTO_RZN1_A5PSW_VALUE,
 	DSA_TAG_PROTO_LAN937X		= DSA_TAG_PROTO_LAN937X_VALUE,
+	DSA_TAG_PROTO_OOB		= DSA_TAG_PROTO_OOB_VALUE,
 };
 
 struct dsa_switch;

net/core/skbuff.c

@@ -61,8 +61,12 @@
 #include <linux/if_vlan.h>
 #include <linux/mpls.h>
 #include <linux/kcov.h>
+#ifdef CONFIG_NET_DSA_TAG_OOB
+#include <linux/dsa/oob.h>
+#endif
 
 #include <net/protocol.h>
+#include <net/dsa.h>
 #include <net/dst.h>
 #include <net/sock.h>
 #include <net/checksum.h>
@@ -4483,6 +4487,9 @@ static const u8 skb_ext_type_len[] = {
 #if IS_ENABLED(CONFIG_MCTP_FLOWS)
 	[SKB_EXT_MCTP] = SKB_EXT_CHUNKSIZEOF(struct mctp_flow),
 #endif
+#if IS_ENABLED(CONFIG_NET_DSA_TAG_OOB)
+	[SKB_EXT_DSA_OOB] = SKB_EXT_CHUNKSIZEOF(struct dsa_oob_tag_info),
+#endif
 };
 
 static __always_inline unsigned int skb_ext_total_length(void)
@@ -4502,6 +4509,9 @@ static __always_inline unsigned int skb_ext_total_length(void)
 #endif
 #if IS_ENABLED(CONFIG_MCTP_FLOWS)
 		skb_ext_type_len[SKB_EXT_MCTP] +
+#endif
+#if IS_ENABLED(CONFIG_NET_DSA_TAG_OOB)
+		skb_ext_type_len[SKB_EXT_DSA_OOB] +
 #endif
 		0;
 }

net/dsa/Kconfig

@@ -113,6 +113,15 @@ config NET_DSA_TAG_OCELOT_8021Q
 	  this mode, less TCAM resources (VCAP IS1, IS2, ES0) are available for
 	  use with tc-flower.
 
+config NET_DSA_TAG_OOB
+	select SKB_EXTENSIONS
+	tristate "Tag driver for Out-of-band tagging drivers"
+	help
+	  Say Y or M if you want to enable support for pairs of embedded
+	  switches and host MAC drivers which perform demultiplexing and
+	  packet steering to ports using out of band metadata processed
+	  by the DSA master, rather than tags present in the packets.
+
 config NET_DSA_TAG_QCA
 	tristate "Tag driver for Qualcomm Atheros QCA8K switches"
 	help

net/dsa/Makefile

@@ -22,6 +22,7 @@ obj-$(CONFIG_NET_DSA_TAG_LAN9303) += tag_lan9303.o
 obj-$(CONFIG_NET_DSA_TAG_MTK) += tag_mtk.o
 obj-$(CONFIG_NET_DSA_TAG_OCELOT) += tag_ocelot.o
 obj-$(CONFIG_NET_DSA_TAG_OCELOT_8021Q) += tag_ocelot_8021q.o
+obj-$(CONFIG_NET_DSA_TAG_OOB) += tag_oob.o
 obj-$(CONFIG_NET_DSA_TAG_QCA) += tag_qca.o
 obj-$(CONFIG_NET_DSA_TAG_RTL4_A) += tag_rtl4_a.o
 obj-$(CONFIG_NET_DSA_TAG_RTL8_4) += tag_rtl8_4.o

net/dsa/tag_oob.c

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/* Copyright (c) 2022, Maxime Chevallier <maxime.chevallier@bootlin.com> */
+
+#include <linux/bitfield.h>
+#include <linux/dsa/oob.h>
+#include <linux/skbuff.h>
+
+#include "dsa_priv.h"
+
+static struct sk_buff *oob_tag_xmit(struct sk_buff *skb,
+				    struct net_device *dev)
+{
+	struct dsa_oob_tag_info *tag_info = skb_ext_add(skb, SKB_EXT_DSA_OOB);
+	struct dsa_port *dp = dsa_slave_to_port(dev);
+
+	tag_info->port = dp->index;
+
+	return skb;
+}
+
+static struct sk_buff *oob_tag_rcv(struct sk_buff *skb,
+				   struct net_device *dev)
+{
+	struct dsa_oob_tag_info *tag_info = skb_ext_find(skb, SKB_EXT_DSA_OOB);
+	if (!tag_info)
+		return NULL;
+
+	skb->dev = dsa_master_find_slave(dev, 0, tag_info->port);
+	if (!skb->dev)
+		return NULL;
+
+	return skb;
+}
+
+const struct dsa_device_ops oob_tag_dsa_ops = {
+	.name	= "oob",
+	.proto	= DSA_TAG_PROTO_OOB,
+	.xmit	= oob_tag_xmit,
+	.rcv	= oob_tag_rcv,
+};
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("DSA tag driver for out-of-band tagging");
+MODULE_AUTHOR("Maxime Chevallier <maxime.chevallier@bootlin.com>");
+MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_OOB);
+
+module_dsa_tag_driver(oob_tag_dsa_ops);