Merge tag 'drm-misc-fixes-2026-09-24' of https://gitlab.freedesktop.org/drm/misc/kernel into drm-fixesgrafted

A number of fixes:
  - bridge:
    - samsung-dsim: fix GPIO lifetime
  - client: Null pointer dereference fix
  - imagination: error handling fix, page handling fix
  - nouveau: fix reference leaks, double-frees, out-of-bounds accesses,
    use-after-frees, don't reject config without SCDC,  a number of
    workarounds
  - virtio: fix memory leak, reference leaks, null pointer dereference,
    add pixel blend mode, cache coherency fix

Signed-off-by: Dave Airlie <airlied@redhat.com>

From: Maxime Ripard <self@mripard.dev>
Link: https://patch.msgid.link/arU22zzqUGDEco1y@houat

8 files changed, 4130 insertions, 0 deletions

diff --git a/drivers/net/mctp/Kconfig b/drivers/net/mctp/Kconfig
new file mode 100644
index 000000000..c40ac9c66
--- /dev/null
+++ b/drivers/net/mctp/Kconfig
@@ -0,0 +1,78 @@
+
+if MCTP
+
+menu "MCTP Device Drivers"
+
+config MCTP_SERIAL
+	tristate "MCTP serial transport"
+	depends on TTY
+	select CRC_CCITT
+	help
+	  This driver provides an MCTP-over-serial interface, through a
+	  serial line-discipline, as defined by DMTF specification "DSP0253 -
+	  MCTP Serial Transport Binding". By attaching the ldisc to a serial
+	  device, we get a new net device to transport MCTP packets.
+
+	  This allows communication with external MCTP endpoints which use
+	  serial as their transport. It can also be used as an easy way to
+	  provide MCTP connectivity between virtual machines, by forwarding
+	  data between simple virtual serial devices.
+
+	  Say y here if you need to connect to MCTP endpoints over serial. To
+	  compile as a module, use m; the module will be called mctp-serial.
+
+config MCTP_SERIAL_TEST
+        bool "MCTP serial tests" if !KUNIT_ALL_TESTS
+        depends on MCTP_SERIAL=y && KUNIT=y
+        default KUNIT_ALL_TESTS
+
+config MCTP_TRANSPORT_I2C
+	tristate "MCTP SMBus/I2C transport"
+	# i2c-mux is optional, but we must build as a module if i2c-mux is a module
+	depends on I2C_MUX || !I2C_MUX
+	depends on I2C
+	depends on I2C_SLAVE
+	select MCTP_FLOWS
+	help
+	  Provides a driver to access MCTP devices over SMBus/I2C transport,
+	  from DMTF specification DSP0237. A MCTP protocol network device is
+	  created for each I2C bus that has been assigned a mctp-i2c device.
+
+config MCTP_TRANSPORT_I3C
+	tristate "MCTP I3C transport"
+	depends on I3C
+	help
+	  Provides a driver to access MCTP devices over I3C transport,
+	  from DMTF specification DSP0233.
+	  A MCTP protocol network device is created for each I3C bus
+	  having a "mctp-controller" devicetree property.
+
+config MCTP_TRANSPORT_USBLIB
+	tristate "MCTP over USB common library"
+	depends on USB
+	help
+	  Common protocol handling functions for MCTP-over-USB transport
+	  implementations, suitable for use in either host- or gadget-side
+	  transport driver
+
+	  This will be automatically enabled by the transport driver.
+
+config MCTP_TRANSPORT_USBLIB_TEST
+        bool "MCTP usblib tests" if !KUNIT_ALL_TESTS
+        depends on MCTP_TRANSPORT_USBLIB=y && KUNIT=y
+        default KUNIT_ALL_TESTS
+
+config MCTP_TRANSPORT_USB
+	tristate "MCTP USB transport"
+	depends on USB
+	select MCTP_TRANSPORT_USBLIB
+	help
+	  Provides a driver to access MCTP devices over USB transport,
+	  defined by DMTF specification DSP0283.
+
+	  MCTP-over-USB interfaces are peer-to-peer, so each interface
+	  represents a physical connection to one remote MCTP endpoint.
+
+endmenu
+
+endif
diff --git a/drivers/net/mctp/Makefile b/drivers/net/mctp/Makefile
new file mode 100644
index 000000000..c870b62d3
--- /dev/null
+++ b/drivers/net/mctp/Makefile
@@ -0,0 +1,5 @@
+obj-$(CONFIG_MCTP_SERIAL) += mctp-serial.o
+obj-$(CONFIG_MCTP_TRANSPORT_I2C) += mctp-i2c.o
+obj-$(CONFIG_MCTP_TRANSPORT_I3C) += mctp-i3c.o
+obj-$(CONFIG_MCTP_TRANSPORT_USB) += mctp-usb.o
+obj-$(CONFIG_MCTP_TRANSPORT_USBLIB) += mctp-usblib.o
diff --git a/drivers/net/mctp/mctp-i2c.c b/drivers/net/mctp/mctp-i2c.c
new file mode 100644
index 000000000..b02476d33
--- /dev/null
+++ b/drivers/net/mctp/mctp-i2c.c
@@ -0,0 +1,1156 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Management Controller Transport Protocol (MCTP)
+ * Implements DMTF specification
+ * "DSP0237 Management Component Transport Protocol (MCTP) SMBus/I2C
+ * Transport Binding"
+ * https://www.dmtf.org/sites/default/files/standards/documents/DSP0237_1.2.0.pdf
+ *
+ * A netdev is created for each I2C bus that handles MCTP. In the case of an I2C
+ * mux topology a single I2C client is attached to the root of the mux topology,
+ * shared between all mux I2C busses underneath. For non-mux cases an I2C client
+ * is attached per netdev.
+ *
+ * mctp-i2c-controller.yml devicetree binding has further details.
+ *
+ * Copyright (c) 2022 Code Construct
+ * Copyright (c) 2022 Google
+ */
+
+#include <linux/module.h>
+#include <linux/netdevice.h>
+#include <linux/i2c.h>
+#include <linux/i2c-mux.h>
+#include <linux/if_arp.h>
+#include <net/mctp.h>
+#include <net/mctpdevice.h>
+
+/* byte_count is limited to u8 */
+#define MCTP_I2C_MAXBLOCK 255
+/* One byte is taken by source_slave */
+#define MCTP_I2C_MAXMTU (MCTP_I2C_MAXBLOCK - 1)
+#define MCTP_I2C_MINMTU (64 + 4)
+/* Allow space for dest_address, command, byte_count, data, PEC */
+#define MCTP_I2C_BUFSZ (3 + MCTP_I2C_MAXBLOCK + 1)
+#define MCTP_I2C_MINLEN 8
+#define MCTP_I2C_COMMANDCODE 0x0f
+#define MCTP_I2C_TX_WORK_LEN 100
+/* Sufficient for 64kB at min mtu */
+#define MCTP_I2C_TX_QUEUE_LEN 1100
+
+#define MCTP_I2C_OF_PROP "mctp-controller"
+
+enum {
+	MCTP_I2C_FLOW_STATE_NEW = 0,
+	MCTP_I2C_FLOW_STATE_ACTIVE,
+	MCTP_I2C_FLOW_STATE_INVALID,
+};
+
+/* List of all struct mctp_i2c_client
+ * Lock protects driver_clients and also prevents adding/removing adapters
+ * during mctp_i2c_client probe/remove.
+ */
+static DEFINE_MUTEX(driver_clients_lock);
+static LIST_HEAD(driver_clients);
+
+struct mctp_i2c_client;
+
+/* The netdev structure. One of these per I2C adapter. */
+struct mctp_i2c_dev {
+	struct net_device *ndev;
+	struct i2c_adapter *adapter;
+	struct mctp_i2c_client *client;
+	struct list_head list; /* For mctp_i2c_client.devs */
+
+	size_t rx_pos;
+	u8 rx_buffer[MCTP_I2C_BUFSZ];
+	struct completion rx_done;
+
+	struct task_struct *tx_thread;
+	wait_queue_head_t tx_wq;
+	struct sk_buff_head tx_queue;
+	u8 tx_scratch[MCTP_I2C_BUFSZ];
+
+	/* A fake entry in our tx queue to perform an unlock operation */
+	struct sk_buff unlock_marker;
+
+	/* Spinlock protects i2c_lock_count, release_count, allow_rx */
+	spinlock_t lock;
+	int i2c_lock_count;
+	int release_count;
+	/* Indicates that the netif is ready to receive incoming packets */
+	bool allow_rx;
+
+};
+
+/* The i2c client structure. One per hardware i2c bus at the top of the
+ * mux tree, shared by multiple netdevs
+ */
+struct mctp_i2c_client {
+	struct i2c_client *client;
+	u8 lladdr;
+
+	struct mctp_i2c_dev *sel;
+	struct list_head devs;
+	spinlock_t sel_lock; /* Protects sel and devs */
+
+	struct list_head list; /* For driver_clients */
+};
+
+/* Header on the wire. */
+struct mctp_i2c_hdr {
+	u8 dest_slave;
+	u8 command;
+	/* Count of bytes following byte_count, excluding PEC */
+	u8 byte_count;
+	u8 source_slave;
+};
+
+static int mctp_i2c_recv(struct mctp_i2c_dev *midev);
+static int mctp_i2c_slave_cb(struct i2c_client *client,
+			     enum i2c_slave_event event, u8 *val);
+static void mctp_i2c_ndo_uninit(struct net_device *dev);
+static int mctp_i2c_ndo_open(struct net_device *dev);
+
+static struct i2c_adapter *mux_root_adapter(struct i2c_adapter *adap)
+{
+#if IS_ENABLED(CONFIG_I2C_MUX)
+	return i2c_root_adapter(&adap->dev);
+#else
+	/* In non-mux config all i2c adapters are root adapters */
+	return adap;
+#endif
+}
+
+/* Creates a new i2c slave device attached to the root adapter.
+ * Sets up the slave callback.
+ * Must be called with a client on a root adapter.
+ */
+static struct mctp_i2c_client *mctp_i2c_new_client(struct i2c_client *client)
+{
+	struct mctp_i2c_client *mcli = NULL;
+	struct i2c_adapter *root = NULL;
+	int rc;
+
+	if (client->flags & I2C_CLIENT_TEN) {
+		dev_err(&client->dev, "failed, MCTP requires a 7-bit I2C address, addr=0x%x\n",
+			client->addr);
+		rc = -EINVAL;
+		goto err;
+	}
+
+	root = mux_root_adapter(client->adapter);
+	if (!root) {
+		dev_err(&client->dev, "failed to find root adapter\n");
+		rc = -ENOENT;
+		goto err;
+	}
+	if (root != client->adapter) {
+		dev_err(&client->dev,
+			"A mctp-i2c-controller client cannot be placed on an I2C mux adapter.\n"
+			" It should be placed on the mux tree root adapter\n"
+			" then set mctp-controller property on adapters to attach\n");
+		rc = -EINVAL;
+		goto err;
+	}
+
+	mcli = kzalloc_obj(*mcli);
+	if (!mcli) {
+		rc = -ENOMEM;
+		goto err;
+	}
+	spin_lock_init(&mcli->sel_lock);
+	INIT_LIST_HEAD(&mcli->devs);
+	INIT_LIST_HEAD(&mcli->list);
+	mcli->lladdr = client->addr & 0xff;
+	mcli->client = client;
+	i2c_set_clientdata(client, mcli);
+
+	rc = i2c_slave_register(mcli->client, mctp_i2c_slave_cb);
+	if (rc < 0) {
+		dev_err(&client->dev, "i2c register failed %d\n", rc);
+		mcli->client = NULL;
+		i2c_set_clientdata(client, NULL);
+		goto err;
+	}
+
+	return mcli;
+err:
+	if (mcli) {
+		i2c_unregister_device(mcli->client);
+		kfree(mcli);
+	}
+	return ERR_PTR(rc);
+}
+
+static void mctp_i2c_free_client(struct mctp_i2c_client *mcli)
+{
+	int rc;
+
+	WARN_ON(!mutex_is_locked(&driver_clients_lock));
+	WARN_ON(!list_empty(&mcli->devs));
+	WARN_ON(mcli->sel); /* sanity check, no locking */
+
+	rc = i2c_slave_unregister(mcli->client);
+	/* Leak if it fails, we can't propagate errors upwards */
+	if (rc < 0)
+		dev_err(&mcli->client->dev, "i2c unregister failed %d\n", rc);
+	else
+		kfree(mcli);
+}
+
+/* Switch the mctp i2c device to receive responses.
+ * Call with sel_lock held
+ */
+static void __mctp_i2c_device_select(struct mctp_i2c_client *mcli,
+				     struct mctp_i2c_dev *midev)
+{
+	assert_spin_locked(&mcli->sel_lock);
+	if (midev)
+		dev_hold(midev->ndev);
+	if (mcli->sel)
+		dev_put(mcli->sel->ndev);
+	mcli->sel = midev;
+}
+
+/* Switch the mctp i2c device to receive responses */
+static void mctp_i2c_device_select(struct mctp_i2c_client *mcli,
+				   struct mctp_i2c_dev *midev)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&mcli->sel_lock, flags);
+	__mctp_i2c_device_select(mcli, midev);
+	spin_unlock_irqrestore(&mcli->sel_lock, flags);
+}
+
+static int mctp_i2c_slave_cb(struct i2c_client *client,
+			     enum i2c_slave_event event, u8 *val)
+{
+	struct mctp_i2c_client *mcli = i2c_get_clientdata(client);
+	struct mctp_i2c_dev *midev = NULL;
+	unsigned long flags;
+	int rc = 0;
+
+	spin_lock_irqsave(&mcli->sel_lock, flags);
+	midev = mcli->sel;
+	if (midev)
+		dev_hold(midev->ndev);
+	spin_unlock_irqrestore(&mcli->sel_lock, flags);
+
+	if (!midev)
+		return 0;
+
+	switch (event) {
+	case I2C_SLAVE_READ_REQUESTED:
+	case I2C_SLAVE_READ_PROCESSED:
+		/* MCTP I2C transport only uses writes */
+		midev->rx_pos = 0;
+		*val = 0xff;
+		break;
+	case I2C_SLAVE_WRITE_RECEIVED:
+		if (midev->rx_pos < MCTP_I2C_BUFSZ) {
+			midev->rx_buffer[midev->rx_pos] = *val;
+			midev->rx_pos++;
+		} else {
+			midev->ndev->stats.rx_over_errors++;
+		}
+
+		break;
+	case I2C_SLAVE_WRITE_REQUESTED:
+		/* dest_slave as first byte */
+		midev->rx_buffer[0] = mcli->lladdr << 1;
+		midev->rx_pos = 1;
+		break;
+	case I2C_SLAVE_STOP:
+		rc = mctp_i2c_recv(midev);
+		break;
+	default:
+		break;
+	}
+
+	dev_put(midev->ndev);
+	return rc;
+}
+
+/* Processes incoming data that has been accumulated by the slave cb */
+static int mctp_i2c_recv(struct mctp_i2c_dev *midev)
+{
+	struct net_device *ndev = midev->ndev;
+	struct mctp_i2c_hdr *hdr;
+	struct mctp_skb_cb *cb;
+	struct sk_buff *skb;
+	unsigned long flags;
+	u8 pec, calc_pec;
+	size_t recvlen;
+	int status;
+
+	if (midev->rx_pos == 0)
+		return 0;
+
+	/* + 1 for the PEC */
+	if (midev->rx_pos < MCTP_I2C_MINLEN + 1) {
+		ndev->stats.rx_length_errors++;
+		return -EINVAL;
+	}
+	/* recvlen excludes PEC */
+	recvlen = midev->rx_pos - 1;
+
+	hdr = (void *)midev->rx_buffer;
+	if (hdr->command != MCTP_I2C_COMMANDCODE) {
+		ndev->stats.rx_dropped++;
+		return -EINVAL;
+	}
+
+	if (hdr->byte_count + offsetof(struct mctp_i2c_hdr, source_slave) != recvlen) {
+		ndev->stats.rx_length_errors++;
+		return -EINVAL;
+	}
+
+	pec = midev->rx_buffer[midev->rx_pos - 1];
+	calc_pec = i2c_smbus_pec(0, midev->rx_buffer, recvlen);
+	if (pec != calc_pec) {
+		ndev->stats.rx_crc_errors++;
+		return -EINVAL;
+	}
+
+	skb = netdev_alloc_skb(ndev, recvlen);
+	if (!skb) {
+		ndev->stats.rx_dropped++;
+		return -ENOMEM;
+	}
+
+	skb->protocol = htons(ETH_P_MCTP);
+	skb_put_data(skb, midev->rx_buffer, recvlen);
+	skb_reset_mac_header(skb);
+	skb_pull(skb, sizeof(struct mctp_i2c_hdr));
+	skb_reset_network_header(skb);
+
+	cb = __mctp_cb(skb);
+	cb->halen = 1;
+	cb->haddr[0] = hdr->source_slave >> 1;
+
+	/* We need to ensure that the netif is not used once netdev
+	 * unregister occurs
+	 */
+	spin_lock_irqsave(&midev->lock, flags);
+	if (midev->allow_rx) {
+		reinit_completion(&midev->rx_done);
+		spin_unlock_irqrestore(&midev->lock, flags);
+
+		status = netif_rx(skb);
+		complete(&midev->rx_done);
+	} else {
+		status = NET_RX_DROP;
+		spin_unlock_irqrestore(&midev->lock, flags);
+		kfree_skb(skb);
+	}
+
+	if (status == NET_RX_SUCCESS) {
+		ndev->stats.rx_packets++;
+		ndev->stats.rx_bytes += recvlen;
+	} else {
+		ndev->stats.rx_dropped++;
+	}
+	return 0;
+}
+
+enum mctp_i2c_flow_state {
+	MCTP_I2C_TX_FLOW_INVALID,
+	MCTP_I2C_TX_FLOW_NONE,
+	MCTP_I2C_TX_FLOW_NEW,
+	MCTP_I2C_TX_FLOW_EXISTING,
+};
+
+static enum mctp_i2c_flow_state
+mctp_i2c_get_tx_flow_state(struct mctp_i2c_dev *midev, struct sk_buff *skb)
+{
+	enum mctp_i2c_flow_state state;
+	struct mctp_sk_key *key;
+	struct mctp_flow *flow;
+	unsigned long flags;
+
+	flow = skb_ext_find(skb, SKB_EXT_MCTP);
+	if (!flow)
+		return MCTP_I2C_TX_FLOW_NONE;
+
+	key = flow->key;
+	if (!key)
+		return MCTP_I2C_TX_FLOW_NONE;
+
+	spin_lock_irqsave(&key->lock, flags);
+	/* If the key is present but invalid, we're unlikely to be able
+	 * to handle the flow at all; just drop now
+	 */
+	if (!key->valid) {
+		state = MCTP_I2C_TX_FLOW_INVALID;
+	} else {
+		switch (key->dev_flow_state) {
+		case MCTP_I2C_FLOW_STATE_NEW:
+			key->dev_flow_state = MCTP_I2C_FLOW_STATE_ACTIVE;
+			state = MCTP_I2C_TX_FLOW_NEW;
+			break;
+		case MCTP_I2C_FLOW_STATE_ACTIVE:
+			state = MCTP_I2C_TX_FLOW_EXISTING;
+			break;
+		default:
+			state = MCTP_I2C_TX_FLOW_INVALID;
+		}
+	}
+
+	spin_unlock_irqrestore(&key->lock, flags);
+
+	return state;
+}
+
+/* We're not contending with ourselves here; we only need to exclude other
+ * i2c clients from using the bus. refcounts are simply to prevent
+ * recursive locking.
+ */
+static void mctp_i2c_lock_nest(struct mctp_i2c_dev *midev)
+{
+	unsigned long flags;
+	bool lock;
+
+	spin_lock_irqsave(&midev->lock, flags);
+	lock = midev->i2c_lock_count == 0;
+	midev->i2c_lock_count++;
+	spin_unlock_irqrestore(&midev->lock, flags);
+
+	if (lock)
+		i2c_lock_bus(midev->adapter, I2C_LOCK_SEGMENT);
+}
+
+static void mctp_i2c_unlock_nest(struct mctp_i2c_dev *midev)
+{
+	unsigned long flags;
+	bool unlock;
+
+	spin_lock_irqsave(&midev->lock, flags);
+	if (!WARN_ONCE(midev->i2c_lock_count == 0, "lock count underflow!"))
+		midev->i2c_lock_count--;
+	unlock = midev->i2c_lock_count == 0;
+	spin_unlock_irqrestore(&midev->lock, flags);
+
+	if (unlock)
+		i2c_unlock_bus(midev->adapter, I2C_LOCK_SEGMENT);
+}
+
+/* Unlocks the bus if was previously locked, used for cleanup */
+static void mctp_i2c_unlock_reset(struct mctp_i2c_dev *midev)
+{
+	unsigned long flags;
+	bool unlock;
+
+	spin_lock_irqsave(&midev->lock, flags);
+	unlock = midev->i2c_lock_count > 0;
+	midev->i2c_lock_count = 0;
+	spin_unlock_irqrestore(&midev->lock, flags);
+
+	if (unlock)
+		i2c_unlock_bus(midev->adapter, I2C_LOCK_SEGMENT);
+}
+
+static void mctp_i2c_invalidate_tx_flow(struct mctp_i2c_dev *midev,
+					struct sk_buff *skb)
+{
+	struct mctp_sk_key *key;
+	struct mctp_flow *flow;
+	unsigned long flags;
+	bool release;
+
+	flow = skb_ext_find(skb, SKB_EXT_MCTP);
+	if (!flow)
+		return;
+
+	key = flow->key;
+	if (!key)
+		return;
+
+	spin_lock_irqsave(&key->lock, flags);
+	if (key->manual_alloc) {
+		/* we don't have control over lifetimes for manually-allocated
+		 * keys, so cannot assume we can invalidate all future flows
+		 * that would use this key.
+		 */
+		release = false;
+	} else {
+		release = key->dev_flow_state == MCTP_I2C_FLOW_STATE_ACTIVE;
+		key->dev_flow_state = MCTP_I2C_FLOW_STATE_INVALID;
+	}
+	spin_unlock_irqrestore(&key->lock, flags);
+
+	/* if we have changed state from active, the flow held a reference on
+	 * the lock; release that now.
+	 */
+	if (release)
+		mctp_i2c_unlock_nest(midev);
+}
+
+static void mctp_i2c_xmit(struct mctp_i2c_dev *midev, struct sk_buff *skb)
+{
+	struct net_device_stats *stats = &midev->ndev->stats;
+	enum mctp_i2c_flow_state fs;
+	struct mctp_i2c_hdr *hdr;
+	struct i2c_msg msg = {0};
+	u8 *pecp;
+	int rc;
+
+	hdr = (void *)skb_mac_header(skb);
+	/* Sanity check that packet contents matches skb length,
+	 * and can't exceed MCTP_I2C_BUFSZ
+	 */
+	if (skb->len != hdr->byte_count + 3) {
+		dev_warn_ratelimited(&midev->adapter->dev,
+				     "Bad tx length %d vs skb %u\n",
+				     hdr->byte_count + 3, skb->len);
+		return;
+	}
+
+	fs = mctp_i2c_get_tx_flow_state(midev, skb);
+
+	if (skb_tailroom(skb) >= 1) {
+		/* Linear case with space, we can just append the PEC */
+		skb_put(skb, 1);
+	} else {
+		/* Otherwise need to copy the buffer */
+		skb_copy_bits(skb, 0, midev->tx_scratch, skb->len);
+		hdr = (void *)midev->tx_scratch;
+	}
+
+	pecp = (void *)&hdr->source_slave + hdr->byte_count;
+	*pecp = i2c_smbus_pec(0, (u8 *)hdr, hdr->byte_count + 3);
+	msg.buf = (void *)&hdr->command;
+	/* command, bytecount, data, pec */
+	msg.len = 2 + hdr->byte_count + 1;
+	msg.addr = hdr->dest_slave >> 1;
+
+	switch (fs) {
+	case MCTP_I2C_TX_FLOW_NONE:
+		/* no flow: full lock & unlock */
+		mctp_i2c_lock_nest(midev);
+		mctp_i2c_device_select(midev->client, midev);
+		rc = __i2c_transfer(midev->adapter, &msg, 1);
+		mctp_i2c_unlock_nest(midev);
+		break;
+
+	case MCTP_I2C_TX_FLOW_NEW:
+		/* new flow: lock, tx, but don't unlock; that will happen
+		 * on flow release
+		 */
+		mctp_i2c_lock_nest(midev);
+		mctp_i2c_device_select(midev->client, midev);
+		fallthrough;
+
+	case MCTP_I2C_TX_FLOW_EXISTING:
+		/* existing flow: we already have the lock; just tx */
+		rc = __i2c_transfer(midev->adapter, &msg, 1);
+
+		/* on tx errors, the flow can no longer be considered valid */
+		if (rc < 0)
+			mctp_i2c_invalidate_tx_flow(midev, skb);
+
+		break;
+
+	case MCTP_I2C_TX_FLOW_INVALID:
+		return;
+	}
+
+	if (rc < 0) {
+		dev_warn_ratelimited(&midev->adapter->dev,
+				     "__i2c_transfer failed %d\n", rc);
+		stats->tx_errors++;
+	} else {
+		stats->tx_bytes += skb->len;
+		stats->tx_packets++;
+	}
+}
+
+static void mctp_i2c_flow_release(struct mctp_i2c_dev *midev)
+{
+	unsigned long flags;
+	bool unlock;
+
+	spin_lock_irqsave(&midev->lock, flags);
+	if (midev->release_count > midev->i2c_lock_count) {
+		WARN_ONCE(1, "release count overflow");
+		midev->release_count = midev->i2c_lock_count;
+	}
+
+	midev->i2c_lock_count -= midev->release_count;
+	unlock = midev->i2c_lock_count == 0 && midev->release_count > 0;
+	midev->release_count = 0;
+	spin_unlock_irqrestore(&midev->lock, flags);
+
+	if (unlock)
+		i2c_unlock_bus(midev->adapter, I2C_LOCK_SEGMENT);
+}
+
+static int mctp_i2c_header_create(struct sk_buff *skb, struct net_device *dev,
+				  unsigned short type, const void *daddr,
+	   const void *saddr, unsigned int len)
+{
+	struct mctp_i2c_hdr *hdr;
+	struct mctp_hdr *mhdr;
+	u8 lldst, llsrc;
+	int rc;
+
+	if (len > MCTP_I2C_MAXMTU)
+		return -EMSGSIZE;
+
+	if (!daddr || !saddr)
+		return -EINVAL;
+
+	lldst = *((u8 *)daddr);
+	llsrc = *((u8 *)saddr);
+
+	rc = skb_cow_head(skb, sizeof(struct mctp_i2c_hdr));
+	if (rc)
+		return rc;
+
+	skb_push(skb, sizeof(struct mctp_i2c_hdr));
+	skb_reset_mac_header(skb);
+	hdr = (void *)skb_mac_header(skb);
+	mhdr = mctp_hdr(skb);
+	hdr->dest_slave = (lldst << 1) & 0xff;
+	hdr->command = MCTP_I2C_COMMANDCODE;
+	hdr->byte_count = len + 1;
+	hdr->source_slave = ((llsrc << 1) & 0xff) | 0x01;
+	mhdr->ver = 0x01;
+
+	return sizeof(struct mctp_i2c_hdr);
+}
+
+static int mctp_i2c_tx_thread(void *data)
+{
+	struct mctp_i2c_dev *midev = data;
+	struct sk_buff *skb;
+	unsigned long flags;
+
+	for (;;) {
+		if (kthread_should_stop())
+			break;
+
+		spin_lock_irqsave(&midev->tx_queue.lock, flags);
+		skb = __skb_dequeue(&midev->tx_queue);
+		if (netif_queue_stopped(midev->ndev))
+			netif_wake_queue(midev->ndev);
+		spin_unlock_irqrestore(&midev->tx_queue.lock, flags);
+
+		if (skb == &midev->unlock_marker) {
+			mctp_i2c_flow_release(midev);
+
+		} else if (skb) {
+			mctp_i2c_xmit(midev, skb);
+			kfree_skb(skb);
+
+		} else {
+			wait_event_idle(midev->tx_wq,
+					!skb_queue_empty(&midev->tx_queue) ||
+				   kthread_should_stop());
+		}
+	}
+
+	return 0;
+}
+
+static netdev_tx_t mctp_i2c_start_xmit(struct sk_buff *skb,
+				       struct net_device *dev)
+{
+	struct mctp_i2c_dev *midev = netdev_priv(dev);
+	unsigned long flags;
+
+	spin_lock_irqsave(&midev->tx_queue.lock, flags);
+	if (skb_queue_len(&midev->tx_queue) >= MCTP_I2C_TX_WORK_LEN) {
+		netif_stop_queue(dev);
+		spin_unlock_irqrestore(&midev->tx_queue.lock, flags);
+		netdev_err(dev, "BUG! Tx Ring full when queue awake!\n");
+		return NETDEV_TX_BUSY;
+	}
+
+	__skb_queue_tail(&midev->tx_queue, skb);
+	if (skb_queue_len(&midev->tx_queue) == MCTP_I2C_TX_WORK_LEN)
+		netif_stop_queue(dev);
+	spin_unlock_irqrestore(&midev->tx_queue.lock, flags);
+
+	wake_up(&midev->tx_wq);
+	return NETDEV_TX_OK;
+}
+
+static void mctp_i2c_release_flow(struct mctp_dev *mdev,
+				  struct mctp_sk_key *key)
+
+{
+	struct mctp_i2c_dev *midev = netdev_priv(mdev->dev);
+	bool queue_release = false;
+	unsigned long flags;
+
+	spin_lock_irqsave(&midev->lock, flags);
+	/* if we have seen the flow/key previously, we need to pair the
+	 * original lock with a release
+	 */
+	if (key->dev_flow_state == MCTP_I2C_FLOW_STATE_ACTIVE) {
+		midev->release_count++;
+		queue_release = true;
+	}
+	key->dev_flow_state = MCTP_I2C_FLOW_STATE_INVALID;
+	spin_unlock_irqrestore(&midev->lock, flags);
+
+	if (queue_release) {
+		/* Ensure we have a release operation queued, through the fake
+		 * marker skb
+		 */
+		spin_lock(&midev->tx_queue.lock);
+		if (!midev->unlock_marker.next)
+			__skb_queue_tail(&midev->tx_queue,
+					 &midev->unlock_marker);
+		spin_unlock(&midev->tx_queue.lock);
+		wake_up(&midev->tx_wq);
+	}
+}
+
+static const struct net_device_ops mctp_i2c_ops = {
+	.ndo_start_xmit = mctp_i2c_start_xmit,
+	.ndo_uninit = mctp_i2c_ndo_uninit,
+	.ndo_open = mctp_i2c_ndo_open,
+};
+
+static const struct header_ops mctp_i2c_headops = {
+	.create = mctp_i2c_header_create,
+};
+
+static const struct mctp_netdev_ops mctp_i2c_mctp_ops = {
+	.release_flow = mctp_i2c_release_flow,
+};
+
+static void mctp_i2c_net_setup(struct net_device *dev)
+{
+	dev->type = ARPHRD_MCTP;
+
+	dev->mtu = MCTP_I2C_MAXMTU;
+	dev->min_mtu = MCTP_I2C_MINMTU;
+	dev->max_mtu = MCTP_I2C_MAXMTU;
+	dev->tx_queue_len = MCTP_I2C_TX_QUEUE_LEN;
+
+	dev->hard_header_len = sizeof(struct mctp_i2c_hdr);
+	dev->addr_len = 1;
+
+	dev->netdev_ops		= &mctp_i2c_ops;
+	dev->header_ops		= &mctp_i2c_headops;
+}
+
+/* Populates the mctp_i2c_dev priv struct for a netdev.
+ * Returns an error pointer on failure.
+ */
+static struct mctp_i2c_dev *mctp_i2c_midev_init(struct net_device *dev,
+						struct mctp_i2c_client *mcli,
+						struct i2c_adapter *adap)
+{
+	struct mctp_i2c_dev *midev = netdev_priv(dev);
+	unsigned long flags;
+
+	midev->tx_thread = kthread_create(mctp_i2c_tx_thread, midev,
+					  "%s/tx", dev->name);
+	if (IS_ERR(midev->tx_thread))
+		return ERR_CAST(midev->tx_thread);
+
+	midev->ndev = dev;
+	get_device(&adap->dev);
+	midev->adapter = adap;
+	get_device(&mcli->client->dev);
+	midev->client = mcli;
+	INIT_LIST_HEAD(&midev->list);
+	spin_lock_init(&midev->lock);
+	midev->i2c_lock_count = 0;
+	midev->release_count = 0;
+	init_completion(&midev->rx_done);
+	complete(&midev->rx_done);
+	init_waitqueue_head(&midev->tx_wq);
+	skb_queue_head_init(&midev->tx_queue);
+
+	/* Add to the parent mcli */
+	spin_lock_irqsave(&mcli->sel_lock, flags);
+	list_add(&midev->list, &mcli->devs);
+	/* Select a device by default */
+	if (!mcli->sel)
+		__mctp_i2c_device_select(mcli, midev);
+	spin_unlock_irqrestore(&mcli->sel_lock, flags);
+
+	/* Start the worker thread */
+	wake_up_process(midev->tx_thread);
+
+	return midev;
+}
+
+/* Counterpart of mctp_i2c_midev_init */
+static void mctp_i2c_midev_free(struct mctp_i2c_dev *midev)
+{
+	struct mctp_i2c_client *mcli = midev->client;
+	unsigned long flags;
+
+	if (midev->tx_thread) {
+		kthread_stop(midev->tx_thread);
+		midev->tx_thread = NULL;
+	}
+
+	/* Unconditionally unlock on close */
+	mctp_i2c_unlock_reset(midev);
+
+	/* Remove the netdev from the parent i2c client. */
+	spin_lock_irqsave(&mcli->sel_lock, flags);
+	list_del(&midev->list);
+	if (mcli->sel == midev) {
+		struct mctp_i2c_dev *first;
+
+		first = list_first_entry_or_null(&mcli->devs, struct mctp_i2c_dev, list);
+		__mctp_i2c_device_select(mcli, first);
+	}
+	spin_unlock_irqrestore(&mcli->sel_lock, flags);
+
+	skb_queue_purge(&midev->tx_queue);
+	put_device(&midev->adapter->dev);
+	put_device(&mcli->client->dev);
+}
+
+/* Stops, unregisters, and frees midev */
+static void mctp_i2c_unregister(struct mctp_i2c_dev *midev)
+{
+	unsigned long flags;
+
+	/* Stop tx thread prior to unregister, it uses netif_() functions */
+	kthread_stop(midev->tx_thread);
+	midev->tx_thread = NULL;
+
+	/* Prevent any new rx in mctp_i2c_recv(), let any pending work finish */
+	spin_lock_irqsave(&midev->lock, flags);
+	midev->allow_rx = false;
+	spin_unlock_irqrestore(&midev->lock, flags);
+	wait_for_completion(&midev->rx_done);
+
+	mctp_unregister_netdev(midev->ndev);
+	/* midev has been freed now by mctp_i2c_ndo_uninit callback */
+
+	free_netdev(midev->ndev);
+}
+
+static void mctp_i2c_ndo_uninit(struct net_device *dev)
+{
+	struct mctp_i2c_dev *midev = netdev_priv(dev);
+
+	/* Perform cleanup here to ensure that mcli->sel isn't holding
+	 * a reference that would prevent unregister_netdevice()
+	 * from completing.
+	 */
+	mctp_i2c_midev_free(midev);
+}
+
+static int mctp_i2c_ndo_open(struct net_device *dev)
+{
+	struct mctp_i2c_dev *midev = netdev_priv(dev);
+	unsigned long flags;
+
+	/* i2c rx handler can only pass packets once the netdev is registered */
+	spin_lock_irqsave(&midev->lock, flags);
+	midev->allow_rx = true;
+	spin_unlock_irqrestore(&midev->lock, flags);
+
+	return 0;
+}
+
+static int mctp_i2c_add_netdev(struct mctp_i2c_client *mcli,
+			       struct i2c_adapter *adap)
+{
+	struct mctp_i2c_dev *midev = NULL;
+	struct net_device *ndev = NULL;
+	struct i2c_adapter *root;
+	unsigned long flags;
+	char namebuf[30];
+	int rc;
+
+	root = mux_root_adapter(adap);
+	if (root != mcli->client->adapter) {
+		dev_err(&mcli->client->dev,
+			"I2C adapter %s is not a child bus of %s\n",
+			mcli->client->adapter->name, root->name);
+		return -EINVAL;
+	}
+
+	WARN_ON(!mutex_is_locked(&driver_clients_lock));
+	snprintf(namebuf, sizeof(namebuf), "mctpi2c%d", adap->nr);
+	ndev = alloc_netdev(sizeof(*midev), namebuf, NET_NAME_ENUM, mctp_i2c_net_setup);
+	if (!ndev) {
+		dev_err(&mcli->client->dev, "alloc netdev failed\n");
+		rc = -ENOMEM;
+		goto err;
+	}
+	dev_net_set(ndev, current->nsproxy->net_ns);
+	SET_NETDEV_DEV(ndev, &adap->dev);
+	dev_addr_set(ndev, &mcli->lladdr);
+
+	midev = mctp_i2c_midev_init(ndev, mcli, adap);
+	if (IS_ERR(midev)) {
+		rc = PTR_ERR(midev);
+		midev = NULL;
+		goto err;
+	}
+
+	rc = mctp_register_netdev(ndev, &mctp_i2c_mctp_ops,
+				  MCTP_PHYS_BINDING_SMBUS);
+	if (rc < 0) {
+		dev_err(&mcli->client->dev,
+			"register netdev \"%s\" failed %d\n",
+			ndev->name, rc);
+		goto err;
+	}
+
+	spin_lock_irqsave(&midev->lock, flags);
+	midev->allow_rx = false;
+	spin_unlock_irqrestore(&midev->lock, flags);
+
+	return 0;
+err:
+	if (midev)
+		mctp_i2c_midev_free(midev);
+	if (ndev)
+		free_netdev(ndev);
+	return rc;
+}
+
+/* Removes any netdev for adap. mcli is the parent root i2c client */
+static void mctp_i2c_remove_netdev(struct mctp_i2c_client *mcli,
+				   struct i2c_adapter *adap)
+{
+	struct mctp_i2c_dev *midev = NULL, *m = NULL;
+	unsigned long flags;
+
+	WARN_ON(!mutex_is_locked(&driver_clients_lock));
+	spin_lock_irqsave(&mcli->sel_lock, flags);
+	/* List size is limited by number of MCTP netdevs on a single hardware bus */
+	list_for_each_entry(m, &mcli->devs, list)
+		if (m->adapter == adap) {
+			midev = m;
+			break;
+		}
+	spin_unlock_irqrestore(&mcli->sel_lock, flags);
+
+	if (midev)
+		mctp_i2c_unregister(midev);
+}
+
+/* Determines whether a device is an i2c adapter.
+ * Optionally returns the root i2c_adapter
+ */
+static struct i2c_adapter *mctp_i2c_get_adapter(struct device *dev,
+						struct i2c_adapter **ret_root)
+{
+	struct i2c_adapter *root, *adap;
+
+	if (dev->type != &i2c_adapter_type)
+		return NULL;
+	adap = to_i2c_adapter(dev);
+	root = mux_root_adapter(adap);
+	WARN_ONCE(!root, "MCTP I2C failed to find root adapter for %s\n",
+		  dev_name(dev));
+	if (!root)
+		return NULL;
+	if (ret_root)
+		*ret_root = root;
+	return adap;
+}
+
+/* Determines whether a device is an i2c adapter with the "mctp-controller"
+ * devicetree property set. If adap is not an OF node, returns match_no_of
+ */
+static bool mctp_i2c_adapter_match(struct i2c_adapter *adap, bool match_no_of)
+{
+	if (!adap->dev.of_node)
+		return match_no_of;
+	return of_property_read_bool(adap->dev.of_node, MCTP_I2C_OF_PROP);
+}
+
+/* Called for each existing i2c device (adapter or client) when a
+ * new mctp-i2c client is probed.
+ */
+static int mctp_i2c_client_try_attach(struct device *dev, void *data)
+{
+	struct i2c_adapter *adap = NULL, *root = NULL;
+	struct mctp_i2c_client *mcli = data;
+
+	adap = mctp_i2c_get_adapter(dev, &root);
+	if (!adap)
+		return 0;
+	if (mcli->client->adapter != root)
+		return 0;
+	/* Must either have mctp-controller property on the adapter, or
+	 * be a root adapter if it's non-devicetree
+	 */
+	if (!mctp_i2c_adapter_match(adap, adap == root))
+		return 0;
+
+	return mctp_i2c_add_netdev(mcli, adap);
+}
+
+static void mctp_i2c_notify_add(struct device *dev)
+{
+	struct mctp_i2c_client *mcli = NULL, *m = NULL;
+	struct i2c_adapter *root = NULL, *adap = NULL;
+	int rc;
+
+	adap = mctp_i2c_get_adapter(dev, &root);
+	if (!adap)
+		return;
+	/* Check for mctp-controller property on the adapter */
+	if (!mctp_i2c_adapter_match(adap, false))
+		return;
+
+	/* Find an existing mcli for adap's root */
+	mutex_lock(&driver_clients_lock);
+	list_for_each_entry(m, &driver_clients, list) {
+		if (m->client->adapter == root) {
+			mcli = m;
+			break;
+		}
+	}
+
+	if (mcli) {
+		rc = mctp_i2c_add_netdev(mcli, adap);
+		if (rc < 0)
+			dev_warn(dev, "Failed adding mctp-i2c net device\n");
+	}
+	mutex_unlock(&driver_clients_lock);
+}
+
+static void mctp_i2c_notify_del(struct device *dev)
+{
+	struct i2c_adapter *root = NULL, *adap = NULL;
+	struct mctp_i2c_client *mcli = NULL;
+
+	adap = mctp_i2c_get_adapter(dev, &root);
+	if (!adap)
+		return;
+
+	mutex_lock(&driver_clients_lock);
+	list_for_each_entry(mcli, &driver_clients, list) {
+		if (mcli->client->adapter == root) {
+			mctp_i2c_remove_netdev(mcli, adap);
+			break;
+		}
+	}
+	mutex_unlock(&driver_clients_lock);
+}
+
+static int mctp_i2c_probe(struct i2c_client *client)
+{
+	struct mctp_i2c_client *mcli = NULL;
+	int rc;
+
+	mutex_lock(&driver_clients_lock);
+	mcli = mctp_i2c_new_client(client);
+	if (IS_ERR(mcli)) {
+		rc = PTR_ERR(mcli);
+		mcli = NULL;
+		goto out;
+	} else {
+		list_add(&mcli->list, &driver_clients);
+	}
+
+	/* Add a netdev for adapters that have a 'mctp-controller' property */
+	i2c_for_each_dev(mcli, mctp_i2c_client_try_attach);
+	rc = 0;
+out:
+	mutex_unlock(&driver_clients_lock);
+	return rc;
+}
+
+static void mctp_i2c_remove(struct i2c_client *client)
+{
+	struct mctp_i2c_client *mcli = i2c_get_clientdata(client);
+	struct mctp_i2c_dev *midev = NULL, *tmp = NULL;
+
+	mutex_lock(&driver_clients_lock);
+	list_del(&mcli->list);
+	/* Remove all child adapter netdevs */
+	list_for_each_entry_safe(midev, tmp, &mcli->devs, list)
+		mctp_i2c_unregister(midev);
+
+	mctp_i2c_free_client(mcli);
+	mutex_unlock(&driver_clients_lock);
+}
+
+/* We look for a 'mctp-controller' property on I2C busses as they are
+ * added/deleted, creating/removing netdevs as required.
+ */
+static int mctp_i2c_notifier_call(struct notifier_block *nb,
+				  unsigned long action, void *data)
+{
+	struct device *dev = data;
+
+	switch (action) {
+	case BUS_NOTIFY_ADD_DEVICE:
+		mctp_i2c_notify_add(dev);
+		break;
+	case BUS_NOTIFY_DEL_DEVICE:
+		mctp_i2c_notify_del(dev);
+		break;
+	}
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block mctp_i2c_notifier = {
+	.notifier_call = mctp_i2c_notifier_call,
+};
+
+static const struct i2c_device_id mctp_i2c_id[] = {
+	{ .name = "mctp-i2c-interface" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, mctp_i2c_id);
+
+static const struct of_device_id mctp_i2c_of_match[] = {
+	{ .compatible = "mctp-i2c-controller" },
+	{},
+};
+MODULE_DEVICE_TABLE(of, mctp_i2c_of_match);
+
+static struct i2c_driver mctp_i2c_driver = {
+	.driver = {
+		.name = "mctp-i2c-interface",
+		.of_match_table = mctp_i2c_of_match,
+	},
+	.probe = mctp_i2c_probe,
+	.remove = mctp_i2c_remove,
+	.id_table = mctp_i2c_id,
+};
+
+static __init int mctp_i2c_mod_init(void)
+{
+	int rc;
+
+	pr_info("MCTP I2C interface driver\n");
+	rc = i2c_add_driver(&mctp_i2c_driver);
+	if (rc < 0)
+		return rc;
+	rc = bus_register_notifier(&i2c_bus_type, &mctp_i2c_notifier);
+	if (rc < 0) {
+		i2c_del_driver(&mctp_i2c_driver);
+		return rc;
+	}
+	return 0;
+}
+
+static __exit void mctp_i2c_mod_exit(void)
+{
+	int rc;
+
+	rc = bus_unregister_notifier(&i2c_bus_type, &mctp_i2c_notifier);
+	if (rc < 0)
+		pr_warn("MCTP I2C could not unregister notifier, %d\n", rc);
+	i2c_del_driver(&mctp_i2c_driver);
+}
+
+module_init(mctp_i2c_mod_init);
+module_exit(mctp_i2c_mod_exit);
+
+MODULE_DESCRIPTION("MCTP I2C device");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Matt Johnston <matt@codeconstruct.com.au>");
diff --git a/drivers/net/mctp/mctp-i3c.c b/drivers/net/mctp/mctp-i3c.c
new file mode 100644
index 000000000..4e857dd5d
--- /dev/null
+++ b/drivers/net/mctp/mctp-i3c.c
@@ -0,0 +1,773 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Implements DMTF specification
+ * "DSP0233 Management Component Transport Protocol (MCTP) I3C Transport
+ * Binding"
+ * https://www.dmtf.org/sites/default/files/standards/documents/DSP0233_1.0.0.pdf
+ *
+ * Copyright (c) 2023 Code Construct
+ */
+
+#include <linux/module.h>
+#include <linux/netdevice.h>
+#include <linux/i3c/device.h>
+#include <linux/i3c/master.h>
+#include <linux/if_arp.h>
+#include <linux/unaligned.h>
+#include <net/mctp.h>
+#include <net/mctpdevice.h>
+
+#define MCTP_I3C_MAXBUF 65536
+/* 48 bit Provisioned Id */
+#define PID_SIZE 6
+
+/* 64 byte payload, 4 byte MCTP header */
+static const int MCTP_I3C_MINMTU = 64 + 4;
+/* One byte less to allow for the PEC */
+static const int MCTP_I3C_MAXMTU = MCTP_I3C_MAXBUF - 1;
+/* 4 byte MCTP header, no data, 1 byte PEC */
+static const int MCTP_I3C_MINLEN = 4 + 1;
+
+/* Sufficient for 64kB at min mtu */
+static const int MCTP_I3C_TX_QUEUE_LEN = 1100;
+
+/* Somewhat arbitrary */
+static const int MCTP_I3C_IBI_SLOTS = 8;
+
+/* Mandatory Data Byte in an IBI, from DSP0233 */
+#define I3C_MDB_MCTP 0xAE
+/* From MIPI Device Characteristics Register (DCR) Assignments */
+#define I3C_DCR_MCTP 0xCC
+
+static const char *MCTP_I3C_OF_PROP = "mctp-controller";
+
+/* List of mctp_i3c_busdev */
+static LIST_HEAD(busdevs);
+/* Protects busdevs, as well as mctp_i3c_bus.devs lists */
+static DEFINE_MUTEX(busdevs_lock);
+
+struct mctp_i3c_bus {
+	struct net_device *ndev;
+
+	struct task_struct *tx_thread;
+	wait_queue_head_t tx_wq;
+	/* tx_lock protects tx_skb and devs */
+	spinlock_t tx_lock;
+	/* Next skb to transmit */
+	struct sk_buff *tx_skb;
+	/* Scratch buffer for xmit */
+	u8 tx_scratch[MCTP_I3C_MAXBUF];
+
+	/* Element of busdevs */
+	struct list_head list;
+
+	/* Provisioned ID of our controller */
+	u64 pid;
+
+	struct i3c_bus *bus;
+	/* Head of mctp_i3c_device.list. Protected by busdevs_lock */
+	struct list_head devs;
+};
+
+struct mctp_i3c_device {
+	struct i3c_device *i3c;
+	struct mctp_i3c_bus *mbus;
+	struct list_head list; /* Element of mctp_i3c_bus.devs */
+
+	/* Held while tx_thread is using this device */
+	struct mutex lock;
+
+	/* Whether BCR indicates MDB is present in IBI */
+	bool have_mdb;
+	/* I3C dynamic address */
+	u8 addr;
+	/* Maximum read length */
+	u16 mrl;
+	/* Maximum write length */
+	u16 mwl;
+	/* Provisioned ID */
+	u64 pid;
+};
+
+/* We synthesise a mac header using the Provisioned ID.
+ * Used to pass dest to mctp_i3c_start_xmit.
+ */
+struct mctp_i3c_internal_hdr {
+	u8 dest[PID_SIZE];
+	u8 source[PID_SIZE];
+} __packed;
+
+static int mctp_i3c_read(struct mctp_i3c_device *mi)
+{
+	struct i3c_xfer xfer = { .rnw = 1, .len = mi->mrl };
+	struct net_device_stats *stats = &mi->mbus->ndev->stats;
+	struct mctp_i3c_internal_hdr *ihdr = NULL;
+	struct sk_buff *skb = NULL;
+	struct mctp_skb_cb *cb;
+	int net_status, rc;
+	u8 pec, addr;
+
+	skb = netdev_alloc_skb(mi->mbus->ndev,
+			       mi->mrl + sizeof(struct mctp_i3c_internal_hdr));
+	if (!skb) {
+		stats->rx_dropped++;
+		rc = -ENOMEM;
+		goto err;
+	}
+
+	skb->protocol = htons(ETH_P_MCTP);
+	/* Create a header for internal use */
+	skb_reset_mac_header(skb);
+	ihdr = skb_put(skb, sizeof(struct mctp_i3c_internal_hdr));
+	put_unaligned_be48(mi->pid, ihdr->source);
+	put_unaligned_be48(mi->mbus->pid, ihdr->dest);
+	skb_pull(skb, sizeof(struct mctp_i3c_internal_hdr));
+
+	xfer.data.in = skb_put(skb, mi->mrl);
+
+	/* Make sure netif_rx() is read in the same order as i3c. */
+	mutex_lock(&mi->lock);
+	rc = i3c_device_do_xfers(mi->i3c, &xfer, 1, I3C_SDR);
+	if (rc < 0)
+		goto err;
+
+	if (WARN_ON_ONCE(xfer.len > mi->mrl)) {
+		/* Bad i3c bus driver */
+		rc = -EIO;
+		goto err;
+	}
+	if (xfer.len < MCTP_I3C_MINLEN) {
+		stats->rx_length_errors++;
+		rc = -EIO;
+		goto err;
+	}
+
+	/* check PEC, including address byte */
+	addr = mi->addr << 1 | 1;
+	pec = i2c_smbus_pec(0, &addr, 1);
+	pec = i2c_smbus_pec(pec, xfer.data.in, xfer.len - 1);
+	if (pec != ((u8 *)xfer.data.in)[xfer.len - 1]) {
+		stats->rx_crc_errors++;
+		rc = -EINVAL;
+		goto err;
+	}
+
+	/* Remove PEC */
+	skb_trim(skb, xfer.len - 1);
+
+	cb = __mctp_cb(skb);
+	cb->halen = PID_SIZE;
+	put_unaligned_be48(mi->pid, cb->haddr);
+
+	net_status = netif_rx(skb);
+
+	if (net_status == NET_RX_SUCCESS) {
+		stats->rx_packets++;
+		stats->rx_bytes += xfer.len - 1;
+	} else {
+		stats->rx_dropped++;
+	}
+
+	mutex_unlock(&mi->lock);
+	return 0;
+err:
+	mutex_unlock(&mi->lock);
+	kfree_skb(skb);
+	return rc;
+}
+
+static void mctp_i3c_ibi_handler(struct i3c_device *i3c,
+				 const struct i3c_ibi_payload *payload)
+{
+	struct mctp_i3c_device *mi = i3cdev_get_drvdata(i3c);
+
+	if (WARN_ON_ONCE(!mi))
+		return;
+
+	if (mi->have_mdb) {
+		if (payload->len > 0) {
+			if (((u8 *)payload->data)[0] != I3C_MDB_MCTP) {
+				/* Not a mctp-i3c interrupt, ignore it */
+				return;
+			}
+		} else {
+			/* The BCR advertised a Mandatory Data Byte but the
+			 * device didn't send one.
+			 */
+			dev_warn_once(i3cdev_to_dev(i3c), "IBI with missing MDB");
+		}
+	}
+
+	mctp_i3c_read(mi);
+}
+
+static int mctp_i3c_setup(struct mctp_i3c_device *mi)
+{
+	const struct i3c_ibi_setup ibi = {
+		.max_payload_len = 1,
+		.num_slots = MCTP_I3C_IBI_SLOTS,
+		.handler = mctp_i3c_ibi_handler,
+	};
+	struct i3c_device_info info;
+	int rc;
+
+	i3c_device_get_info(mi->i3c, &info);
+	mi->have_mdb = info.bcr & BIT(2);
+	mi->addr = info.dyn_addr;
+	mi->mwl = info.max_write_len;
+	mi->mrl = info.max_read_len;
+	mi->pid = info.pid;
+
+	rc = i3c_device_request_ibi(mi->i3c, &ibi);
+	if (rc == -ENOTSUPP) {
+		/* This driver only supports In-Band Interrupt mode.
+		 * Support for Polling Mode could be added if required.
+		 * (ENOTSUPP is from the i3c layer, not EOPNOTSUPP).
+		 */
+		dev_warn(i3cdev_to_dev(mi->i3c),
+			 "Failed, bus driver doesn't support In-Band Interrupts");
+		goto err;
+	} else if (rc < 0) {
+		dev_err(i3cdev_to_dev(mi->i3c),
+			"Failed requesting IBI (%d)\n", rc);
+		goto err;
+	}
+
+	rc = i3c_device_enable_ibi(mi->i3c);
+	if (rc < 0) {
+		/* Assume a driver supporting request_ibi also
+		 * supports enable_ibi.
+		 */
+		dev_err(i3cdev_to_dev(mi->i3c), "Failed enabling IBI (%d)\n", rc);
+		goto err_free_ibi;
+	}
+
+	return 0;
+
+err_free_ibi:
+	i3c_device_free_ibi(mi->i3c);
+
+err:
+	return rc;
+}
+
+/* Adds a new MCTP i3c_device to a bus */
+static int mctp_i3c_add_device(struct mctp_i3c_bus *mbus,
+			       struct i3c_device *i3c)
+__must_hold(&busdevs_lock)
+{
+	struct mctp_i3c_device *mi = NULL;
+	int rc;
+
+	mi = kzalloc_obj(*mi);
+	if (!mi) {
+		rc = -ENOMEM;
+		goto err;
+	}
+	mi->mbus = mbus;
+	mi->i3c = i3c;
+	mutex_init(&mi->lock);
+	list_add(&mi->list, &mbus->devs);
+
+	i3cdev_set_drvdata(i3c, mi);
+	rc = mctp_i3c_setup(mi);
+	if (rc < 0)
+		goto err_free;
+
+	return 0;
+
+err_free:
+	list_del(&mi->list);
+	kfree(mi);
+
+err:
+	dev_warn(i3cdev_to_dev(i3c), "Error adding mctp-i3c device, %d\n", rc);
+	return rc;
+}
+
+static int mctp_i3c_probe(struct i3c_device *i3c)
+{
+	struct mctp_i3c_bus *b = NULL, *mbus = NULL;
+	int rc;
+
+	/* Look for a known bus */
+	mutex_lock(&busdevs_lock);
+	list_for_each_entry(b, &busdevs, list)
+		if (b->bus == i3c->bus) {
+			mbus = b;
+			break;
+		}
+
+	if (!mbus) {
+		/* probably no "mctp-controller" property on the i3c bus */
+		rc = -ENODEV;
+	} else {
+		rc = mctp_i3c_add_device(mbus, i3c);
+	}
+	mutex_unlock(&busdevs_lock);
+
+	return rc;
+}
+
+static void mctp_i3c_remove_device(struct mctp_i3c_device *mi)
+__must_hold(&busdevs_lock)
+{
+	/* Ensure the tx thread isn't using the device */
+	mutex_lock(&mi->lock);
+
+	/* Counterpart of mctp_i3c_setup */
+	i3c_device_disable_ibi(mi->i3c);
+	i3c_device_free_ibi(mi->i3c);
+
+	/* Counterpart of mctp_i3c_add_device */
+	i3cdev_set_drvdata(mi->i3c, NULL);
+	list_del(&mi->list);
+
+	/* Safe to unlock after removing from the list */
+	mutex_unlock(&mi->lock);
+	kfree(mi);
+}
+
+static void mctp_i3c_remove(struct i3c_device *i3c)
+{
+	struct mctp_i3c_device *mi = i3cdev_get_drvdata(i3c);
+
+	/* We my have received a Bus Remove notify prior to device remove,
+	 * so mi will already be removed.
+	 */
+	if (!mi)
+		return;
+
+	mutex_lock(&busdevs_lock);
+	mctp_i3c_remove_device(mi);
+	mutex_unlock(&busdevs_lock);
+}
+
+/* Returns the device for an address, with mi->lock held */
+static struct mctp_i3c_device *
+mctp_i3c_lookup(struct mctp_i3c_bus *mbus, u64 pid)
+{
+	struct mctp_i3c_device *mi = NULL, *ret = NULL;
+
+	mutex_lock(&busdevs_lock);
+	list_for_each_entry(mi, &mbus->devs, list)
+		if (mi->pid == pid) {
+			ret = mi;
+			mutex_lock(&mi->lock);
+			break;
+		}
+	mutex_unlock(&busdevs_lock);
+	return ret;
+}
+
+static void mctp_i3c_xmit(struct mctp_i3c_bus *mbus, struct sk_buff *skb)
+{
+	struct net_device_stats *stats = &mbus->ndev->stats;
+	struct i3c_xfer xfer = { .rnw = false };
+	struct mctp_i3c_internal_hdr *ihdr = NULL;
+	struct mctp_i3c_device *mi = NULL;
+	unsigned int data_len;
+	u8 *data = NULL;
+	u8 addr, pec;
+	int rc = 0;
+	u64 pid;
+
+	skb_pull(skb, sizeof(struct mctp_i3c_internal_hdr));
+	data_len = skb->len;
+
+	ihdr = (void *)skb_mac_header(skb);
+
+	pid = get_unaligned_be48(ihdr->dest);
+	mi = mctp_i3c_lookup(mbus, pid);
+	if (!mi) {
+		/* I3C endpoint went away after the packet was enqueued? */
+		stats->tx_dropped++;
+		goto out;
+	}
+
+	if (WARN_ON_ONCE(data_len + 1 > MCTP_I3C_MAXBUF))
+		goto out;
+
+	if (data_len + 1 > (unsigned int)mi->mwl) {
+		/* Route MTU was larger than supported by the endpoint */
+		stats->tx_dropped++;
+		goto out;
+	}
+
+	/* Need a linear buffer with space for the PEC */
+	xfer.len = data_len + 1;
+	if (skb_tailroom(skb) >= 1) {
+		skb_put(skb, 1);
+		data = skb->data;
+	} else {
+		/* Otherwise need to copy the buffer */
+		skb_copy_bits(skb, 0, mbus->tx_scratch, skb->len);
+		data = mbus->tx_scratch;
+	}
+
+	/* PEC calculation */
+	addr = mi->addr << 1;
+	pec = i2c_smbus_pec(0, &addr, 1);
+	pec = i2c_smbus_pec(pec, data, data_len);
+	data[data_len] = pec;
+
+	xfer.data.out = data;
+	rc = i3c_device_do_xfers(mi->i3c, &xfer, 1, I3C_SDR);
+	if (rc == 0) {
+		stats->tx_bytes += data_len;
+		stats->tx_packets++;
+	} else {
+		stats->tx_errors++;
+	}
+
+out:
+	if (mi)
+		mutex_unlock(&mi->lock);
+}
+
+static int mctp_i3c_tx_thread(void *data)
+{
+	struct mctp_i3c_bus *mbus = data;
+	struct sk_buff *skb;
+
+	for (;;) {
+		if (kthread_should_stop())
+			break;
+
+		spin_lock_bh(&mbus->tx_lock);
+		skb = mbus->tx_skb;
+		mbus->tx_skb = NULL;
+		spin_unlock_bh(&mbus->tx_lock);
+
+		if (netif_queue_stopped(mbus->ndev))
+			netif_wake_queue(mbus->ndev);
+
+		if (skb) {
+			mctp_i3c_xmit(mbus, skb);
+			kfree_skb(skb);
+		} else {
+			wait_event_idle(mbus->tx_wq,
+					mbus->tx_skb || kthread_should_stop());
+		}
+	}
+
+	return 0;
+}
+
+static netdev_tx_t mctp_i3c_start_xmit(struct sk_buff *skb,
+				       struct net_device *ndev)
+{
+	struct mctp_i3c_bus *mbus = netdev_priv(ndev);
+	netdev_tx_t ret;
+
+	spin_lock(&mbus->tx_lock);
+	netif_stop_queue(ndev);
+	if (mbus->tx_skb) {
+		dev_warn_ratelimited(&ndev->dev, "TX with queue stopped");
+		ret = NETDEV_TX_BUSY;
+	} else {
+		mbus->tx_skb = skb;
+		ret = NETDEV_TX_OK;
+	}
+	spin_unlock(&mbus->tx_lock);
+
+	if (ret == NETDEV_TX_OK)
+		wake_up(&mbus->tx_wq);
+
+	return ret;
+}
+
+static void mctp_i3c_bus_free(struct mctp_i3c_bus *mbus)
+__must_hold(&busdevs_lock)
+{
+	struct mctp_i3c_device *mi = NULL, *tmp = NULL;
+
+	if (mbus->tx_thread) {
+		kthread_stop(mbus->tx_thread);
+		mbus->tx_thread = NULL;
+	}
+
+	/* Remove any child devices */
+	list_for_each_entry_safe(mi, tmp, &mbus->devs, list) {
+		mctp_i3c_remove_device(mi);
+	}
+
+	kfree_skb(mbus->tx_skb);
+	list_del(&mbus->list);
+}
+
+static void mctp_i3c_ndo_uninit(struct net_device *ndev)
+{
+	struct mctp_i3c_bus *mbus = netdev_priv(ndev);
+
+	/* Perform cleanup here to ensure there are no remaining references */
+	mctp_i3c_bus_free(mbus);
+}
+
+static int mctp_i3c_header_create(struct sk_buff *skb, struct net_device *dev,
+				  unsigned short type, const void *daddr,
+	   const void *saddr, unsigned int len)
+{
+	struct mctp_i3c_internal_hdr *ihdr;
+	int rc;
+
+	if (!daddr || !saddr)
+		return -EINVAL;
+
+	rc = skb_cow_head(skb, sizeof(struct mctp_i3c_internal_hdr));
+	if (rc)
+		return rc;
+
+	skb_push(skb, sizeof(struct mctp_i3c_internal_hdr));
+	skb_reset_mac_header(skb);
+	ihdr = (void *)skb_mac_header(skb);
+	memcpy(ihdr->dest, daddr, PID_SIZE);
+	memcpy(ihdr->source, saddr, PID_SIZE);
+	return 0;
+}
+
+static const struct net_device_ops mctp_i3c_ops = {
+	.ndo_start_xmit = mctp_i3c_start_xmit,
+	.ndo_uninit = mctp_i3c_ndo_uninit,
+};
+
+static const struct header_ops mctp_i3c_headops = {
+	.create = mctp_i3c_header_create,
+};
+
+static void mctp_i3c_net_setup(struct net_device *dev)
+{
+	dev->type = ARPHRD_MCTP;
+
+	dev->mtu = MCTP_I3C_MAXMTU;
+	dev->min_mtu = MCTP_I3C_MINMTU;
+	dev->max_mtu = MCTP_I3C_MAXMTU;
+	dev->tx_queue_len = MCTP_I3C_TX_QUEUE_LEN;
+
+	dev->hard_header_len = sizeof(struct mctp_i3c_internal_hdr);
+	dev->addr_len = PID_SIZE;
+
+	dev->netdev_ops	= &mctp_i3c_ops;
+	dev->header_ops	= &mctp_i3c_headops;
+}
+
+static bool mctp_i3c_is_mctp_controller(struct i3c_bus *bus)
+{
+	struct i3c_dev_desc *master = bus->cur_master;
+
+	if (!master)
+		return false;
+
+	return of_property_read_bool(master->common.master->dev.of_node,
+				     MCTP_I3C_OF_PROP);
+}
+
+/* Returns the Provisioned Id of a local bus master */
+static int mctp_i3c_bus_local_pid(struct i3c_bus *bus, u64 *ret_pid)
+{
+	struct i3c_dev_desc *master;
+
+	master = bus->cur_master;
+	if (WARN_ON_ONCE(!master))
+		return -ENOENT;
+	*ret_pid = master->info.pid;
+
+	return 0;
+}
+
+/* Returns an ERR_PTR on failure */
+static struct mctp_i3c_bus *mctp_i3c_bus_add(struct i3c_bus *bus)
+__must_hold(&busdevs_lock)
+{
+	struct mctp_i3c_bus *mbus = NULL;
+	struct net_device *ndev = NULL;
+	char namebuf[IFNAMSIZ];
+	u8 addr[PID_SIZE];
+	int rc;
+
+	if (!mctp_i3c_is_mctp_controller(bus))
+		return ERR_PTR(-ENOENT);
+
+	snprintf(namebuf, sizeof(namebuf), "mctpi3c%d", bus->id);
+	ndev = alloc_netdev(sizeof(*mbus), namebuf, NET_NAME_ENUM,
+			    mctp_i3c_net_setup);
+	if (!ndev) {
+		rc = -ENOMEM;
+		goto err;
+	}
+
+	mbus = netdev_priv(ndev);
+	mbus->ndev = ndev;
+	mbus->bus = bus;
+	INIT_LIST_HEAD(&mbus->devs);
+	list_add(&mbus->list, &busdevs);
+
+	rc = mctp_i3c_bus_local_pid(bus, &mbus->pid);
+	if (rc < 0) {
+		dev_err(&ndev->dev, "No I3C PID available\n");
+		goto err_free_uninit;
+	}
+	put_unaligned_be48(mbus->pid, addr);
+	dev_addr_set(ndev, addr);
+
+	init_waitqueue_head(&mbus->tx_wq);
+	spin_lock_init(&mbus->tx_lock);
+	mbus->tx_thread = kthread_run(mctp_i3c_tx_thread, mbus,
+				      "%s/tx", ndev->name);
+	if (IS_ERR(mbus->tx_thread)) {
+		dev_warn(&ndev->dev, "Error creating thread: %pe\n",
+			 mbus->tx_thread);
+		rc = PTR_ERR(mbus->tx_thread);
+		mbus->tx_thread = NULL;
+		goto err_free_uninit;
+	}
+
+	rc = mctp_register_netdev(ndev, NULL, MCTP_PHYS_BINDING_I3C);
+	if (rc < 0) {
+		dev_warn(&ndev->dev, "netdev register failed: %d\n", rc);
+		goto err_free_netdev;
+	}
+	return mbus;
+
+err_free_uninit:
+	/* uninit will not get called if a netdev has not been registered,
+	 * so we perform the same mbus cleanup manually.
+	 */
+	mctp_i3c_bus_free(mbus);
+
+err_free_netdev:
+	free_netdev(ndev);
+
+err:
+	return ERR_PTR(rc);
+}
+
+static void mctp_i3c_bus_remove(struct mctp_i3c_bus *mbus)
+__must_hold(&busdevs_lock)
+{
+	/* Unregister calls through to ndo_uninit -> mctp_i3c_bus_free() */
+	mctp_unregister_netdev(mbus->ndev);
+
+	free_netdev(mbus->ndev);
+	/* mbus is deallocated */
+}
+
+/* Removes all mctp-i3c busses */
+static void mctp_i3c_bus_remove_all(void)
+{
+	struct mctp_i3c_bus *mbus = NULL, *tmp = NULL;
+
+	mutex_lock(&busdevs_lock);
+	list_for_each_entry_safe(mbus, tmp, &busdevs, list) {
+		mctp_i3c_bus_remove(mbus);
+	}
+	mutex_unlock(&busdevs_lock);
+}
+
+/* Adds a i3c_bus if it isn't already in the busdevs list.
+ * Suitable as an i3c_for_each_bus_locked callback.
+ */
+static int mctp_i3c_bus_add_new(struct i3c_bus *bus, void *data)
+{
+	struct mctp_i3c_bus *mbus = NULL, *tmp = NULL;
+	bool exists = false;
+
+	mutex_lock(&busdevs_lock);
+	list_for_each_entry_safe(mbus, tmp, &busdevs, list)
+		if (mbus->bus == bus)
+			exists = true;
+
+	/* It is OK for a bus to already exist. That can occur due to
+	 * the race in mod_init between notifier and for_each_bus
+	 */
+	if (!exists)
+		mctp_i3c_bus_add(bus);
+	mutex_unlock(&busdevs_lock);
+	return 0;
+}
+
+static void mctp_i3c_notify_bus_remove(struct i3c_bus *bus)
+{
+	struct mctp_i3c_bus *mbus = NULL, *tmp;
+
+	mutex_lock(&busdevs_lock);
+	list_for_each_entry_safe(mbus, tmp, &busdevs, list)
+		if (mbus->bus == bus)
+			mctp_i3c_bus_remove(mbus);
+	mutex_unlock(&busdevs_lock);
+}
+
+static int mctp_i3c_notifier_call(struct notifier_block *nb,
+				  unsigned long action, void *data)
+{
+	switch (action) {
+	case I3C_NOTIFY_BUS_ADD:
+		mctp_i3c_bus_add_new((struct i3c_bus *)data, NULL);
+		break;
+	case I3C_NOTIFY_BUS_REMOVE:
+		mctp_i3c_notify_bus_remove((struct i3c_bus *)data);
+		break;
+	}
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block mctp_i3c_notifier = {
+	.notifier_call = mctp_i3c_notifier_call,
+};
+
+static const struct i3c_device_id mctp_i3c_ids[] = {
+	I3C_CLASS(I3C_DCR_MCTP, NULL),
+	{ 0 },
+};
+
+static struct i3c_driver mctp_i3c_driver = {
+	.driver = {
+		.name = "mctp-i3c",
+	},
+	.probe = mctp_i3c_probe,
+	.remove = mctp_i3c_remove,
+	.id_table = mctp_i3c_ids,
+};
+
+static __init int mctp_i3c_mod_init(void)
+{
+	int rc;
+
+	rc = i3c_register_notifier(&mctp_i3c_notifier);
+	if (rc < 0)
+		return rc;
+
+	i3c_for_each_bus_locked(mctp_i3c_bus_add_new, NULL);
+
+	rc = i3c_driver_register(&mctp_i3c_driver);
+	if (rc < 0)
+		goto err_unregister_notifier;
+
+	return 0;
+
+err_unregister_notifier:
+	i3c_unregister_notifier(&mctp_i3c_notifier);
+	mctp_i3c_bus_remove_all();
+	return rc;
+}
+
+static __exit void mctp_i3c_mod_exit(void)
+{
+	int rc;
+
+	i3c_driver_unregister(&mctp_i3c_driver);
+
+	rc = i3c_unregister_notifier(&mctp_i3c_notifier);
+	if (rc < 0)
+		pr_warn("MCTP I3C could not unregister notifier, %d\n", rc);
+
+	mctp_i3c_bus_remove_all();
+}
+
+module_init(mctp_i3c_mod_init);
+module_exit(mctp_i3c_mod_exit);
+
+MODULE_DEVICE_TABLE(i3c, mctp_i3c_ids);
+MODULE_DESCRIPTION("MCTP I3C device");
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Matt Johnston <matt@codeconstruct.com.au>");
diff --git a/drivers/net/mctp/mctp-serial.c b/drivers/net/mctp/mctp-serial.c
new file mode 100644
index 000000000..a5070ffa9
--- /dev/null
+++ b/drivers/net/mctp/mctp-serial.c
@@ -0,0 +1,634 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Management Component Transport Protocol (MCTP) - serial transport
+ * binding. This driver is an implementation of the DMTF specificiation
+ * "DSP0253 - Management Component Transport Protocol (MCTP) Serial Transport
+ * Binding", available at:
+ *
+ *  https://www.dmtf.org/sites/default/files/standards/documents/DSP0253_1.0.0.pdf
+ *
+ * This driver provides DSP0253-type MCTP-over-serial transport using a Linux
+ * tty device, by setting the N_MCTP line discipline on the tty.
+ *
+ * Copyright (c) 2021 Code Construct
+ */
+
+#include <linux/idr.h>
+#include <linux/if_arp.h>
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <linux/tty.h>
+#include <linux/workqueue.h>
+#include <linux/crc-ccitt.h>
+
+#include <linux/mctp.h>
+#include <net/mctp.h>
+#include <net/mctpdevice.h>
+#include <net/pkt_sched.h>
+
+#define MCTP_SERIAL_MTU		68 /* base mtu (64) + mctp header */
+#define MCTP_SERIAL_FRAME_MTU	(MCTP_SERIAL_MTU + 6) /* + serial framing */
+
+#define MCTP_SERIAL_VERSION	0x1 /* DSP0253 defines a single version: 1 */
+
+#define BUFSIZE			MCTP_SERIAL_FRAME_MTU
+
+#define BYTE_FRAME		0x7e
+#define BYTE_ESC		0x7d
+
+#define FCS_INIT		0xffff
+
+static DEFINE_IDA(mctp_serial_ida);
+
+enum mctp_serial_state {
+	STATE_IDLE,
+	STATE_START,
+	STATE_HEADER,
+	STATE_DATA,
+	STATE_ESCAPE,
+	STATE_TRAILER,
+	STATE_DONE,
+	STATE_ERR,
+};
+
+struct mctp_serial {
+	struct net_device	*netdev;
+	struct tty_struct	*tty;
+
+	int			idx;
+
+	/* protects our rx & tx state machines; held during both paths */
+	spinlock_t		lock;
+
+	struct work_struct	tx_work;
+	enum mctp_serial_state	txstate, rxstate;
+	u16			txfcs, rxfcs, rxfcs_rcvd;
+	unsigned int		txlen, rxlen;
+	unsigned int		txpos, rxpos;
+	u8			txbuf[BUFSIZE],
+				rxbuf[BUFSIZE];
+};
+
+static bool needs_escape(u8 c)
+{
+	return c == BYTE_ESC || c == BYTE_FRAME;
+}
+
+static unsigned int next_chunk_len(struct mctp_serial *dev)
+{
+	unsigned int i;
+
+	/* either we have no bytes to send ... */
+	if (dev->txpos == dev->txlen)
+		return 0;
+
+	/* ... or the next byte to send is an escaped byte; requiring a
+	 * single-byte chunk...
+	 */
+	if (needs_escape(dev->txbuf[dev->txpos]))
+		return 1;
+
+	/* ... or we have one or more bytes up to the next escape - this chunk
+	 * will be those non-escaped bytes, and does not include the escaped
+	 * byte.
+	 */
+	for (i = 1; i + dev->txpos < dev->txlen; i++) {
+		if (needs_escape(dev->txbuf[dev->txpos + i]))
+			break;
+	}
+
+	return i;
+}
+
+static ssize_t write_chunk(struct mctp_serial *dev, u8 *buf, size_t len)
+{
+	return dev->tty->ops->write(dev->tty, buf, len);
+}
+
+static void mctp_serial_tx_work(struct work_struct *work)
+{
+	struct mctp_serial *dev = container_of(work, struct mctp_serial,
+					       tx_work);
+	unsigned long flags;
+	ssize_t txlen;
+	unsigned int len;
+	u8 c, buf[3];
+
+	spin_lock_irqsave(&dev->lock, flags);
+
+	/* txstate represents the next thing to send */
+	switch (dev->txstate) {
+	case STATE_START:
+		dev->txpos = 0;
+		fallthrough;
+	case STATE_HEADER:
+		buf[0] = BYTE_FRAME;
+		buf[1] = MCTP_SERIAL_VERSION;
+		buf[2] = dev->txlen;
+
+		if (!dev->txpos)
+			dev->txfcs = crc_ccitt(FCS_INIT, buf + 1, 2);
+
+		txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
+		if (txlen <= 0) {
+			dev->txstate = STATE_ERR;
+		} else {
+			dev->txpos += txlen;
+			if (dev->txpos == 3) {
+				dev->txstate = STATE_DATA;
+				dev->txpos = 0;
+			}
+		}
+		break;
+
+	case STATE_ESCAPE:
+		buf[0] = dev->txbuf[dev->txpos] & ~0x20;
+		txlen = write_chunk(dev, buf, 1);
+		if (txlen <= 0) {
+			dev->txstate = STATE_ERR;
+		} else {
+			dev->txpos += txlen;
+			if (dev->txpos == dev->txlen) {
+				dev->txstate = STATE_TRAILER;
+				dev->txpos = 0;
+			}
+		}
+
+		break;
+
+	case STATE_DATA:
+		len = next_chunk_len(dev);
+		if (len) {
+			c = dev->txbuf[dev->txpos];
+			if (len == 1 && needs_escape(c)) {
+				buf[0] = BYTE_ESC;
+				buf[1] = c & ~0x20;
+				dev->txfcs = crc_ccitt_byte(dev->txfcs, c);
+				txlen = write_chunk(dev, buf, 2);
+				if (txlen == 2)
+					dev->txpos++;
+				else if (txlen == 1)
+					dev->txstate = STATE_ESCAPE;
+				else
+					dev->txstate = STATE_ERR;
+			} else {
+				txlen = write_chunk(dev,
+						    dev->txbuf + dev->txpos,
+						    len);
+				if (txlen <= 0) {
+					dev->txstate = STATE_ERR;
+				} else {
+					dev->txfcs = crc_ccitt(dev->txfcs,
+							       dev->txbuf +
+							       dev->txpos,
+							       txlen);
+					dev->txpos += txlen;
+				}
+			}
+			if (dev->txstate == STATE_DATA &&
+			    dev->txpos == dev->txlen) {
+				dev->txstate = STATE_TRAILER;
+				dev->txpos = 0;
+			}
+			break;
+		}
+		dev->txstate = STATE_TRAILER;
+		dev->txpos = 0;
+		fallthrough;
+
+	case STATE_TRAILER:
+		buf[0] = dev->txfcs >> 8;
+		buf[1] = dev->txfcs & 0xff;
+		buf[2] = BYTE_FRAME;
+		txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
+		if (txlen <= 0) {
+			dev->txstate = STATE_ERR;
+		} else {
+			dev->txpos += txlen;
+			if (dev->txpos == 3) {
+				dev->txstate = STATE_DONE;
+				dev->txpos = 0;
+			}
+		}
+		break;
+	default:
+		netdev_err_once(dev->netdev, "invalid tx state %d\n",
+				dev->txstate);
+	}
+
+	if (dev->txstate == STATE_DONE) {
+		dev->netdev->stats.tx_packets++;
+		dev->netdev->stats.tx_bytes += dev->txlen;
+		dev->txlen = 0;
+		dev->txpos = 0;
+		clear_bit(TTY_DO_WRITE_WAKEUP, &dev->tty->flags);
+		dev->txstate = STATE_IDLE;
+		spin_unlock_irqrestore(&dev->lock, flags);
+
+		netif_wake_queue(dev->netdev);
+	} else {
+		spin_unlock_irqrestore(&dev->lock, flags);
+	}
+}
+
+static netdev_tx_t mctp_serial_tx(struct sk_buff *skb, struct net_device *ndev)
+{
+	struct mctp_serial *dev = netdev_priv(ndev);
+	unsigned long flags;
+
+	WARN_ON(dev->txstate != STATE_IDLE);
+
+	if (skb->len > MCTP_SERIAL_MTU) {
+		dev->netdev->stats.tx_dropped++;
+		goto out;
+	}
+
+	spin_lock_irqsave(&dev->lock, flags);
+	netif_stop_queue(dev->netdev);
+	skb_copy_bits(skb, 0, dev->txbuf, skb->len);
+	dev->txpos = 0;
+	dev->txlen = skb->len;
+	dev->txstate = STATE_START;
+	spin_unlock_irqrestore(&dev->lock, flags);
+
+	set_bit(TTY_DO_WRITE_WAKEUP, &dev->tty->flags);
+	schedule_work(&dev->tx_work);
+
+out:
+	kfree_skb(skb);
+	return NETDEV_TX_OK;
+}
+
+static void mctp_serial_tty_write_wakeup(struct tty_struct *tty)
+{
+	struct mctp_serial *dev = tty->disc_data;
+
+	schedule_work(&dev->tx_work);
+}
+
+static void mctp_serial_rx(struct mctp_serial *dev)
+{
+	struct mctp_skb_cb *cb;
+	struct sk_buff *skb;
+
+	if (dev->rxfcs != dev->rxfcs_rcvd) {
+		dev->netdev->stats.rx_dropped++;
+		dev->netdev->stats.rx_crc_errors++;
+		return;
+	}
+
+	skb = netdev_alloc_skb(dev->netdev, dev->rxlen);
+	if (!skb) {
+		dev->netdev->stats.rx_dropped++;
+		return;
+	}
+
+	skb->protocol = htons(ETH_P_MCTP);
+	skb_put_data(skb, dev->rxbuf, dev->rxlen);
+	skb_reset_network_header(skb);
+
+	cb = __mctp_cb(skb);
+	cb->halen = 0;
+
+	netif_rx(skb);
+	dev->netdev->stats.rx_packets++;
+	dev->netdev->stats.rx_bytes += dev->rxlen;
+}
+
+static void mctp_serial_push_header(struct mctp_serial *dev, u8 c)
+{
+	switch (dev->rxpos) {
+	case 0:
+		if (c == BYTE_FRAME)
+			dev->rxpos++;
+		else
+			dev->rxstate = STATE_ERR;
+		break;
+	case 1:
+		if (c == MCTP_SERIAL_VERSION) {
+			dev->rxpos++;
+			dev->rxfcs = crc_ccitt_byte(FCS_INIT, c);
+		} else {
+			dev->rxstate = STATE_ERR;
+		}
+		break;
+	case 2:
+		if (c > MCTP_SERIAL_FRAME_MTU) {
+			dev->rxstate = STATE_ERR;
+		} else {
+			dev->rxlen = c;
+			dev->rxpos = 0;
+			dev->rxstate = c > 0 ? STATE_DATA : STATE_TRAILER;
+			dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c);
+		}
+		break;
+	}
+}
+
+static void mctp_serial_push_trailer(struct mctp_serial *dev, u8 c)
+{
+	switch (dev->rxpos) {
+	case 0:
+		dev->rxfcs_rcvd = c << 8;
+		dev->rxpos++;
+		break;
+	case 1:
+		dev->rxfcs_rcvd |= c;
+		dev->rxpos++;
+		break;
+	case 2:
+		if (c != BYTE_FRAME) {
+			dev->rxstate = STATE_ERR;
+		} else {
+			mctp_serial_rx(dev);
+			dev->rxlen = 0;
+			dev->rxpos = 0;
+			dev->rxstate = STATE_IDLE;
+		}
+		break;
+	}
+}
+
+static void mctp_serial_push(struct mctp_serial *dev, u8 c)
+{
+	switch (dev->rxstate) {
+	case STATE_IDLE:
+		dev->rxstate = STATE_HEADER;
+		fallthrough;
+	case STATE_HEADER:
+		mctp_serial_push_header(dev, c);
+		break;
+
+	case STATE_ESCAPE:
+		c |= 0x20;
+		fallthrough;
+	case STATE_DATA:
+		if (dev->rxstate != STATE_ESCAPE && c == BYTE_ESC) {
+			dev->rxstate = STATE_ESCAPE;
+		} else {
+			dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c);
+			dev->rxbuf[dev->rxpos] = c;
+			dev->rxpos++;
+			dev->rxstate = STATE_DATA;
+			if (dev->rxpos == dev->rxlen) {
+				dev->rxpos = 0;
+				dev->rxstate = STATE_TRAILER;
+			}
+		}
+		break;
+
+	case STATE_TRAILER:
+		mctp_serial_push_trailer(dev, c);
+		break;
+
+	case STATE_ERR:
+		if (c == BYTE_FRAME)
+			dev->rxstate = STATE_IDLE;
+		break;
+
+	default:
+		netdev_err_once(dev->netdev, "invalid rx state %d\n",
+				dev->rxstate);
+	}
+}
+
+static void mctp_serial_tty_receive_buf(struct tty_struct *tty, const u8 *c,
+					const u8 *f, size_t len)
+{
+	struct mctp_serial *dev = tty->disc_data;
+	size_t i;
+
+	if (!netif_running(dev->netdev))
+		return;
+
+	/* we don't (currently) use the flag bytes, just data. */
+	for (i = 0; i < len; i++)
+		mctp_serial_push(dev, c[i]);
+}
+
+static void mctp_serial_uninit(struct net_device *ndev)
+{
+	struct mctp_serial *dev = netdev_priv(ndev);
+
+	cancel_work_sync(&dev->tx_work);
+}
+
+static const struct net_device_ops mctp_serial_netdev_ops = {
+	.ndo_start_xmit = mctp_serial_tx,
+	.ndo_uninit = mctp_serial_uninit,
+};
+
+static void mctp_serial_setup(struct net_device *ndev)
+{
+	ndev->type = ARPHRD_MCTP;
+
+	/* we limit at the fixed MTU, which is also the MCTP-standard
+	 * baseline MTU, so is also our minimum
+	 */
+	ndev->mtu = MCTP_SERIAL_MTU;
+	ndev->max_mtu = MCTP_SERIAL_MTU;
+	ndev->min_mtu = MCTP_SERIAL_MTU;
+
+	ndev->hard_header_len = 0;
+	ndev->addr_len = 0;
+	ndev->tx_queue_len = DEFAULT_TX_QUEUE_LEN;
+	ndev->flags = IFF_NOARP;
+	ndev->netdev_ops = &mctp_serial_netdev_ops;
+	ndev->needs_free_netdev = true;
+}
+
+static int mctp_serial_open(struct tty_struct *tty)
+{
+	struct mctp_serial *dev;
+	struct net_device *ndev;
+	char name[32];
+	int idx, rc;
+
+	if (!capable(CAP_NET_ADMIN))
+		return -EPERM;
+
+	if (!tty->ops->write)
+		return -EOPNOTSUPP;
+
+	idx = ida_alloc(&mctp_serial_ida, GFP_KERNEL);
+	if (idx < 0)
+		return idx;
+
+	snprintf(name, sizeof(name), "mctpserial%d", idx);
+	ndev = alloc_netdev(sizeof(*dev), name, NET_NAME_ENUM,
+			    mctp_serial_setup);
+	if (!ndev) {
+		rc = -ENOMEM;
+		goto free_ida;
+	}
+
+	dev = netdev_priv(ndev);
+	dev->idx = idx;
+	dev->tty = tty;
+	dev->netdev = ndev;
+	dev->txstate = STATE_IDLE;
+	dev->rxstate = STATE_IDLE;
+	spin_lock_init(&dev->lock);
+	INIT_WORK(&dev->tx_work, mctp_serial_tx_work);
+
+	rc = mctp_register_netdev(ndev, NULL, MCTP_PHYS_BINDING_SERIAL);
+	if (rc)
+		goto free_netdev;
+
+	tty->receive_room = 64 * 1024;
+	tty->disc_data = dev;
+
+	return 0;
+
+free_netdev:
+	free_netdev(ndev);
+
+free_ida:
+	ida_free(&mctp_serial_ida, idx);
+	return rc;
+}
+
+static void mctp_serial_close(struct tty_struct *tty)
+{
+	struct mctp_serial *dev = tty->disc_data;
+	int idx = dev->idx;
+
+	mctp_unregister_netdev(dev->netdev);
+	ida_free(&mctp_serial_ida, idx);
+}
+
+static struct tty_ldisc_ops mctp_ldisc = {
+	.owner		= THIS_MODULE,
+	.num		= N_MCTP,
+	.name		= "mctp",
+	.open		= mctp_serial_open,
+	.close		= mctp_serial_close,
+	.receive_buf	= mctp_serial_tty_receive_buf,
+	.write_wakeup	= mctp_serial_tty_write_wakeup,
+};
+
+static int __init mctp_serial_init(void)
+{
+	return tty_register_ldisc(&mctp_ldisc);
+}
+
+static void __exit mctp_serial_exit(void)
+{
+	tty_unregister_ldisc(&mctp_ldisc);
+}
+
+module_init(mctp_serial_init);
+module_exit(mctp_serial_exit);
+
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Jeremy Kerr <jk@codeconstruct.com.au>");
+MODULE_DESCRIPTION("MCTP Serial transport");
+
+#if IS_ENABLED(CONFIG_MCTP_SERIAL_TEST)
+#include <kunit/test.h>
+
+#define MAX_CHUNKS 6
+struct test_chunk_tx {
+	u8 input_len;
+	u8 input[MCTP_SERIAL_MTU];
+	u8 chunks[MAX_CHUNKS];
+};
+
+static void test_next_chunk_len(struct kunit *test)
+{
+	struct mctp_serial devx;
+	struct mctp_serial *dev = &devx;
+	int next;
+
+	const struct test_chunk_tx *params = test->param_value;
+
+	memset(dev, 0x0, sizeof(*dev));
+	memcpy(dev->txbuf, params->input, params->input_len);
+	dev->txlen = params->input_len;
+
+	for (size_t i = 0; i < MAX_CHUNKS; i++) {
+		next = next_chunk_len(dev);
+		dev->txpos += next;
+		KUNIT_EXPECT_EQ(test, next, params->chunks[i]);
+
+		if (next == 0) {
+			KUNIT_EXPECT_EQ(test, dev->txpos, dev->txlen);
+			return;
+		}
+	}
+
+	KUNIT_FAIL_AND_ABORT(test, "Ran out of chunks");
+}
+
+static struct test_chunk_tx chunk_tx_tests[] = {
+	{
+		.input_len = 5,
+		.input = { 0x00, 0x11, 0x22, 0x7e, 0x80 },
+		.chunks = { 3, 1, 1, 0},
+	},
+	{
+		.input_len = 5,
+		.input = { 0x00, 0x11, 0x22, 0x7e, 0x7d },
+		.chunks = { 3, 1, 1, 0},
+	},
+	{
+		.input_len = 3,
+		.input = { 0x7e, 0x11, 0x22, },
+		.chunks = { 1, 2, 0},
+	},
+	{
+		.input_len = 3,
+		.input = { 0x7e, 0x7e, 0x7d, },
+		.chunks = { 1, 1, 1, 0},
+	},
+	{
+		.input_len = 4,
+		.input = { 0x7e, 0x7e, 0x00, 0x7d, },
+		.chunks = { 1, 1, 1, 1, 0},
+	},
+	{
+		.input_len = 6,
+		.input = { 0x7e, 0x7e, 0x00, 0x7d, 0x10, 0x10},
+		.chunks = { 1, 1, 1, 1, 2, 0},
+	},
+	{
+		.input_len = 1,
+		.input = { 0x7e },
+		.chunks = { 1, 0 },
+	},
+	{
+		.input_len = 1,
+		.input = { 0x80 },
+		.chunks = { 1, 0 },
+	},
+	{
+		.input_len = 3,
+		.input = { 0x80, 0x80, 0x00 },
+		.chunks = { 3, 0 },
+	},
+	{
+		.input_len = 7,
+		.input = { 0x01, 0x00, 0x08, 0xc8, 0x00, 0x80, 0x02 },
+		.chunks = { 7, 0 },
+	},
+	{
+		.input_len = 7,
+		.input = { 0x01, 0x00, 0x08, 0xc8, 0x7e, 0x80, 0x02 },
+		.chunks = { 4, 1, 2, 0 },
+	},
+};
+
+KUNIT_ARRAY_PARAM(chunk_tx, chunk_tx_tests, NULL);
+
+static struct kunit_case mctp_serial_test_cases[] = {
+	KUNIT_CASE_PARAM(test_next_chunk_len, chunk_tx_gen_params),
+};
+
+static struct kunit_suite mctp_serial_test_suite = {
+	.name = "mctp_serial",
+	.test_cases = mctp_serial_test_cases,
+};
+
+kunit_test_suite(mctp_serial_test_suite);
+
+#endif /* CONFIG_MCTP_SERIAL_TEST */
diff --git a/drivers/net/mctp/mctp-usb.c b/drivers/net/mctp/mctp-usb.c
new file mode 100644
index 000000000..542a570c7
--- /dev/null
+++ b/drivers/net/mctp/mctp-usb.c
@@ -0,0 +1,447 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * mctp-usb.c - MCTP-over-USB (DMTF DSP0283) transport binding driver.
+ *
+ * DSP0283 is available at:
+ * https://www.dmtf.org/sites/default/files/standards/documents/DSP0283_1.1.0.pdf
+ *
+ * Copyright (C) 2024-2026 Code Construct Pty Ltd
+ */
+
+#include <linux/module.h>
+#include <linux/netdevice.h>
+#include <linux/usb.h>
+#include <linux/usb/mctp-usb.h>
+
+#include <net/mctp.h>
+#include <net/mctpdevice.h>
+#include <net/pkt_sched.h>
+
+#include <uapi/linux/if_arp.h>
+
+struct mctp_usb {
+	struct usb_device *usbdev;
+	struct usb_interface *intf;
+	bool span;
+
+	struct net_device *netdev;
+
+	u8 ep_in;
+	u8 ep_out;
+
+	struct mctp_usblib_rx rx;
+	struct urb *rx_urb;
+	int in_err_count;
+	int in_err_orig;
+	bool clear_halt;
+
+	/* enforces atomic access to rx_stopped and requeuing the retry work */
+	spinlock_t rx_lock;
+	bool rx_stopped;
+	struct delayed_work rx_retry_work;
+
+	struct mctp_usblib_tx tx;
+	struct usb_anchor tx_anchor;
+	/* serialises tx_qmem updates to netdev queue states */
+	spinlock_t tx_qmem_lock;
+	int tx_qmem;
+};
+
+enum {
+	MCTP_USB_SUBCLASS_BASE = 0x00,
+	MCTP_USB_SUBCLASS_SPAN = 0x02,
+};
+
+/* We use a total-size limit for outstanding URBs, as the transfer counts
+ * may vary a lot between spanning- and non-spanning modes. In spanning mode,
+ * this will allow for a couple of max-sized transfers to be in flight. In
+ * non-spanning mode, 32.
+ *
+ * We want to avoid disabling the tx queue if possible; doing so will end up
+ * requeueing to gso_skb, and we only dequeue from that one skb at a time,
+ * so can no longer perform transfer packing.
+ */
+static const unsigned int TX_QMEM_MAX = 16384;
+
+static void mctp_usb_out_complete(struct urb *urb)
+{
+	struct mctp_usblib_tx_ctx *tx_ctx = urb->context;
+	struct mctp_usb *mctp_usb = mctp_usblib_tx_ctx_priv(tx_ctx);
+	unsigned int len = urb->transfer_buffer_length;
+	struct net_device *netdev = mctp_usb->netdev;
+	unsigned long flags;
+
+	mctp_usblib_tx_send_complete(tx_ctx, netdev, urb->status == 0);
+
+	usb_free_urb(urb);
+
+	spin_lock_irqsave(&mctp_usb->tx_qmem_lock, flags);
+	mctp_usb->tx_qmem -= len;
+	if (mctp_usb->tx_qmem < TX_QMEM_MAX && netif_running(netdev))
+		netif_wake_queue(netdev);
+	spin_unlock_irqrestore(&mctp_usb->tx_qmem_lock, flags);
+}
+
+static int mctp_usb_tx_send(struct mctp_usblib_tx_ctx *tx_ctx,
+			    void *data, size_t len)
+{
+	struct mctp_usb *mctp_usb = mctp_usblib_tx_ctx_priv(tx_ctx);
+	unsigned long flags;
+	struct urb *urb;
+	int rc;
+
+	urb = usb_alloc_urb(0, GFP_ATOMIC);
+	if (!urb)
+		return -ENOMEM;
+
+	usb_fill_bulk_urb(urb, mctp_usb->usbdev,
+			  usb_sndbulkpipe(mctp_usb->usbdev, mctp_usb->ep_out),
+			  data, len, mctp_usb_out_complete, tx_ctx);
+
+	if (mctp_usb->span)
+		urb->transfer_flags |= URB_ZERO_PACKET;
+
+	usb_anchor_urb(urb, &mctp_usb->tx_anchor);
+
+	rc = usb_submit_urb(urb, GFP_ATOMIC);
+	if (rc) {
+		netdev_dbg(mctp_usb->netdev, "TX urb submit failed, %d\n", rc);
+		usb_unanchor_urb(urb);
+		usb_free_urb(urb);
+	} else {
+		spin_lock_irqsave(&mctp_usb->tx_qmem_lock, flags);
+		mctp_usb->tx_qmem += len;
+		if (mctp_usb->tx_qmem >= TX_QMEM_MAX)
+			netif_stop_queue(mctp_usb->netdev);
+		spin_unlock_irqrestore(&mctp_usb->tx_qmem_lock, flags);
+	}
+
+	return rc;
+}
+
+static const struct mctp_usblib_tx_ops tx_ops = {
+	.send = mctp_usb_tx_send,
+};
+
+static netdev_tx_t mctp_usb_start_xmit(struct sk_buff *skb,
+				       struct net_device *dev)
+{
+	struct mctp_usb *mctp_usb = netdev_priv(dev);
+	bool more = netdev_xmit_more();
+
+	mctp_usblib_tx_push(dev, &mctp_usb->tx, skb, more);
+
+	return NETDEV_TX_OK;
+}
+
+static void mctp_usb_in_complete(struct urb *urb);
+
+/* If we fail to queue an in urb atomically (either due to skb allocation or
+ * urb submission), we will schedule a rx queue in nonatomic context
+ * after a delay, specified in jiffies
+ */
+static const unsigned long RX_RETRY_DELAY = HZ / 4;
+
+static int mctp_usb_rx_queue(struct mctp_usb *mctp_usb, gfp_t gfp)
+{
+	unsigned long flags;
+	size_t len;
+	void *buf;
+	int rc;
+
+	rc = mctp_usblib_rx_prepare(mctp_usb->netdev, &mctp_usb->rx,
+				    &buf, &len, gfp);
+	if (rc)
+		goto err_retry;
+
+	usb_fill_bulk_urb(mctp_usb->rx_urb, mctp_usb->usbdev,
+			  usb_rcvbulkpipe(mctp_usb->usbdev, mctp_usb->ep_in),
+			  buf, len, mctp_usb_in_complete, mctp_usb);
+
+	rc = usb_submit_urb(mctp_usb->rx_urb, gfp);
+	if (rc) {
+		netdev_dbg(mctp_usb->netdev, "rx urb submit failure: %d\n", rc);
+		mctp_usblib_rx_cancel(&mctp_usb->rx);
+		if (rc == -ENOMEM)
+			goto err_retry;
+	}
+
+	return rc;
+
+err_retry:
+	spin_lock_irqsave(&mctp_usb->rx_lock, flags);
+	if (!mctp_usb->rx_stopped)
+		schedule_delayed_work(&mctp_usb->rx_retry_work, RX_RETRY_DELAY);
+	spin_unlock_irqrestore(&mctp_usb->rx_lock, flags);
+	return 0;
+}
+
+static const unsigned int rx_err_max = 10;
+
+/* Returns -1 if we have hit excessive errors, zero otherwise. */
+static int mctp_usb_in_urb_err(struct mctp_usb *mctp_usb, int status,
+			       bool stalled)
+{
+	mctp_usblib_rx_cancel(&mctp_usb->rx);
+
+	if (!mctp_usb->in_err_count++)
+		mctp_usb->in_err_orig = status;
+
+	if (mctp_usb->in_err_count >= rx_err_max) {
+		netdev_err(mctp_usb->netdev,
+			   "excessive errors from%s IN EP, first: %d\n",
+			   stalled ? " (stalled)" : "",
+			   mctp_usb->in_err_orig);
+		return -1;
+	}
+
+	return 0;
+}
+
+static void mctp_usb_in_complete(struct urb *urb)
+{
+	struct mctp_usb *mctp_usb = urb->context;
+	struct net_device *netdev = mctp_usb->netdev;
+	unsigned long flags;
+	int rc, status;
+
+	status = urb->status;
+
+	switch (status) {
+	case -ENOENT:
+	case -ECONNRESET:
+	case -ESHUTDOWN:
+		/* device shutdown, don't resubmit */
+		mctp_usblib_rx_cancel(&mctp_usb->rx);
+		return;
+
+	case -EPIPE:
+		/* endpoint stall: clear halt, which will cause a resubmit */
+		rc = mctp_usb_in_urb_err(mctp_usb, status, true);
+		if (rc)
+			return;
+
+		mctp_usb->clear_halt = true;
+		spin_lock_irqsave(&mctp_usb->rx_lock, flags);
+		if (!mctp_usb->rx_stopped)
+			schedule_delayed_work(&mctp_usb->rx_retry_work,
+					      RX_RETRY_DELAY);
+		spin_unlock_irqrestore(&mctp_usb->rx_lock, flags);
+		return;
+
+	default:
+		netdev_dbg(netdev, "unexpected rx urb status: %d\n", status);
+		fallthrough;
+	case -ETIME:
+	case -EPROTO:
+	case -EILSEQ:
+	case -EOVERFLOW:
+		/* possibly transient; record first failure, resubmit */
+		rc = mctp_usb_in_urb_err(mctp_usb, status, false);
+		if (rc)
+			return;
+		break;
+
+	case 0:
+		mctp_usblib_rx_complete(netdev, &mctp_usb->rx, urb->actual_length);
+		mctp_usb->in_err_count = 0;
+		break;
+	}
+
+	mctp_usb_rx_queue(mctp_usb, GFP_ATOMIC);
+}
+
+static void mctp_usb_rx_retry_work(struct work_struct *work)
+{
+	struct mctp_usb *mctp_usb = container_of(work, struct mctp_usb,
+						 rx_retry_work.work);
+	unsigned long flags;
+	int rc;
+
+	/* We are only called when rx completions are suspended */
+	if (mctp_usb->clear_halt) {
+		int pipe = usb_rcvbulkpipe(mctp_usb->usbdev, mctp_usb->ep_in);
+
+		rc = usb_clear_halt(mctp_usb->usbdev, pipe);
+		if (rc) {
+			netdev_err(mctp_usb->netdev,
+				   "can't clear IN EP halt: %d\n", rc);
+
+			if (++mctp_usb->in_err_count >= rx_err_max)
+				return;
+
+			spin_lock_irqsave(&mctp_usb->rx_lock, flags);
+			if (!mctp_usb->rx_stopped)
+				schedule_delayed_work(&mctp_usb->rx_retry_work,
+						      RX_RETRY_DELAY);
+			spin_unlock_irqrestore(&mctp_usb->rx_lock, flags);
+			return;
+		}
+		mctp_usb->clear_halt = false;
+	}
+
+	mctp_usb_rx_queue(mctp_usb, GFP_KERNEL);
+}
+
+static int mctp_usb_open(struct net_device *dev)
+{
+	struct mctp_usb *mctp_usb = netdev_priv(dev);
+
+	WRITE_ONCE(mctp_usb->rx_stopped, false);
+	mctp_usb->clear_halt = false;
+	mctp_usb->in_err_count = 0;
+
+	netif_start_queue(dev);
+
+	return mctp_usb_rx_queue(mctp_usb, GFP_KERNEL);
+}
+
+static int mctp_usb_stop(struct net_device *dev)
+{
+	struct mctp_usb *mctp_usb = netdev_priv(dev);
+	unsigned long flags;
+
+	netif_stop_queue(dev);
+
+	/* prevent RX submission retry */
+	spin_lock_irqsave(&mctp_usb->rx_lock, flags);
+	mctp_usb->rx_stopped = true;
+	cancel_delayed_work(&mctp_usb->rx_retry_work);
+	spin_unlock_irqrestore(&mctp_usb->rx_lock, flags);
+
+	flush_delayed_work(&mctp_usb->rx_retry_work);
+
+	usb_kill_urb(mctp_usb->rx_urb);
+
+	usb_kill_anchored_urbs(&mctp_usb->tx_anchor);
+
+	mctp_usblib_tx_cancel(&mctp_usb->tx, dev, SKB_DROP_REASON_DEV_READY);
+	mctp_usblib_rx_cancel(&mctp_usb->rx);
+
+	return 0;
+}
+
+static const struct net_device_ops mctp_usb_netdev_ops = {
+	.ndo_start_xmit = mctp_usb_start_xmit,
+	.ndo_open = mctp_usb_open,
+	.ndo_stop = mctp_usb_stop,
+};
+
+static void mctp_usb_netdev_setup(struct net_device *dev)
+{
+	dev->type = ARPHRD_MCTP;
+
+	dev->mtu = MCTP_USB_MTU_MIN;
+	dev->min_mtu = MCTP_USB_MTU_MIN;
+	dev->max_mtu = MCTP_USB_1_0_MTU_MAX;
+
+	dev->hard_header_len = sizeof(struct mctp_usb_hdr);
+	dev->tx_queue_len = DEFAULT_TX_QUEUE_LEN;
+	dev->flags = IFF_NOARP;
+	dev->netdev_ops = &mctp_usb_netdev_ops;
+	dev->pcpu_stat_type = NETDEV_PCPU_STAT_DSTATS;
+}
+
+static int mctp_usb_probe(struct usb_interface *intf,
+			  const struct usb_device_id *id)
+{
+	struct usb_endpoint_descriptor *ep_in, *ep_out;
+	struct usb_host_interface *iface_desc;
+	struct net_device *netdev;
+	struct mctp_usb *dev;
+	bool span;
+	int rc;
+
+	/* only one alternate */
+	iface_desc = intf->cur_altsetting;
+
+	rc = usb_find_common_endpoints(iface_desc, &ep_in, &ep_out, NULL, NULL);
+	if (rc) {
+		dev_err(&intf->dev, "invalid endpoints on device?\n");
+		return rc;
+	}
+
+	span = iface_desc->desc.bInterfaceSubClass == MCTP_USB_SUBCLASS_SPAN;
+
+	netdev = alloc_netdev(sizeof(*dev), "mctpusb%d", NET_NAME_ENUM,
+			      mctp_usb_netdev_setup);
+	if (!netdev)
+		return -ENOMEM;
+
+	SET_NETDEV_DEV(netdev, &intf->dev);
+	dev = netdev_priv(netdev);
+	dev->span = span;
+	dev->netdev = netdev;
+	dev->usbdev = interface_to_usbdev(intf);
+	dev->intf = intf;
+	spin_lock_init(&dev->rx_lock);
+	if (dev->span)
+		netdev->max_mtu = MCTP_USB_1_1_MTU_MAX;
+	spin_lock_init(&dev->tx_qmem_lock);
+	usb_set_intfdata(intf, dev);
+
+	rc = mctp_usblib_rx_init(&dev->rx, le16_to_cpu(ep_in->wMaxPacketSize),
+				 dev->span);
+	if (rc)
+		goto err_free_netdev;
+	mctp_usblib_tx_init(&dev->tx, &tx_ops, dev, dev->span);
+	init_usb_anchor(&dev->tx_anchor);
+
+	dev->ep_in = ep_in->bEndpointAddress;
+	dev->ep_out = ep_out->bEndpointAddress;
+
+	dev->rx_urb = usb_alloc_urb(0, GFP_KERNEL);
+	if (!dev->rx_urb) {
+		rc = -ENOMEM;
+		goto err_fini_rxtx;
+	}
+
+	INIT_DELAYED_WORK(&dev->rx_retry_work, mctp_usb_rx_retry_work);
+
+	rc = mctp_register_netdev(netdev, NULL, MCTP_PHYS_BINDING_USB);
+	if (rc)
+		goto err_free_urb;
+
+	return 0;
+
+err_free_urb:
+	usb_free_urb(dev->rx_urb);
+err_fini_rxtx:
+	mctp_usblib_tx_fini(&dev->tx);
+	mctp_usblib_rx_fini(&dev->rx);
+err_free_netdev:
+	free_netdev(netdev);
+	return rc;
+}
+
+static void mctp_usb_disconnect(struct usb_interface *intf)
+{
+	struct mctp_usb *dev = usb_get_intfdata(intf);
+
+	mctp_unregister_netdev(dev->netdev);
+	mctp_usblib_rx_fini(&dev->rx);
+	mctp_usblib_tx_fini(&dev->tx);
+	usb_free_urb(dev->rx_urb);
+	free_netdev(dev->netdev);
+}
+
+static const struct usb_device_id mctp_usb_devices[] = {
+	{ USB_INTERFACE_INFO(USB_CLASS_MCTP, MCTP_USB_SUBCLASS_BASE, 0x1) },
+	{ USB_INTERFACE_INFO(USB_CLASS_MCTP, MCTP_USB_SUBCLASS_SPAN, 0x1) },
+	{ 0 },
+};
+
+MODULE_DEVICE_TABLE(usb, mctp_usb_devices);
+
+static struct usb_driver mctp_usb_driver = {
+	.name		= "mctp-usb",
+	.id_table	= mctp_usb_devices,
+	.probe		= mctp_usb_probe,
+	.disconnect	= mctp_usb_disconnect,
+};
+
+module_usb_driver(mctp_usb_driver)
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Jeremy Kerr <jk@codeconstruct.com.au>");
+MODULE_DESCRIPTION("MCTP USB transport");
diff --git a/drivers/net/mctp/mctp-usblib-test.c b/drivers/net/mctp/mctp-usblib-test.c
new file mode 100644
index 000000000..9df401a91
--- /dev/null
+++ b/drivers/net/mctp/mctp-usblib-test.c
@@ -0,0 +1,412 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * mctp-usblib-test.c - MCTP-over-USB (DMTF DSP0283) transport helper library,
+ * unit test definitions.
+ *
+ * Copyright (C) 2026 Code Construct Pty Ltd
+ */
+
+#include <uapi/linux/netdevice.h>
+#include <linux/netdevice.h>
+#include <kunit/test.h>
+#include <linux/if_arp.h>
+#include <net/mctp.h>
+#include <net/mctpdevice.h>
+#include <linux/usb/mctp-usb.h>
+
+struct mctp_usblib_test_dev {
+	struct net_device *ndev;
+	struct mctp_dev *mdev;
+	struct sk_buff_head rx_pkts;
+};
+
+struct mctp_usblib_test_ctx {
+	struct mctp_usblib_test_dev *dev;
+	struct mctp_route rt;
+};
+
+static netdev_tx_t mctp_usblib_dev_tx(struct sk_buff *skb,
+				      struct net_device *ndev)
+{
+	/* we don't track any TXed packets at present */
+	kfree_skb(skb);
+	return NETDEV_TX_OK;
+}
+
+static const struct net_device_ops mctp_test_netdev_ops = {
+	.ndo_start_xmit = mctp_usblib_dev_tx,
+};
+
+static const u16 ep_maxpacket = 512;
+static const mctp_eid_t local_eid = 8;
+
+static void mctp_usblib_dev_setup(struct net_device *ndev)
+{
+	ndev->type = ARPHRD_MCTP;
+	ndev->mtu = 8192;
+	ndev->flags = IFF_NOARP;
+	ndev->netdev_ops = &mctp_test_netdev_ops;
+	ndev->needs_free_netdev = true;
+	ndev->pcpu_stat_type = NETDEV_PCPU_STAT_DSTATS;
+}
+
+static void mctp_usblib_test_dev_action(void *data)
+{
+	struct mctp_usblib_test_dev *dev = data;
+
+	skb_queue_purge(&dev->rx_pkts);
+	if (dev->mdev)
+		mctp_dev_put(dev->mdev);
+	unregister_netdev(dev->ndev);
+}
+
+static struct mctp_usblib_test_dev *
+mctp_usblib_test_create_dev(struct kunit *test)
+{
+	struct mctp_usblib_test_dev *dev;
+	struct net_device *ndev;
+	int rc;
+
+	ndev = alloc_netdev(sizeof(*dev), "mctptest%d", NET_NAME_ENUM,
+			    mctp_usblib_dev_setup);
+	if (!ndev)
+		return NULL;
+
+	dev = netdev_priv(ndev);
+	dev->ndev = ndev;
+	skb_queue_head_init(&dev->rx_pkts);
+
+	rc = register_netdev(ndev);
+	if (rc) {
+		free_netdev(ndev);
+		return NULL;
+	}
+
+	rc = kunit_add_action_or_reset(test, mctp_usblib_test_dev_action, dev);
+	if (rc)
+		return NULL;
+
+	rcu_read_lock();
+	dev->mdev = __mctp_dev_get(ndev);
+	if (dev->mdev)
+		dev->mdev->net = mctp_default_net(dev_net(ndev));
+	rcu_read_unlock();
+
+	if (!dev->mdev)
+		return NULL;
+
+	rtnl_lock();
+	rc = dev_open(ndev, NULL);
+	rtnl_unlock();
+	if (rc)
+		return NULL;
+
+	return dev;
+}
+
+static int mctp_usblib_test_dst_output(struct mctp_dst *dst,
+				       struct sk_buff *skb)
+{
+	struct mctp_usblib_test_dev *dev = netdev_priv(skb->dev);
+
+	skb_queue_tail(&dev->rx_pkts, skb);
+
+	return 0;
+}
+
+static void mctp_usblib_test_fini_action(void *data)
+{
+	struct mctp_usblib_test_ctx *ctx = data;
+
+	/* The device will have been destroyed, so ->rt will be unlinked.
+	 * Just ensure that the refcount is as expected.
+	 */
+	KUNIT_EXPECT_TRUE(current->kunit_test,
+			  refcount_dec_and_test(&ctx->rt.refs));
+
+	kfree(ctx);
+}
+
+static struct mctp_usblib_test_ctx *mctp_usblib_test_init(struct kunit *test)
+{
+	struct mctp_usblib_test_ctx *ctx;
+	struct mctp_route *rt;
+	int rc;
+
+	ctx = kzalloc_obj(*ctx);
+	KUNIT_ASSERT_NOT_NULL(test, ctx);
+
+	INIT_LIST_HEAD(&ctx->rt.list);
+	rt = &ctx->rt;
+	refcount_set(&rt->refs, 1);
+
+	rc = kunit_add_action_or_reset(test, mctp_usblib_test_fini_action, ctx);
+	KUNIT_ASSERT_EQ(test, rc, 0);
+
+	ctx->dev = mctp_usblib_test_create_dev(test);
+	KUNIT_ASSERT_NOT_NULL(test, ctx->dev);
+
+	rt->min = local_eid;
+	rt->max = local_eid;
+	rt->dst_type = MCTP_ROUTE_DIRECT;
+	rt->type = RTN_LOCAL;
+	rt->dev = ctx->dev->mdev;
+	rt->output = mctp_usblib_test_dst_output;
+
+	rtnl_lock();
+	list_add_rcu(&ctx->rt.list, &init_net.mctp.routes);
+	refcount_inc(&rt->refs);
+	rtnl_unlock();
+
+	return ctx;
+}
+
+/* Init a MCTP-over-USB packet within a buffer. @len is the length of the
+ * buffer to write, @payload_len is the reported size of the MCTP-over-USB
+ * packet.
+ */
+static void mctp_usblib_test_init_pkt(void *data, size_t len,
+				      size_t payload_len)
+{
+	struct {
+		struct mctp_usb_hdr usb;
+		struct mctp_hdr mctp;
+	} hdr;
+
+	hdr.usb.id = cpu_to_be16(MCTP_USB_DMTF_ID);
+	hdr.usb.len = cpu_to_be16(payload_len);
+	hdr.mctp.ver = 1;
+	hdr.mctp.dest = local_eid;
+	hdr.mctp.src = 0;
+	hdr.mctp.flags_seq_tag = 0;
+
+	memcpy(data, &hdr, min(len, sizeof(hdr)));
+	if (len > sizeof(hdr))
+		memset(data + sizeof(hdr), 0, len - sizeof(hdr));
+}
+
+static void action_rx_fini(void *data)
+{
+	struct mctp_usblib_rx *rx = data;
+
+	mctp_usblib_rx_fini(rx);
+	kfree(rx);
+}
+
+static struct mctp_usblib_rx *
+mctp_usblib_test_rx_init(struct kunit *test, bool span)
+{
+	struct mctp_usblib_rx *rx;
+	int rc;
+
+	rx = kzalloc_obj(*rx);
+	if (rx) {
+		rc = kunit_add_action_or_reset(test, action_rx_fini, rx);
+		KUNIT_ASSERT_EQ(test, rc, 0);
+	}
+	KUNIT_ASSERT_NOT_NULL(test, rx);
+
+	rc = mctp_usblib_rx_init(rx, ep_maxpacket, span);
+	KUNIT_ASSERT_EQ(test, rc, 0);
+
+	return rx;
+}
+
+/* Wrappers for usblib's rx_complete callback, which is intended to be called
+ * from atomic context
+ */
+static int mctp_usblib_test_rx_complete(struct net_device *netdev,
+					struct mctp_usblib_rx *rx, size_t len)
+{
+	int rc;
+
+	local_bh_disable();
+	rc = mctp_usblib_rx_complete(netdev, rx, len);
+	local_bh_enable();
+
+	return rc;
+}
+
+/* Single packet, starting on a transfer boundary, contained entirely within
+ * the transfer
+ */
+static void mctp_usblib_test_rx_single(struct kunit *test)
+{
+	struct mctp_usblib_test_dev *dev;
+	struct mctp_usblib_test_ctx *ctx;
+	struct mctp_usblib_rx *rx;
+	struct sk_buff *skb;
+	size_t len;
+	void *buf;
+	int rc;
+
+	ctx = mctp_usblib_test_init(test);
+	dev = ctx->dev;
+
+	rx = mctp_usblib_test_rx_init(test, true);
+
+	rc = mctp_usblib_rx_prepare(dev->ndev, rx,
+				    &buf, &len, GFP_KERNEL);
+	KUNIT_ASSERT_EQ(test, rc, 0);
+
+	/* we should always have a maxpacket of transfer available */
+	KUNIT_ASSERT_GE(test, len, ep_maxpacket);
+
+	mctp_usblib_test_init_pkt(buf, 8, 8);
+
+	rc = mctp_usblib_test_rx_complete(dev->ndev, rx, 8);
+	KUNIT_ASSERT_EQ(test, rc, 0);
+
+	skb = __skb_dequeue(&dev->rx_pkts);
+	KUNIT_EXPECT_NOT_NULL(test, skb);
+	if (skb)
+		KUNIT_EXPECT_EQ(test, skb->len, 4);
+	kfree_skb(skb);
+}
+
+struct mctp_usblib_test_pkt_span {
+	const char *name;
+	size_t n_pkts;
+	size_t pkts[6];
+	size_t n_xfers;
+	size_t xfers[6];
+};
+
+static void
+mctp_usblib_test_pkt_span_to_desc(const struct mctp_usblib_test_pkt_span *t,
+				  char *desc)
+{
+	strscpy(desc, t->name, KUNIT_PARAM_DESC_SIZE);
+}
+
+static void
+mctp_usblib_test_pkt_span_validate(struct kunit *test,
+				   const struct mctp_usblib_test_pkt_span *span,
+				   size_t *len)
+{
+	size_t pkt_len = 0, xfer_len = 0;
+	unsigned int i;
+
+	for (i = 0; i < span->n_pkts; i++) {
+		KUNIT_ASSERT_GE_MSG(test, span->pkts[i], 8,
+				    "pkt[%u] len too small (%zu) for %s",
+				    i, span->pkts[i], span->name);
+		pkt_len += span->pkts[i];
+	}
+
+	for (i = 0; i < span->n_xfers; i++)
+		xfer_len += span->xfers[i];
+
+	KUNIT_ASSERT_EQ_MSG(test, pkt_len, xfer_len,
+			    "invalid pkt_len (%zu) != xfer_len (%zu) for %s",
+			    pkt_len, xfer_len, span->name);
+
+	*len = pkt_len;
+}
+
+static void mctp_usblib_test_rx_pkt_span(struct kunit *test)
+{
+	const struct mctp_usblib_test_pkt_span *pkt_span = test->param_value;
+	size_t len, xfer_len, off, xfer_off;
+	struct mctp_usblib_test_dev *dev;
+	struct mctp_usblib_test_ctx *ctx;
+	struct mctp_usblib_rx *rx;
+	unsigned int i;
+	u8 *pktbuf;
+	void *buf;
+	int rc;
+
+	mctp_usblib_test_pkt_span_validate(test, pkt_span, &len);
+	pktbuf = kunit_kmalloc_array(test, 1, len, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, pktbuf);
+
+	/* lay out packets */
+	for (off = 0, i = 0; i < pkt_span->n_pkts; i++) {
+		len = pkt_span->pkts[i];
+		mctp_usblib_test_init_pkt(pktbuf + off, len, len);
+		off += len;
+	}
+
+	ctx = mctp_usblib_test_init(test);
+	dev = ctx->dev;
+
+	rx = mctp_usblib_test_rx_init(test, true);
+
+	/* feed transfers */
+	for (off = 0, xfer_off = 0, i = 0; i < pkt_span->n_xfers;) {
+		xfer_len = pkt_span->xfers[i] - xfer_off;
+		rc = mctp_usblib_rx_prepare(dev->ndev, rx,
+					    &buf, &len, GFP_KERNEL);
+		KUNIT_ASSERT_EQ(test, rc, 0);
+
+		KUNIT_ASSERT_GE(test, len, ep_maxpacket);
+
+		len = min(len, xfer_len);
+		memcpy(buf, pktbuf + off, len);
+
+		if (len == xfer_len) {
+			/* whole/end xfer, proceed to next */
+			xfer_off = 0;
+			i++;
+		} else {
+			/* partial */
+			xfer_off += len;
+		}
+
+		rc = mctp_usblib_test_rx_complete(dev->ndev, rx, len);
+		KUNIT_ASSERT_EQ(test, rc, 0);
+		off += len;
+	}
+
+	/* check received packets */
+	KUNIT_EXPECT_EQ(test, dev->rx_pkts.qlen, pkt_span->n_pkts);
+	for (i = 0; ; i++) {
+		struct sk_buff *skb = __skb_dequeue(&dev->rx_pkts);
+
+		if (!skb)
+			break;
+
+		if (i < pkt_span->n_pkts)
+			KUNIT_EXPECT_EQ(test, skb->len, pkt_span->pkts[i] - 4);
+
+		kfree_skb(skb);
+	}
+}
+
+static const struct mctp_usblib_test_pkt_span mctp_usblib_test_pkt_spans[] = {
+	/* One packet completely within a transfer */
+	{ "1p1x-complete", 1, { 8 }, 1, { 8 } },
+	/* Two small packets combined within one transfer */
+	{ "2p1x-combined", 2, { 8, 8 }, 1, { 16 } },
+	/* A packet split over two transfers, at the MCTP payload */
+	{ "1p2x-split-payload", 1, { 16 }, 2, { 8, 8 } },
+	/* A packet split over two transfers, at the USB transport header */
+	{ "1p2x-split-usbhdr", 1, { 16 }, 2, { 2, 14 } },
+	/* A packet split over two transfers, at the MCTP header */
+	{ "1p2x-split-mctphdr", 1, { 16 }, 2, { 6, 10 } },
+	/* Single packet split over 3 transfers, middle entirely continuation */
+	{ "1p3x-split", 1, { 12 }, 3, { 4, 4, 4 } },
+	/* Max-sized single transfer */
+	{ "1p1x-large", 1, { 8191 }, 1, { 8191 } },
+	/* Two large packets, split at the worst-case for allocation, with a
+	 * single byte continuing the span
+	 */
+	{ "2p2x-large-split", 2, { 8190, 8190 }, 2, { 8191, 8189 } },
+};
+
+KUNIT_ARRAY_PARAM(mctp_usblib_test_rx_pkt_span, mctp_usblib_test_pkt_spans,
+		  mctp_usblib_test_pkt_span_to_desc);
+
+static struct kunit_case mctp_usblib_test_cases[] = {
+	KUNIT_CASE(mctp_usblib_test_rx_single),
+	KUNIT_CASE_PARAM(mctp_usblib_test_rx_pkt_span,
+			 mctp_usblib_test_rx_pkt_span_gen_params),
+	{}
+};
+
+static struct kunit_suite mctp_usblib_test_suite = {
+	.name = "mctp-usblib",
+	.test_cases = mctp_usblib_test_cases,
+};
+
+kunit_test_suite(mctp_usblib_test_suite);
diff --git a/drivers/net/mctp/mctp-usblib.c b/drivers/net/mctp/mctp-usblib.c
new file mode 100644
index 000000000..31997f989
--- /dev/null
+++ b/drivers/net/mctp/mctp-usblib.c
@@ -0,0 +1,625 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * mctp-usblib.c - MCTP-over-USB (DMTF DSP0283) transport helper library
+ *
+ * DSP0283 is available at:
+ * https://www.dmtf.org/sites/default/files/standards/documents/DSP0283_1.1.0.pdf
+ *
+ * Copyright (C) 2024-2026 Code Construct Pty Ltd
+ */
+
+#include <linux/module.h>
+#include <linux/netdevice.h>
+#include <linux/skbuff.h>
+#include <linux/usb/ch9.h>
+#include <linux/usb/mctp-usb.h>
+#include <net/mctp.h>
+
+int mctp_usblib_rx_init(struct mctp_usblib_rx *rx, u16 ep_pktlen, bool span)
+{
+	if (!ep_pktlen)
+		return -EINVAL;
+
+	if (ep_pktlen & ~USB_ENDPOINT_MAXP_MASK)
+		return -EINVAL;
+
+	memset(rx, 0, sizeof(*rx));
+	rx->span = span;
+	rx->ep_pktlen = ep_pktlen;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_rx_init);
+
+void mctp_usblib_rx_fini(struct mctp_usblib_rx *rx)
+{
+	kfree_skb(rx->skb);
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_rx_fini);
+
+/*
+ * Prepare a transfer buffer for future completion; *bufp and *lenp will
+ * be populated on success.
+ */
+int mctp_usblib_rx_prepare(struct net_device *netdev,
+			   struct mctp_usblib_rx *rx,
+			   void **bufp, size_t *lenp, gfp_t gfp)
+{
+	struct sk_buff *skb = rx->skb;
+	unsigned int len = 0;
+
+	if (skb && skb->len >= MCTP_USB_1_1_PKTLEN_MAX) {
+		/* something must have gone terribly wrong. clear and restart */
+		mctp_usblib_rx_cancel(rx);
+		skb = NULL;
+	}
+
+	len = rx->span ? roundup(MCTP_USB_1_1_PKTLEN_MAX, rx->ep_pktlen)
+		: MCTP_USB_1_0_XFER_SIZE;
+
+	if (!skb) {
+		skb = __netdev_alloc_skb(netdev, len, gfp);
+		if (!skb)
+			return -ENOMEM;
+
+	} else if (skb->cloned || skb_tailroom(skb) < rx->ep_pktlen) {
+		/* We always need to realloc if ->cloned, as we cannot
+		 * resubmit the (now-shared) skb buffer for possible DMA.
+		 *
+		 * Otherwise (if we have an un-cloned SKB): just ensure we
+		 * have sufficient space to prevent babble. Since we allocated
+		 * for max size in the last prepare (and have not consumed any
+		 * of that space for a prior MCTP packet, because !cloned), we
+		 * have sufficient data to finish the current MCTP packet.
+		 */
+		struct sk_buff *skb2;
+
+		skb2 = skb_copy_expand(skb, 0, len, gfp);
+		if (!skb2)
+			return -ENOMEM;
+		dev_kfree_skb_any(skb);
+		skb = skb2;
+	}
+
+	rx->skb = skb;
+
+	/* Spanning mode allows ZLPs, so we don't require exactly one
+	 * transfer packet. If we have extra tailroom, may as well use it,
+	 * and we have ensured that the tailroom >= ep_pktlen.
+	 */
+	if (rx->span)
+		len = rounddown(skb_tailroom(skb), rx->ep_pktlen);
+
+	*bufp = skb_tail_pointer(skb);
+	*lenp = len;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_rx_prepare);
+
+static void mctp_usblib_rx(struct net_device *netdev, struct sk_buff *skb)
+{
+	struct pcpu_dstats *dstats = this_cpu_ptr(netdev->dstats);
+	struct mctp_skb_cb *cb;
+	unsigned long flags;
+
+	skb_reset_mac_header(skb);
+	skb_pull(skb, sizeof(struct mctp_usb_hdr));
+
+	/* we're called from an URB completion handler, and cannot assume local
+	 * irqs are always disabled
+	 */
+	flags = u64_stats_update_begin_irqsave(&dstats->syncp);
+	u64_stats_inc(&dstats->rx_packets);
+	u64_stats_add(&dstats->rx_bytes, skb->len);
+	u64_stats_update_end_irqrestore(&dstats->syncp, flags);
+
+	skb->protocol = htons(ETH_P_MCTP);
+	skb_reset_network_header(skb);
+	cb = __mctp_cb(skb);
+	cb->halen = 0;
+	netif_rx(skb);
+}
+
+static void mctp_usblib_rx_stats_single_drop(struct net_device *dev)
+{
+	struct pcpu_dstats *dstats = this_cpu_ptr(dev->dstats);
+	unsigned long flags;
+
+	flags = u64_stats_update_begin_irqsave(&dstats->syncp);
+	u64_stats_inc(&dstats->rx_drops);
+	u64_stats_update_end_irqrestore(&dstats->syncp, flags);
+}
+
+/*
+ * Receive a USB completion of @len bytes of incoming data. We will then split
+ * this into packets and netif_rx() each. Intended to be called in atomic
+ * contexts - ie., URB completion.
+ *
+ * Assumes @netdev uses dstats.
+ */
+int mctp_usblib_rx_complete(struct net_device *netdev,
+			    struct mctp_usblib_rx *rx, size_t len)
+{
+	struct sk_buff *skb = rx->skb;
+	int rc = 0;
+
+	__skb_put(skb, len);
+
+	for (;;) {
+		struct mctp_usb_hdr *hdr;
+		struct sk_buff *skb2;
+		/* length of MCTP packet, including USB header */
+		u16 pkt_len;
+
+		/* no header yet, resubmit for the rest of the packet */
+		if (skb->len < sizeof(*hdr)) {
+			if (!rx->span) {
+				netdev_dbg(netdev,
+					   "rx: tiny xfer (%d) in non-span mode",
+					   skb->len);
+				rc = -ENOMSG;
+				goto err_reset;
+			}
+			break;
+		}
+
+		hdr = (struct mctp_usb_hdr *)skb->data;
+
+		if (be16_to_cpu(hdr->id) != MCTP_USB_DMTF_ID) {
+			/* By resetting here, will start the next IN transfer
+			 * at the beginning of the new skb. This will mean
+			 * we re-sync when we next see a spanned packet aligned
+			 * with the start of a transfer.
+			 *
+			 * In non-spanning mode, this just means we'll drop
+			 * the current transfer only
+			 */
+			netdev_dbg(netdev, "rx: invalid id %04x\n",
+				   be16_to_cpu(hdr->id));
+			rc = -EPROTO;
+			goto err_reset;
+		}
+
+		pkt_len = be16_to_cpu(hdr->len);
+		/* v1.1, with span enabled, has a 13-bit length */
+		pkt_len &= rx->span ?
+			MCTP_USB_1_1_PKTLEN_MAX : MCTP_USB_1_0_PKTLEN_MAX;
+		if (pkt_len < sizeof(*hdr) + sizeof(struct mctp_hdr)) {
+			netdev_dbg(netdev, "rx: invalid len %d\n", pkt_len);
+			rc = -EPROTO;
+			goto err_reset;
+		}
+
+		/* span continues to the next transfer, resubmit */
+		if (pkt_len > skb->len) {
+			if (!rx->span) {
+				netdev_dbg(netdev,
+					   "rx: short xfer (%d vs %d) in non-span mode",
+					   pkt_len, skb->len);
+				rc = -EPROTO;
+				goto err_reset;
+			}
+			break;
+		}
+
+		/* we have (exactly) a complete packet, RX it directly */
+		if (pkt_len == skb->len) {
+			mctp_usblib_rx(netdev, skb);
+			rx->skb = NULL;
+			break;
+		}
+
+		/* more packets follow - RX a clone so that we can continue
+		 * processing the current SKB, which may be the start of a
+		 * span.
+		 */
+		skb2 = skb_clone(skb, GFP_ATOMIC);
+		if (skb2) {
+			skb_trim(skb2, pkt_len);
+			mctp_usblib_rx(netdev, skb2);
+		} else {
+			mctp_usblib_rx_stats_single_drop(netdev);
+		}
+		skb_pull(skb, pkt_len);
+	}
+
+	return 0;
+
+err_reset:
+	dev_kfree_skb_any(rx->skb);
+	rx->skb = NULL;
+	return rc;
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_rx_complete);
+
+/*
+ * Cancel a rx context; subsequent prepare/complete calls will not be a
+ * continuation of any data already received.
+ */
+void mctp_usblib_rx_cancel(struct mctp_usblib_rx *rx)
+{
+	dev_kfree_skb_any(rx->skb);
+	rx->skb = NULL;
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_rx_cancel);
+
+/* transmit context: encapsulates one transfer */
+struct mctp_usblib_tx_ctx {
+	struct mctp_usblib_tx *tx;
+	struct sk_buff_head skbs;
+	unsigned int buf_len, len;
+	enum mctp_usblib_tx_buf_type {
+		TX_SINGLE,
+		TX_FLAT,
+	} buf_type;
+	u8 buf[] ____cacheline_aligned;
+};
+
+void mctp_usblib_tx_init(struct mctp_usblib_tx *tx,
+			 const struct mctp_usblib_tx_ops *ops,
+			 void *priv, bool span)
+{
+	memset(tx, 0, sizeof(*tx));
+	tx->ops = *ops;
+	tx->priv = priv;
+	tx->span = span;
+	spin_lock_init(&tx->lock);
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_tx_init);
+
+static int mctp_usblib_tx_avail(struct mctp_usblib_tx_ctx *ctx)
+{
+	return ctx->buf_type == TX_SINGLE ? 0 : ctx->buf_len - ctx->len;
+}
+
+static bool mctp_usblib_tx_should_send(struct mctp_usblib_tx_ctx *ctx)
+{
+	/* Use the baseline length (ie, BTU) as an approximate
+	 * "reasonably-sized" packet we could expect. If there is
+	 * insufficient capacity for that, then send.
+	 */
+	const size_t pkt_len = MCTP_USB_BTU + sizeof(struct mctp_usb_hdr);
+
+	return mctp_usblib_tx_avail(ctx) < pkt_len;
+}
+
+/*
+ * Returns zero on success, non-zero on failure - indicating that the new skb
+ * could not be appended. So, errors reported here to the TX path will result
+ * in the TX being transmitted.
+ */
+static int mctp_usblib_tx_append(struct mctp_usblib_tx_ctx *ctx,
+				 struct sk_buff *skb)
+{
+	if (ctx->buf_type == TX_SINGLE)
+		return -EINVAL;
+
+	if (mctp_usblib_tx_avail(ctx) < skb->len)
+		return -ENOBUFS;
+
+	__skb_queue_tail(&ctx->skbs, skb);
+
+	ctx->len += skb->len;
+
+	return 0;
+}
+
+static int mctp_usblib_tx_send(struct mctp_usblib_tx_ctx *ctx)
+{
+	void *buf;
+
+	/* If we have a qlen of 1, we only ended up packing a single skb,
+	 * despite allocating for multiple. Skip the copy and send directly
+	 * from the skb data.
+	 */
+	if (ctx->buf_type == TX_SINGLE || ctx->skbs.qlen == 1) {
+		buf = ctx->skbs.next->data;
+
+	} else if (ctx->buf_type == TX_FLAT) {
+		struct sk_buff *skb;
+		size_t pos = 0;
+
+		skb_queue_walk(&ctx->skbs, skb) {
+			skb_copy_bits(skb, 0, ctx->buf + pos, skb->len);
+			pos += skb->len;
+		}
+
+		buf = ctx->buf;
+	} else {
+		return -EINVAL;
+	}
+
+	return ctx->tx->ops.send(ctx, buf, ctx->len);
+}
+
+static void mctp_usblib_tx_ctx_free(struct mctp_usblib_tx_ctx *ctx,
+				    enum skb_drop_reason reason)
+{
+	struct sk_buff *skb;
+
+	if (!ctx)
+		return;
+
+	while ((skb = __skb_dequeue(&ctx->skbs)) != NULL)
+		dev_kfree_skb_any_reason(skb, reason);
+	kfree(ctx);
+}
+
+void *mctp_usblib_tx_ctx_priv(struct mctp_usblib_tx_ctx *tx_ctx)
+{
+	return tx_ctx->tx->priv;
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_tx_ctx_priv);
+
+/* caller must ensure the tx & completion path is quiesced */
+void mctp_usblib_tx_fini(struct mctp_usblib_tx *tx)
+{
+	mctp_usblib_tx_ctx_free(tx->cur_ctx, SKB_DROP_REASON_NOT_SPECIFIED);
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_tx_fini);
+
+/* Max size of a spanned TX. Since we allocate a separate span buffer, limit
+ * the tx-time allocations to 4k. Larger packets will be sent as single
+ * transfers.
+ */
+static const unsigned int TX_SPAN_MAX = 4096 - sizeof(struct mctp_usblib_tx_ctx);
+
+static struct mctp_usblib_tx_ctx *
+mctp_usblib_tx_ctx_create(struct mctp_usblib_tx *tx, struct sk_buff *skb,
+			  bool single)
+{
+	enum mctp_usblib_tx_buf_type type;
+	struct mctp_usblib_tx_ctx *ctx;
+	size_t sz = 0;
+
+	if (single || skb->len > TX_SPAN_MAX) {
+		type = TX_SINGLE;
+	} else {
+		type = TX_FLAT;
+		sz = tx->span ? TX_SPAN_MAX : MCTP_USB_1_0_XFER_SIZE;
+	}
+
+	ctx = kzalloc_flex(*ctx, buf, sz, GFP_ATOMIC);
+	if (!ctx)
+		return NULL;
+
+	ctx->tx = tx;
+	ctx->buf_type = type;
+	ctx->buf_len = sz;
+	ctx->len = skb->len;
+	skb_queue_head_init(&ctx->skbs);
+	__skb_queue_tail(&ctx->skbs, skb);
+
+	return ctx;
+}
+
+static void mctp_usblib_tx_stats_update(struct mctp_usblib_tx_ctx *ctx,
+					struct net_device *dev,
+					bool ok)
+{
+	struct pcpu_dstats *dstats = get_cpu_ptr(dev->dstats);
+	unsigned long flags;
+
+	flags = u64_stats_update_begin_irqsave(&dstats->syncp);
+	if (ok) {
+		/* Only include the network-layer data in tx stats; we know
+		 * that there is a 4-byte header pushed to all skbs in
+		 * tx_skb_prepare()
+		 */
+		u64 n = ctx->skbs.qlen;
+		s64 len = ctx->len - (n * sizeof(struct mctp_usb_hdr));
+
+		u64_stats_add(&dstats->tx_packets, n);
+		u64_stats_add(&dstats->tx_bytes, len);
+	} else {
+		u64_stats_add(&dstats->tx_drops, ctx->skbs.qlen);
+	}
+	u64_stats_update_end_irqrestore(&dstats->syncp, flags);
+	put_cpu_ptr(dev->dstats);
+}
+
+static void mctp_usblib_tx_stats_single_drop(struct net_device *dev)
+{
+	struct pcpu_dstats *dstats = get_cpu_ptr(dev->dstats);
+	unsigned long flags;
+
+	flags = u64_stats_update_begin_irqsave(&dstats->syncp);
+	u64_stats_inc(&dstats->tx_drops);
+	u64_stats_update_end_irqrestore(&dstats->syncp, flags);
+	put_cpu_ptr(dev->dstats);
+}
+
+/*
+ * Completion for the ->send() op. This will update netdev stats and
+ * free the tx context.
+ *
+ * Likely called from (atomic) URB completion context.
+ */
+void mctp_usblib_tx_send_complete(struct mctp_usblib_tx_ctx *tx_ctx,
+				  struct net_device *dev, bool ok)
+{
+	enum skb_drop_reason reason =
+		ok ? SKB_CONSUMED : SKB_DROP_REASON_NOT_SPECIFIED;
+
+	mctp_usblib_tx_stats_update(tx_ctx, dev, ok);
+	mctp_usblib_tx_ctx_free(tx_ctx, reason);
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_tx_send_complete);
+
+/* Prepare a skb for push()
+ *
+ * On error, populates @reason.
+ */
+static int mctp_usblib_tx_skb_prepare(struct sk_buff *skb, bool span,
+				      enum skb_drop_reason *reason)
+{
+	unsigned long plen, max_len;
+	struct mctp_usb_hdr *hdr;
+	int rc;
+
+	max_len = span ? MCTP_USB_1_1_PKTLEN_MAX : MCTP_USB_1_0_PKTLEN_MAX;
+
+	plen = skb->len;
+	if (plen + sizeof(*hdr) > max_len) {
+		*reason = SKB_DROP_REASON_PKT_TOO_BIG;
+		return -EMSGSIZE;
+	}
+
+	rc = skb_cow_head(skb, sizeof(*hdr));
+	if (rc) {
+		*reason = SKB_DROP_REASON_NOMEM;
+		return rc;
+	}
+
+	hdr = skb_push(skb, sizeof(*hdr));
+	if (!hdr) {
+		*reason = SKB_DROP_REASON_NOMEM;
+		return -ENOMEM;
+	}
+
+	hdr->id = cpu_to_be16(MCTP_USB_DMTF_ID);
+	hdr->len = cpu_to_be16(plen + sizeof(*hdr));
+
+	return 0;
+}
+
+/*
+ * Push a new skb to the transfer. May result in zero or more calls to
+ * ops->send().
+ *
+ * Takes ownership of @skb, including on error.
+ */
+int mctp_usblib_tx_push(struct net_device *dev,
+			struct mctp_usblib_tx *tx,
+			struct sk_buff *skb, bool more)
+{
+	struct mctp_usblib_tx_ctx *ctx, *send_ctx = NULL;
+	enum skb_drop_reason reason;
+	const int max_tries = 3;
+	unsigned long flags;
+	int try = 1, rc;
+
+	rc = mctp_usblib_tx_skb_prepare(skb, tx->span, &reason);
+	if (rc) {
+		mctp_usblib_tx_stats_single_drop(dev);
+		kfree_skb_reason(skb, reason);
+		/* we may still need to proceed, in case an existing ctx
+		 * is now sendable (ie.: !more).
+		 */
+		skb = NULL;
+	}
+
+	reason = SKB_DROP_REASON_NOT_SPECIFIED;
+retry:
+	/* Try and queue to the current context. We exit this critical section
+	 * with a few bits of state:
+	 *  - send_ctx: indicating a prior context that needs to be sent
+	 *  - skb: indicating that a skb still needs to be queued/sent
+	 */
+	spin_lock_irqsave(&tx->lock, flags);
+	ctx = tx->cur_ctx;
+	if (ctx) {
+		if (skb) {
+			rc = mctp_usblib_tx_append(ctx, skb);
+			if (rc) {
+				/* can't append to the pending tx - detach for
+				 * sending, and we'll create a new tx below.
+				 */
+				swap(tx->cur_ctx, send_ctx);
+			} else {
+				/* we have queued */
+				skb = NULL;
+				if (!more || mctp_usblib_tx_should_send(ctx))
+					swap(tx->cur_ctx, send_ctx);
+			}
+		} else if (!more) {
+			swap(tx->cur_ctx, send_ctx);
+		}
+	}
+	spin_unlock_irqrestore(&tx->lock, flags);
+
+	if (send_ctx) {
+		rc = mctp_usblib_tx_send(send_ctx);
+		if (rc) {
+			mctp_usblib_tx_stats_update(send_ctx, dev, false);
+			mctp_usblib_tx_ctx_free(send_ctx, reason);
+		}
+		send_ctx = NULL;
+	}
+
+	/* we have either queued, or the prepare failed; nothing more to do */
+	if (!skb)
+		return 0;
+
+	ctx = mctp_usblib_tx_ctx_create(tx, skb, !more);
+	if (!ctx) {
+		netdev_dbg(dev, "TX context create failed\n");
+		mctp_usblib_tx_stats_single_drop(dev);
+		kfree_skb(skb);
+		return -ENOMEM;
+	}
+
+	/* if we're ready to send now, no need to enqueue */
+	if (!more || mctp_usblib_tx_should_send(ctx)) {
+		rc = mctp_usblib_tx_send(ctx);
+		if (rc) {
+			mctp_usblib_tx_stats_update(ctx, dev, false);
+			mctp_usblib_tx_ctx_free(ctx, reason);
+		}
+		return 0;
+	}
+
+	spin_lock_irqsave(&tx->lock, flags);
+	if (!tx->cur_ctx) {
+		tx->cur_ctx = ctx;
+		ctx = NULL;
+	}
+	spin_unlock_irqrestore(&tx->lock, flags);
+
+	/* we may have lost the race with a concurrent tx; shouldn't happen, as
+	 * ndo_start_xmit should be serialised over one queue, but try again
+	 * from the top, as we may be able to queue the skb to that context.
+	 */
+	if (ctx) {
+		/* unlink the new (sole) skb, we don't want it freed with ctx */
+		__skb_queue_head_init(&ctx->skbs);
+		mctp_usblib_tx_ctx_free(ctx, reason);
+		if (++try > max_tries) {
+			kfree_skb(skb);
+			mctp_usblib_tx_stats_single_drop(dev);
+			return -EBUSY;
+		}
+		goto retry;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_tx_push);
+
+/* Cancel a tx: any un-sent context is released. */
+void mctp_usblib_tx_cancel(struct mctp_usblib_tx *tx, struct net_device *dev,
+			   enum skb_drop_reason reason)
+{
+	struct mctp_usblib_tx_ctx *ctx = NULL;
+	unsigned long flags;
+
+	spin_lock_irqsave(&tx->lock, flags);
+	swap(tx->cur_ctx, ctx);
+	spin_unlock_irqrestore(&tx->lock, flags);
+
+	if (!ctx)
+		return;
+
+	mctp_usblib_tx_stats_update(ctx, dev, false);
+	mctp_usblib_tx_ctx_free(ctx, reason);
+}
+EXPORT_SYMBOL_GPL(mctp_usblib_tx_cancel);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Jeremy Kerr <jk@codeconstruct.com.au>");
+MODULE_DESCRIPTION("MCTP USB transport library");
+
+#if IS_ENABLED(CONFIG_MCTP_TRANSPORT_USBLIB_TEST)
+#include "mctp-usblib-test.c"
+#endif