mctp i2c: MCTP I2C binding driver

Provides MCTP network transport over an I2C bus, as specified in
DMTF DSP0237. All messages between nodes are sent as SMBus Block Writes.

Each I2C bus to be used for MCTP is flagged in devicetree by a
'mctp-controller' property on the bus node. Each flagged bus gets a
mctpi2cX net device created based on the bus number. A
'mctp-i2c-controller' I2C client needs to be added under the adapter. In
an I2C mux situation the mctp-i2c-controller node must be attached only
to the root I2C bus. The I2C client will handle incoming I2C slave block
write data for subordinate busses as well as its own bus.

In configurations without devicetree a driver instance can be attached
to a bus using the I2C slave new_device mechanism.

The MCTP core will hold/release the MCTP I2C device while responses
are pending (a 6 second timeout or once a socket is closed, response
received etc). While held the MCTP I2C driver will lock the I2C bus so
that the correct I2C mux remains selected while responses are received.

(Ideally we would just lock the mux to keep the current bus selected for
the response rather than a full I2C bus lock, but that isn't exposed in
the I2C mux API)

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Reviewed-by: Wolfram Sang <wsa@kernel.org> # I2C transport parts
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 1094 insertions, 0 deletions

diff --git a/drivers/net/mctp/Kconfig b/drivers/net/mctp/Kconfig
index 2929471395ae..dc71657d9184 100644
--- a/drivers/net/mctp/Kconfig
+++ b/drivers/net/mctp/Kconfig
@@ -21,6 +21,18 @@ config MCTP_SERIAL
 	  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_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.
+
 endmenu
 
 endif
diff --git a/drivers/net/mctp/Makefile b/drivers/net/mctp/Makefile
index d32622613ce4..1ca3e6028f77 100644
--- a/drivers/net/mctp/Makefile
+++ b/drivers/net/mctp/Makefile
@@ -1 +1,2 @@
 obj-$(CONFIG_MCTP_SERIAL) += mctp-serial.o
+obj-$(CONFIG_MCTP_TRANSPORT_I2C) += mctp-i2c.o
diff --git a/drivers/net/mctp/mctp-i2c.c b/drivers/net/mctp/mctp-i2c.c
new file mode 100644
index 000000000000..365c3dfd4034
--- /dev/null
+++ b/drivers/net/mctp/mctp-i2c.c
@@ -0,0 +1,1081 @@
+// 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,
+};
+
+/* 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(sizeof(*mcli), GFP_KERNEL);
+	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) {
+		if (mcli->client)
+			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_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;
+
+	/* + 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);
+	}
+
+	if (status == NET_RX_SUCCESS) {
+		ndev->stats.rx_packets++;
+		ndev->stats.rx_bytes += skb->len;
+	} 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 if (key->dev_flow_state == MCTP_I2C_FLOW_STATE_NEW) {
+		key->dev_flow_state = MCTP_I2C_FLOW_STATE_ACTIVE;
+		state = MCTP_I2C_TX_FLOW_NEW;
+	} else {
+		state = MCTP_I2C_TX_FLOW_EXISTING;
+	}
+
+	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_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;
+
+	fs = mctp_i2c_get_tx_flow_state(midev, skb);
+
+	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;
+	}
+
+	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);
+		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;
+
+	lldst = *((u8 *)daddr);
+	llsrc = *((u8 *)saddr);
+
+	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;
+	if (hdr->byte_count > MCTP_I2C_MAXBLOCK)
+		return -EMSGSIZE;
+	hdr->source_slave = ((llsrc << 1) & 0xff) | 0x01;
+	mhdr->ver = 0x01;
+
+	return 0;
+}
+
+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);
+	unsigned long flags;
+
+	spin_lock_irqsave(&midev->lock, flags);
+	midev->release_count++;
+	spin_unlock_irqrestore(&midev->lock, flags);
+
+	/* 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);
+	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 int 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);
+	/* Callers ignore return code */
+	return 0;
+}
+
+/* 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[] = {
+	{ "mctp-i2c-interface", 0 },
+	{},
+};
+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_new = 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>");