[PATCH v1 0/4] i2c: SMBus ARP support

[PATCH v1 0/4] i2c: SMBus ARP support

[Heikki Krogerus]

Hi,

I know there has been a couple of PoCs for ARP over the years, but for
what ever reason none of them were ever made part of the kernel. I
don't know how common ARP-capable devices are, but since they do
exist, we really should enumerate them as intended. I'm guessing that
in Linux ARP-capable devices have so far been left mostly undetected.

The problem I'm trying to tackle with these is to detect SMBus devices
on discrete components like PCIe cards. We obviously won't have any
kind ACPI or DT description for those, so we would always have to
create the device for them in the drivers. Unfortunately in some cases
it's actually very difficult to know what exactly is attached to the
I2C on those cards because the vendors can put what ever they like
there - this is the case at least with some of the GPUs it seems. But
luckily those I2C/SMBus devices are at least in some cases fully
ARP-capable.

I'm including a patch that binds the detected ARP-capable MCTP devices
to the driver. There is also a target mode test driver.

thanks,

Heikki Krogerus (4):
  i2c: SMBus Address Resolution Protocol implementation for host side
  i2c: Sysfs attribute files for the Unique Device Identifier fields
  mctp i2c: Enable SMBus ARP discovery
  i2c: Add SMBus ARP target mode test driver

 Documentation/ABI/testing/sysfs-bus-i2c |  53 ++++
 drivers/i2c/Kconfig                     |   6 +
 drivers/i2c/Makefile                    |   3 +-
 drivers/i2c/i2c-core-arp.c              | 334 ++++++++++++++++++++++++
 drivers/i2c/i2c-core-base.c             | 154 ++++++++++-
 drivers/i2c/i2c-core.h                  |   8 +
 drivers/i2c/i2c-target-arp.c            | 201 ++++++++++++++
 drivers/net/mctp/mctp-i2c.c             |   8 +
 include/linux/i2c-smbus.h               |  67 +++++
 include/linux/i2c.h                     |  10 +
 include/linux/mod_devicetable.h         |  13 +
 scripts/mod/devicetable-offsets.c       |   8 +
 scripts/mod/file2alias.c                |  24 ++
 13 files changed, 878 insertions(+), 11 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-i2c
 create mode 100644 drivers/i2c/i2c-core-arp.c
 create mode 100644 drivers/i2c/i2c-target-arp.c

-- 
2.50.1

[PATCH v1 1/4] i2c: SMBus Address Resolution Protocol implementation for host side

[Heikki Krogerus]

SMBus Address Resolution Protocol (ARP) is a standard (but
optional) method to detect and identify devices on SMBus.
The ARP is higher level message protocol that can be fully
emulated. ARP will therefore always be supported, and the
ARP-capable devices will be scanned automatically for every
adapter that is registered.

The ARP devices are identified with the Unique Device
Identifier (UDID). The UDID can be used for device matching
in drivers with a new struct smbus_device_id.

The function i2c_handle_smbus_host_notify() is also modified
so that the hotplugged ARP devices that send Notify ARP
Controller command can be detected.

Link: https://www.smbus.org/specs/smbus20.pdf
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
---
 drivers/i2c/Makefile              |   2 +-
 drivers/i2c/i2c-core-arp.c        | 334 ++++++++++++++++++++++++++++++
 drivers/i2c/i2c-core-base.c       |  47 ++++-
 drivers/i2c/i2c-core.h            |   8 +
 include/linux/i2c-smbus.h         |  67 ++++++
 include/linux/i2c.h               |  10 +
 include/linux/mod_devicetable.h   |  13 ++
 scripts/mod/devicetable-offsets.c |   8 +
 scripts/mod/file2alias.c          |  24 +++
 9 files changed, 503 insertions(+), 10 deletions(-)
 create mode 100644 drivers/i2c/i2c-core-arp.c

diff --git a/drivers/i2c/Makefile b/drivers/i2c/Makefile
index d27de18de46f..4fdb37b94aa4 100644
--- a/drivers/i2c/Makefile
+++ b/drivers/i2c/Makefile
@@ -5,7 +5,7 @@
 
 obj-$(CONFIG_I2C_BOARDINFO)	+= i2c-boardinfo.o
 obj-$(CONFIG_I2C)		+= i2c-core.o
-i2c-core-y			:= i2c-core-base.o i2c-core-smbus.o
+i2c-core-y			:= i2c-core-base.o i2c-core-smbus.o i2c-core-arp.o
 i2c-core-$(CONFIG_ACPI)		+= i2c-core-acpi.o
 i2c-core-$(CONFIG_I2C_SLAVE)	+= i2c-core-slave.o
 i2c-core-$(CONFIG_OF)		+= i2c-core-of.o
diff --git a/drivers/i2c/i2c-core-arp.c b/drivers/i2c/i2c-core-arp.c
new file mode 100644
index 000000000000..706508e5dd37
--- /dev/null
+++ b/drivers/i2c/i2c-core-arp.c
@@ -0,0 +1,334 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * SMBus Address Resolution Protocol
+ *
+ * Copyright (C) 2026 Intel Corporation
+ */
+
+#include <linux/bits.h>
+#include <linux/bitfield.h>
+#include <linux/bitmap.h>
+#include <linux/dev_printk.h>
+#include <linux/device/devres.h>
+#include <linux/idr.h>
+#include <linux/i2c.h>
+#include <linux/i2c-smbus.h>
+#include <linux/list.h>
+#include <linux/mod_devicetable.h>
+#include <linux/mutex.h>
+#include <linux/slab.h>
+#include <linux/sprintf.h>
+#include <linux/string.h>
+#include <linux/types.h>
+#include <linux/workqueue.h>
+
+#include "i2c-core.h"
+
+#define I2C_SMBUS_DEFAULT_ADDR	0x61
+
+#define ARP_CMD_PREPARE_TO_ARP	0x01
+#define ARP_CMD_RESET_DEVICE	0x02
+#define ARP_CMD_GET_UDID	0x03
+#define ARP_CMD_ASSIGN_ADDRESS	0x04
+
+struct get_udid_data {
+	struct i2c_arp_udid	udid;
+	u8			target_address;
+} __packed;
+
+struct arp_work {
+	struct work_struct	work;
+	struct i2c_adapter	*adapter;
+	u8			target;
+};
+
+struct i2c_smbus_arp {
+	struct list_head	clients;
+	struct mutex		lock; /* ARP controller lock */
+};
+
+/* SMBus Specification Appendix C. */
+static const DECLARE_BITMAP(reserved_addrs, (1 << 7)) = {
+	[BIT_WORD(0)] = GENMASK(12, 0) |	/* SMBus v1.1 */
+			BIT_MASK(0x28) |	/* PMBus ZONE READ */
+			BIT_MASK(0x2c) |	/* Reserved */
+			BIT_MASK(0x2d) |	/* Reserved */
+			BIT_MASK(0x55),		/* PMBus ZONE WRITE */
+
+	[BIT_WORD(64)] = BIT_MASK(0x40) |	/* Reserved */
+			 BIT_MASK(0x41) |	/* Reserved */
+			 BIT_MASK(0x42) |	/* Reserved */
+			 BIT_MASK(0x43) |	/* Reserved */
+			 BIT_MASK(0x44) |	/* Reserved */
+			 BIT_MASK(0x48) |	/* Prototype Address */
+			 BIT_MASK(0x49) |	/* Prototype Address */
+			 BIT_MASK(0x4a) |	/* Prototype Address */
+			 BIT_MASK(0x4b) |	/* Prototype Address */
+			 BIT_MASK(0x61) |	/* SMBus Default Address */
+			 BIT_MASK(0x78) |	/* 10-bit target addressing */
+			 BIT_MASK(0x79) |	/* 10-bit target addressing */
+			 BIT_MASK(0x7a) |	/* 10-bit target addressing */
+			 BIT_MASK(0x7b) |	/* 10-bit target addressing */
+			 BIT_MASK(0x7c) |	/* Reserved */
+			 BIT_MASK(0x7d) |	/* Reserved */
+			 BIT_MASK(0x7e) |	/* Reserved */
+			 BIT_MASK(0x7f)		/* Reserved */
+};
+
+const struct smbus_device_id *i2c_smbus_match_id(const struct i2c_client *client,
+						 const struct smbus_device_id *id)
+{
+	struct i2c_arp_udid *udid;
+
+	if (!(id && client && client->udid))
+		return NULL;
+
+	udid = client->udid;
+
+	while (id->vendor) {
+		if ((id->vendor == SMBUS_ANY_ID || id->vendor == udid->vendor) &&
+		    (id->device == SMBUS_ANY_ID || id->device == udid->device) &&
+		    /* The interface is split into "version" and "protocol". */
+		    (id->interface == SMBUS_ANY_ID ||
+		      /* minimum version must be supported */
+		     (((id->interface & 0xf) <= (udid->interface & 0xf)) &&
+		      /* the protocol must match */
+		      ((id->interface & 0xf0) == (udid->interface & 0xf0)))) &&
+		    (id->subvendor == SMBUS_ANY_ID || id->subvendor == udid->subvendor) &&
+		    (id->subdevice == SMBUS_ANY_ID || id->subdevice == udid->subdevice) &&
+		    (id->vendor_specific_id == SMBUS_ANY_VENDOR_SPECIFIC_ID ||
+		     id->vendor_specific_id == udid->vendor_specific_id))
+			return id;
+		id++;
+	}
+
+	return NULL;
+}
+
+static int i2c_smbus_arp_get_udid(struct i2c_adapter *adapter, u8 target,
+				  struct get_udid_data *get_udid)
+{
+	union i2c_smbus_data data;
+	u8 command;
+	int ret;
+
+	if (target)
+		command = (target << 1) | 1;
+	else
+		command = ARP_CMD_GET_UDID;
+
+	ret = i2c_smbus_xfer(adapter, I2C_SMBUS_DEFAULT_ADDR,
+			     I2C_CLIENT_PEC, I2C_SMBUS_READ,
+			     command, I2C_SMBUS_BLOCK_DATA, &data);
+	if (ret)
+		return ret;
+
+	memcpy(get_udid, &data.block[1], sizeof(*get_udid));
+
+	return 0;
+}
+
+static int i2c_smbus_arp_verify_address(struct i2c_adapter *adapter,
+					struct get_udid_data *get_udid)
+{
+	u8 addr_type = FIELD_GET(ARP_CAP_ADDRESS_TYPE_MASK,
+				 get_udid->udid.capabilities);
+	u8 addr = get_udid->target_address;
+
+	if (addr_type == ARP_CAP_ADDRESS_TYPE_FIXED)
+		return 0;
+
+	/* Find a free address if necessary. */
+	if (addr == 0xff || i2c_check_addr_busy(adapter, addr)) {
+		for_each_clear_bit(addr, reserved_addrs, 128)
+			if (!i2c_check_addr_busy(adapter, addr))
+				break;
+		if (addr == 128)
+			return -EBUSY;
+
+		get_udid->target_address = addr;
+	}
+
+	return 0;
+}
+
+static int i2c_smbus_arp_assign_address(struct i2c_adapter *adapter,
+					struct get_udid_data *get_udid)
+{
+	union i2c_smbus_data data;
+	int ret;
+
+	ret = i2c_smbus_arp_verify_address(adapter, get_udid);
+	if (ret)
+		return ret;
+
+	data.block[0] = sizeof(*get_udid);
+	memcpy(&data.block[1], get_udid, data.block[0]);
+
+	ret = i2c_smbus_xfer(adapter, I2C_SMBUS_DEFAULT_ADDR,
+			     I2C_CLIENT_PEC, I2C_SMBUS_WRITE,
+			     ARP_CMD_ASSIGN_ADDRESS, I2C_SMBUS_BLOCK_DATA, &data);
+	if (ret)
+		return -EAGAIN;
+
+	return 0;
+}
+
+static void i2c_smbus_arp_remove_client(void *udid)
+{
+	kfree(udid);
+}
+
+static int i2c_smbus_arp_new_client(struct i2c_adapter *adapter,
+				    struct get_udid_data *data)
+{
+	struct i2c_board_info info = { };
+	struct i2c_client *client;
+	int ret;
+
+	info.udid = kmemdup(&data->udid, sizeof(data->udid), GFP_KERNEL);
+	if (!info.udid)
+		return -ENOMEM;
+
+	info.addr = data->target_address;
+	info.flags = I2C_CLIENT_HOST_NOTIFY;
+
+	if (data->udid.capabilities & ARP_CAP_PEC_SUPPORTED)
+		info.flags |= I2C_CLIENT_PEC;
+
+	sprintf(info.type, "%d:arp-%zu", i2c_adapter_id(adapter),
+		list_count_nodes(&adapter->arp->clients));
+	info.dev_name = info.type;
+
+	client = i2c_new_client_device(adapter, &info);
+	if (IS_ERR(client)) {
+		kfree(info.udid);
+		return PTR_ERR(client);
+	}
+
+	ret = devm_add_action_or_reset(&client->dev, i2c_smbus_arp_remove_client, info.udid);
+	if (ret)
+		return ret;
+
+	list_add_tail(&client->detected, &adapter->arp->clients);
+
+	return 0;
+}
+
+static void i2c_smbus_arp_work(struct work_struct *work)
+{
+	struct arp_work *awork = container_of(work, struct arp_work, work);
+	struct i2c_adapter *adapter = awork->adapter;
+	u8 target = awork->target;
+	struct get_udid_data data;
+	int ret;
+
+	mutex_lock(&adapter->arp->lock);
+
+	do {
+		if (i2c_smbus_arp_get_udid(adapter, target, &data))
+			break;
+
+		ret = i2c_smbus_arp_assign_address(adapter, &data);
+		if (ret == -EAGAIN)
+			continue;
+		if (ret < 0) {
+			dev_warn(&adapter->dev, "out of addresses\n");
+			break;
+		}
+
+		if (i2c_smbus_arp_new_client(adapter, &data))
+			break;
+	} while (!target);
+
+	mutex_unlock(&adapter->arp->lock);
+	kfree(awork);
+}
+
+/**
+ * i2c_smbus_arp_detect - Schedule detection and registration of ARP devices
+ * @adapter: ARP Controller
+ * @target_address: Address for directed ARP commands
+ *
+ * Registers ARP-capable devices attached to @adapter. If @target_address is
+ * supplied, directed Get UDID command will be used. Otherwise, if
+ * @target_address is 0, the general Get UDID command is used until there are no
+ * more responses.
+ *
+ * Returns 0 on success or errno.
+ */
+int i2c_smbus_arp_detect(struct i2c_adapter *adapter, u8 target_address)
+{
+	struct arp_work *awork;
+
+	if (!adapter->arp)
+		return -ENXIO;
+
+	awork = kzalloc_obj(*awork, GFP_KERNEL);
+	if (!awork)
+		return -ENOMEM;
+
+	INIT_WORK(&awork->work, i2c_smbus_arp_work);
+	awork->target = target_address;
+	awork->adapter = adapter;
+
+	queue_work(system_long_wq, &awork->work);
+
+	return 0;
+}
+
+/**
+ * i2c_smbus_arp_probe - Declare adapter as ARP controller
+ * @adapter: ARP Controller
+ *
+ * Declare @adapter as the ARP controller with the Prepare to ARP command, and
+ * then detect all available ARP devices with the general Get UDID command.
+ *
+ * Returns 0 on success or errno.
+ */
+int i2c_smbus_arp_probe(struct i2c_adapter *adapter)
+{
+	int ret;
+
+	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE | I2C_FUNC_SMBUS_BLOCK_DATA))
+		return 0;
+
+	adapter->arp = devm_kzalloc(&adapter->dev, sizeof(*adapter->arp), GFP_KERNEL);
+	if (!adapter->arp)
+		return -ENOMEM;
+
+	mutex_init(&adapter->arp->lock);
+	INIT_LIST_HEAD(&adapter->arp->clients);
+
+	/* Broadcast "Prepare to ARP" command. */
+	ret = i2c_smbus_xfer(adapter, I2C_SMBUS_DEFAULT_ADDR, I2C_CLIENT_PEC,
+			     I2C_SMBUS_WRITE, ARP_CMD_PREPARE_TO_ARP,
+			     I2C_SMBUS_BYTE, NULL);
+	if (ret)
+		return 0;
+
+	return i2c_smbus_arp_detect(adapter, 0);
+}
+
+/**
+ * i2c_smbus_arp_remove - Unregister all ARP devices
+ * @adapter: ARP Controller
+ *
+ * Unregister all ARP devices attached to @adapter.
+ */
+void i2c_smbus_arp_remove(struct i2c_adapter *adapter)
+{
+	struct i2c_client *client, *next;
+
+	if (!adapter->arp)
+		return;
+
+	mutex_lock(&adapter->arp->lock);
+
+	list_for_each_entry_safe(client, next, &adapter->arp->clients, detected) {
+		list_del(&client->detected);
+		i2c_unregister_device(client);
+	}
+
+	mutex_unlock(&adapter->arp->lock);
+}
diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
index ae7e9c8b65a6..2e61f1be8de6 100644
--- a/drivers/i2c/i2c-core-base.c
+++ b/drivers/i2c/i2c-core-base.c
@@ -120,19 +120,23 @@ EXPORT_SYMBOL_GPL(i2c_match_id);
 const void *i2c_get_match_data(const struct i2c_client *client)
 {
 	struct i2c_driver *driver = to_i2c_driver(client->dev.driver);
+	const struct smbus_device_id *smbus_match;
 	const struct i2c_device_id *match;
 	const void *data;
 
 	data = device_get_match_data(&client->dev);
-	if (!data) {
-		match = i2c_match_id(driver->id_table, client);
-		if (!match)
-			return NULL;
+	if (data)
+		return data;
 
-		data = (const void *)match->driver_data;
-	}
+	smbus_match = i2c_smbus_match_id(client, driver->smbus_id_table);
+	if (smbus_match)
+		return (const void *)smbus_match->driver_data;
+
+	match = i2c_match_id(driver->id_table, client);
+	if (match)
+		return (const void *)match->driver_data;
 
-	return data;
+	return NULL;
 }
 EXPORT_SYMBOL(i2c_get_match_data);
 
@@ -152,6 +156,10 @@ static int i2c_device_match(struct device *dev, const struct device_driver *drv)
 
 	driver = to_i2c_driver(drv);
 
+	/* SMBus Unique Device Identifier match */
+	if (i2c_smbus_match_id(client, driver->smbus_id_table))
+		return 1;
+
 	/* Finally an I2C match */
 	if (i2c_match_id(driver->id_table, client))
 		return 1;
@@ -172,6 +180,13 @@ static int i2c_device_uevent(const struct device *dev, struct kobj_uevent_env *e
 	if (rc != -ENODEV)
 		return rc;
 
+	if (client->udid)
+		return add_uevent_var(env, "MODALIAS=smbus:v%04xd%04xi%04xsv%04xsd%04xvsi%08x",
+				  client->udid->vendor, client->udid->device,
+				  client->udid->interface,
+				  client->udid->subvendor, client->udid->subdevice,
+				  client->udid->vendor_specific_id);
+
 	return add_uevent_var(env, "MODALIAS=%s%s", I2C_MODULE_PREFIX, client->name);
 }
 
@@ -684,6 +699,13 @@ modalias_show(struct device *dev, struct device_attribute *attr, char *buf)
 	if (len != -ENODEV)
 		return len;
 
+	if (client->udid)
+		return sysfs_emit(buf, "smbus:v%04xd%04xi%04xsv%04xsd%04xvsi%08x\n",
+				  client->udid->vendor, client->udid->device,
+				  client->udid->interface,
+				  client->udid->subvendor, client->udid->subdevice,
+				  client->udid->vendor_specific_id);
+
 	return sprintf(buf, "%s%s\n", I2C_MODULE_PREFIX, client->name);
 }
 static DEVICE_ATTR_RO(modalias);
@@ -822,7 +844,7 @@ static int i2c_check_mux_children(struct device *dev, void *addrp)
 	return result;
 }
 
-static int i2c_check_addr_busy(struct i2c_adapter *adapter, int addr)
+int i2c_check_addr_busy(struct i2c_adapter *adapter, int addr)
 {
 	struct i2c_adapter *parent = i2c_parent_is_i2c_adapter(adapter);
 	int result = 0;
@@ -973,6 +995,7 @@ i2c_new_client_device(struct i2c_adapter *adap, struct i2c_board_info const *inf
 	client->dev.platform_data = info->platform_data;
 	client->flags = info->flags;
 	client->addr = info->addr;
+	client->udid = info->udid;
 
 	client->init_irq = info->irq;
 	if (!client->init_irq)
@@ -1508,7 +1531,7 @@ int i2c_handle_smbus_host_notify(struct i2c_adapter *adap, unsigned short addr)
 
 	irq = irq_find_mapping(adap->host_notify_domain, addr);
 	if (irq <= 0)
-		return -ENXIO;
+		return i2c_smbus_arp_detect(adap, addr);
 
 	generic_handle_irq_safe(irq);
 
@@ -1602,6 +1625,11 @@ static int i2c_register_adapter(struct i2c_adapter *adap)
 	bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);
 	mutex_unlock(&core_lock);
 
+	/* Detect ARP clients. */
+	res = i2c_smbus_arp_probe(adap);
+	if (res)
+		dev_err(&adap->dev, "ARP device registration failed\n");
+
 	return 0;
 
 out_reg:
@@ -1766,6 +1794,7 @@ void i2c_del_adapter(struct i2c_adapter *adap)
 		return;
 	}
 
+	i2c_smbus_arp_remove(adap);
 	i2c_acpi_remove_space_handler(adap);
 	/* Tell drivers about this removal */
 	mutex_lock(&core_lock);
diff --git a/drivers/i2c/i2c-core.h b/drivers/i2c/i2c-core.h
index 4797ba88331c..c8f7d8e49621 100644
--- a/drivers/i2c/i2c-core.h
+++ b/drivers/i2c/i2c-core.h
@@ -60,6 +60,8 @@ static inline int __i2c_check_suspended(struct i2c_adapter *adap)
 	return 0;
 }
 
+int i2c_check_addr_busy(struct i2c_adapter *adapter, int addr);
+
 #ifdef CONFIG_ACPI
 void i2c_acpi_register_devices(struct i2c_adapter *adap);
 
@@ -106,3 +108,9 @@ static inline int i2c_setup_smbus_alert(struct i2c_adapter *adap)
 	return 0;
 }
 #endif
+
+const struct smbus_device_id *i2c_smbus_match_id(const struct i2c_client *client,
+						 const struct smbus_device_id *id);
+int i2c_smbus_arp_detect(struct i2c_adapter *adapter, u8 target_address);
+int i2c_smbus_arp_probe(struct i2c_adapter *adapter);
+void i2c_smbus_arp_remove(struct i2c_adapter *adapter);
diff --git a/include/linux/i2c-smbus.h b/include/linux/i2c-smbus.h
index dc1bd2ab4c13..c08b35d2858a 100644
--- a/include/linux/i2c-smbus.h
+++ b/include/linux/i2c-smbus.h
@@ -51,4 +51,71 @@ static inline void i2c_register_spd_write_disable(struct i2c_adapter *adap) { }
 static inline void i2c_register_spd_write_enable(struct i2c_adapter *adap) { }
 #endif
 
+/**
+ * struct i2c_arp_udid - ARP Unique Device Identifier
+ * @capabilities:
+ * @version:
+ * @vendor:
+ * @device:
+ * @interface:
+ * @subvendor:
+ * @subdevice:
+ * @vendor_specific_id:
+ */
+struct i2c_arp_udid {
+	u8 capabilities;
+#define ARP_CAP_PEC_SUPPORTED				BIT(0)
+#define ARP_CAP_ADDRESS_TYPE_MASK			GENMASK(7, 6)
+#define   ARP_CAP_ADDRESS_TYPE_FIXED			0
+#define   ARP_CAP_ADDRESS_TYPE_DYNAMIC_PERSISTENT	1
+#define   ARP_CAP_ADDRESS_TYPE_DYNAMIC_VOLATILE		2
+#define   ARP_CAP_ADDRESS_TYPE_RANDOM_NUMBER		3
+	u8 version;
+	u16 vendor;
+	u16 device;
+	u16 interface;
+	u16 subvendor;
+	u16 subdevice;
+	u32 vendor_specific_id;
+} __packed;
+
+/**
+ * SMBUS_DEVICE - macro used to describe a specific SMBus device
+ * @v: the 16 bit UDID Vendor ID
+ * @d: the 16 bit UDID Device ID
+ *
+ * This macro is used to create a struct smbus_device_id that matches a
+ * specific device. The interface, subvendor and subdevice fields will be set to
+ * SMBUS_ANY_ID.
+ */
+#define SMBUS_DEVICE(v, d) \
+	.vendor = (v), .device = (d), \
+	.interface = SMBUS_ANY_ID, \
+	.subvendor = SMBUS_ANY_ID, .subdevice = SMBUS_ANY_ID, \
+	.vendor_specific_id = SMBUS_ANY_VENDOR_SPECIFIC_ID,
+
+/**
+ * SMBUS_INTERFACE - macro used to describe a specific SMBus interface
+ * @i: the 16 bit UDID interface
+ *
+ * This macro is used to create a struct smbus_device_id that matches a
+ * specific interface. The vendor, device, subvendor and subdevice fields will be
+ * set to SMBUS_ANY_ID.
+ */
+#define SMBUS_INTERFACE(i) \
+	.vendor = SMBUS_ANY_ID, .device = SMBUS_ANY_ID, \
+	.interface = (i), \
+	.subvendor = SMBUS_ANY_ID, .subdevice = SMBUS_ANY_ID, \
+	.vendor_specific_id = SMBUS_ANY_VENDOR_SPECIFIC_ID,
+
+/* The version field in the interface - these are used as the minimum. */
+#define SMBUS_INTERFACE_SMBUS_V2_0	0x4
+#define SMBUS_INTERFACE_SMBUS_V3_0	0x5
+
+/* The interfaces defined in the SMBus specification. */
+#define SMBUS_INTERFACE_OEM	SMBUS_INTERFACE(BIT(4) | SMBUS_INTERFACE_SMBUS_V2_0)
+#define SMBUS_INTERFACE_ASF	SMBUS_INTERFACE(BIT(5) | SMBUS_INTERFACE_SMBUS_V2_0)
+#define SMBUS_INTERFACE_IPMI	SMBUS_INTERFACE(BIT(6) | SMBUS_INTERFACE_SMBUS_V2_0)
+#define SMBUS_INTERFACE_ZONE	SMBUS_INTERFACE(BIT(7) | SMBUS_INTERFACE_SMBUS_V3_0)
+
 #endif /* _LINUX_I2C_SMBUS_H */
diff --git a/include/linux/i2c.h b/include/linux/i2c.h
index 20fd41b51d5c..1f0cd285e711 100644
--- a/include/linux/i2c.h
+++ b/include/linux/i2c.h
@@ -34,6 +34,8 @@ struct i2c_adapter;
 struct i2c_client;
 struct i2c_driver;
 struct i2c_device_identity;
+struct i2c_smbus_arp;
+struct i2c_arp_udid;
 union i2c_smbus_data;
 struct i2c_board_info;
 enum i2c_slave_event;
@@ -242,6 +244,7 @@ enum i2c_driver_flags {
  * @command: Callback for bus-wide signaling (optional)
  * @driver: Device driver model driver
  * @id_table: List of I2C devices supported by this driver
+ * @smbus_id_table: SMBus ARP devices supported by this driver
  * @detect: Callback for device detection
  * @address_list: The I2C addresses to probe (for detect)
  * @clients: List of detected clients we created (for i2c-core use only)
@@ -295,6 +298,7 @@ struct i2c_driver {
 
 	struct device_driver driver;
 	const struct i2c_device_id *id_table;
+	const struct smbus_device_id *smbus_id_table;
 
 	/* Device detection callback for automatic device creation */
 	int (*detect)(struct i2c_client *client, struct i2c_board_info *info);
@@ -311,6 +315,7 @@ struct i2c_driver {
  * @addr: Address used on the I2C bus connected to the parent adapter.
  * @name: Indicates the type of the device, usually a chip name that's
  *	generic enough to hide second-sourcing and compatible revisions.
+ * @udid: SMBus ARP Unique Device Identifier
  * @adapter: manages the bus segment hosting this I2C device
  * @dev: Driver model device node for the slave.
  * @init_irq: IRQ that was set at initialization
@@ -343,6 +348,7 @@ struct i2c_client {
 					/* addresses are stored in the	*/
 					/* _LOWER_ 7 bits		*/
 	char name[I2C_NAME_SIZE];
+	struct i2c_arp_udid *udid;	/* Unique Device Identifier	*/
 	struct i2c_adapter *adapter;	/* the adapter we sit on	*/
 	struct device dev;		/* the device structure		*/
 	int init_irq;			/* irq set at initialization	*/
@@ -410,6 +416,7 @@ static inline bool i2c_detect_slave_mode(struct device *dev) { return false; }
  * @resources: resources associated with the device
  * @num_resources: number of resources in the @resources array
  * @irq: stored in i2c_client.irq
+ * @udid: SMBus ARP Unique Device Identifier
  *
  * I2C doesn't actually support hardware probing, although controllers and
  * devices may be able to use I2C_SMBUS_QUICK to tell whether or not there's
@@ -433,6 +440,7 @@ struct i2c_board_info {
 	const struct resource *resources;
 	unsigned int	num_resources;
 	int		irq;
+	struct i2c_arp_udid *udid;
 };
 
 /**
@@ -755,6 +763,8 @@ struct i2c_adapter {
 	struct mutex userspace_clients_lock;
 	struct list_head userspace_clients;
 
+	struct i2c_smbus_arp *arp;
+
 	struct i2c_bus_recovery_info *bus_recovery_info;
 	const struct i2c_adapter_quirks *quirks;
 
diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
index 24eb5a88a5c5..9d75cdb81e25 100644
--- a/include/linux/mod_devicetable.h
+++ b/include/linux/mod_devicetable.h
@@ -480,6 +480,19 @@ struct i2c_device_id {
 	kernel_ulong_t driver_data;	/* Data private to the driver */
 };
 
+#define SMBUS_ANY_ID			0xffff
+#define SMBUS_ANY_VENDOR_SPECIFIC_ID	0xffffffff
+
+struct smbus_device_id {
+	__u16 vendor;
+	__u16 device;
+	__u16 interface;
+	__u16 subvendor;
+	__u16 subdevice;
+	__u32 vendor_specific_id;
+	kernel_ulong_t driver_data;	/* Data private to the driver */
+};
+
 /* pci_epf */
 
 #define PCI_EPF_NAME_SIZE	20
diff --git a/scripts/mod/devicetable-offsets.c b/scripts/mod/devicetable-offsets.c
index b4178c42d08f..cd53628e1470 100644
--- a/scripts/mod/devicetable-offsets.c
+++ b/scripts/mod/devicetable-offsets.c
@@ -147,6 +147,14 @@ int main(void)
 	DEVID(i2c_device_id);
 	DEVID_FIELD(i2c_device_id, name);
 
+	DEVID(smbus_device_id);
+	DEVID_FIELD(smbus_device_id, vendor);
+	DEVID_FIELD(smbus_device_id, device);
+	DEVID_FIELD(smbus_device_id, interface);
+	DEVID_FIELD(smbus_device_id, subvendor);
+	DEVID_FIELD(smbus_device_id, subdevice);
+	DEVID_FIELD(smbus_device_id, vendor_specific_id);
+
 	DEVID(i3c_device_id);
 	DEVID_FIELD(i3c_device_id, match_flags);
 	DEVID_FIELD(i3c_device_id, dcr);
diff --git a/scripts/mod/file2alias.c b/scripts/mod/file2alias.c
index 4e99393a35f1..66efff6d3623 100644
--- a/scripts/mod/file2alias.c
+++ b/scripts/mod/file2alias.c
@@ -840,6 +840,29 @@ static void do_i2c_entry(struct module *mod, void *symval)
 	module_alias_printf(mod, false, I2C_MODULE_PREFIX "%s", *name);
 }
 
+/* Looks like: smbus:vNdNiNsvNsdNvsiN*/
+static void do_smbus_entry(struct module *mod, void *symval)
+{
+	char alias[256] = {};
+
+	DEF_FIELD(symval, smbus_device_id, vendor);
+	DEF_FIELD(symval, smbus_device_id, device);
+	DEF_FIELD(symval, smbus_device_id, interface);
+	DEF_FIELD(symval, smbus_device_id, subvendor);
+	DEF_FIELD(symval, smbus_device_id, subdevice);
+	DEF_FIELD(symval, smbus_device_id, vendor_specific_id);
+
+	ADD(alias, "v", vendor != SMBUS_ANY_ID, vendor);
+	ADD(alias, "d", device != SMBUS_ANY_ID, device);
+	ADD(alias, "i", interface != SMBUS_ANY_ID, interface);
+	ADD(alias, "sv", subvendor != SMBUS_ANY_ID, subvendor);
+	ADD(alias, "sd", subdevice != SMBUS_ANY_ID, subdevice);
+	ADD(alias, "vsi", vendor_specific_id != SMBUS_ANY_VENDOR_SPECIFIC_ID,
+	    vendor_specific_id);
+
+	module_alias_printf(mod, true, "smbus:%s", alias);
+}
+
 static void do_i3c_entry(struct module *mod, void *symval)
 {
 	char alias[256] = {};
@@ -1439,6 +1462,7 @@ static const struct devtable devtable[] = {
 	{"vmbus", SIZE_hv_vmbus_device_id, do_vmbus_entry},
 	{"rpmsg", SIZE_rpmsg_device_id, do_rpmsg_entry},
 	{"i2c", SIZE_i2c_device_id, do_i2c_entry},
+	{"smbus", SIZE_smbus_device_id, do_smbus_entry},
 	{"i3c", SIZE_i3c_device_id, do_i3c_entry},
 	{"slim", SIZE_slim_device_id, do_slim_entry},
 	{"spi", SIZE_spi_device_id, do_spi_entry},
-- 
2.50.1

[PATCH v1 2/4] i2c: Sysfs attribute files for the Unique Device Identifier fields

[Heikki Krogerus]

In order to utilise the SMBus Address Resolution Protocol's
(ARP) Unique Device Identifier (UDID) also in user space,
the UDID details need to be exposed. With ARP the address is
also dynamically assigned (and may be reset) so it also
needs to be exposed to the user space with its own file.

The UDID details are only visible with ARP devices, but the
address file is made always visible for all I2C client
devices.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
---
 Documentation/ABI/testing/sysfs-bus-i2c |  53 ++++++++++++
 drivers/i2c/i2c-core-base.c             | 107 +++++++++++++++++++++++-
 2 files changed, 159 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-i2c

diff --git a/Documentation/ABI/testing/sysfs-bus-i2c b/Documentation/ABI/testing/sysfs-bus-i2c
new file mode 100644
index 000000000000..b9ebfc2e1b9d
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-i2c
@@ -0,0 +1,53 @@
+What:		/sys/bus/i2c/devices/.../address
+Date:		September 2025
+Contact:	Heikki Krogerus <heikki.krogerus@linux.intel.com>
+Description:
+		This file has the target address the I2C client responds to.
+
+What:		/sys/bus/i2c/devices/.../vendor
+Date:		September 2025
+Contact:	Heikki Krogerus <heikki.krogerus@linux.intel.com>
+Description:
+		This file is only visible for ARP devices. It returns the Vendor
+		ID field from the SMBus ARP Unique Device Identifier. It is the
+		device manufacturer's ID assigned by the SBS Implementers Forum
+		or the PCI SIG.
+
+What:		/sys/bus/i2c/devices/.../device
+Date:		September 2025
+Contact:	Heikki Krogerus <heikki.krogerus@linux.intel.com>
+Description:
+		This file is only visible for ARP devices. It returns the Device
+		ID field from the SMBus ARP Unique Device Identifier. The
+		device ID is assigned by the device manufacturer.
+
+What:		/sys/bus/i2c/devices/.../interface
+Date:		September 2025
+Contact:	Heikki Krogerus <heikki.krogerus@linux.intel.com>
+Description:
+		This file is only visible for ARP devices. It returns the
+		Interface field from the SMBus ARP Unique Device Identifier.
+
+What:		/sys/bus/i2c/devices/.../subsystem_vendor
+Date:		September 2025
+Contact:	Heikki Krogerus <heikki.krogerus@linux.intel.com>
+Description:
+		This file is only visible for ARP devices. It returns the
+		Subsystem Vendor ID field from the SMBus ARP Unique Device
+		Identifier.
+
+What:		/sys/bus/i2c/devices/.../subsystem_device
+Date:		September 2025
+Contact:	Heikki Krogerus <heikki.krogerus@linux.intel.com>
+Description:
+		This file is only visible for ARP devices. It returns the
+		Subsystem Device ID field from the SMBus ARP Unique Device
+		Identifier.
+
+What:		/sys/bus/i2c/devices/.../vendor_specific_id
+Date:		September 2025
+Contact:	Heikki Krogerus <heikki.krogerus@linux.intel.com>
+Description:
+		This file is only visible for ARP devices. It returns the
+		Vendor Specific ID field from the SMBus ARP Unique Device
+		Identifier.
diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
index 2e61f1be8de6..6a3452a25972 100644
--- a/drivers/i2c/i2c-core-base.c
+++ b/drivers/i2c/i2c-core-base.c
@@ -685,6 +685,13 @@ name_show(struct device *dev, struct device_attribute *attr, char *buf)
 }
 static DEVICE_ATTR_RO(name);
 
+static ssize_t
+address_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "0x%02x\n", to_i2c_client(dev)->addr);
+}
+static DEVICE_ATTR_RO(address);
+
 static ssize_t
 modalias_show(struct device *dev, struct device_attribute *attr, char *buf)
 {
@@ -712,11 +719,109 @@ static DEVICE_ATTR_RO(modalias);
 
 static struct attribute *i2c_dev_attrs[] = {
 	&dev_attr_name.attr,
+	&dev_attr_address.attr,
 	/* modalias helps coldplug:  modprobe $(cat .../modalias) */
 	&dev_attr_modalias.attr,
 	NULL
 };
-ATTRIBUTE_GROUPS(i2c_dev);
+
+static const struct attribute_group i2c_dev_group = {
+	.attrs = i2c_dev_attrs,
+};
+
+static ssize_t
+capabilities_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+
+	return sysfs_emit(buf, "0x%02x\n", client->udid->capabilities);
+}
+static DEVICE_ATTR_RO(capabilities);
+
+static ssize_t
+vendor_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+
+	return sysfs_emit(buf, "0x%04x\n", client->udid->vendor);
+}
+static DEVICE_ATTR_RO(vendor);
+
+static ssize_t
+device_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+
+	return sysfs_emit(buf, "0x%04x\n", client->udid->device);
+}
+static DEVICE_ATTR_RO(device);
+
+static ssize_t
+interface_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+
+	return sysfs_emit(buf, "0x%04x\n", client->udid->interface);
+}
+static DEVICE_ATTR_RO(interface);
+
+static ssize_t
+subsystem_vendor_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+
+	return sysfs_emit(buf, "0x%04x\n", client->udid->subvendor);
+}
+static DEVICE_ATTR_RO(subsystem_vendor);
+
+static ssize_t
+subsystem_device_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+
+	return sysfs_emit(buf, "0x%04x\n", client->udid->subdevice);
+}
+static DEVICE_ATTR_RO(subsystem_device);
+
+static ssize_t
+vendor_specific_id_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+
+	return sysfs_emit(buf, "0x%08x\n", client->udid->vendor_specific_id);
+}
+static DEVICE_ATTR_RO(vendor_specific_id);
+
+static struct attribute *udid_attrs[] = {
+	&dev_attr_capabilities.attr,
+	&dev_attr_vendor.attr,
+	&dev_attr_device.attr,
+	&dev_attr_interface.attr,
+	&dev_attr_subsystem_vendor.attr,
+	&dev_attr_subsystem_device.attr,
+	&dev_attr_vendor_specific_id.attr,
+	NULL
+};
+
+static umode_t
+udid_is_visible(struct kobject *kobj, struct attribute *attr, int n)
+{
+	if (to_i2c_client(kobj_to_dev(kobj))->udid)
+		return attr->mode;
+
+	return 0;
+}
+
+static const struct attribute_group udid_group = {
+	.is_visible = udid_is_visible,
+	.attrs = udid_attrs,
+};
+
+static const struct attribute_group *i2c_dev_groups[] = {
+	&i2c_dev_group,
+	&udid_group,
+	NULL
+};
 
 const struct bus_type i2c_bus_type = {
 	.name		= "i2c",
-- 
2.50.1

[PATCH v1 3/4] mctp i2c: Enable SMBus ARP discovery

[Heikki Krogerus]

Since the SMBus Address Resolution Protocol (ARP) is now
supported with all I2C host adapters, every ARP-capable MCTP
device will get automatically enumerated. Those devices just
need to be bind to this driver.

The SMBus ARP-capable MCTP devices are identified by
checking the interface (MCTP SMBus/I2C Transport Binding
Specification section 6.5). The interface must match the ASF
protocol, so all devices that use the ASF protocol as their
interface will be probed by this driver.

Link: https://www.dmtf.org/sites/default/files/standards/documents/DSP0237_1.2.0.pdf
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
---
 drivers/net/mctp/mctp-i2c.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/mctp/mctp-i2c.c b/drivers/net/mctp/mctp-i2c.c
index f782d93f826e..3a0ffe941c76 100644
--- a/drivers/net/mctp/mctp-i2c.c
+++ b/drivers/net/mctp/mctp-i2c.c
@@ -21,6 +21,7 @@
 #include <linux/netdevice.h>
 #include <linux/i2c.h>
 #include <linux/i2c-mux.h>
+#include <linux/i2c-smbus.h>
 #include <linux/if_arp.h>
 #include <net/mctp.h>
 #include <net/mctpdevice.h>
@@ -1096,6 +1097,12 @@ static const struct i2c_device_id mctp_i2c_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, mctp_i2c_id);
 
+static const struct smbus_device_id mctp_smbus_id[] = {
+	{ SMBUS_INTERFACE_ASF },
+	{}
+};
+MODULE_DEVICE_TABLE(smbus, mctp_smbus_id);
+
 static const struct of_device_id mctp_i2c_of_match[] = {
 	{ .compatible = "mctp-i2c-controller" },
 	{},
@@ -1110,6 +1117,7 @@ static struct i2c_driver mctp_i2c_driver = {
 	.probe = mctp_i2c_probe,
 	.remove = mctp_i2c_remove,
 	.id_table = mctp_i2c_id,
+	.smbus_id_table = mctp_smbus_id,
 };
 
 static __init int mctp_i2c_mod_init(void)
-- 
2.50.1

[PATCH v1 4/4] i2c: Add SMBus ARP target mode test driver

[Heikki Krogerus]

Backend that the hosts can enumerate by using the SMBus
Address Resolution Protocol.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
---
 drivers/i2c/Kconfig          |   6 ++
 drivers/i2c/Makefile         |   1 +
 drivers/i2c/i2c-target-arp.c | 201 +++++++++++++++++++++++++++++++++++
 3 files changed, 208 insertions(+)
 create mode 100644 drivers/i2c/i2c-target-arp.c

diff --git a/drivers/i2c/Kconfig b/drivers/i2c/Kconfig
index c232054fddd6..4b97a66fc698 100644
--- a/drivers/i2c/Kconfig
+++ b/drivers/i2c/Kconfig
@@ -134,6 +134,12 @@ config I2C_SLAVE_TESTUNIT
 	  multi-master, SMBus Host Notify, etc. Please read
 	  Documentation/i2c/slave-testunit-backend.rst for further details.
 
+config I2C_TARGET_ARP
+	tristate "SMBus ARP target mode test driver"
+	help
+	  This backend makes Linux respond to SMBus Address Resolution Protocol
+	  commands.
+
 endif
 
 config I2C_DEBUG_CORE
diff --git a/drivers/i2c/Makefile b/drivers/i2c/Makefile
index 4fdb37b94aa4..3ea292dab065 100644
--- a/drivers/i2c/Makefile
+++ b/drivers/i2c/Makefile
@@ -19,5 +19,6 @@ obj-y				+= algos/ busses/ muxes/
 obj-$(CONFIG_I2C_STUB)		+= i2c-stub.o
 obj-$(CONFIG_I2C_SLAVE_EEPROM)	+= i2c-slave-eeprom.o
 obj-$(CONFIG_I2C_SLAVE_TESTUNIT)	+= i2c-slave-testunit.o
+obj-$(CONFIG_I2C_TARGET_ARP)	+= i2c-target-arp.o
 
 ccflags-$(CONFIG_I2C_DEBUG_CORE) := -DDEBUG
diff --git a/drivers/i2c/i2c-target-arp.c b/drivers/i2c/i2c-target-arp.c
new file mode 100644
index 000000000000..458749e4681e
--- /dev/null
+++ b/drivers/i2c/i2c-target-arp.c
@@ -0,0 +1,201 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * SMBus Address Resolution Protocol target mode test driver
+ *
+ * Copyright (C) 2026 Intel Corporation
+ */
+#include <linux/bits.h>
+#include <linux/device/devres.h>
+#include <linux/i2c.h>
+#include <linux/i2c-smbus.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/string.h>
+
+#define ARP_ADDRESS_VALID	BIT(0)
+#define ARP_ADDRESS_RESOLVED	BIT(1)
+
+#define I2C_SMBUS_HOST		0x08
+#define I2C_SMBUS_DEFAULT_ADDR	0x61
+
+#define ARP_CMD_PREPARE_TO_ARP	0x01
+#define ARP_CMD_RESET_DEVICE	0x02
+#define ARP_CMD_GET_UDID	0x03
+#define ARP_CMD_ASSIGN_ADDRESS	0x04
+
+struct get_udid_data {
+	struct i2c_arp_udid	udid;
+	u8			target_addr;
+} __packed;
+
+struct arp_target {
+	struct i2c_client target;
+	struct i2c_client *client;
+
+	int read_count;
+	int write_count;
+	unsigned char buf[24];
+
+	struct get_udid_data data;
+	unsigned char flag;
+};
+
+static u8 i2c_arp_pec(struct arp_target *arp, bool read)
+{
+	u8 addr = I2C_SMBUS_DEFAULT_ADDR << 1;
+	u8 len = sizeof(arp->data);
+	u8 pec = 0;
+
+	pec = i2c_smbus_pec(pec, &addr, 1);
+	pec = i2c_smbus_pec(pec, arp->buf, arp->write_count);
+
+	if (read) {
+		addr |= 1;
+		pec = i2c_smbus_pec(pec, &addr, 1);
+		pec = i2c_smbus_pec(pec, &len, 1);
+		pec = i2c_smbus_pec(pec, (void *)&arp->data, len);
+	}
+
+	return pec;
+}
+
+static int i2c_target_arp_cb(struct i2c_client *target, enum i2c_slave_event event, u8 *val)
+{
+	struct arp_target *arp = container_of(target, struct arp_target, target);
+
+	switch (event) {
+	case I2C_SLAVE_READ_PROCESSED:
+		if (arp->flag)
+			break;
+
+		if (arp->read_count == sizeof(arp->data))
+			*val = i2c_arp_pec(arp, 1);
+		else
+			*val = ((u8 *)&arp->data)[arp->read_count];
+
+		arp->read_count++;
+		break;
+	case I2C_SLAVE_READ_REQUESTED:
+		if (arp->flag)
+			break;
+
+		*val = sizeof(arp->data);
+		break;
+	case I2C_SLAVE_STOP:
+		switch (arp->buf[0]) {
+		case ARP_CMD_PREPARE_TO_ARP:
+			arp->flag = 0;
+			break;
+		case ARP_CMD_ASSIGN_ADDRESS:
+			/* If the UDID matches, this address is for us. */
+			if (!memcmp(&arp->buf[2], &arp->data.udid, sizeof(arp->data.udid)))
+				arp->flag = ARP_ADDRESS_VALID | ARP_ADDRESS_RESOLVED;
+			break;
+		default:
+			break;
+		}
+
+		arp->read_count = 0;
+		arp->write_count = 0;
+		break;
+	case I2C_SLAVE_WRITE_REQUESTED:
+		arp->read_count = 0;
+		arp->write_count = 0;
+		break;
+	case I2C_SLAVE_WRITE_RECEIVED:
+		arp->buf[arp->write_count] = *val;
+		arp->write_count++;
+		break;
+	}
+
+	return 0;
+}
+
+static void i2c_notify_arp_controller(struct i2c_client *client)
+{
+	union i2c_smbus_data data = { };
+	int ret;
+
+	if (!i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_WORD_DATA))
+		return;
+
+	ret = i2c_smbus_xfer(client->adapter, I2C_SMBUS_HOST, client->flags,
+			     I2C_SMBUS_WRITE, client->addr,
+			     I2C_SMBUS_WORD_DATA, &data);
+	if (ret)
+		dev_warn(&client->dev, "Failed to Notify ARP Controller (%d)\n", ret);
+}
+
+static int i2c_target_arp_probe(struct i2c_client *client)
+{
+	struct arp_target *arp;
+	int ret;
+
+	arp = devm_kzalloc(&client->dev, sizeof(*arp), GFP_KERNEL);
+	if (!arp)
+		return -ENOMEM;
+
+	arp->client = client;
+	i2c_set_clientdata(client, arp);
+
+	arp->target = *client;
+	arp->target.addr = I2C_SMBUS_DEFAULT_ADDR;
+	arp->target.flags |= I2C_CLIENT_SLAVE;
+
+	/*
+	 * The vendor and device ID are left undefined. Some hosts may not
+	 * support this. If that's the case, supply values for them.
+	 *
+	 * arp->data.udid.vendor = 0xXXXX;
+	 * arp->data.udid.device = 0xXXXX;
+	 *
+	 * The capabilities are also 0. That makes this a fixed address device
+	 * without support for PEC.
+	 */
+	arp->data.udid.version = BIT(3); /* UDID Version 1 */
+	arp->data.udid.interface = SMBUS_INTERFACE_SMBUS_V2_0;
+	arp->data.target_addr = client->addr;
+
+	/*
+	 * NOTE: This target is only for the ARP. After the address has been
+	 * assigned, another target should be registered for the actual device,
+	 * but this test code does _not_ register it.
+	 */
+	ret = i2c_slave_register(&arp->target, i2c_target_arp_cb);
+	if (ret)
+		return ret;
+
+	i2c_notify_arp_controller(client);
+
+	return 0;
+}
+
+static void i2c_target_arp_remove(struct i2c_client *client)
+{
+	struct arp_target *arp;
+
+	if (IS_ERR_OR_NULL(client))
+		return;
+
+	arp = i2c_get_clientdata(client);
+	i2c_slave_unregister(&arp->target);
+}
+
+static const struct i2c_device_id i2c_target_arp_id[] = {
+	{ "target-arp" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, i2c_target_arp_id);
+
+static struct i2c_driver i2c_target_arp_driver = {
+	.driver = {
+		.name = "i2c-target-arp",
+	},
+	.probe = i2c_target_arp_probe,
+	.remove = i2c_target_arp_remove,
+	.id_table = i2c_target_arp_id,
+};
+module_i2c_driver(i2c_target_arp_driver);
+
+MODULE_DESCRIPTION("SMBus ARP target mode test driver");
+MODULE_LICENSE("GPL");
-- 
2.50.1