@@ -19,6 +19,17 @@ config PSE_REALTEK_MCU
 	  Shared core for the Realtek PSE MCU driver. This is selected
 	  automatically by the transport options below.
 
+config PSE_REALTEK_MCU_I2C
+	tristate "Realtek PSE MCU driver (I2C transport)"
+	depends on I2C
+	select PSE_REALTEK_MCU
+	help
+	  Driver for the microcontroller (MCU) that fronts the PSE
+	  hardware on various Realtek-based managed switches, attached
+	  via I2C/SMBus. The MCU exposes a message-based protocol; the actual
+	  PSE silicon is not accessed directly. To compile this driver as a
+	  module, choose M here: the module will be called realtek-pse-mcu-i2c.
+
 config PSE_REGULATOR
 	tristate "Regulator based PSE controller"
 	help

@@ -4,6 +4,7 @@
 obj-$(CONFIG_PSE_CONTROLLER) += pse_core.o
 
 obj-$(CONFIG_PSE_REALTEK_MCU) += realtek-pse-mcu-core.o
+obj-$(CONFIG_PSE_REALTEK_MCU_I2C) += realtek-pse-mcu-i2c.o
 obj-$(CONFIG_PSE_REGULATOR) += pse_regulator.o
 obj-$(CONFIG_PSE_PD692X0) += pd692x0.o
 obj-$(CONFIG_PSE_SI3474) += si3474.o

@@ -0,0 +1,170 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <linux/delay.h>
+#include <linux/i2c.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/pse-pd/pse.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+
+#include "realtek-pse-mcu.h"
+
+/*
+ * The core has already waited RTPSE_MCU_RESPONSE_MS before calling us, so
+ * the response is normally ready on the very first read. For commands the
+ * MCU produces more slowly, keep polling at the typical response cadence
+ * up to the worst-case ceiling.
+ */
+#define RTPSE_MCU_I2C_RETRY_MS	RTPSE_MCU_RESPONSE_MS
+#define RTPSE_MCU_I2C_MAX_TRIES	(RTPSE_MCU_RESPONSE_MAX_MS / RTPSE_MCU_I2C_RETRY_MS)
+
+static int rtpse_mcu_i2c_smbus_send(struct rtpse_mcu_ctrl *pse, const struct rtpse_mcu_msg *req)
+{
+	struct i2c_client *client = to_i2c_client(pse->dev);
+
+	/* Send opcode as SMBus command byte; remaining 11 bytes as block data */
+	return i2c_smbus_write_i2c_block_data(client, req->opcode, RTPSE_MCU_MSG_SIZE - 1,
+					      (u8 *)req + 1);
+}
+
+static int rtpse_mcu_i2c_smbus_recv(struct rtpse_mcu_ctrl *pse, const struct rtpse_mcu_msg *req,
+				    struct rtpse_mcu_msg *resp)
+{
+	struct i2c_client *client = to_i2c_client(pse->dev);
+	int tries, ret;
+
+	for (tries = 0; tries < RTPSE_MCU_I2C_MAX_TRIES; tries++) {
+		if (tries > 0)
+			msleep(RTPSE_MCU_I2C_RETRY_MS);
+
+		/* MCU needs 0x00 as command byte for read */
+		ret = i2c_smbus_read_i2c_block_data(client, 0x00,
+						    RTPSE_MCU_MSG_SIZE,
+						    (u8 *)resp);
+		if (ret < 0)
+			return ret;
+		if (ret == RTPSE_MCU_MSG_SIZE && rtpse_mcu_resp_is_final(req, resp))
+			return 0;
+	}
+
+	return -ETIMEDOUT;
+}
+
+static const struct rtpse_mcu_transport_ops rtpse_mcu_i2c_smbus_ops = {
+	.send = rtpse_mcu_i2c_smbus_send,
+	.recv = rtpse_mcu_i2c_smbus_recv,
+};
+
+static int rtpse_mcu_i2c_native_send(struct rtpse_mcu_ctrl *pse, const struct rtpse_mcu_msg *req)
+{
+	struct i2c_client *client = to_i2c_client(pse->dev);
+	void *buf;
+	int ret;
+
+	/* i2c_master_send() may DMA, so the buffer must not be on the stack. */
+	buf = kmemdup(req, RTPSE_MCU_MSG_SIZE, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	ret = i2c_master_send(client, buf, RTPSE_MCU_MSG_SIZE);
+	kfree(buf);
+	if (ret < 0)
+		return ret;
+	return ret == RTPSE_MCU_MSG_SIZE ? 0 : -EIO;
+}
+
+static int rtpse_mcu_i2c_native_recv(struct rtpse_mcu_ctrl *pse, const struct rtpse_mcu_msg *req,
+				     struct rtpse_mcu_msg *resp)
+{
+	struct i2c_client *client = to_i2c_client(pse->dev);
+	int tries, ret;
+	u8 *buf;
+
+	/* i2c_master_recv() may DMA, so read into an off-stack buffer. */
+	buf = kmalloc(RTPSE_MCU_MSG_SIZE, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	for (tries = 0; tries < RTPSE_MCU_I2C_MAX_TRIES; tries++) {
+		if (tries > 0)
+			msleep(RTPSE_MCU_I2C_RETRY_MS);
+
+		ret = i2c_master_recv(client, buf, RTPSE_MCU_MSG_SIZE);
+		if (ret < 0)
+			goto out;
+		if (ret == RTPSE_MCU_MSG_SIZE) {
+			memcpy(resp, buf, RTPSE_MCU_MSG_SIZE);
+			if (rtpse_mcu_resp_is_final(req, resp)) {
+				ret = 0;
+				goto out;
+			}
+		}
+	}
+	ret = -ETIMEDOUT;
+out:
+	kfree(buf);
+	return ret;
+}
+
+static const struct rtpse_mcu_transport_ops rtpse_mcu_i2c_native_ops = {
+	.send = rtpse_mcu_i2c_native_send,
+	.recv = rtpse_mcu_i2c_native_recv,
+};
+
+static int rtpse_mcu_i2c_probe(struct i2c_client *client)
+{
+	struct device *dev = &client->dev;
+	const struct rtpse_mcu_match_data *match;
+	struct rtpse_mcu_ctrl *pse;
+	bool use_native;
+
+	match = device_get_match_data(dev);
+	if (!match)
+		return dev_err_probe(dev, -ENODEV, "missing match data\n");
+
+	/* The framing (raw I2C vs SMBus) is carried by the match data. */
+	use_native = match->native_i2c;
+	if (use_native) {
+		if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))
+			return dev_err_probe(dev, -EOPNOTSUPP,
+				"plain-I2C MCU protocol requires I2C-capable adapter\n");
+	} else {
+		if (!i2c_check_functionality(client->adapter,
+					     I2C_FUNC_SMBUS_WRITE_I2C_BLOCK |
+					     I2C_FUNC_SMBUS_READ_I2C_BLOCK))
+			return dev_err_probe(dev, -EOPNOTSUPP,
+				"SMBus MCU protocol requires SMBus I2C-block support\n");
+	}
+
+	pse = devm_kzalloc(dev, sizeof(*pse), GFP_KERNEL);
+	if (!pse)
+		return -ENOMEM;
+
+	pse->dev = dev;
+	pse->pcdev.owner = THIS_MODULE;
+	pse->transport = use_native ? &rtpse_mcu_i2c_native_ops : &rtpse_mcu_i2c_smbus_ops;
+
+	return rtpse_mcu_register(pse);
+}
+
+static const struct of_device_id rtpse_mcu_i2c_of_match[] = {
+	{ .compatible = "realtek,pse-mcu-gen1-smbus", .data = &rtpse_mcu_gen1_data },
+	{ .compatible = "realtek,pse-mcu-gen2-smbus", .data = &rtpse_mcu_gen2_data },
+	{ .compatible = "realtek,pse-mcu-gen2-i2c", .data = &rtpse_mcu_gen2_i2c_data },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, rtpse_mcu_i2c_of_match);
+
+static struct i2c_driver rtpse_mcu_i2c_driver = {
+	.driver = {
+		.name		= "realtek-pse-mcu-i2c",
+		.of_match_table	= rtpse_mcu_i2c_of_match,
+	},
+	.probe		= rtpse_mcu_i2c_probe,
+};
+module_i2c_driver(rtpse_mcu_i2c_driver);
+
+MODULE_AUTHOR("Jonas Jelonek <jelonek.jonas@gmail.com>");
+MODULE_DESCRIPTION("Realtek PSE MCU driver (I2C transport)");
+MODULE_LICENSE("GPL");