diff --git a/MAINTAINERS b/MAINTAINERS
index 3e7359ac005c..0fbd2d953da4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -18654,6 +18654,12 @@ L:	linux-media@vger.kernel.org
 S:	Odd Fixes
 F:	drivers/media/tuners/qm1d1c0042*
 
+QNAP MCU DRIVER
+M:	Heiko Stuebner <heiko@sntech.de>
+S:	Maintained
+F:	drivers/mfd/qnap-mcu.c
+F:	include/linux/qnap-mcu.h
+
 QNX4 FILESYSTEM
 M:	Anders Larsen <al@alarsen.net>
 S:	Maintained
diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index bc8be2e593b6..d43d2747ac4e 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -2362,6 +2362,19 @@ config MFD_INTEL_M10_BMC_PMCI
 	  additional drivers must be enabled in order to use the functionality
 	  of the device.
 
+config MFD_QNAP_MCU
+	tristate "QNAP microcontroller unit core driver"
+	depends on SERIAL_DEV_BUS
+	select MFD_CORE
+	help
+	  Select this to get support for the QNAP MCU device found in
+	  several devices of QNAP network attached storage devices that
+	  implements additional functionality for the device, like
+	  fan- and LED control.
+
+	  This driver implements the base serial protocol to talk to the
+	  device and provides functions for the other parts to hook into.
+
 config MFD_RSMU_I2C
 	tristate "Renesas Synchronization Management Unit with I2C"
 	depends on I2C && OF
diff --git a/drivers/mfd/Makefile b/drivers/mfd/Makefile
index 02b651cd7535..fc8b825725ff 100644
--- a/drivers/mfd/Makefile
+++ b/drivers/mfd/Makefile
@@ -286,5 +286,7 @@ obj-$(CONFIG_MFD_INTEL_M10_BMC_PMCI)   += intel-m10-bmc-pmci.o
 obj-$(CONFIG_MFD_ATC260X)	+= atc260x-core.o
 obj-$(CONFIG_MFD_ATC260X_I2C)	+= atc260x-i2c.o
 
+obj-$(CONFIG_MFD_QNAP_MCU)	+= qnap-mcu.o
+
 obj-$(CONFIG_MFD_RSMU_I2C)	+= rsmu_i2c.o rsmu_core.o
 obj-$(CONFIG_MFD_RSMU_SPI)	+= rsmu_spi.o rsmu_core.o
diff --git a/drivers/mfd/qnap-mcu.c b/drivers/mfd/qnap-mcu.c
new file mode 100644
index 000000000000..8abe1414da54
--- /dev/null
+++ b/drivers/mfd/qnap-mcu.c
@@ -0,0 +1,347 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Core driver for the microcontroller unit in QNAP NAS devices that is
+ * connected via a dedicated UART port.
+ *
+ * Copyright (C) 2024 Heiko Stuebner <heiko@sntech.de>
+ */
+
+#include <linux/cleanup.h>
+#include <linux/export.h>
+#include <linux/mfd/core.h>
+#include <linux/mfd/qnap-mcu.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/reboot.h>
+#include <linux/serdev.h>
+#include <linux/slab.h>
+
+/* The longest command found so far is 5 bytes long */
+#define QNAP_MCU_MAX_CMD_SIZE		5
+#define QNAP_MCU_MAX_DATA_SIZE		36
+#define QNAP_MCU_CHECKSUM_SIZE		1
+
+#define QNAP_MCU_RX_BUFFER_SIZE		\
+		(QNAP_MCU_MAX_DATA_SIZE + QNAP_MCU_CHECKSUM_SIZE)
+
+#define QNAP_MCU_TX_BUFFER_SIZE		\
+		(QNAP_MCU_MAX_CMD_SIZE + QNAP_MCU_CHECKSUM_SIZE)
+
+#define QNAP_MCU_ACK_LEN		2
+#define QNAP_MCU_VERSION_LEN		4
+
+/**
+ * struct qnap_mcu_reply - Reply to a command
+ *
+ * @data:	Buffer to store reply payload in
+ * @length:	Expected reply length, including the checksum
+ * @received:	Received number of bytes, so far
+ * @done:	Triggered when the entire reply has been received
+ */
+struct qnap_mcu_reply {
+	u8 *data;
+	size_t length;
+	size_t received;
+	struct completion done;
+};
+
+/**
+ * struct qnap_mcu - QNAP NAS embedded controller
+ *
+ * @serdev:	Pointer to underlying serdev
+ * @bus_lock:	Lock to serialize access to the device
+ * @reply_lock:	Lock protecting @reply
+ * @reply:	Pointer to memory to store reply payload
+ * @variant:	Device variant specific information
+ * @version:	MCU firmware version
+ */
+struct qnap_mcu {
+	struct serdev_device *serdev;
+	struct mutex bus_lock;
+	struct qnap_mcu_reply reply;
+	const struct qnap_mcu_variant *variant;
+	u8 version[QNAP_MCU_VERSION_LEN];
+};
+
+/*
+ * The QNAP-MCU uses a basic XOR checksum.
+ * It is always the last byte and XORs the whole previous message.
+ */
+static u8 qnap_mcu_csum(const u8 *buf, size_t size)
+{
+	u8 csum = 0;
+
+	while (size--)
+		csum ^= *buf++;
+
+	return csum;
+}
+
+static int qnap_mcu_write(struct qnap_mcu *mcu, const u8 *data, u8 data_size)
+{
+	unsigned char tx[QNAP_MCU_TX_BUFFER_SIZE];
+	size_t length = data_size + QNAP_MCU_CHECKSUM_SIZE;
+
+	if (length > sizeof(tx)) {
+		dev_err(&mcu->serdev->dev, "data too big for transmit buffer");
+		return -EINVAL;
+	}
+
+	memcpy(tx, data, data_size);
+	tx[data_size] = qnap_mcu_csum(data, data_size);
+
+	return serdev_device_write(mcu->serdev, tx, length, HZ);
+}
+
+static size_t qnap_mcu_receive_buf(struct serdev_device *serdev, const u8 *buf, size_t size)
+{
+	struct device *dev = &serdev->dev;
+	struct qnap_mcu *mcu = dev_get_drvdata(dev);
+	struct qnap_mcu_reply *reply = &mcu->reply;
+	const u8 *src = buf;
+	const u8 *end = buf + size;
+
+	if (!reply->length) {
+		dev_warn(dev, "Received %zu bytes, we were not waiting for\n",
+			 size);
+		return size;
+	}
+
+	while (src < end) {
+		reply->data[reply->received] = *src++;
+		reply->received++;
+
+		if (reply->received == reply->length) {
+			complete(&reply->done);
+
+			/*
+			 * We report the consumed number of bytes. If there
+			 * are still bytes remaining (though there shouldn't)
+			 * the serdev layer will re-execute this handler with
+			 * the remainder of the Rx bytes.
+			 */
+			return src - buf;
+		}
+	}
+
+	/*
+	 * The only way to get out of the above loop and end up here
+	 * is through consuming all of the supplied data, so here we
+	 * report that we processed it all.
+	 */
+	return size;
+}
+
+static const struct serdev_device_ops qnap_mcu_serdev_device_ops = {
+	.receive_buf  = qnap_mcu_receive_buf,
+	.write_wakeup = serdev_device_write_wakeup,
+};
+
+int qnap_mcu_exec(struct qnap_mcu *mcu,
+		  const u8 *cmd_data, size_t cmd_data_size,
+		  u8 *reply_data, size_t reply_data_size)
+{
+	unsigned char rx[QNAP_MCU_RX_BUFFER_SIZE];
+	size_t length = reply_data_size + QNAP_MCU_CHECKSUM_SIZE;
+	struct qnap_mcu_reply *reply = &mcu->reply;
+	int ret;
+
+	if (length > sizeof(rx)) {
+		dev_err(&mcu->serdev->dev, "expected data too big for receive buffer");
+		return -EINVAL;
+	}
+
+	mutex_lock(&mcu->bus_lock);
+
+	/* Initialize reply fields */
+	reply->data = rx,
+	reply->length = length,
+	reply->received = 0,
+	reinit_completion(&reply->done);
+
+	qnap_mcu_write(mcu, cmd_data, cmd_data_size);
+
+	if (!wait_for_completion_timeout(&reply->done,
+					 msecs_to_jiffies(1000))) {
+		dev_err(&mcu->serdev->dev, "Command timeout\n");
+		ret = -ETIMEDOUT;
+	} else {
+		u8 crc = qnap_mcu_csum(rx, reply_data_size);
+
+		if (crc != rx[reply_data_size]) {
+			dev_err(&mcu->serdev->dev,
+				"Checksum 0x%02x wrong for data\n", crc);
+			ret = -EIO;
+		} else {
+			memcpy(reply_data, rx, reply_data_size);
+			ret = 0;
+		}
+	}
+
+	/* We don't expect any characters from the device now */
+	reply->length = 0;
+
+	mutex_unlock(&mcu->bus_lock);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(qnap_mcu_exec);
+
+int qnap_mcu_exec_with_ack(struct qnap_mcu *mcu,
+			   const u8 *cmd_data, size_t cmd_data_size)
+{
+	u8 ack[QNAP_MCU_ACK_LEN];
+	int ret;
+
+	ret = qnap_mcu_exec(mcu, cmd_data, cmd_data_size, ack, sizeof(ack));
+	if (ret)
+		return ret;
+
+	/* Should return @0 */
+	if (ack[0] != 0x40 || ack[1] != 0x30) {
+		dev_err(&mcu->serdev->dev, "Did not receive ack\n");
+		return -EIO;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(qnap_mcu_exec_with_ack);
+
+const struct qnap_mcu_variant *qnap_mcu_get_variant_data(struct qnap_mcu *mcu)
+{
+	return mcu->variant;
+}
+EXPORT_SYMBOL_GPL(qnap_mcu_get_variant_data);
+
+static int qnap_mcu_get_version(struct qnap_mcu *mcu)
+{
+	const u8 cmd[] = {
+		[0] = 0x25, /* % */
+		[1] = 0x56  /* V */
+	};
+	u8 rx[14];
+	int ret;
+
+	/* Reply is the 2 command-bytes + 4 bytes describing the version */
+	ret = qnap_mcu_exec(mcu, cmd, sizeof(cmd), rx, QNAP_MCU_VERSION_LEN + 2);
+	if (ret)
+		return ret;
+
+	memcpy(mcu->version, &rx[2], QNAP_MCU_VERSION_LEN);
+
+	return 0;
+}
+
+/*
+ * The MCU controls power to the peripherals but not the CPU.
+ *
+ * So using the pmic to power off the system keeps the MCU and hard-drives
+ * running. This also then prevents the system from turning back on until
+ * the MCU is turned off by unplugging the power-cable.
+ * Turning off the MCU alone on the other hand turns off the hard-drives,
+ * LEDs, etc while the main SoC stays running - including its network ports.
+ */
+static int qnap_mcu_power_off(struct sys_off_data *data)
+{
+	struct qnap_mcu *mcu = data->cb_data;
+	int ret;
+	const u8 cmd[] = {
+		[0] = 0x40, /* @ */
+		[1] = 0x43, /* C */
+		[2] = 0x30  /* 0 */
+	};
+
+	ret = qnap_mcu_exec_with_ack(mcu, cmd, sizeof(cmd));
+	if (ret) {
+		dev_err(&mcu->serdev->dev, "MCU poweroff failed %d\n", ret);
+		return NOTIFY_STOP;
+	}
+
+	return NOTIFY_DONE;
+}
+
+static const struct qnap_mcu_variant qnap_ts433_mcu = {
+	.baud_rate = 115200,
+	.num_drives = 4,
+	.fan_pwm_min = 51,  /* Specified in original model.conf */
+	.fan_pwm_max = 255,
+	.usb_led = true,
+};
+
+static const struct of_device_id qnap_mcu_dt_ids[] = {
+	{ .compatible = "qnap,ts433-mcu", .data = &qnap_ts433_mcu },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, qnap_mcu_dt_ids);
+
+static const struct mfd_cell qnap_mcu_subdevs[] = {
+	{ .name = "qnap-mcu-input", },
+	{ .name = "qnap-mcu-leds", },
+	{ .name = "qnap-mcu-hwmon", }
+};
+
+static int qnap_mcu_probe(struct serdev_device *serdev)
+{
+	struct device *dev = &serdev->dev;
+	struct qnap_mcu *mcu;
+	int ret;
+
+	mcu = devm_kzalloc(dev, sizeof(*mcu), GFP_KERNEL);
+	if (!mcu)
+		return -ENOMEM;
+
+	mcu->serdev = serdev;
+	dev_set_drvdata(dev, mcu);
+
+	mcu->variant = of_device_get_match_data(dev);
+	if (!mcu->variant)
+		return -ENODEV;
+
+	mutex_init(&mcu->bus_lock);
+	init_completion(&mcu->reply.done);
+
+	serdev_device_set_client_ops(serdev, &qnap_mcu_serdev_device_ops);
+	ret = devm_serdev_device_open(dev, serdev);
+	if (ret)
+		return ret;
+
+	serdev_device_set_baudrate(serdev, mcu->variant->baud_rate);
+	serdev_device_set_flow_control(serdev, false);
+
+	ret = serdev_device_set_parity(serdev, SERDEV_PARITY_NONE);
+	if (ret) {
+		dev_err(dev, "Failed to set parity\n");
+		return ret;
+	}
+
+	ret = qnap_mcu_get_version(mcu);
+	if (ret)
+		return ret;
+
+	ret = devm_register_sys_off_handler(dev,
+					    SYS_OFF_MODE_POWER_OFF_PREPARE,
+					    SYS_OFF_PRIO_DEFAULT,
+					    &qnap_mcu_power_off, mcu);
+	if (ret)
+		return dev_err_probe(dev, ret,
+				     "Failed to register poweroff handler\n");
+
+	ret = devm_mfd_add_devices(dev, PLATFORM_DEVID_AUTO, qnap_mcu_subdevs,
+				   ARRAY_SIZE(qnap_mcu_subdevs), NULL, 0, NULL);
+	if (ret)
+		return dev_err_probe(dev, ret, "Failed to add child devices\n");
+
+	return 0;
+}
+
+static struct serdev_device_driver qnap_mcu_drv = {
+	.probe = qnap_mcu_probe,
+	.driver = {
+		.name = "qnap-mcu",
+		.of_match_table = qnap_mcu_dt_ids,
+	},
+};
+module_serdev_device_driver(qnap_mcu_drv);
+
+MODULE_AUTHOR("Heiko Stuebner <heiko@sntech.de>");
+MODULE_DESCRIPTION("QNAP MCU core driver");
+MODULE_LICENSE("GPL");
diff --git a/include/linux/mfd/qnap-mcu.h b/include/linux/mfd/qnap-mcu.h
new file mode 100644
index 000000000000..903b923537c7
--- /dev/null
+++ b/include/linux/mfd/qnap-mcu.h
@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Core definitions for QNAP MCU MFD driver.
+ * Copyright (C) 2024 Heiko Stuebner <heiko@sntech.de>
+ */
+
+#ifndef _LINUX_QNAP_MCU_H_
+#define _LINUX_QNAP_MCU_H_
+
+struct qnap_mcu;
+
+struct qnap_mcu_variant {
+	u32 baud_rate;
+	int num_drives;
+	int fan_pwm_min;
+	int fan_pwm_max;
+	bool usb_led;
+};
+
+int qnap_mcu_exec(struct qnap_mcu *mcu,
+		  const u8 *cmd_data, size_t cmd_data_size,
+		  u8 *reply_data, size_t reply_data_size);
+int qnap_mcu_exec_with_ack(struct qnap_mcu *mcu,
+			   const u8 *cmd_data, size_t cmd_data_size);
+const struct qnap_mcu_variant *qnap_mcu_get_variant_data(struct qnap_mcu *mcu);
+
+#endif /* _LINUX_QNAP_MCU_H_ */
