[PATCH 0/2] power: supply: Add GL.iNet GL-E750 MCU battery driver

[PATCH 0/2] power: supply: Add GL.iNet GL-E750 MCU battery driver

[Serv Pol via B4 Relay]

The GL.iNet GL-E750 (Mudi) is a battery powered LTE travel router
(QCA9531). A microcontroller on the SoC UART drives the OLED status
screen, monitors the battery and watches the power switch. The host talks
to it in text lines of JSON; on request it reports the battery state, and
when the power switch is turned off it sends a notification a few seconds
before cutting the power.

This series adds a binding and a serdev driver that exposes the battery
through the power_supply class and shuts the system down cleanly when the
switch is turned off. The screen content (SSIDs, clients, cellular state,
free-form vendor JSON) is deliberately left out.

The board itself is supported by OpenWrt (ath79), not by an in-tree DTS,
so there is no in-tree user of the binding yet. The OpenWrt side is at
https://github.com/openwrt/openwrt/pull/25649

Testing: OpenWrt main with kernel 6.18 on a GL-E750 V1. Battery
properties read back as expected (capacity, status, temperature, cycle
count), the module was unloaded and reloaded repeatedly without
warnings, and turning the power switch off logs "power switch turned off,
shutting down" and leads to a clean shutdown before the MCU removes
power. On this tree: checkpatch --strict, W=1 and sparse builds, and
dt_binding_check are clean.

Tool use: the driver, the binding and the changelogs were written with
the help of an AI coding assistant (Claude, claude-opus-5-5), based on
the vendor protocol description at
https://github.com/gl-inet/GL-E750-MCU-instruction and on traces of the
MCU traffic on the device. I reviewed the code, tested it on the
hardware as described above and am responsible for it.

---
Serv Pol (2):
      dt-bindings: power: supply: Add GL.iNet GL-E750 MCU
      power: supply: Add GL.iNet GL-E750 MCU battery driver

 .../bindings/power/supply/glinet,gl-e750-mcu.yaml  |  37 +++
 MAINTAINERS                                        |   7 +
 drivers/power/supply/Kconfig                       |  13 +
 drivers/power/supply/Makefile                      |   1 +
 drivers/power/supply/gl-e750-mcu.c                 | 352 +++++++++++++++++++++
 5 files changed, 410 insertions(+)
---
base-commit: 5c1c602463d597c5189ddc5a73dece588ba2c57a
change-id: 20261005-gl-e750-mcu-f606ccd66dec

Best regards,
--  
Serv Pol <servpol@tuta.io>

[PATCH 1/2] dt-bindings: power: supply: Add GL.iNet GL-E750 MCU

[Serv Pol via B4 Relay]

From: Serv Pol <servpol@tuta.io>

The GL.iNet GL-E750 (Mudi) travel router has a microcontroller on the SoC
UART. It drives the OLED status screen, monitors the battery and reports
when the power switch is turned off, a few seconds before it cuts the
power. Add a binding for it.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Serv Pol <servpol@tuta.io>
---
 .../bindings/power/supply/glinet,gl-e750-mcu.yaml  | 37 ++++++++++++++++++++++
 MAINTAINERS                                        |  6 ++++
 2 files changed, 43 insertions(+)

diff --git a/Documentation/devicetree/bindings/power/supply/glinet,gl-e750-mcu.yaml b/Documentation/devicetree/bindings/power/supply/glinet,gl-e750-mcu.yaml
new file mode 100644
index 000000000..a7ea4dde8
--- /dev/null
+++ b/Documentation/devicetree/bindings/power/supply/glinet,gl-e750-mcu.yaml
@@ -0,0 +1,37 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/power/supply/glinet,gl-e750-mcu.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: GL.iNet GL-E750 status MCU
+
+maintainers:
+  - Serv Pol <servpol@tuta.io>
+
+description:
+  The GL.iNet GL-E750 (Mudi) travel router has a microcontroller connected
+  to the SoC UART. It drives the OLED status screen, monitors the battery
+  and reports when the power switch is turned off, a few seconds before it
+  cuts the power.
+
+allOf:
+  - $ref: power-supply.yaml#
+  - $ref: /schemas/serial/serial-peripheral-props.yaml#
+
+properties:
+  compatible:
+    const: glinet,gl-e750-mcu
+
+required:
+  - compatible
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    serial {
+        mcu {
+            compatible = "glinet,gl-e750-mcu";
+        };
+    };
diff --git a/MAINTAINERS b/MAINTAINERS
index 3a19da74d..247432449 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -11105,6 +11105,12 @@ L:	platform-driver-x86@vger.kernel.org
 S:	Maintained
 F:	drivers/platform/x86/gigabyte-wmi.c
 
+GL.INET GL-E750 MCU BATTERY DRIVER
+M:	Serv Pol <servpol@tuta.io>
+L:	linux-pm@vger.kernel.org
+S:	Maintained
+F:	Documentation/devicetree/bindings/power/supply/glinet,gl-e750-mcu.yaml
+
 GNSS SUBSYSTEM
 M:	Johan Hovold <johan@kernel.org>
 S:	Maintained

-- 
2.47.3

[PATCH 2/2] power: supply: Add GL.iNet GL-E750 MCU battery driver

[Serv Pol via B4 Relay]

From: Serv Pol <servpol@tuta.io>

The status MCU of the GL.iNet GL-E750 (Mudi) travel router sits on the
SoC UART and talks in text lines: the host sends JSON objects whose values
are all strings, and a request containing "mcu_status": "1" is answered
with

  {OK},<percent>,<temperature>,<charging>,<cycles>

Poll that every 30 seconds and expose the result as a power_supply.

When the power switch is turned off, the MCU sends {"shut_down": "1"},
without a line ending, and cuts the power a few seconds later. Shut the
system down cleanly in that case.

The MCU also renders the status screen from JSON fields (SSIDs, clients,
cellular state...). That vendor-specific content is not handled here.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Serv Pol <servpol@tuta.io>
---
 MAINTAINERS                        |   1 +
 drivers/power/supply/Kconfig       |  13 ++
 drivers/power/supply/Makefile      |   1 +
 drivers/power/supply/gl-e750-mcu.c | 352 +++++++++++++++++++++++++++++++++++++
 4 files changed, 367 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index 247432449..505aa205c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -11110,6 +11110,7 @@ M:	Serv Pol <servpol@tuta.io>
 L:	linux-pm@vger.kernel.org
 S:	Maintained
 F:	Documentation/devicetree/bindings/power/supply/glinet,gl-e750-mcu.yaml
+F:	drivers/power/supply/gl-e750-mcu.c
 
 GNSS SUBSYSTEM
 M:	Johan Hovold <johan@kernel.org>
diff --git a/drivers/power/supply/Kconfig b/drivers/power/supply/Kconfig
index ee7dc5f26..e87c76c18 100644
--- a/drivers/power/supply/Kconfig
+++ b/drivers/power/supply/Kconfig
@@ -1203,4 +1203,17 @@ config MACSMC_POWER
 
 	  Say Y or M here if you have an Apple Silicon based Mac.
 
+config BATTERY_GL_E750_MCU
+	tristate "GL.iNet GL-E750 status MCU battery driver"
+	depends on SERIAL_DEV_BUS
+	depends on OF || COMPILE_TEST
+	help
+	  Say Y or M here to enable the battery driver for the status
+	  microcontroller of the GL.iNet GL-E750 (Mudi) travel router.
+	  It reports the battery state and shuts the system down when
+	  the power switch is turned off.
+
+	  To compile this driver as a module, choose M here: the module
+	  will be called gl-e750-mcu.
+
 endif # POWER_SUPPLY
diff --git a/drivers/power/supply/Makefile b/drivers/power/supply/Makefile
index f3c0d1006..5d405556e 100644
--- a/drivers/power/supply/Makefile
+++ b/drivers/power/supply/Makefile
@@ -78,6 +78,7 @@ obj-$(CONFIG_CHARGER_TWL4030)	+= twl4030_charger.o
 obj-$(CONFIG_CHARGER_TWL6030)	+= twl6030_charger.o
 obj-$(CONFIG_CHARGER_LP8727)	+= lp8727_charger.o
 obj-$(CONFIG_CHARGER_LP8788)	+= lp8788-charger.o
+obj-$(CONFIG_BATTERY_GL_E750_MCU)	+= gl-e750-mcu.o
 obj-$(CONFIG_CHARGER_GPIO)	+= gpio-charger.o
 obj-$(CONFIG_CHARGER_MANAGER)	+= charger-manager.o
 obj-$(CONFIG_CHARGER_LT3651)	+= lt3651-charger.o
diff --git a/drivers/power/supply/gl-e750-mcu.c b/drivers/power/supply/gl-e750-mcu.c
new file mode 100644
index 000000000..0799e41d5
--- /dev/null
+++ b/drivers/power/supply/gl-e750-mcu.c
@@ -0,0 +1,352 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * GL.iNet GL-E750 (Mudi) status MCU battery driver
+ *
+ * The MCU sits on the SoC UART. It drives the OLED screen, monitors the
+ * battery and watches the power switch. The host sends one JSON object per
+ * line, where every value is a string. When a line contains
+ * "mcu_status": "1", the MCU answers with
+ *
+ *	{OK},<percent>,<temperature>,<charging>,<cycles>
+ *
+ * When the power switch is turned off, the MCU sends {"shut_down": "1"} and
+ * cuts the power a few seconds later.
+ *
+ * Copyright (C) 2026 Serv Pol <servpol@tuta.io>
+ */
+
+#include <linux/completion.h>
+#include <linux/ctype.h>
+#include <linux/devm-helpers.h>
+#include <linux/math.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <linux/power_supply.h>
+#include <linux/property.h>
+#include <linux/reboot.h>
+#include <linux/serdev.h>
+#include <linux/spinlock.h>
+#include <linux/string.h>
+#include <linux/workqueue.h>
+
+#define MCU_BAUDRATE		115200
+#define MCU_RX_LINE_MAX		128
+#define MCU_POLL_INTERVAL_MS	30000
+#define MCU_REPLY_TIMEOUT_MS	1500
+
+static const u8 mcu_status_req[] = "{ \"mcu_status\": \"1\" }\n";
+
+struct gl_e750_mcu {
+	struct serdev_device *serdev;
+	struct power_supply *psy;
+	struct delayed_work poll_work;
+	struct work_struct poweroff_work;
+	struct completion reply;
+
+	/* receive buffer and battery state, filled from the serdev callback */
+	spinlock_t lock;
+	char rx_line[MCU_RX_LINE_MAX];
+	size_t rx_len;
+	bool valid;
+	int capacity;		/* percent */
+	int temp;		/* tenths of a degree Celsius */
+	bool charging;
+	int cycles;
+};
+
+/* "37.4", "40" or "-1.5" -> tenths of a degree */
+static int mcu_parse_tenths(const char *s, int *out)
+{
+	const char *dot = strchr(s, '.');
+	char whole_str[8];
+	int whole, frac = 0;
+
+	if (!dot) {
+		if (kstrtoint(s, 10, &whole))
+			return -EINVAL;
+		*out = whole * 10;
+		return 0;
+	}
+
+	if (dot == s || dot - s >= sizeof(whole_str))
+		return -EINVAL;
+	memcpy(whole_str, s, dot - s);
+	whole_str[dot - s] = '\0';
+	if (kstrtoint(whole_str, 10, &whole))
+		return -EINVAL;
+	if (isdigit(dot[1]))
+		frac = dot[1] - '0';
+	*out = whole * 10 + (*s == '-' ? -frac : frac);
+	return 0;
+}
+
+/*
+ * "{OK},<percent>,<temperature>,<charging>,<cycles>", called with the lock
+ * held. Returns true if the line was a valid status reply.
+ */
+static bool mcu_parse_status(struct gl_e750_mcu *mcu, char *line)
+{
+	char *p = line + 4, *f[4];
+	int capacity, temp, charging, cycles, i;
+	bool changed;
+
+	if (*p++ != ',')
+		return false;
+	for (i = 0; i < ARRAY_SIZE(f); i++) {
+		f[i] = strsep(&p, ",");
+		if (!f[i] || !*f[i])
+			return false;
+	}
+
+	if (kstrtoint(f[0], 10, &capacity) || mcu_parse_tenths(f[1], &temp) ||
+	    kstrtoint(f[2], 10, &charging) || kstrtoint(f[3], 10, &cycles))
+		return false;
+	if (capacity < 0 || capacity > 100)
+		return false;
+
+	changed = !mcu->valid || mcu->capacity != capacity ||
+		  mcu->charging != !!charging || mcu->cycles != cycles ||
+		  abs(mcu->temp - temp) >= 10;
+	mcu->capacity = capacity;
+	mcu->temp = temp;
+	mcu->charging = !!charging;
+	mcu->cycles = cycles;
+	mcu->valid = true;
+
+	complete(&mcu->reply);
+	if (changed)
+		power_supply_changed(mcu->psy);
+	return true;
+}
+
+/* called with the lock held */
+static void mcu_handle_line(struct gl_e750_mcu *mcu, char *line)
+{
+	if (!strncmp(line, "{OK}", 4))
+		mcu_parse_status(mcu, line);
+	else if (strstr(line, "\"shut_down\""))
+		schedule_work(&mcu->poweroff_work);
+	else
+		dev_dbg_ratelimited(&mcu->serdev->dev, "unhandled: %s\n", line);
+}
+
+static size_t mcu_receive_buf(struct serdev_device *serdev, const u8 *data,
+			      size_t count)
+{
+	struct gl_e750_mcu *mcu = serdev_device_get_drvdata(serdev);
+	unsigned long flags;
+	size_t i;
+
+	spin_lock_irqsave(&mcu->lock, flags);
+	for (i = 0; i < count; i++) {
+		char c = data[i];
+
+		if (c == '\r' || c == '\n') {
+			if (mcu->rx_len) {
+				mcu->rx_line[mcu->rx_len] = '\0';
+				mcu_handle_line(mcu, mcu->rx_line);
+				mcu->rx_len = 0;
+			}
+			continue;
+		}
+		if (mcu->rx_len < MCU_RX_LINE_MAX - 1)
+			mcu->rx_line[mcu->rx_len++] = c;
+
+		/* JSON events from the MCU come without a line ending */
+		if (c == '}' && mcu->rx_line[0] == '{' &&
+		    strncmp(mcu->rx_line, "{OK}", 4)) {
+			mcu->rx_line[mcu->rx_len] = '\0';
+			mcu_handle_line(mcu, mcu->rx_line);
+			mcu->rx_len = 0;
+		}
+	}
+	spin_unlock_irqrestore(&mcu->lock, flags);
+
+	return count;
+}
+
+static const struct serdev_device_ops mcu_serdev_ops = {
+	.receive_buf = mcu_receive_buf,
+	.write_wakeup = serdev_device_write_wakeup,
+};
+
+static void mcu_poll_work(struct work_struct *work)
+{
+	struct gl_e750_mcu *mcu = container_of(to_delayed_work(work),
+					       struct gl_e750_mcu, poll_work);
+	unsigned long timeout = msecs_to_jiffies(MCU_REPLY_TIMEOUT_MS);
+	unsigned long flags;
+	int ret;
+
+	spin_lock_irqsave(&mcu->lock, flags);
+	mcu->rx_len = 0;
+	reinit_completion(&mcu->reply);
+	spin_unlock_irqrestore(&mcu->lock, flags);
+
+	ret = serdev_device_write(mcu->serdev, mcu_status_req,
+				  sizeof(mcu_status_req) - 1, timeout);
+	if (ret < 0) {
+		dev_warn_ratelimited(&mcu->serdev->dev, "write failed: %d\n", ret);
+	} else if (!wait_for_completion_timeout(&mcu->reply, timeout)) {
+		/* the reply may also come without a line ending */
+		spin_lock_irqsave(&mcu->lock, flags);
+		mcu->rx_line[mcu->rx_len] = '\0';
+		if (!(mcu->rx_len > 4 && !strncmp(mcu->rx_line, "{OK}", 4) &&
+		      mcu_parse_status(mcu, mcu->rx_line)))
+			dev_dbg(&mcu->serdev->dev, "no status reply\n");
+		mcu->rx_len = 0;
+		spin_unlock_irqrestore(&mcu->lock, flags);
+	}
+
+	schedule_delayed_work(&mcu->poll_work,
+			      msecs_to_jiffies(MCU_POLL_INTERVAL_MS));
+}
+
+static void mcu_poweroff_work(struct work_struct *work)
+{
+	struct gl_e750_mcu *mcu = container_of(work, struct gl_e750_mcu,
+					       poweroff_work);
+
+	/* the MCU cuts the power a few seconds after reporting the switch */
+	dev_info(&mcu->serdev->dev, "power switch turned off, shutting down\n");
+	orderly_poweroff(true);
+}
+
+static const enum power_supply_property mcu_battery_props[] = {
+	POWER_SUPPLY_PROP_PRESENT,
+	POWER_SUPPLY_PROP_STATUS,
+	POWER_SUPPLY_PROP_TECHNOLOGY,
+	POWER_SUPPLY_PROP_CAPACITY,
+	POWER_SUPPLY_PROP_TEMP,
+	POWER_SUPPLY_PROP_CYCLE_COUNT,
+};
+
+static int mcu_battery_get_property(struct power_supply *psy,
+				    enum power_supply_property psp,
+				    union power_supply_propval *val)
+{
+	struct gl_e750_mcu *mcu = power_supply_get_drvdata(psy);
+	unsigned long flags;
+	int ret = 0;
+
+	spin_lock_irqsave(&mcu->lock, flags);
+	switch (psp) {
+	case POWER_SUPPLY_PROP_PRESENT:
+		val->intval = 1;
+		break;
+	case POWER_SUPPLY_PROP_TECHNOLOGY:
+		val->intval = POWER_SUPPLY_TECHNOLOGY_LION;
+		break;
+	case POWER_SUPPLY_PROP_STATUS:
+		if (!mcu->valid)
+			val->intval = POWER_SUPPLY_STATUS_UNKNOWN;
+		else if (mcu->charging)
+			val->intval = mcu->capacity >= 100 ?
+				      POWER_SUPPLY_STATUS_FULL :
+				      POWER_SUPPLY_STATUS_CHARGING;
+		else
+			val->intval = POWER_SUPPLY_STATUS_DISCHARGING;
+		break;
+	case POWER_SUPPLY_PROP_CAPACITY:
+		if (mcu->valid)
+			val->intval = mcu->capacity;
+		else
+			ret = -ENODATA;
+		break;
+	case POWER_SUPPLY_PROP_TEMP:
+		if (mcu->valid)
+			val->intval = mcu->temp;
+		else
+			ret = -ENODATA;
+		break;
+	case POWER_SUPPLY_PROP_CYCLE_COUNT:
+		if (mcu->valid)
+			val->intval = mcu->cycles;
+		else
+			ret = -ENODATA;
+		break;
+	default:
+		ret = -EINVAL;
+	}
+	spin_unlock_irqrestore(&mcu->lock, flags);
+
+	return ret;
+}
+
+static const struct power_supply_desc mcu_battery_desc = {
+	.name = "gl-e750-battery",
+	.type = POWER_SUPPLY_TYPE_BATTERY,
+	.properties = mcu_battery_props,
+	.num_properties = ARRAY_SIZE(mcu_battery_props),
+	.get_property = mcu_battery_get_property,
+};
+
+static int gl_e750_mcu_probe(struct serdev_device *serdev)
+{
+	struct device *dev = &serdev->dev;
+	struct power_supply_config psy_cfg = {};
+	struct gl_e750_mcu *mcu;
+	int ret;
+
+	mcu = devm_kzalloc(dev, sizeof(*mcu), GFP_KERNEL);
+	if (!mcu)
+		return -ENOMEM;
+
+	mcu->serdev = serdev;
+	spin_lock_init(&mcu->lock);
+	init_completion(&mcu->reply);
+	serdev_device_set_drvdata(serdev, mcu);
+	serdev_device_set_client_ops(serdev, &mcu_serdev_ops);
+
+	/*
+	 * devm teardown runs in reverse: the poll work is cancelled before the
+	 * port is closed, the power-off work (queued from the receive path)
+	 * and the battery only after it.
+	 */
+	psy_cfg.drv_data = mcu;
+	psy_cfg.fwnode = dev_fwnode(dev);
+	mcu->psy = devm_power_supply_register(dev, &mcu_battery_desc, &psy_cfg);
+	if (IS_ERR(mcu->psy))
+		return dev_err_probe(dev, PTR_ERR(mcu->psy),
+				     "failed to register battery\n");
+
+	ret = devm_work_autocancel(dev, &mcu->poweroff_work, mcu_poweroff_work);
+	if (ret)
+		return ret;
+
+	ret = devm_serdev_device_open(dev, serdev);
+	if (ret)
+		return dev_err_probe(dev, ret, "failed to open serdev\n");
+
+	serdev_device_set_baudrate(serdev, MCU_BAUDRATE);
+	serdev_device_set_flow_control(serdev, false);
+	ret = serdev_device_set_parity(serdev, SERDEV_PARITY_NONE);
+	if (ret)
+		return dev_err_probe(dev, ret, "failed to set parity\n");
+
+	ret = devm_delayed_work_autocancel(dev, &mcu->poll_work, mcu_poll_work);
+	if (ret)
+		return ret;
+
+	schedule_delayed_work(&mcu->poll_work, 0);
+	return 0;
+}
+
+static const struct of_device_id gl_e750_mcu_of_match[] = {
+	{ .compatible = "glinet,gl-e750-mcu" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, gl_e750_mcu_of_match);
+
+static struct serdev_device_driver gl_e750_mcu_driver = {
+	.probe = gl_e750_mcu_probe,
+	.driver = {
+		.name = "gl-e750-mcu",
+		.of_match_table = gl_e750_mcu_of_match,
+	},
+};
+module_serdev_device_driver(gl_e750_mcu_driver);
+
+MODULE_AUTHOR("Serv Pol <servpol@tuta.io>");
+MODULE_DESCRIPTION("GL.iNet GL-E750 status MCU battery driver");
+MODULE_LICENSE("GPL");

-- 
2.47.3

Re: [PATCH 1/2] dt-bindings: power: supply: Add GL.iNet GL-E750 MCU

[Conor Dooley]

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Acked-by: Conor Dooley <conor.dooley@microchip.com>
pw-bot: not-applicable

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