Re: [PATCH v2 2/2] power: supply: Add GL.iNet GL-E750 MCU battery driver

[Sebastian Reichel <sebastian.reichel@collabora.com>]

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Hi,

On Wed, Oct 21, 2026 at 07:33:44PM +0000, Serv Pol via B4 Relay wrote:
> 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>
> ---

The Sashiko analysis looks correct:

https://sashiko.dev/#/patchset/20261005-gl-e750-mcu-v1-0-499d2e31f786%40tuta.io

Greetings,

-- Sebastian

>  MAINTAINERS                        |   1 +
>  drivers/power/supply/Kconfig       |  13 ++
>  drivers/power/supply/Makefile      |   1 +
>  drivers/power/supply/gl-e750-mcu.c | 369 +++++++++++++++++++++++++++++++++++++
>  4 files changed, 384 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..37541915e
> --- /dev/null
> +++ b/drivers/power/supply/gl-e750-mcu.c
> @@ -0,0 +1,369 @@
> +// 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/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);
> +}
> +
> +/* hand the buffered message over, called with the lock held */
> +static void mcu_flush_line(struct gl_e750_mcu *mcu)
> +{
> +	mcu->rx_line[mcu->rx_len] = '\0';
> +	mcu_handle_line(mcu, mcu->rx_line);
> +	mcu->rx_len = 0;
> +}
> +
> +/* a status reply is waiting in the buffer, called with the lock held */
> +static bool mcu_status_pending(struct gl_e750_mcu *mcu)
> +{
> +	return mcu->rx_len > 4 && !strncmp(mcu->rx_line, "{OK}", 4);
> +}
> +
> +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_flush_line(mcu);
> +			continue;
> +		}
> +
> +		/*
> +		 * Status replies and JSON events may come without a line
> +		 * ending. Every message starts with '{', so a new one ends
> +		 * the previous message, and a JSON event ends with '}'.
> +		 */
> +		if (c == '{' && mcu->rx_len)
> +			mcu_flush_line(mcu);
> +		if (mcu->rx_len < MCU_RX_LINE_MAX - 1)
> +			mcu->rx_line[mcu->rx_len++] = c;
> +		if (c == '}' && mcu->rx_line[0] == '{' &&
> +		    strncmp(mcu->rx_line, "{OK}", 4))
> +			mcu_flush_line(mcu);
> +	}
> +	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;
> +
> +	/*
> +	 * Only an earlier status reply is consumed here; a partially received
> +	 * event stays in the buffer.
> +	 */
> +	spin_lock_irqsave(&mcu->lock, flags);
> +	if (mcu_status_pending(mcu))
> +		mcu_flush_line(mcu);
> +	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 come without a line ending */
> +		spin_lock_irqsave(&mcu->lock, flags);
> +		if (mcu_status_pending(mcu))
> +			mcu_flush_line(mcu);
> +		else
> +			dev_dbg(&mcu->serdev->dev, "no status reply\n");
> +		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
> 
> 

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