hwmon: (yogafan) Add support for Lenovo Yoga/Legion fan monitoring

This driver provides fan speed monitoring for Lenovo Yoga, Legion, and
IdeaPad laptops by interfacing with the Embedded Controller (EC) via ACPI.

To address low-resolution sampling in Lenovo EC firmware, a Rate-Limited
Lag (RLLag) filter is implemented. The filter ensures a consistent physical
curve regardless of userspace polling frequency.

Hardware identification is performed via DMI-based quirk tables, which
map specific ACPI object paths and register widths (8-bit vs 16-bit)
deterministically.

Signed-off-by: Sergio Melas <sergiomelas@gmail.com>
Link: https://lore.kernel.org/r/20260327221602.18832-1-sergiomelas@gmail.com
[groeck: Dropped double empty line in Kconfig]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

3 files changed, 288 insertions, 0 deletions

diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
index 9d49cfd4ef3d..152ab31298d9 100644
--- a/drivers/hwmon/Kconfig
+++ b/drivers/hwmon/Kconfig
@@ -2651,6 +2651,18 @@ config SENSORS_XGENE
 	  If you say yes here you get support for the temperature
 	  and power sensors for APM X-Gene SoC.
 
+config SENSORS_YOGAFAN
+	tristate "Lenovo Yoga Fan Hardware Monitoring"
+	depends on ACPI && HWMON && DMI
+	help
+	  If you say yes here you get support for fan speed monitoring
+	  on Lenovo Yoga, Legion, IdeaPad, Slim and LOQ laptops.
+	  The driver interfaces with the Embedded Controller via ACPI
+	  and uses a Rate-Limited Lag filter to smooth RPM readings.
+
+	  This driver can also be built as a module. If so, the module
+	  will be called yogafan.
+
 config SENSORS_INTEL_M10_BMC_HWMON
 	tristate "Intel MAX10 BMC Hardware Monitoring"
 	depends on MFD_INTEL_M10_BMC_CORE
diff --git a/drivers/hwmon/Makefile b/drivers/hwmon/Makefile
index 556e86d277b1..0fce31b43eb1 100644
--- a/drivers/hwmon/Makefile
+++ b/drivers/hwmon/Makefile
@@ -245,6 +245,7 @@ obj-$(CONFIG_SENSORS_W83L786NG)	+= w83l786ng.o
 obj-$(CONFIG_SENSORS_WM831X)	+= wm831x-hwmon.o
 obj-$(CONFIG_SENSORS_WM8350)	+= wm8350-hwmon.o
 obj-$(CONFIG_SENSORS_XGENE)	+= xgene-hwmon.o
+obj-$(CONFIG_SENSORS_YOGAFAN)	+= yogafan.o
 
 obj-$(CONFIG_SENSORS_OCC)	+= occ/
 obj-$(CONFIG_SENSORS_PECI)	+= peci/
diff --git a/drivers/hwmon/yogafan.c b/drivers/hwmon/yogafan.c
new file mode 100644
index 000000000000..605cc928f21f
--- /dev/null
+++ b/drivers/hwmon/yogafan.c
@@ -0,0 +1,275 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/**
+ * yoga_fan.c - Lenovo Yoga/Legion Fan Hardware Monitoring Driver
+ *
+ * Provides fan speed monitoring for Lenovo Yoga, Legion, and IdeaPad
+ * laptops by interfacing with the Embedded Controller (EC) via ACPI.
+ *
+ * The driver implements a passive discrete-time first-order lag filter
+ * with slew-rate limiting (RLLag). This addresses low-resolution
+ * tachometer sampling in the EC by smoothing RPM readings based on
+ * the time delta (dt) between userspace requests, ensuring physical
+ * consistency without background task overhead or race conditions.
+ * The filter implements multirate filtering with autoreset in case
+ * of large sampling time.
+ *
+ * Copyright (C) 2021-2026 Sergio Melas <sergiomelas@gmail.com>
+ */
+#include <linux/acpi.h>
+#include <linux/dmi.h>
+#include <linux/err.h>
+#include <linux/hwmon.h>
+#include <linux/ktime.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+#include <linux/math64.h>
+
+/* Driver Configuration Constants */
+#define DRVNAME			"yogafan"
+#define MAX_FANS		8
+
+/* Filter Configuration Constants */
+#define TAU_MS			1000	/* Time constant for the first-order lag (ms) */
+#define MAX_SLEW_RPM_S		1500	/* Maximum allowed change in RPM per second */
+#define MAX_SAMPLING		5000	/* Maximum allowed Ts for reset (ms) */
+#define MIN_SAMPLING		100	/* Minimum interval between filter updates (ms) */
+
+/* RPM Sanitation Constants */
+#define RPM_FLOOR_LIMIT		50	/* Snap filtered value to 0 if raw is 0 */
+
+struct yogafan_config {
+	int multiplier;
+	int fan_count;
+	const char *paths[2];
+};
+
+struct yoga_fan_data {
+	acpi_handle active_handles[MAX_FANS];
+	long filtered_val[MAX_FANS];
+	ktime_t last_sample[MAX_FANS];
+	int multiplier;
+	int fan_count;
+};
+
+/* Specific configurations mapped via DMI */
+static const struct yogafan_config yoga_8bit_fans_cfg = {
+	.multiplier = 100,
+	.fan_count = 1,
+	.paths = { "\\_SB.PCI0.LPC0.EC0.FANS", NULL }
+};
+
+static const struct yogafan_config ideapad_8bit_fan0_cfg = {
+	.multiplier = 100,
+	.fan_count = 1,
+	.paths = { "\\_SB.PCI0.LPC0.EC0.FAN0", NULL }
+};
+
+static const struct yogafan_config legion_16bit_dual_cfg = {
+	.multiplier = 1,
+	.fan_count = 2,
+	.paths = { "\\_SB.PCI0.LPC0.EC0.FANS", "\\_SB.PCI0.LPC0.EC0.FA2S" }
+};
+
+static void apply_rllag_filter(struct yoga_fan_data *data, int idx, long raw_rpm)
+{
+	ktime_t now = ktime_get_boottime();
+	s64 dt_ms = ktime_to_ms(ktime_sub(now, data->last_sample[idx]));
+	long delta, step, limit, alpha;
+	s64 temp_num;
+
+	if (raw_rpm < RPM_FLOOR_LIMIT) {
+		data->filtered_val[idx] = 0;
+		data->last_sample[idx] = now;
+		return;
+	}
+
+	if (data->last_sample[idx] == 0 || dt_ms > MAX_SAMPLING) {
+		data->filtered_val[idx] = raw_rpm;
+		data->last_sample[idx] = now;
+		return;
+	}
+
+	if (dt_ms < MIN_SAMPLING)
+		return;
+
+	delta = raw_rpm - data->filtered_val[idx];
+	if (delta == 0) {
+		data->last_sample[idx] = now;
+		return;
+	}
+
+	temp_num = dt_ms << 12;
+	alpha = (long)div64_s64(temp_num, (s64)(TAU_MS + dt_ms));
+	step = (delta * alpha) >> 12;
+
+	if (step == 0 && delta != 0)
+		step = (delta > 0) ? 1 : -1;
+
+	limit = (MAX_SLEW_RPM_S * (long)dt_ms) / 1000;
+	if (limit < 1)
+		limit = 1;
+
+	if (step > limit)
+		step = limit;
+	else if (step < -limit)
+		step = -limit;
+
+	data->filtered_val[idx] += step;
+	data->last_sample[idx] = now;
+}
+
+static int yoga_fan_read(struct device *dev, enum hwmon_sensor_types type,
+			 u32 attr, int channel, long *val)
+{
+	struct yoga_fan_data *data = dev_get_drvdata(dev);
+	unsigned long long raw_acpi;
+	acpi_status status;
+
+	if (type != hwmon_fan || attr != hwmon_fan_input)
+		return -EOPNOTSUPP;
+
+	status = acpi_evaluate_integer(data->active_handles[channel], NULL, NULL, &raw_acpi);
+	if (ACPI_FAILURE(status))
+		return -EIO;
+
+	apply_rllag_filter(data, channel, (long)raw_acpi * data->multiplier);
+	*val = data->filtered_val[channel];
+
+	return 0;
+}
+
+static umode_t yoga_fan_is_visible(const void *data, enum hwmon_sensor_types type,
+				   u32 attr, int channel)
+{
+	const struct yoga_fan_data *fan_data = data;
+
+	if (type == hwmon_fan && channel < fan_data->fan_count)
+		return 0444;
+
+	return 0;
+}
+
+static const struct hwmon_ops yoga_fan_hwmon_ops = {
+	.is_visible = yoga_fan_is_visible,
+	.read = yoga_fan_read,
+};
+
+static const struct hwmon_channel_info *yoga_fan_info[] = {
+	HWMON_CHANNEL_INFO(fan,
+			   HWMON_F_INPUT, HWMON_F_INPUT,
+			   HWMON_F_INPUT, HWMON_F_INPUT,
+			   HWMON_F_INPUT, HWMON_F_INPUT,
+			   HWMON_F_INPUT, HWMON_F_INPUT),
+	NULL
+};
+
+static const struct hwmon_chip_info yoga_fan_chip_info = {
+	.ops = &yoga_fan_hwmon_ops,
+	.info = yoga_fan_info,
+};
+
+static const struct dmi_system_id yogafan_quirks[] = {
+	{
+		.ident = "Lenovo Yoga",
+		.matches = {
+			DMI_MATCH(DMI_SYS_VENDOR, "LENOVO"),
+			DMI_MATCH(DMI_PRODUCT_FAMILY, "Yoga"),
+		},
+		.driver_data = (void *)&yoga_8bit_fans_cfg,
+	},
+	{
+		.ident = "Lenovo Legion",
+		.matches = {
+			DMI_MATCH(DMI_SYS_VENDOR, "LENOVO"),
+			DMI_MATCH(DMI_PRODUCT_FAMILY, "Legion"),
+		},
+		.driver_data = (void *)&legion_16bit_dual_cfg,
+	},
+	{
+		.ident = "Lenovo IdeaPad",
+		.matches = {
+			DMI_MATCH(DMI_SYS_VENDOR, "LENOVO"),
+			DMI_MATCH(DMI_PRODUCT_FAMILY, "IdeaPad"),
+		},
+		.driver_data = (void *)&ideapad_8bit_fan0_cfg,
+	},
+	{ }
+};
+MODULE_DEVICE_TABLE(dmi, yogafan_quirks);
+
+static int yoga_fan_probe(struct platform_device *pdev)
+{
+	const struct dmi_system_id *dmi_id;
+	const struct yogafan_config *cfg;
+	struct yoga_fan_data *data;
+	struct device *hwmon_dev;
+	int i;
+
+	dmi_id = dmi_first_match(yogafan_quirks);
+	if (!dmi_id)
+		return -ENODEV;
+
+	cfg = dmi_id->driver_data;
+	data = devm_kzalloc(&pdev->dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	data->multiplier = cfg->multiplier;
+
+	for (i = 0; i < cfg->fan_count; i++) {
+		acpi_status status;
+
+		status = acpi_get_handle(NULL, (char *)cfg->paths[i],
+					 &data->active_handles[data->fan_count]);
+		if (ACPI_SUCCESS(status))
+			data->fan_count++;
+	}
+
+	if (data->fan_count == 0)
+		return -ENODEV;
+
+	hwmon_dev = devm_hwmon_device_register_with_info(&pdev->dev, DRVNAME,
+							 data, &yoga_fan_chip_info, NULL);
+
+	return PTR_ERR_OR_ZERO(hwmon_dev);
+}
+
+static struct platform_driver yoga_fan_driver = {
+	.driver = { .name = DRVNAME },
+	.probe = yoga_fan_probe,
+};
+
+static struct platform_device *yoga_fan_device;
+
+static int __init yoga_fan_init(void)
+{
+	int ret;
+
+	if (!dmi_check_system(yogafan_quirks))
+		return -ENODEV;
+
+	ret = platform_driver_register(&yoga_fan_driver);
+	if (ret)
+		return ret;
+
+	yoga_fan_device = platform_device_register_simple(DRVNAME, -1, NULL, 0);
+	if (IS_ERR(yoga_fan_device)) {
+		platform_driver_unregister(&yoga_fan_driver);
+		return PTR_ERR(yoga_fan_device);
+	}
+	return 0;
+}
+
+static void __exit yoga_fan_exit(void)
+{
+	platform_device_unregister(yoga_fan_device);
+	platform_driver_unregister(&yoga_fan_driver);
+}
+
+module_init(yoga_fan_init);
+module_exit(yoga_fan_exit);
+
+MODULE_AUTHOR("Sergio Melas <sergiomelas@gmail.com>");
+MODULE_DESCRIPTION("Lenovo Yoga/Legion Fan Monitor Driver");
+MODULE_LICENSE("GPL");