[PATCH v7] hwmon: (yogafan) Add support for Lenovo Yoga/Legion fan monitoring

[Sergio Melas <sergiomelas@gmail.com>]

This driver provides fan speed monitoring for modern Lenovo Yoga,
Legion, and IdeaPad laptops. It interfaces with the Embedded
Controller (EC) via ACPI to retrieve tachometer data.

To address low-resolution sampling in the Lenovo EC firmware, the
driver implements a Rate-Limited Lag (RLLag) filter using a passive
discrete-time first-order model. This ensures physical consistency
of the RPM signal regardless of userspace polling rates.

Signed-off-by: Sergio Melas <sergiomelas@gmail.com>

---
v7:
  - Fixed Kconfig: Removed non-existent 'select MATH64'.
  - Fixed unused macro: Utilized RPM_FLOOR_LIMIT to implement an 
    immediate 0-RPM bypass in the filter.
  - Clarification: Previous "unified structure" comment meant that all 
    6 files (driver, docs, metadata) are now in this single atomic patch.
v6:
  - Unified patch structure (6 files changed).
  - Verified FOPTD (First-Order Plus Time Delay) model against hardware 
     traces (Yoga 14c) to ensure physical accuracy of the 1000ms time constant.
  - Fixed a rounding stall: added a +/- 1 RPM floor to the step calculation 
    to ensure convergence even at high polling frequencies.
  - Set MAX_SLEW_RPM_S to 1500 to match physical motor inertia.
  - Documentation: Updated to clarify 100-RPM hardware step resolution.
  - 32-bit safety: Implemented div64_s64 for coefficient precision.
v5:
  - Fixed 32-bit build failures by using div64_s64 for 64-bit division.
  - Extracted magic numbers into constants (RPM_UNIT_THRESHOLD, etc.).
  - Fixed filter stall by ensuring a minimum slew limit (limit = 1).
  - Refined RPM floor logic to trigger only when hardware reports 0 RPM.
  - Resolved 255/256 unit-jump bug by adjusting heuristic thresholds.
v4:
  - Rebased on groeck/hwmon-next branch for clean application.
  - Corrected alphabetical sorting in Kconfig and Makefile.
  - Technical Validation & FOPTD Verification:
    - Implemented RLLag (Rate-Limited Lag) first-order modeling.
    - Used 10-bit fixed-point math for alpha calculation to avoid
      floating point overhead in the kernel.
    - Added 5000ms filter reset for resume/long-polling sanitation.
v3:
  - Added MAINTAINERS entry and full Documentation/hwmon/yogafan.rst.
  - Fixed integer overflow in filter math.
  - Added support for secondary fan paths (FA2S) for Legion laptops.
v2:
  - Migrated from background worker to passive multirate filtering.
  - Implemented dt-based scaling to maximize CPU sleep states.
  - Restricted driver to Lenovo hardware via DMI matching.
v1:
  - Initial submission with basic ACPI fan path support.
---
---
 Documentation/hwmon/index.rst   |   1 +
 Documentation/hwmon/yogafan.rst |  48 +++++++
 MAINTAINERS                     |   8 ++
 drivers/hwmon/Kconfig           |   8 ++
 drivers/hwmon/Makefile          |   1 +
 drivers/hwmon/yogafan.c         | 230 ++++++++++++++++++++++++++++++++
 6 files changed, 296 insertions(+)
 create mode 100644 Documentation/hwmon/yogafan.rst
 create mode 100644 drivers/hwmon/yogafan.c

diff --git a/Documentation/hwmon/index.rst b/Documentation/hwmon/index.rst
index 559c32344cd3..199f35a75282 100644
--- a/Documentation/hwmon/index.rst
+++ b/Documentation/hwmon/index.rst
@@ -282,4 +282,5 @@ Hardware Monitoring Kernel Drivers
    xdp710
    xdpe12284
    xdpe152c4
+   yogafan
    zl6100
diff --git a/Documentation/hwmon/yogafan.rst b/Documentation/hwmon/yogafan.rst
new file mode 100644
index 000000000000..e0f2b060aabc
--- /dev/null
+++ b/Documentation/hwmon/yogafan.rst
@@ -0,0 +1,48 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+Kernel driver yogafan
+=====================
+
+Supported chips:
+
+  * Lenovo Yoga, Legion, and IdeaPad Embedded Controllers
+    Prefix: 'yogafan'
+    Addresses: ACPI handle (see probe list in driver)
+
+Author: Sergio Melas <sergiomelas@gmail.com>
+
+Description
+-----------
+
+This driver provides fan speed monitoring for modern Lenovo laptops.
+Most Lenovo laptops do not provide fan tachometer data through standard
+ISA/LPC hardware monitoring chips. Instead, the data is stored in the
+Embedded Controller (EC) and exposed via ACPI.
+
+The driver implements a **Rate-Limited Lag (RLLag)** filter to handle
+the low-resolution and jittery sampling found in Lenovo EC firmware.
+
+Filter Details:
+---------------
+
+The RLLag filter is a discrete-time first-order lag model that ensures:
+  - **Smoothing:** Jittery 1000-RPM step increments are smoothed into 1-RPM increments.
+  - **Slew-Rate Limiting:** Prevents "teleporting" readings by capping the change
+    to 1500 RPM/s, matching physical fan inertia.
+  - **Polling Independence:** The filter math scales based on the time delta
+    between userspace reads, ensuring the same physical curve regardless
+    of whether you poll at 1Hz or 1000Hz.
+
+Usage
+-----
+
+The driver exposes standard hwmon sysfs attributes:
+
+================  =============================================================
+Attribute         Description
+================  =============================================================
+fanX_input        Filtered fan speed in RPM.
+================  =============================================================
+
+Note: If the hardware reports 0 RPM, the filter is bypassed and 0 is reported
+immediately to ensure the user knows the fan has stopped.
diff --git a/MAINTAINERS b/MAINTAINERS
index 830c6f076b00..94416af57b28 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -14873,6 +14873,14 @@ W:	https://linuxtv.org
 Q:	http://patchwork.linuxtv.org/project/linux-media/list/
 F:	drivers/media/usb/dvb-usb-v2/lmedm04*
 
+LENOVO YOGA FAN DRIVER
+M:	Sergio Melas <sergiomelas@gmail.com>
+L:	linux-hwmon@vger.kernel.org
+W:	https://github.com/sergiomelas
+S:	Maintained
+F:	Documentation/hwmon/yogafan.rst
+F:	drivers/hwmon/yogafan.c
+
 LOADPIN SECURITY MODULE
 M:	Kees Cook <kees@kernel.org>
 S:	Supported
diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
index 7dd8381ba0d0..644b52c6ba66 100644
--- a/drivers/hwmon/Kconfig
+++ b/drivers/hwmon/Kconfig
@@ -2653,6 +2653,14 @@ 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
+	help
+	  Say Y here if you want to monitor fan speeds on Lenovo Yoga
+	  and Legion laptops.
+
+
 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..4c27f6884b4a
--- /dev/null
+++ b/drivers/hwmon/yogafan.c
@@ -0,0 +1,230 @@
+// 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/init.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) */
+/* RPM Heuristic and Sanitation Constants */
+#define RPM_UNIT_THRESHOLD 500  /* Values below this treated as units of 100 */
+#define RPM_UNIT_MULT      100  /* Multiplier for low-res EC readings */
+#define RPM_FLOOR_LIMIT    50   /* Snap filtered value to 0 if raw is 0 */
+
+struct yoga_fan_data {
+	const char *active_paths[MAX_FANS];
+	long filtered_val[MAX_FANS];
+	ktime_t last_sample[MAX_FANS]; /* Renamed from last_update for consistency */
+	int fan_count;
+};
+/**
+ * apply_rllag_filter - Discrete-time filter update (Passive Multirate)
+ * @data: pointer to driver data
+ * @idx: fan index
+ * @raw_rpm: new raw value from ACPI
+ */
+static void apply_rllag_filter(struct yoga_fan_data *data, int idx, long raw_rpm)
+{
+	ktime_t now = ktime_get();
+	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;
+	}
+	/* Initialize on first run or after long sleep/stall */
+	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 <= 0) return;
+
+	delta = raw_rpm - data->filtered_val[idx];
+	if (delta == 0) {
+		data->last_sample[idx] = now;
+		return;
+	}
+	/* Alpha with 12-bit precision to prevent alpha=0 on fast polls */
+	temp_num = dt_ms << 12;
+	alpha = (long)div64_s64(temp_num, (s64)(TAU_MS + dt_ms));
+	step = (delta * alpha) >> 12;
+	/* FIX THE STALL: Force a move of 1 RPM if alpha*delta rounds to zero */
+	if (step == 0 && delta != 0)
+		step = (delta > 0) ? 1 : -1;
+	/* SLEW RATE LIMITING: Scaled by time delta */
+	limit = (MAX_SLEW_RPM_S * (long)dt_ms) / 1000;
+	if (limit < 1) limit = 1;
+	/* Clamp step to physical slew rate */
+	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;
+	long rpm;
+
+	if (type != hwmon_fan || attr != hwmon_fan_input)
+		return -EOPNOTSUPP;
+	status = acpi_evaluate_integer(NULL, (acpi_string)data->active_paths[channel],
+					NULL, &raw_acpi);
+	if (ACPI_FAILURE(status))
+		return -EIO;
+	/* * Heuristic: Convert units-of-100 to raw RPM.
+	 * Most Yoga/Legion ECs return a single byte (0-255).
+	 * We use 500 as a safety threshold to distinguish from raw 16-bit RPM.
+	 */
+	rpm = (long)raw_acpi;
+	if (rpm > 0 && rpm < RPM_UNIT_THRESHOLD)
+		rpm *= RPM_UNIT_MULT;
+	apply_rllag_filter(data, channel, rpm);
+	*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 int yoga_fan_probe(struct platform_device *pdev)
+{
+	struct yoga_fan_data *data;
+	struct device *hwmon_dev;
+	acpi_handle handle;
+	int i;
+	static const char * const fan_paths[] = {
+		"\\_SB.PCI0.LPC0.EC0.FANS",  /* Primary Fan (Yoga 14c) */
+		"\\_SB.PCI0.LPC0.EC0.FA2S",  /* Secondary Fan (Legion) */
+		"\\_SB.PCI0.LPC0.EC0.FAN0",  /* IdeaPad / Slim */
+		"\\_SB.PCI0.LPC.EC.FAN0",    /* Legacy */
+		"\\_SB.PCI0.LPC0.EC.FAN0",   /* Alternate */
+	};
+	data = devm_kzalloc(&pdev->dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+	data->fan_count = 0;
+	for (i = 0; i < ARRAY_SIZE(fan_paths); i++) {
+		if (ACPI_SUCCESS(acpi_get_handle(NULL, (char *)fan_paths[i], &handle))) {
+			data->active_paths[data->fan_count] = fan_paths[i];
+			data->fan_count++;
+			if (data->fan_count >= MAX_FANS)
+				break;
+		}
+	}
+
+	if (data->fan_count == 0)
+		return -ENODEV;
+	platform_set_drvdata(pdev, data);
+	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 const struct dmi_system_id yoga_dmi_table[] __initconst = {
+	{
+		.ident = "Lenovo",
+		.matches = {
+			DMI_MATCH(DMI_SYS_VENDOR, "LENOVO"),
+		},
+	},
+	{ }
+};
+MODULE_DEVICE_TABLE(dmi, yoga_dmi_table);
+
+static int __init yoga_fan_init(void)
+{
+	int ret;
+
+	if (!dmi_check_system(yoga_dmi_table))
+		return -ENODEV;
+	ret = platform_driver_register(&yoga_fan_driver);
+	if (ret)
+		return ret;
+	yoga_fan_device = platform_device_register_simple(DRVNAME, 0, 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");
-- 
2.53.0