Re: [PATCH v6] hwmon: (yogafan) Add support for Lenovo Yoga/Legion fan monitoring
From: Guenter Roeck
Date: Tue Mar 24 2026 - 20:13:18 EST
On 3/24/26 15:10, Sergio Melas wrote:
This driver provides fan speed monitoring for modern Lenovo Yoga,One more:
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@xxxxxxxxx>
https://sashiko.dev/#/patchset/20260324221008.212753-1-sergiomelas%40gmail.com
In short, there is no "MATH64" config symbol, and RPM_FLOOR_LIMIT is unused.
Thanks,
Guenter
---
v6:
- Unified patch structure (6 files changed).
Side note: I have no idea what this means ??
- 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 | 9 ++
drivers/hwmon/Makefile | 1 +
drivers/hwmon/yogafan.c | 225 ++++++++++++++++++++++++++++++++
6 files changed, 292 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@xxxxxxxxx>
+
+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@xxxxxxxxx>
+L: linux-hwmon@xxxxxxxxxxxxxxx
+W: https://github.com/sergiomelas
+S: Maintained
+F: Documentation/hwmon/yogafan.rst
+F: drivers/hwmon/yogafan.c
+
LOADPIN SECURITY MODULE
M: Kees Cook <kees@xxxxxxxxxx>
S: Supported
diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
index 7dd8381ba0d0..f87023a0066e 100644
--- a/drivers/hwmon/Kconfig
+++ b/drivers/hwmon/Kconfig
@@ -2653,6 +2653,15 @@ 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
+ select MATH64
+ 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..44b6a7fd2deb
--- /dev/null
+++ b/drivers/hwmon/yogafan.c
@@ -0,0 +1,225 @@
+// 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@xxxxxxxxx>
+ */
+#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;
+
+ /* 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@xxxxxxxxx>");
+MODULE_DESCRIPTION("Lenovo Yoga/Legion Fan Monitor Driver");
+MODULE_LICENSE("GPL");