Re: [PATCH v6] hwmon: (yogafan) Add support for Lenovo Yoga/Legion fan monitoring

Guenter Roeck
2026-03-25T00:13:12Z

On 3/24/26 15:10, Sergio Melas wrote:
> 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>
>
One more:

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@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..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@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;
> +
> + /* 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");