Re: [PATCH v8] hwmon: (yogafan) Add support for Lenovo Yoga/Legion fan monitoring

Guenter Roeck
2026-03-26T00:21:41Z

Hi again.

On 3/25/26 15:06, 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>
>

Thanks a lot for the quick turnaround.

We are down to optimizations. Please take a look.

https://sashiko.dev/#/patchset/20260325220624.102847-1-sergiomelas%40gmail.com

Both suggestions make sense to me.

Thanks,
Guenter

> ---
> v8:
> - Replaced heuristic multiplier with deterministic DMI Quirk Table.
> - Added 'depends on DMI' to Kconfig.
> - Verified FOPTD model (1000ms TAU / 1500 RPM/s slew) against hardware traces.
> - Increased filter precision to 12-bit fixed-point.
> 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 | 96 +++++++++++
> MAINTAINERS | 8 +
> drivers/hwmon/Kconfig | 8 +
> drivers/hwmon/Makefile | 1 +
> drivers/hwmon/yogafan.c | 284 ++++++++++++++++++++++++++++++++
> 6 files changed, 398 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..c9ca9f6a53b2
> --- /dev/null
> +++ b/Documentation/hwmon/yogafan.rst
> @@ -0,0 +1,96 @@
> +.. SPDX-License-Identifier: GPL-2.0-only
> +
> +Kernel driver yogafan
> +=====================
> +
> +Supported chips:
> +
> + * Lenovo Yoga, Legion, IdeaPad, Slim, Flex, and LOQ 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 consumer 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.
> +
> +Hardware Identification and Multiplier Logic
> +--------------------------------------------
> +
> +The driver supports two distinct EC architectures. Differentiation is handled
> +deterministically via a DMI Product Family quirk table during the probe phase,
> +eliminating the need for runtime heuristics.
> +
> +1. 8-bit EC Architecture (Multiplier: 100)
> + - **Families:** Yoga, IdeaPad, Slim, Flex.
> + - **Technical Detail:** These models allocate a single 8-bit register for
> + tachometer data. Since 8-bit fields are limited to a value of 255, the
> + BIOS stores fan speed in units of 100 RPM (e.g., 42 = 4200 RPM).
> +
> +2. 16-bit EC Architecture (Multiplier: 1)
> + - **Families:** Legion, LOQ.
> + - **Technical Detail:** High-performance gaming models require greater
> + precision for fans exceeding 6000 RPM. These use a 16-bit word (2 bytes)
> + storing the raw RPM value directly.
> +
> +Filter Details:
> +---------------
> +
> +The RLLag filter is a passive discrete-time first-order lag model that ensures:
> + - **Smoothing:** Low-resolution step increments are smoothed into 1-RPM increments.
> + - **Slew-Rate Limiting:** Prevents unrealistic 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 a consistent physical curve regardless
> + of polling frequency.
> +
> +Suspend and Resume
> +------------------
> +
> +The driver utilizes the boottime clock (ktime_get_boottime()) to calculate the
> +sampling delta. This ensures that time spent in system suspend is accounted
> +for. If the delta exceeds 5 seconds (e.g., after waking the laptop), the
> +filter automatically resets to the current hardware value to prevent
> +reporting "ghost" RPM data from before the sleep state.
> +
> +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.
> +
> +References
> +----------
> +
> +1. **ACPI Specification (Field Objects):** Documentation on how 8-bit vs 16-bit
> + fields are accessed in OperationRegions.
> + https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html#field-objects
> +
> +2. **LegionFanControl & NBFC Projects:** Community-driven reverse engineering
> + of Lenovo Legion/LOQ EC memory maps (16-bit raw registers).
> + https://www.legionfancontrol.com/
> + https://github.com/hirschmann/nbfc/tree/master/Configs
> +
> +3. **Linux Kernel Timekeeping API:** Documentation for ktime_get_boottime() and
> + handling deltas across suspend states.
> + https://www.kernel.org/doc/html/latest/core-api/timekeeping.html
> +
> +4. **Lenovo IdeaPad Laptop Driver:** Reference for DMI-based hardware
> + feature gating in Lenovo laptops.
> + https://github.com/torvalds/linux/blob/master/drivers/platform/x86/ideapad-laptop.c
> 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..ca1ed3e63d4a 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 && DMI
> + 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..f5a5689ba1a0
> --- /dev/null
> +++ b/drivers/hwmon/yogafan.c
> @@ -0,0 +1,284 @@
> +// 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 Sanitation Constants */
> +#define RPM_FLOOR_LIMIT 50 /* Snap filtered value to 0 if raw is 0 */
> +
> +struct yogafan_config {
> + int multiplier;
> +};
> +
> +struct yoga_fan_data {
> + const char *active_paths[MAX_FANS];
> + long filtered_val[MAX_FANS];
> + ktime_t last_sample[MAX_FANS];
> + int multiplier;
> + int fan_count;
> +};
> +
> +/* Known hardware configurations based on EC register bit-width */
> +static const struct yogafan_config yoga_8bit_cfg = { .multiplier = 100 };
> +static const struct yogafan_config legion_16bit_cfg = { .multiplier = 1 };
> +
> +/**
> + * 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_boottime(); /* Fixed for Suspend/Resume safety */
> + 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;
> + 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;
> + /* Deterministic calculation based on DMI-detected multiplier */
> + 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,
> +};
> +
> +/* Quirk table to map families to multipliers deterministically */
> +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_cfg,
> + },
> + {
> + .ident = "Lenovo IdeaPad",
> + .matches = {
> + DMI_MATCH(DMI_SYS_VENDOR, "LENOVO"),
> + DMI_MATCH(DMI_PRODUCT_FAMILY, "IdeaPad"),
> + },
> + .driver_data = (void *)&yoga_8bit_cfg,
> + },
> + {
> + .ident = "Lenovo Slim",
> + .matches = {
> + DMI_MATCH(DMI_SYS_VENDOR, "LENOVO"),
> + DMI_MATCH(DMI_PRODUCT_FAMILY, "Slim"),
> + },
> + .driver_data = (void *)&yoga_8bit_cfg,
> + },
> + {
> + .ident = "Lenovo Flex",
> + .matches = {
> + DMI_MATCH(DMI_SYS_VENDOR, "LENOVO"),
> + DMI_MATCH(DMI_PRODUCT_FAMILY, "Flex"),
> + },
> + .driver_data = (void *)&yoga_8bit_cfg,
> + },
> + {
> + .ident = "Lenovo Legion",
> + .matches = {
> + DMI_MATCH(DMI_SYS_VENDOR, "LENOVO"),
> + DMI_MATCH(DMI_PRODUCT_FAMILY, "Legion"),
> + },
> + .driver_data = (void *)&legion_16bit_cfg,
> + },
> + {
> + .ident = "Lenovo LOQ",
> + .matches = {
> + DMI_MATCH(DMI_SYS_VENDOR, "LENOVO"),
> + DMI_MATCH(DMI_PRODUCT_FAMILY, "LOQ"),
> + },
> + .driver_data = (void *)&legion_16bit_cfg,
> + },
> + { }
> +};
> +
> +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;
> + acpi_handle handle;
> + int i;
> + static const char * const fan_paths[] = {
> + "\\_SB.PCI0.LPC0.EC0.FANS", /* Primary Fan (Yoga) */
> + "\\_SB.PCI0.LPC0.EC0.FA2S", /* Secondary Fan (Legion / LOQ) */
> + "\\_SB.PCI0.LPC0.EC0.FAN0", /* IdeaPad / Slim / Flex */
> + "\\_SB.PCI0.LPC.EC.FAN0", /* Legacy (pre-2020 models) */
> + "\\_SB.PCI0.LPC0.EC.FAN0", /* Alternate (Certain Slim/Flex) */
> + };
> +
> + 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;
> + 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 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, 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");