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Kernel driver yogafan

Documentation/hwmon/yogafan.rst

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.. 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 Database Below)

Author: Sergio Melas [email protected]

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.

==================================================================================================== LENOVO FAN CONTROLLER: MASTER REFERENCE DATABASE (2026)

::

MODEL (DMI PN) | FAMILY / SERIES | EC OFFSET | FULL ACPI OBJECT PATH | WIDTH | MULTiplier

82N7 | Yoga 14cACN | 0x06 | _SB.PCI0.LPC0.EC0.FANS | 8-bit | 100 80V2 / 81C3 | Yoga 710/720 | 0x06 | _SB.PCI0.LPC0.EC0.FAN0 | 8-bit | 100 83E2 / 83DN | Yoga Pro 7/9 | 0xFE | _SB.PCI0.LPC0.EC0.FANS | 8-bit | 100 82A2 / 82A3 | Yoga Slim 7 | 0x06 | _SB.PCI0.LPC0.EC0.FANS | 8-bit | 100 81YM / 82FG | IdeaPad 5 | 0x06 | _SB.PCI0.LPC0.EC0.FAN0 | 8-bit | 100 82JW / 82JU | Legion 5 (AMD) | 0xFE/0xFF | _SB.PCI0.LPC0.EC0.FANS (Fan1) | 16-bit | 1 82JW / 82JU | Legion 5 (AMD) | 0xFE/0xFF | _SB.PCI0.LPC0.EC0.FA2S (Fan2) | 16-bit | 1 82WQ | Legion 7i (Int) | 0xFE/0xFF | _SB.PCI0.LPC0.EC0.FANS (Fan1) | 16-bit | 1 82WQ | Legion 7i (Int) | 0xFE/0xFF | _SB.PCI0.LPC0.EC0.FA2S (Fan2) | 16-bit | 1 82XV / 83DV | LOQ 15/16 | 0xFE/0xFF | _SB.PCI0.LPC0.EC0.FANS /FA2S | 16-bit | 1 83AK | ThinkBook G6 | 0x06 | _SB.PCI0.LPC0.EC0.FAN0 | 8-bit | 100 81X1 | Flex 5 | 0x06 | _SB.PCI0.LPC0.EC0.FAN0 | 8-bit | 100 Legacy | Pre-2020 Models | 0x06 | _SB.PCI0.LPC.EC.FAN0 | 8-bit | 100

METHODOLOGY & IDENTIFICATION:

  1. DSDT ANALYSIS (THE PATH): BIOS ACPI tables were analyzed using 'iasl' and cross-referenced with public dumps. Internal labels (FANS, FAN0, FA2S) are mapped to EmbeddedControl OperationRegion offsets.

  2. EC MEMORY MAPPING (THE OFFSET): Validated by matching NBFC (NoteBook FanControl) XML logic with DSDT Field definitions found in BIOS firmware.

  3. DATA-WIDTH ANALYSIS (THE MULTIPLIER):

    • 8-bit (Multiplier 100): Standard for Yoga/IdeaPad. Raw values (0-255).
    • 16-bit (Multiplier 1): Standard for Legion/LOQ. Two registers (0xFE/0xFF).

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. NBFC Projects: Community-driven reverse engineering of Lenovo Legion/LOQ EC memory maps (16-bit raw registers). 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/lenovo/ideapad-laptop.c