.. 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 <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.


====================================================================================================
                 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
