[RFC PATCH 0/1] platform/x86: thinkpad_acpi: Add support for auto keyboard backlight

[Qiling Zhu <kmatzkaczor244@gmail.com>]

Hi Mark, Ilpo, Rong,

Recent ThinkPads with an ambient light sensor (ALS) support an auto
mode for the keyboard backlight: the EC picks the backlight level
according to ambient light. The mode is EC keyboard backlight level 3,
above the two fixed levels exposed so far, and it is reachable only
through the Fn+Space shortcut, which is handled entirely by the
firmware. Userspace can neither select the mode nor observe whether it
is active.

This series wires the auto mode up as a private hardware control
trigger, "tpacpi-kbd-auto", on top of the hw control trigger framework
that recently landed in mainline [1]. With it:

  - selecting the trigger writes EC level 3 (auto mode on); writing
    "none" turns the backlight off
  - the trigger state (bracketed in the trigger sysfs attribute, and
    the new trigger_may_offload_to_hw attribute) is the userspace
    view of auto mode; reading brightness returns -ENODATA while
    auto mode is active, per the framework semantics
  - Fn+Space keeps cycling through levels and auto mode as before,
    and the trigger state now follows hardware-initiated transitions
    automatically

This is the thinkpad_acpi counterpart of the ideapad-laptop wiring
from the same framework series, which is still queued; this patch
depends only on the LED core parts already in mainline and can be
applied independently.

For background: the February discussion settled that auto mode must
not be exposed as a brightness value [2], but no agreement was
reached back then on how to expose it instead; the hardware control
trigger approach was established later by the framework series [1].

Tested on a ThinkPad X1 Carbon Gen 14 (MT 21V8) running linux-next
next-20261030: 61 scripted user-visible assertions covering the
software paths, the Fn+Space event chain and the unchanged legacy
behavior, plus manual exercising of both the Fn+Space cycling and
desktop-environment brightness controls; checkpatch --strict clean.

Deliberately unchanged (details in the commit message):

  - max_brightness stays 2; auto mode is not multiplexed into the
    brightness value
  - writing 3 to /proc/acpi/ibm/kbdlight keeps returning -EINVAL
  - module unload still turns the keyboard backlight off

Two open questions, the main reason for the RFC tag:

  1. MLCG returns capability bits in the upper 16 bits; does one of
     them indicate auto mode support? If yes, I would gate trigger
     registration on it. Mark, does the EC documentation say?
  2. On models without an ALS, the EC behavior when asked for level 3
     via MLCS is unknown to me. If it is undefined rather than
     rejected, optimistic registration would need a rethink (e.g.
     DMI whitelisting).

Rong, happy to rebase this on top of your series or fold it in, if
you prefer to keep both wirings together.

Comments welcome.

Regards,
Qiling Zhu

This patch was developed with LLM assistance; I reviewed every design
decision against the framework sources and validated the behavior on
real hardware myself.

[1] https://lore.kernel.org/r/20260618-leds-trigger-hw-changed-v2-0-c28c44053cf3@rong.moe/
[2] https://lore.kernel.org/r/20260203232219.11683-1-vishnuocv@gmail.com/

Qiling Zhu (1):
  platform/x86: thinkpad_acpi: Add support for auto keyboard backlight

 drivers/platform/x86/lenovo/Kconfig         |   1 +
 drivers/platform/x86/lenovo/thinkpad_acpi.c | 104 ++++++++++++++++----
 2 files changed, 88 insertions(+), 17 deletions(-)

-- 
2.56.0