Re: [ibm-acpi-devel] [PATCH] thinkpad_acpi: Add Auto mode support with dynamic max_brightness

[Andrea Rabbaglietti]

On Mon, Feb 23, 2026, at 6:19 PM, Mark Pearson wrote:
> Hi Hans - do you have an opinion on the path forwards?

Hi all,

a data point from a user, since this thread seems to have stalled.

On a ThinkPad X9-14 Gen 1 (21QA, BIOS 1.25) running 7.2.15, the
firmware already enters Auto mode on its own: Fn+Space cycles

  0 -> 3 (Auto) -> 1 -> 2 -> 0 -> ...

so with the current driver tpacpi::kbd_backlight reports brightness 3
while max_brightness is 2, and brightness_hw_changed emits 3 as well.

This is not only cosmetic. UPower 1.91.x treats 3 as out of range and
stops watching brightness_hw_changed altogether, so the desktop no
longer follows the hotkey at all after the first time Auto is reached:
https://gitlab.freedesktop.org/upower/upower/-/work_items/367

Whatever interface is chosen for exposing Auto mode (separate attribute
or hw-control trigger), would a minimal fix in the meantime be
acceptable? Following Hans' suggestion earlier in the thread, the driver
would report max_brightness (2) in brightness and brightness_hw_changed
while MLCG says Auto, and leave the ABI otherwise unchanged. That keeps
the 0-2 contract userspace relies on and is easy to backport to stable.

I am happy to write and test such a patch on this machine, or to test
a v2 of this series, whichever you prefer.

Thanks,
Andrea Rabbaglietti