Re: [REGRESSION] Please revert 1191285ecb42 from 7.1.y

[Sasha Levin <sashal@kernel.org>]

On Tue, Aug 11, 2026 at 01:25:14PM -0400, Alex Deucher wrote:
>On Tue, Aug 11, 2026 at 11:25 AM Sergio Callegari
><sergio.callegari@gmail.com> wrote:
>>
>> On 11/08/2026 10:08, Greg KH wrote:
>> > We aim to be "bug compatible" with Linus's branch, otherwise the
>> > "pressure" to fix those bugs goes down.  But I defer to the maintainers
>> > of the code involved if it is being worked on, and they want it
>> > reverted, great, we can do that.  Otherwise we prefer to leave it as-is.
>>
>> I understand. Sorry for asking about things that might be obvious to
>> most people here. My expectation was that only a selected subset of the
>> patches on the Linus' tree were to enter in stable, so do we really need
>> to have stable "bug compatible" wrt this bug? The original behavior was
>> not free of issues, but they were rather minor (a reporting issue). Now
>> we have this very bad behavior where the display remains dark.
>>
>> To make the matter worse, on systems that have encrypted root the screen
>> remains dark also on that password prompt (observed with plymouth).
>>
>> In any case, I managed testing 7.2.0RC7 and it is fine again.
>>
>> Looks like the Linus' tree has more changes wrt backlight, and the
>> interesting ones are probably those in the cumulative fix
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=973fd9493ec21f12e760fbbedeb7b0b2317c02f4.
>>
>
>Thanks for testing.  It sounds like we should revert the change from
>stable.  There must be some dependency we are missing in stable.

Hmm...

In stable kernels, convert_custom_brightness() still scales the user value by
(max - min), but f1b5d8f9cc54 ("drm/amd/display: Fix backlight max_brightness
to match exported range") widened the exported range to max. Past ~(max - min),
scale_input_to_fw() exceeds AMDGPU_MAX_BL_LEVEL and
convert_brightness_from_user() returns >0xFFFF, which wraps in
dce_driver_set_backlight(). With the defaults the cliff lands on 64533, which
is what Sergio sees.

Upstream only hides it: mod_power_set_backlight_percent() clamps at 100%.

Does this make sense?

-- 
Thanks,
Sasha