linux-kernel - Backlight control broken on UM325 (OLED) on 5.15 (bisected)

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Message-ID: <20211114105657.b57pjojiv72iopg5@fastboi.localdomain>
Date:   Sun, 14 Nov 2021 11:56:57 +0100
From:   Samuel Čavoj <samuel@...oj.net>
To:     Alex Deucher <alexander.deucher@....com>
Cc:     Harry Wentland <harry.wentland@....com>,
        Roman Li <Roman.Li@....com>, Leo Li <sunpeng.li@....com>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Backlight control broken on UM325 (OLED) on 5.15 (bisected)

Hello,

the backlight control no longer works on my ASUS UM325 (Ryzen 5700U)
OLED laptop. I have bisected the breakage to commit 7fd13baeb7a3a48.

    commit 7fd13baeb7a3a48cae12c36c52f06bf4e9e7d728 (HEAD, refs/bisect/bad)
    Author: Alex Deucher <alexander.deucher@....com>
    Date:   Thu Jul 8 16:31:10 2021 -0400
     
        drm/amdgpu/display: add support for multiple backlights
        
        On platforms that support multiple backlights, register
        each one separately.  This lets us manage them independently
        rather than registering a single backlight and applying the
        same settings to both.
        
        v2: fix typo:
        Reported-by: kernel test robot <lkp@...el.com>
        
        Reviewed-by: Roman Li <Roman.Li@....com>
        Signed-off-by: Alex Deucher <alexander.deucher@....com>

I have encountered another user with the same issue on reddit[0].

The node in /sys/class/backlight exists, writing to it just does
nothing. I would be glad to help debugging the issue. Thank you very
much.

Regards,
Samuel Čavoj

[0]: https://www.reddit.com/r/AMDLaptops/comments/qst0fm/after_updating_to_linux_515_my_brightness/