linux-kernel - Re: [REGRESSION] Missing nouveau backlight control on 6.2.x kernel

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Date:   Sat, 18 Mar 2023 17:49:28 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     linux-kernel@...r.kernel.org, regressions@...ts.linux.dev,
        dri-devel@...ts.freedesktop.org
Subject: Re: [REGRESSION] Missing nouveau backlight control on 6.2.x kernel

Hi Takashi,

On 3/17/23 11:04, Takashi Iwai wrote:
> Hi,
> 
> we've received a regression report on openSUSE Bugzilla about the
> missing backlight control of nouveau device:
>   https://bugzilla.opensuse.org/show_bug.cgi?id=1209296
> 
> On 6.1, with acpi_video=native option, the system provided
> /sys/class/backlight/nv_backlight entry.
> 
> On 6.2, with acpi_video=native option, there is no entry in
> /sys/class/backlight.  And without acpi_video option, it falls back to
> /sys/class/backlight/acpi_video0, which doesn't work as expected.
> 
> Hans, could you check whether the recent change in video_detect.c (or
> anything else) may influence on this?  My gut feeling is that the
> culprit could be rather some change in nouveau (so dri-devel is
> Cc'ed), but I'm not entirely sure...

As I already told the reporter, who first contacted me by private email about this, all the drivers/acpi/video_detect.c changes which may influence this are also present in 6.1.y (IIRC >= 6.1.5) . So since this is being reported as only happening with 6.2.y I don't think the recent backlight detect rework is involved.

And even if the recent backlight detect rework were involved, specifying acpi_backlight=native on the kernel commandline (as the reporter was doing before) should still work since it overrides all other backlight detection.

So yes this seems to be a nouveau bug and should probably be reported following the instructions from: https://nouveau.freedesktop.org/Bugs.html  (which I havealso told the reporter already).

Regards,

Hans