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From:   AceLan Kao <acelan.kao@canonical.com>
Date:   Thu, 8 Jun 2023 11:04:50 +0800
Message-ID: <CAFv23QmVP4ZFAhAFBSx6ty-4fcQ-u9+uZb0xaD2gCJPCdpPvQg@mail.gmail.com>
Subject: Re: [PATCH] platform/x86: dell-laptop: Add drm module soft dependency
To:     Hans de Goede <hdegoede@redhat.com>
Cc:     Matthew Garrett <mjg59@srcf.ucam.org>,
        =?UTF-8?Q?Pali_Roh=C3=A1r?= <pali@kernel.org>,
        Mark Gross <markgross@kernel.org>,
        platform-driver-x86@vger.kernel.org, linux-kernel@vger.kernel.org,
        Kai-Heng Feng <kai.heng.feng@canonical.com>
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Hans de Goede <hdegoede@redhat.com> =E6=96=BC 2023=E5=B9=B46=E6=9C=888=E6=
=97=A5 =E9=80=B1=E5=9B=9B =E4=B8=8A=E5=8D=883:16=E5=AF=AB=E9=81=93=EF=BC=9A
>
> Hi,
>
> On 6/7/23 09:47, Matthew Garrett wrote:
> > On Wed, Jun 07, 2023 at 03:39:33PM +0800, AceLan Kao wrote:
> >
> >> What do you think if we unregister backlight devices if the backlight =
type
> >> is larger than the current registered one.
> >> Do this check in backlight_device_register() and unregister backlight
> >> devices by the order raw(1) > platform(2) > firmware(3)
> >> And maybe introduce a sticky bit into the backlight device if the back=
light
> >> driver doesn't want to be removed.
> >
> > Hans looked at doing this, but there were some awkward corner cases.
> > When we first introduced this functionality, firmware was preferred to
> > platform was preferred to raw - but on Intel, at least, this behaviour
> > changed with later versions of Windows. I don't think there's a single
> > static policy that works, I think you need to pay attention to the hint=
s
> > the platform gives you. How does Windows know which interface to use on
> > this platform? The simplest solution may actually just be for
> > dell-laptop to refuse to register a backlight if the platform claims to
> > be Windows 8 or later.
>
> I like that idea.
>
> AceLan, I guess that you hit this easy while testing on a (development)
> Meteor Lake platform ?
>
> I have had other/similar reports about Meteor Lake platforms.
>
> On hw from the last 10 years dell-laptop will not register
> its vendor-type backlight class device because
> acpi_video_get_backlight_type() will return acpi_backlight_video
> there (1) so it does not matter if the GPU driver shows up only
> later (2).
>
> But it seems that on Meteor Lake the ACPI tables will no longer
> contain acpi_video backlight control support which causes
> acpi_video_get_backlight_type() to return acpi_backlight_vendor (2).
> triggering the issue you are seeing.
>
> Can you give the attached patch a try please ?
>
> Regards,
>
> Hans
>
>
> 1) Starting with kernel >=3D 6.2 acpi_video.c will only register
> the /sys/class/backlight/acpi_video# node after a drm/kms drivers
> asks it to register it.
>
> 2) The native GPU driver will tell the drivers/acpi/video_detect.c
> code that native backlight control is available changing
> the return of acpi_video_get_backlight_type() to native, which
> is what loading the native GPU driver first also fixes this issue.

Hi Hans,

Yes, this patch works for me, thanks.

BTW, I encountered this issue on the RPL platform.