linux-kernel - Re: [PATCH] nouveau: make backlight support non optional

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Message-ID: <CACO55tsoi2akTKvFdz3p48UHRjFXDW7dUnOM8qVePBFWet-3UQ@mail.gmail.com>
Date:   Sat, 24 Jul 2021 14:10:53 +0200
From:   Karol Herbst <kherbst@...hat.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Daniel Vetter <daniel@...ll.ch>,
        ML nouveau <nouveau@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] nouveau: make backlight support non optional

On Sat, Jul 24, 2021 at 1:56 PM Arnd Bergmann <arnd@...nel.org> wrote:
>
> On Sat, Jul 24, 2021 at 11:55 AM Karol Herbst <kherbst@...hat.com> wrote:
> >
> > On Sat, Jul 24, 2021 at 8:55 AM Arnd Bergmann <arnd@...nel.org> wrote:
> > >
> > > On Sat, Jul 24, 2021 at 12:47 AM Karol Herbst <kherbst@...hat.com> wrote:
> > > >
> > > > In the past this only led to compilation issues. Also the small amount of
> > > > extra .text shouldn't really matter compared to the entire nouveau driver
> > > > anyway.
> > > >
> > >
> > > >         select DRM_TTM_HELPER
> > > > -       select BACKLIGHT_CLASS_DEVICE if DRM_NOUVEAU_BACKLIGHT
> > > > -       select ACPI_VIDEO if ACPI && X86 && BACKLIGHT_CLASS_DEVICE && INPUT
> > > > +       select BACKLIGHT_CLASS_DEVICE
> > > > +       select ACPI_VIDEO if ACPI && X86 && INPUT
> > > >         select X86_PLATFORM_DEVICES if ACPI && X86
> > > >         select ACPI_WMI if ACPI && X86
> > >
> > > I think the logic needs to be the reverse: instead of 'select
> > > BACKLIGHT_CLASS_DEVICE',
> > > this should be 'depends on BACKLIGHT_CLASS_DEVICE', and the same for ACPI_VIDEO.
> > >
> > > We may want to add 'default DRM || FB' to BACKLIGHT_CLASS_DEVICE in the
> > > process so we don't lose it for users doing 'make oldconfig' or 'make defconfig'
> > >
> >
> > I think the problem with
> > "depends" is that the user needs to enable backlight support first
> > before even seeing nouveau and I don't know if that makes sense. But
> > maybe "default" is indeed helping here in this case.
>
> In general, no driver should ever 'select' a subsystem. Otherwise you end up
> with two problems:
>
> - enabling this one driver suddenly makes all other drivers that have
> a dependency
>   on this visible, and some of those might have a 'default y', so you
> end up with
>   a ton of stuff in the kernel that would otherwise not be there.
>
> - It becomes impossible to turn it off as long as some driver has that 'select'.
>   This is the pretty much the same problem as the one you describe, just
>    the other side of it.
>
> - You run into dependency loops that prevent a successful build when some
>    other driver has a 'depends on'. Preventing these loops was the main
>    reason I said we should do this change.
>
> In theory we could change the other 85 drivers that use 'depends on' today,
> and make BACKLIGHT_CLASS_DEVICE a hidden symbol that only ever
> selected by the drivers that need it. This would avoid the third problem but
> not the other one.
>
>       Arnd
>

I see. Yeah, I guess we can do it this way then. I just wasn't aware
of the bigger picture here. Thanks for explaining.