Re: [PATCH] nouveau: make backlight support non optional

From: Karol Herbst <kherbst@redhat.com>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Lyude Paul <lyude@redhat.com>, Ben Skeggs <bskeggs@redhat.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Daniel Vetter <daniel@ffwll.ch>,
	ML nouveau <nouveau@lists.freedesktop.org>,
	dri-devel <dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH] nouveau: make backlight support non optional
Date: Sat, 24 Jul 2021 14:51:47 +0200	[thread overview]
Message-ID: <CACO55tuceMUz2pgOM23wvcmtaTqbo6S6rCB+mfLptqJRt=fMWA@mail.gmail.com> (raw)
In-Reply-To: <CACO55tsoi2akTKvFdz3p48UHRjFXDW7dUnOM8qVePBFWet-3UQ@mail.gmail.com>

On Sat, Jul 24, 2021 at 2:10 PM Karol Herbst <kherbst@redhat.com> wrote:
>
> On Sat, Jul 24, 2021 at 1:56 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > On Sat, Jul 24, 2021 at 11:55 AM Karol Herbst <kherbst@redhat.com> wrote:
> > >
> > > On Sat, Jul 24, 2021 at 8:55 AM Arnd Bergmann <arnd@kernel.org> wrote:
> > > >
> > > > On Sat, Jul 24, 2021 at 12:47 AM Karol Herbst <kherbst@redhat.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.

yeah... that doesn't work. So the issue is, that X86_PLATFORM_DEVICES
is a little bit in the way. If I remove the select
X86_PLATFORM_DEVICES then I guess problems once ACPI is enabled, but
if I keep it, I get cyclic dep errors :/