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SubjectRE: [PATCH] drm/nouveau/kms/nv50-: fix build failure with CONFIG_BACKLIGHT=n
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+Harry

-----Original Message-----
From: Daniel Vetter <daniel@ffwll.ch>
Sent: Friday, July 23, 2021 11:23 AM
To: Arnd Bergmann <arnd@kernel.org>
Cc: Daniel Vetter <daniel@ffwll.ch>; Ben Skeggs <bskeggs@redhat.com>; David Airlie <airlied@linux.ie>; Lyude Paul <lyude@redhat.com>; Arnd Bergmann <arnd@arndb.de>; Ville Syrjälä <ville.syrjala@linux.intel.com>; Cornij, Nikola <Nikola.Cornij@amd.com>; dri-devel <dri-devel@lists.freedesktop.org>; Nouveau Dev <nouveau@lists.freedesktop.org>; Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] drm/nouveau/kms/nv50-: fix build failure with CONFIG_BACKLIGHT=n

On Fri, Jul 23, 2021 at 12:16:31PM +0200, Arnd Bergmann wrote:
> On Fri, Jul 23, 2021 at 11:25 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Fri, Jul 23, 2021 at 11:15 AM Arnd Bergmann <arnd@kernel.org> wrote:
> > >
> > > From: Arnd Bergmann <arnd@arndb.de>
> > >
> > > When the backlight support is disabled, the driver fails to build:
> > >
> > > drivers/gpu/drm/nouveau/dispnv50/disp.c: In function 'nv50_sor_atomic_disable':
> > > drivers/gpu/drm/nouveau/dispnv50/disp.c:1665:59: error: 'struct nouveau_connector' has no member named 'backlight'
> > > 1665 | struct nouveau_backlight *backlight = nv_connector->backlight;
> > > | ^~
> > > drivers/gpu/drm/nouveau/dispnv50/disp.c:1670:35: error: invalid use of undefined type 'struct nouveau_backlight'
> > > 1670 | if (backlight && backlight->uses_dpcd) {
> > > | ^~
> > > drivers/gpu/drm/nouveau/dispnv50/disp.c:1671:64: error: invalid use of undefined type 'struct nouveau_backlight'
> > > 1671 | ret = drm_edp_backlight_disable(aux, &backlight->edp_info);
> > > | ^~
> > >
> > > The patch that introduced the problem already contains some #ifdef
> > > checks, so just add another one that makes it build again.
> > >
> > > Fixes: 6eca310e8924 ("drm/nouveau/kms/nv50-: Add basic DPCD
> > > backlight support for nouveau")
> > > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> >
> > Can we just toss the idea that BACKTLIGHT=n is a reasonable config
> > for drm drivers using backlights, and add depends BACKLIGHT to all
> > of them?
> >
> > I mean this is a perfect source of continued patch streams to keep
> > us all busy, but beyond that I really don't see the point ... I
> > frankly have better things to do, and especially with the big
> > drivers we have making backlight optional saves comparitively nothing.
> > -Daniel
>
> Yes! I'd definitely be in favor of that, I've wasted way too much time
> trying to sort through dependency loops and other problems with backlight support.
>
> Maybe we should leave the drivers/video/fbdev/ drivers untouched in
> this regard, at least for the moment, but for the drivers/gpu/drm
> users of backlight that would be a nice simplification, and even the
> smallest ones are unlikely to be used on systems that are too memory
> constrained to deal with 4KB extra .text.

Yeah I think we can do this entirely ad-hoc, i.e. any time the backlight wheel wobbles off again we nail it down for good for that driver with a depends on BACKGLIGHT and remove any lingering #ifdef all over.

If you want maybe start out with the biggest drm drivers in a series, I think if nouveau/amdgpu/i915 folks ack this you're good to go to just convert as things get in the way.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
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