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Date:   Fri, 23 Jul 2021 17:23:19 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
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 =?iso-8859-1?Q?Syrj=E4l=E4?= 
        <ville.syrjala@linux.intel.com>,
        Nikola Cornij <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
Message-ID: <YPre51p6TirescUu@phenom.ffwll.local>
Mail-Followup-To: Arnd Bergmann <arnd@kernel.org>,
        Ben Skeggs <bskeggs@redhat.com>, David Airlie <airlied@linux.ie>,
        Lyude Paul <lyude@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        Ville =?iso-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>,
        Nikola Cornij <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>
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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
http://blog.ffwll.ch