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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Fri, 23 Jul 2021 12:16:31 +0200
Message-ID: <CAK8P3a3xjS-zJFiQgjYDz2Gja4KBeHWyrQX4PrbkkZfEV2OJgg@mail.gmail.com>
Subject: Re: [PATCH] drm/nouveau/kms/nv50-: fix build failure with CONFIG_BACKLIGHT=n
To:     Daniel Vetter <daniel@ffwll.ch>
Cc:     Ben Skeggs <bskeggs@redhat.com>, David Airlie <airlied@linux.ie>,
        Lyude Paul <lyude@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <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 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.

       Arnd