linux-kernel - Re: [Nouveau] [PATCH v3 2/5] drm/nouveau/kms/nv50-: Explicitly check DPCD backlights for aux enable/brightness

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Date:   Wed, 06 Oct 2021 16:14:31 -0400
From:   Lyude Paul <lyude@...hat.com>
To:     Karol Herbst <kherbst@...hat.com>
Cc:     Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        nouveau <nouveau@...ts.freedesktop.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [Nouveau] [PATCH v3 2/5] drm/nouveau/kms/nv50-: Explicitly
 check DPCD backlights for aux enable/brightness

On Wed, 2021-10-06 at 18:30 +0200, Karol Herbst wrote:
> On Wed, Oct 6, 2021 at 4:41 AM Lyude Paul <lyude@...hat.com> wrote:
> > 
> > Since we don't support hybrid AUX/PWM backlights in nouveau right now,
> > let's add some explicit checks so that we don't break nouveau once we
> > enable support for these backlights in other drivers.
> > 
> > Signed-off-by: Lyude Paul <lyude@...hat.com>
> > ---
> >  drivers/gpu/drm/nouveau/nouveau_backlight.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c
> > b/drivers/gpu/drm/nouveau/nouveau_backlight.c
> > index 1cbd71abc80a..ae2f2abc8f5a 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_backlight.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c
> > @@ -308,7 +308,10 @@ nv50_backlight_init(struct nouveau_backlight *bl,
> >                 if (ret < 0)
> >                         return ret;
> > 
> > -               if (drm_edp_backlight_supported(edp_dpcd)) {
> > +               /* TODO: Add support for hybrid PWM/DPCD panels */
> > +               if (drm_edp_backlight_supported(edp_dpcd) &&
> > +                   (edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP) &&
> 
> where does the DP_EDP_BACKLIGHT_AUX_ENABLE_CAP come from? afaik
> drm_edp_backlight_supported checks for
> DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP and
> DP_EDP_TCON_BACKLIGHT_ADJUSTMENT_CAP so wondering if this was
> intentional or a typo

This is intentional - drm_edp_backlight_supported() does check for these, but
in the patch after this we remove the BRIGHTNESS_AUX_SET_CAP from
drm_edp_backlight_supported() in order to implement support for panels lacking
BRIGHTNESS_AUX_SET_CAP in i915. Since we don't have support for this in
nouveau yet but such backlights are likely to mostly work without the use of
DPCD if we avoid trying to set it up, this patch is just here to make sure
that the changes to drm_edp_backlight_supported() don't result in nouveau
suddenly trying (and failing) to enable DPCD backlight controls on those
backlights.

> 
> > +                   (edp_dpcd[2] &
> > DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP)) {
> >                         NV_DEBUG(drm, "DPCD backlight controls supported
> > on %s\n",
> >                                  nv_conn->base.name);
> > 
> > --
> > 2.31.1
> > 
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat