Re: [PATCH v3 4/5] drm/i915: Don't use VBT for detecting DPCD backlight controls

[Lyude Paul <lyude@redhat.com>]

On Fri, 2020-01-17 at 13:36 +0200, Jani Nikula wrote:
> On Thu, 16 Jan 2020, Lyude Paul <lyude@redhat.com> wrote:
> > Despite the fact that the VBT appears to have a field for specifying
> > that a system is equipped with a panel that supports standard VESA
> > backlight controls over the DP AUX channel, so far every system we've
> > spotted DPCD backlight control support on doesn't actually set this
> > field correctly and all have it set to INTEL_BACKLIGHT_DISPLAY_DDI.
> > 
> > While we don't know the exact reason for this VBT misuse, talking with
> > some vendors indicated that there's a good number of laptop panels out
> > there that supposedly support both PWM backlight controls and DPCD
> > backlight controls as a workaround until Intel supports DPCD backlight
> > controls across platforms universally. This being said, the X1 Extreme
> > 2nd Gen that I have here (note that Lenovo is not the hardware vendor
> > that informed us of this) PWM backlight controls are advertised, but
> > only DPCD controls actually function. I'm going to make an educated
> > guess here and say that on systems like this one, it's likely that PWM
> > backlight controls might have been intended to work but were never
> > really tested by QA.
> > 
> > Since we really need backlights to work without any extra module
> > parameters, let's take the risk here and rely on the standard DPCD caps
> > to tell us whether AUX backlight controls are supported or not. We still
> > check the VBT, but only to make sure that we don't enable DPCD backlight
> > controls on a panel that uses something other then the standard VESA
> > interfaces over AUX. Since panels using such non-standard interfaces
> > should probably have support added to i915, we'll print a warning when
> > seeing this in the VBT. We can remove this warning later if we end up
> > adding support for any custom backlight interfaces.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112376
> > Cc: Jani Nikula <jani.nikula@intel.com>
> > Cc: Perry Yuan <pyuan@redhat.com>
> > Cc: AceLan Kao <acelan.kao@canonical.com>
> > ---
> >  .../drm/i915/display/intel_dp_aux_backlight.c    | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > index 77a759361c5c..3002b600635f 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > @@ -330,13 +330,17 @@ int intel_dp_aux_init_backlight_funcs(struct
> > intel_connector *intel_connector)
> >  	struct intel_panel *panel = &intel_connector->panel;
> >  	struct drm_i915_private *dev_priv = to_i915(intel_connector-
> > >base.dev);
> >  
> > -	if (i915_modparams.enable_dpcd_backlight == 0 ||
> > -	    (i915_modparams.enable_dpcd_backlight == -1 &&
> > -	    dev_priv->vbt.backlight.type !=
> > INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE))
> > -		return -ENODEV;
> > -
> > -	if (!intel_dp_aux_display_control_capable(intel_connector))
> > +	if (i915_modparams.enable_dpcd_backlight == 0)
> >  		return -ENODEV;
> > +	if (i915_modparams.enable_dpcd_backlight == -1) {
> > +		if (dev_priv->vbt.backlight.type
> > +		    == INTEL_BACKLIGHT_PANEL_DRIVER_INTERFACE) {
> > +			DRM_WARN("VBT says panel uses custom panel driver
> > interface, not using DPCD backlight controls\n");
> > +			return -ENODEV;
> > +		}
> > +		if (!intel_dp_aux_display_control_capable(intel_connector))
> > +			return -ENODEV;
> 
> Functionally, I'm fine with trying this. But perhaps we should check aux
> and early return first, and then check what vbt says, to reduce the
> dmesg noise.
> 
> I'll probably want to see a debug message if we're enabling aux
> backlight even if dev_priv->vbt.backlight.type !=
> INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE. It's the kind of debug trace
> you'll really want to get first.

Good point, I'll send a respin of this patch with those changes
> 
> BR,
> Jani.
> 
> 
> 
> > +	}
> >  
> >  	panel->backlight.setup = intel_dp_aux_setup_backlight;
> >  	panel->backlight.enable = intel_dp_aux_enable_backlight;
-- 
Cheers,
	Lyude Paul