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

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Date:   Fri, 17 Jan 2020 13:36:12 +0200
From:   Jani Nikula <jani.nikula@...el.com>
To:     Lyude Paul <lyude@...hat.com>, intel-gfx@...ts.freedesktop.org
Cc:     dri-devel@...ts.freedesktop.org, Perry Yuan <pyuan@...hat.com>,
        AceLan Kao <acelan.kao@...onical.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Juha-Pekka Heikkila <juhapekka.heikkila@...il.com>,
        Lee Shawn C <shawn.c.lee@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/5] drm/i915: Don't use VBT for detecting DPCD backlight controls

On Thu, 16 Jan 2020, Lyude Paul <lyude@...hat.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@...hat.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112376
> Cc: Jani Nikula <jani.nikula@...el.com>
> Cc: Perry Yuan <pyuan@...hat.com>
> Cc: AceLan Kao <acelan.kao@...onical.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.

BR,
Jani.



> +	}
>  
>  	panel->backlight.setup = intel_dp_aux_setup_backlight;
>  	panel->backlight.enable = intel_dp_aux_enable_backlight;

-- 
Jani Nikula, Intel Open Source Graphics Center