[Intel-gfx] [PATCH v2 5/5] drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel

[Jani Nikula]

From: Lyude Paul <lyude at redhat.com>

Annoyingly, the VBT on the ThinkPad X1 Extreme 2nd Gen indicates that
the system uses plain PWM based backlight controls, when in reality the
only backlight controls that work are the standard VESA eDP DPCD
backlight controls.

Honestly, this makes me wonder how many other systems have these issues
or lie about this in their VBT. Not sure we have any good way of finding
out until panels like this become more common place in the laptop
market. For now, just add a DRM DP quirk to indicate that this panel is
telling the truth and is being a good LCD.

v2 by Jani:
- rebase

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112376
Closes: https://gitlab.freedesktop.org/drm/intel/issues/642
Tested-by: AceLan Kao <acelan.kao at canonical.com>
Signed-off-by: Lyude Paul <lyude at redhat.com>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>
---
 drivers/gpu/drm/drm_dp_helper.c                       | 4 ++++
 drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 8 ++++++--
 include/drm/drm_dp_helper.h                           | 8 ++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 5a103e9b3c86..90e122809fa4 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -1179,6 +1179,10 @@ static const struct dpcd_quirk dpcd_quirk_list[] = {
 	{ OUI(0x00, 0x00, 0x00), DEVICE_ID('C', 'H', '7', '5', '1', '1'), false, BIT(DP_DPCD_QUIRK_NO_SINK_COUNT) },
 	/* Synaptics DP1.4 MST hubs can support DSC without virtual DPCD */
 	{ OUI(0x90, 0xCC, 0x24), DEVICE_ID_ANY, true, BIT(DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD) },
+	/* Optional 4K AMOLED panel in the ThinkPad X1 Extreme 2nd Generation
+	 * only supports DPCD backlight controls, despite advertising otherwise
+	 */
+	{ OUI(0xba, 0x41, 0x59), DEVICE_ID_ANY, false, BIT(DP_DPCD_QUIRK_FORCE_DPCD_BACKLIGHT) },
 };
 
 #undef OUI
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..57774003e8c5 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -328,11 +328,15 @@ intel_dp_aux_display_control_capable(struct intel_connector *connector)
 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);
+	struct intel_dp *intel_dp = enc_to_intel_dp(intel_connector->encoder);
+	struct drm_i915_private *i915 = 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))
+	     i915->vbt.backlight.type !=
+	     INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE &&
+	     !drm_dp_has_quirk(&intel_dp->desc,
+			       DP_DPCD_QUIRK_FORCE_DPCD_BACKLIGHT)))
 		return -ENODEV;
 
 	if (!intel_dp_aux_display_control_capable(intel_connector))
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 262faf9e5e94..bb081921f53d 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1532,6 +1532,14 @@ enum drm_dp_quirk {
 	 * The DSC caps can be read from the physical aux instead.
 	 */
 	DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD,
+	/**
+	 * @DP_DPCD_QUIRK_FORCE_DPCD_BACKLIGHT:
+	 *
+	 * The device is telling the truth when it says that it uses DPCD
+	 * backlight controls, even if the system's firmware disagrees.
+	 * The driver should honor the DPCD backlight capabilities advertised.
+	 */
+	DP_DPCD_QUIRK_FORCE_DPCD_BACKLIGHT,
 };
 
 /**
-- 
2.20.1