drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel

The X1 Extreme is one of the systems that lies about which backlight
interface that it uses in its VBIOS as PWM backlight controls don't work
at all on this machine. It's possible that this panel could be one of
the infamous ones that can switch between PWM mode and DPCD backlight
control mode, but we haven't gotten any more details on this from Lenovo
just yet. For the time being though, making sure the backlight 'just
works' is a bit more important.

So, add a quirk to force DPCD backlight controls on for these systems
based on EDID (since this panel doesn't appear to fill in the device ID).
Hopefully in the future we'll figure out a better way of probing this.

Changes since v2:
* The bugzilla URL is deprecated, bug reporting happens on gitlab now.
  Update the messages we print to reflect this
* Also, take the opportunity to move FDO_BUG_URL out of i915_utils.c and
  into i915_utils.h so that other places which print things that aren't
  traditional errors but are worth filing bugs about, can actually use
  it.
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303215320.93491-1-lyude@redhat.com

drivers/gpu/drm/drm_dp_helper.c

@@ -1236,6 +1236,10 @@ struct edid_quirk {
  * DP quirks in such cases.
  */
 static const struct edid_quirk edid_quirk_list[] = {
+	/* Optional 4K AMOLED panel in the ThinkPad X1 Extreme 2nd Generation
+	 * only supports DPCD backlight controls
+	 */
+	{ MFG(0x4c, 0x83), PROD_ID(0x41, 0x41), BIT(DP_QUIRK_FORCE_DPCD_BACKLIGHT) },
 };
 
 #undef MFG

drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c

@@ -328,15 +328,31 @@ 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_device *dev = intel_connector->base.dev;
+	struct drm_i915_private *dev_priv = to_i915(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))
+	    !intel_dp_aux_display_control_capable(intel_connector))
 		return -ENODEV;
 
-	if (!intel_dp_aux_display_control_capable(intel_connector))
+	/*
+	 * There are a lot of machines that don't advertise the backlight
+	 * control interface to use properly in their VBIOS, :\
+	 */
+	if (dev_priv->vbt.backlight.type !=
+	    INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE &&
+	    !drm_dp_has_quirk(&intel_dp->desc, intel_dp->edid_quirks,
+			      DP_QUIRK_FORCE_DPCD_BACKLIGHT)) {
+		DRM_DEV_INFO(dev->dev,
+			     "Panel advertises DPCD backlight support, but "
+			     "VBT disagrees. If your backlight controls "
+			     "don't work try booting with "
+			     "i915.enable_dpcd_backlight=1. If your machine "
+			     "needs this, please file a _new_ bug report on "
+			     "drm/i915, see " FDO_BUG_URL " for details.\n");
 		return -ENODEV;
+	}
 
 	panel->backlight.setup = intel_dp_aux_setup_backlight;
 	panel->backlight.enable = intel_dp_aux_enable_backlight;

drivers/gpu/drm/i915/i915_utils.c

@@ -8,7 +8,6 @@
 #include "i915_drv.h"
 #include "i915_utils.h"
 
-#define FDO_BUG_URL "https://gitlab.freedesktop.org/drm/intel/-/wikis/How-to-file-i915-bugs"
 #define FDO_BUG_MSG "Please file a bug on drm/i915; see " FDO_BUG_URL " for details."
 
 void

drivers/gpu/drm/i915/i915_utils.h

@@ -34,6 +34,8 @@
 struct drm_i915_private;
 struct timer_list;
 
+#define FDO_BUG_URL "https://gitlab.freedesktop.org/drm/intel/-/wikis/How-to-file-i915-bugs"
+
 #undef WARN_ON
 /* Many gcc seem to no see through this and fall over :( */
 #if 0

include/drm/drm_dp_helper.h

@@ -1535,6 +1535,16 @@ enum drm_dp_quirk {
 	 * The DSC caps can be read from the physical aux instead.
 	 */
 	DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD,
+	/**
+	 * @DP_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. This
+	 * quirk should be checked against both the ident and panel EDID.
+	 * When present, the driver should honor the DPCD backlight
+	 * capabilities advertised.
+	 */
+	DP_QUIRK_FORCE_DPCD_BACKLIGHT,
 };
 
 /**