drm/i915: Also check for VGA converter in eDP probe

Unfortunately even the HPD based detection added in
commit cfe5bdfb ("drm/i915: Check HPD live state during eDP probe")
fails to detect that the VBT's eDP/DDI-A is a ghost on
Asus B360M-A (CFL+CNP). On that board eDP/DDI-A has its HPD
asserted despite nothing being actually connected there :(
The straps/fuses also indicate that the eDP port is present.

So if one boots with a VGA monitor connected the eDP probe will
mistake the DP->VGA converter hooked to DDI-E for an eDP panel
on DDI-A.

As a last resort check what kind of DP device we've detected,
and if it looks like a DP->VGA converter then conclude that
the eDP port should be ignored.

Cc: stable@vger.kernel.org
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/9636
Fixes: cfe5bdfb ("drm/i915: Check HPD live state during eDP probe")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231114142333.15799-1-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

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

@@ -6270,8 +6270,7 @@ static bool intel_edp_init_connector(struct intel_dp *intel_dp,
 	 * (eg. Acer Chromebook C710), so we'll check it only if multiple
 	 * ports are attempting to use the same AUX CH, according to VBT.
 	 */
-	if (intel_bios_dp_has_shared_aux_ch(encoder->devdata) &&
-	    !intel_digital_port_connected(encoder)) {
+	if (intel_bios_dp_has_shared_aux_ch(encoder->devdata)) {
 		/*
 		 * If this fails, presume the DPCD answer came
 		 * from some other port using the same AUX CH.
@@ -6279,10 +6278,27 @@ static bool intel_edp_init_connector(struct intel_dp *intel_dp,
 		 * FIXME maybe cleaner to check this before the
 		 * DPCD read? Would need sort out the VDD handling...
 		 */
-		drm_info(&dev_priv->drm,
-			 "[ENCODER:%d:%s] HPD is down, disabling eDP\n",
-			 encoder->base.base.id, encoder->base.name);
-		goto out_vdd_off;
+		if (!intel_digital_port_connected(encoder)) {
+			drm_info(&dev_priv->drm,
+				 "[ENCODER:%d:%s] HPD is down, disabling eDP\n",
+				 encoder->base.base.id, encoder->base.name);
+			goto out_vdd_off;
+		}
+
+		/*
+		 * Unfortunately even the HPD based detection fails on
+		 * eg. Asus B360M-A (CFL+CNP), so as a last resort fall
+		 * back to checking for a VGA branch device. Only do this
+		 * on known affected platforms to minimize false positives.
+		 */
+		if (DISPLAY_VER(dev_priv) == 9 && drm_dp_is_branch(intel_dp->dpcd) &&
+		    (intel_dp->dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_TYPE_MASK) ==
+		    DP_DWN_STRM_PORT_TYPE_ANALOG) {
+			drm_info(&dev_priv->drm,
+				 "[ENCODER:%d:%s] VGA converter detected, disabling eDP\n",
+				 encoder->base.base.id, encoder->base.name);
+			goto out_vdd_off;
+		}
 	}
 
 	mutex_lock(&dev_priv->drm.mode_config.mutex);