drm/i915/vbt: ignore extraneous child devices for a port

Ever since we've parsed VBT child devices, starting from 6acab15a
("drm/i915: use the HDMI DDI buffer translations from VBT"), we've
ignored the child device information if more than one child device
references the same port. The rationale for this seems lost in time.

Since commit 311a2094 ("drm/i915: don't init DP or HDMI when not
supported by DDI port") we started using this information more to skip
HDMI/DP init if the port wasn't there per VBT child devices. However, at
the same time it added port defaults without further explanation.

Thus, if the child device info was skipped due to multiple child devices
referencing the same port, the device info would be retrieved from the
somewhat arbitrary defaults.

Finally, when commit bb1d1329 ("drm/i915/vbt: split out defaults
that are set when there is no VBT") stopped initializing the defaults
whenever VBT is present, thus trusting the VBT more, we stopped
initializing ports which were referenced by more than one child device.

Apparently at least Asus UX305UA, UX305U, and UX306U laptops have VBT
child device blocks which cause this behaviour. Arguably they were
shipped with a broken VBT.

Relax the rules for multiple references to the same port, and use the
first child device info to reference a port. Retain the logic to debug
log about this, though.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101745
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=196233
Fixes: bb1d1329 ("drm/i915/vbt: split out defaults that are set when there is no VBT")
Tested-by: NOliver Weißbarth <mail@oweissbarth.de>
Reported-by: NOliver Weißbarth <mail@oweissbarth.de>
Reported-by: NDidier G <didierg-divers@orange.fr>
Reported-by: NGiles Anderson <agander@gmail.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: <stable@vger.kernel.org> # v4.12+
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170811113907.6716-1-jani.nikula@intel.comSigned-off-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_bios.c

@@ -1120,8 +1120,8 @@ static void parse_ddi_port(struct drm_i915_private *dev_priv, enum port port,
 	bool is_dvi, is_hdmi, is_dp, is_edp, is_crt;
 	uint8_t aux_channel, ddc_pin;
 	/* Each DDI port can have more than one value on the "DVO Port" field,
-	 * so look for all the possible values for each port and abort if more
-	 * than one is found. */
+	 * so look for all the possible values for each port.
+	 */
 	int dvo_ports[][3] = {
 		{DVO_PORT_HDMIA, DVO_PORT_DPA, -1},
 		{DVO_PORT_HDMIB, DVO_PORT_DPB, -1},
@@ -1130,7 +1130,10 @@ static void parse_ddi_port(struct drm_i915_private *dev_priv, enum port port,
 		{DVO_PORT_CRT, DVO_PORT_HDMIE, DVO_PORT_DPE},
 	};
 
-	/* Find the child device to use, abort if more than one found. */
+	/*
+	 * Find the first child device to reference the port, report if more
+	 * than one found.
+	 */
 	for (i = 0; i < dev_priv->vbt.child_dev_num; i++) {
 		it = dev_priv->vbt.child_dev + i;
 
@@ -1140,11 +1143,11 @@ static void parse_ddi_port(struct drm_i915_private *dev_priv, enum port port,
 
 			if (it->common.dvo_port == dvo_ports[port][j]) {
 				if (child) {
-					DRM_DEBUG_KMS("More than one child device for port %c in VBT.\n",
+					DRM_DEBUG_KMS("More than one child device for port %c in VBT, using the first.\n",
 						      port_name(port));
-					return;
+				} else {
+					child = it;
 				}
-				child = it;
 			}
 		}
 	}