drm/i915: try another possible DDC bus for the SDVO device with multiple outputs

There exist multiple DDC buses for the SDVO cards with multiple outputs.
When we can't get the EDID by using the select DDC bus, we can try the other
possible DDC bus to see whether the EDID can be obtained.

https://bugs.freedesktop.org/show_bug.cgi?id=23842

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Tested-by: Sebastien Caty <sebastien.caty@mrnf.gouv.qc.ca>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_sdvo.c

@@ -1628,6 +1628,32 @@ intel_sdvo_hdmi_sink_detect(struct drm_connector *connector, u16 response)
 	edid = drm_get_edid(&intel_output->base,
 			    intel_output->ddc_bus);
 
+	/* This is only applied to SDVO cards with multiple outputs */
+	if (edid == NULL && intel_sdvo_multifunc_encoder(intel_output)) {
+		uint8_t saved_ddc, temp_ddc;
+		saved_ddc = sdvo_priv->ddc_bus;
+		temp_ddc = sdvo_priv->ddc_bus >> 1;
+		/*
+		 * Don't use the 1 as the argument of DDC bus switch to get
+		 * the EDID. It is used for SDVO SPD ROM.
+		 */
+		while(temp_ddc > 1) {
+			sdvo_priv->ddc_bus = temp_ddc;
+			edid = drm_get_edid(&intel_output->base,
+				intel_output->ddc_bus);
+			if (edid) {
+				/*
+				 * When we can get the EDID, maybe it is the
+				 * correct DDC bus. Update it.
+				 */
+				sdvo_priv->ddc_bus = temp_ddc;
+				break;
+			}
+			temp_ddc >>= 1;
+		}
+		if (edid == NULL)
+			sdvo_priv->ddc_bus = saved_ddc;
+	}
 	/* when there is no edid and no monitor is connected with VGA
 	 * port, try to use the CRT ddc to read the EDID for DVI-connector
 	 */