drm/amd/display: implement DSC pass-through support

Currently, we only attempt to setup DSC at the virtual DPCD port,
however many modern displays come with DSC support and MST hubs can now
support DSC pass-through. So, to more optimally make use of the
available bandwidth, use DSC pass-through when possible by adding DSC
pass-through enablement support into the DSC enable sequence.
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c

@@ -729,8 +729,14 @@ bool dm_helpers_dp_write_dsc_enable(
 		const struct dc_stream_state *stream,
 		bool enable)
 {
-	uint8_t enable_dsc = enable ? 1 : 0;
+	static const uint8_t DSC_DISABLE;
+	static const uint8_t DSC_DECODING = 0x01;
+	static const uint8_t DSC_PASSTHROUGH = 0x02;
+
 	struct amdgpu_dm_connector *aconnector;
+	struct drm_dp_mst_port *port;
+	uint8_t enable_dsc = enable ? DSC_DECODING : DSC_DISABLE;
+	uint8_t enable_passthrough = enable ? DSC_PASSTHROUGH : DSC_DISABLE;
 	uint8_t ret = 0;
 
 	if (!stream)
@@ -750,8 +756,39 @@ bool dm_helpers_dp_write_dsc_enable(
 				aconnector->dsc_aux, stream, enable_dsc);
 #endif
 
-		ret = drm_dp_dpcd_write(aconnector->dsc_aux, DP_DSC_ENABLE, &enable_dsc, 1);
-		DC_LOG_DC("Send DSC %s to MST RX\n", enable_dsc ? "enable" : "disable");
+		port = aconnector->port;
+
+		if (enable) {
+			if (port->passthrough_aux) {
+				ret = drm_dp_dpcd_write(port->passthrough_aux,
+							DP_DSC_ENABLE,
+							&enable_passthrough, 1);
+				DC_LOG_DC("Sent DSC pass-through enable to virtual dpcd port, ret = %u\n",
+					  ret);
+			}
+
+			ret = drm_dp_dpcd_write(aconnector->dsc_aux,
+						DP_DSC_ENABLE, &enable_dsc, 1);
+			DC_LOG_DC("Sent DSC decoding enable to %s port, ret = %u\n",
+				  (port->passthrough_aux) ? "remote RX" :
+				  "virtual dpcd",
+				  ret);
+		} else {
+			ret = drm_dp_dpcd_write(aconnector->dsc_aux,
+						DP_DSC_ENABLE, &enable_dsc, 1);
+			DC_LOG_DC("Sent DSC decoding disable to %s port, ret = %u\n",
+				  (port->passthrough_aux) ? "remote RX" :
+				  "virtual dpcd",
+				  ret);
+
+			if (port->passthrough_aux) {
+				ret = drm_dp_dpcd_write(port->passthrough_aux,
+							DP_DSC_ENABLE,
+							&enable_passthrough, 1);
+				DC_LOG_DC("Sent DSC pass-through disable to virtual dpcd port, ret = %u\n",
+					  ret);
+			}
+		}
 	}
 
 	if (stream->signal == SIGNAL_TYPE_DISPLAY_PORT || stream->signal == SIGNAL_TYPE_EDP) {
@@ -768,7 +805,7 @@ bool dm_helpers_dp_write_dsc_enable(
 #endif
 	}
 
-	return (ret > 0);
+	return ret;
 }
 
 bool dm_helpers_is_dp_sink_present(struct dc_link *link)