drm/amd/display: do not set RX back to SST mode for non 0 mst stream count

[why]
a recent regression has caused us to mistakenly switch RX back to SST mode
when there are remaining mst stream enabled to the link. We are missing
a condition check for stream count before setting RX back to SST mode.

[how]
Add stream count check condition back and do some further refactor so the
logic is easier to understand to prevent future coding error in this sequence.
Reviewed-by: Samson Tam <Samson.Tam@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/link/link_dpms.c

@@ -1782,37 +1782,36 @@ enum dc_status link_increase_mst_payload(struct pipe_ctx *pipe_ctx, uint32_t bw_
 	return DC_OK;
 }
 
-static void disable_link(struct dc_link *link, const struct link_resource *link_res,
+static void disable_link_dp(struct dc_link *link,
+		const struct link_resource *link_res,
 		enum signal_type signal)
 {
-	/*
-	 * TODO: implement call for dp_set_hw_test_pattern
-	 * it is needed for compliance testing
-	 */
+	struct dc_link_settings link_settings = link->cur_link_settings;
 
-	/* Here we need to specify that encoder output settings
-	 * need to be calculated as for the set mode,
-	 * it will lead to querying dynamic link capabilities
-	 * which should be done before enable output
-	 */
+	if (signal == SIGNAL_TYPE_DISPLAY_PORT_MST &&
+			link->mst_stream_alloc_table.stream_count > 0)
+		/* disable MST link only when last vc payload is deallocated */
+		return;
 
+	dp_disable_link_phy(link, link_res, signal);
+
+	if (signal == SIGNAL_TYPE_DISPLAY_PORT_MST)
+		/* set the sink to SST mode after disabling the link */
+		enable_mst_on_sink(link, false);
+
+	if (link_dp_get_encoding_format(&link_settings) ==
+			DP_8b_10b_ENCODING) {
+		dp_set_fec_enable(link, false);
+		dp_set_fec_ready(link, link_res, false);
+	}
+}
+
+static void disable_link(struct dc_link *link,
+		const struct link_resource *link_res,
+		enum signal_type signal)
+{
 	if (dc_is_dp_signal(signal)) {
-		/* SST DP, eDP */
-		struct dc_link_settings link_settings = link->cur_link_settings;
-		if (dc_is_dp_sst_signal(signal)) {
-			dp_disable_link_phy(link, link_res, signal);
-		} else {
-			dp_disable_link_phy_mst(link, link_res, signal);
-			/* set the sink to SST mode after disabling the link */
-			enable_mst_on_sink(link, false);
-		}
-		if (dc_is_dp_sst_signal(signal) ||
-				link->mst_stream_alloc_table.stream_count == 0) {
-			if (link_dp_get_encoding_format(&link_settings) == DP_8b_10b_ENCODING) {
-				dp_set_fec_enable(link, false);
-				dp_set_fec_ready(link, link_res, false);
-			}
-		}
+		disable_link_dp(link, link_res, signal);
 	} else if (signal != SIGNAL_TYPE_VIRTUAL) {
 		link->dc->hwss.disable_link_output(link, link_res, signal);
 	}

drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_phy.c

@@ -85,17 +85,6 @@ void dp_disable_link_phy(struct dc_link *link,
 		dc->clk_mgr->funcs->notify_link_rate_change(dc->clk_mgr, link);
 }
 
-void dp_disable_link_phy_mst(struct dc_link *link,
-		const struct link_resource *link_res,
-		enum signal_type signal)
-{
-	/* MST disable link only when no stream use the link */
-	if (link->mst_stream_alloc_table.stream_count > 0)
-		return;
-
-	dp_disable_link_phy(link, link_res, signal);
-}
-
 static inline bool is_immediate_downstream(struct dc_link *link, uint32_t offset)
 {
 	return (dp_parse_lttpr_repeater_count(link->dpcd_caps.lttpr_caps.phy_repeater_cnt) ==

drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_phy.h

@@ -38,10 +38,6 @@ void dp_disable_link_phy(struct dc_link *link,
 		const struct link_resource *link_res,
 		enum signal_type signal);
 
-void dp_disable_link_phy_mst(struct dc_link *link,
-		const struct link_resource *link_res,
-		enum signal_type signal);
-
 void dp_set_hw_lane_settings(
 		struct dc_link *link,
 		const struct link_resource *link_res,