drm/amd/display: Update link training fallback behaviour.

[Why]
Some displays may need several link training attempts before
link training succeeds.

[How]
If training succeeds after falling back to lower link bandwidth,
retry at original link bandwidth instead of abandoning link training
whenever link bandwidth is less than stream bandwidth.
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@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/core/dc_link_dp.c

@@ -2783,31 +2783,37 @@ bool perform_link_training_with_retries(
 	struct dc_link *link = stream->link;
 	enum dp_panel_mode panel_mode = dp_get_panel_mode(link);
 	enum link_training_result status = LINK_TRAINING_CR_FAIL_LANE0;
-	struct dc_link_settings current_setting = *link_setting;
+	struct dc_link_settings cur_link_settings = *link_setting;
 	const struct link_hwss *link_hwss = get_link_hwss(link, &pipe_ctx->link_res);
 	int fail_count = 0;
+	bool is_link_bw_low = false; /* link bandwidth < stream bandwidth */
+	bool is_link_bw_min = /* RBR x 1 */
+		(cur_link_settings.link_rate <= LINK_RATE_LOW) &&
+		(cur_link_settings.lane_count <= LANE_COUNT_ONE);
 
 	dp_trace_commit_lt_init(link);
 
 
-	if (dp_get_link_encoding_format(&current_setting) == DP_8b_10b_ENCODING)
+	if (dp_get_link_encoding_format(&cur_link_settings) == DP_8b_10b_ENCODING)
 		/* We need to do this before the link training to ensure the idle
 		 * pattern in SST mode will be sent right after the link training
 		 */
 		link_hwss->setup_stream_encoder(pipe_ctx);
 
 	dp_trace_set_lt_start_timestamp(link, false);
-	for (j = 0; j < attempts; ++j) {
+	j = 0;
+	while (j < attempts && fail_count < (attempts * 10)) {
 
-		DC_LOG_HW_LINK_TRAINING("%s: Beginning link training attempt %u of %d\n",
-			__func__, (unsigned int)j + 1, attempts);
+		DC_LOG_HW_LINK_TRAINING("%s: Beginning link training attempt %u of %d @ rate(%d) x lane(%d)\n",
+			__func__, (unsigned int)j + 1, attempts, cur_link_settings.link_rate,
+			cur_link_settings.lane_count);
 
 		dp_enable_link_phy(
 			link,
 			&pipe_ctx->link_res,
 			signal,
 			pipe_ctx->clock_source->id,
-			&current_setting);
+			&cur_link_settings);
 
 		if (stream->sink_patches.dppowerup_delay > 0) {
 			int delay_dp_power_up_in_ms = stream->sink_patches.dppowerup_delay;
@@ -2832,30 +2838,30 @@ bool perform_link_training_with_retries(
 		dp_set_panel_mode(link, panel_mode);
 
 		if (link->aux_access_disabled) {
-			dc_link_dp_perform_link_training_skip_aux(link, &pipe_ctx->link_res, &current_setting);
+			dc_link_dp_perform_link_training_skip_aux(link, &pipe_ctx->link_res, &cur_link_settings);
 			return true;
 		} else {
 			/** @todo Consolidate USB4 DP and DPx.x training. */
 			if (link->ep_type == DISPLAY_ENDPOINT_USB4_DPIA) {
 				status = dc_link_dpia_perform_link_training(link,
 						&pipe_ctx->link_res,
-						&current_setting,
+						&cur_link_settings,
 						skip_video_pattern);
 
 				/* Transmit idle pattern once training successful. */
-				if (status == LINK_TRAINING_SUCCESS)
+				if (status == LINK_TRAINING_SUCCESS && !is_link_bw_low)
 					dp_set_hw_test_pattern(link, &pipe_ctx->link_res, DP_TEST_PATTERN_VIDEO_MODE, NULL, 0);
 			} else {
 				status = dc_link_dp_perform_link_training(link,
 						&pipe_ctx->link_res,
-						&current_setting,
+						&cur_link_settings,
 						skip_video_pattern);
 			}
 
 			dp_trace_lt_total_count_increment(link, false);
 			dp_trace_lt_result_update(link, status, false);
 			dp_trace_set_lt_end_timestamp(link, false);
-			if (status == LINK_TRAINING_SUCCESS)
+			if (status == LINK_TRAINING_SUCCESS && !is_link_bw_low)
 				return true;
 		}
 
@@ -2866,8 +2872,9 @@ bool perform_link_training_with_retries(
 		if (j == (attempts - 1) && link->ep_type == DISPLAY_ENDPOINT_PHY)
 			break;
 
-		DC_LOG_WARNING("%s: Link training attempt %u of %d failed\n",
-			__func__, (unsigned int)j + 1, attempts);
+		DC_LOG_WARNING("%s: Link training attempt %u of %d failed @ rate(%d) x lane(%d)\n",
+			__func__, (unsigned int)j + 1, attempts, cur_link_settings.link_rate,
+			cur_link_settings.lane_count);
 
 		dp_disable_link_phy(link, &pipe_ctx->link_res, signal);
 
@@ -2876,27 +2883,49 @@ bool perform_link_training_with_retries(
 			enum dc_connection_type type = dc_connection_none;
 
 			dc_link_detect_sink(link, &type);
-			if (type == dc_connection_none)
+			if (type == dc_connection_none) {
+				DC_LOG_HW_LINK_TRAINING("%s: Aborting training because sink unplugged\n", __func__);
 				break;
-		} else if (do_fallback) {
+			}
+		}
+
+		/* Try to train again at original settings if:
+		 * - not falling back between training attempts;
+		 * - aborted previous attempt due to reasons other than sink unplug;
+		 * - successfully trained but at a link rate lower than that required by stream;
+		 * - reached minimum link bandwidth.
+		 */
+		if (!do_fallback || (status == LINK_TRAINING_ABORT) ||
+				(status == LINK_TRAINING_SUCCESS && is_link_bw_low) ||
+				is_link_bw_min) {
+			j++;
+			cur_link_settings = *link_setting;
+			delay_between_attempts += LINK_TRAINING_RETRY_DELAY;
+			is_link_bw_low = false;
+			is_link_bw_min = (cur_link_settings.link_rate <= LINK_RATE_LOW) &&
+				(cur_link_settings.lane_count <= LANE_COUNT_ONE);
+
+		} else if (do_fallback) { /* Try training at lower link bandwidth if doing fallback. */
 			uint32_t req_bw;
 			uint32_t link_bw;
 
-			decide_fallback_link_setting(link, *link_setting, &current_setting, status);
-			/* Fail link training if reduced link bandwidth no longer meets
-			 * stream requirements.
+			decide_fallback_link_setting(link, *link_setting, &cur_link_settings, status);
+			/* Flag if reduced link bandwidth no longer meets stream requirements or fallen back to
+			 * minimum link bandwidth.
 			 */
 			req_bw = dc_bandwidth_in_kbps_from_timing(&stream->timing);
-			link_bw = dc_link_bandwidth_kbps(link, &current_setting);
-			if (req_bw > link_bw)
-				break;
+			link_bw = dc_link_bandwidth_kbps(link, &cur_link_settings);
+			is_link_bw_low = (req_bw > link_bw);
+			is_link_bw_min = ((cur_link_settings.link_rate <= LINK_RATE_LOW) &&
+				(cur_link_settings.lane_count <= LANE_COUNT_ONE));
+
+			if (is_link_bw_low)
+				DC_LOG_WARNING("%s: Link bandwidth too low after fallback req_bw(%d) > link_bw(%d)\n",
+					__func__, req_bw, link_bw);
 		}
 
 		msleep(delay_between_attempts);
-
-		delay_between_attempts += LINK_TRAINING_RETRY_DELAY;
 	}
-
 	return false;
 }