drm/amd/display: add a debug option to force dp2 lt fallback method

[why]
A debug option is needed to temporarily force dp2 new link training
fallback method for debugging purpose.
Reviewed-by: NGeorge Shen <George.Shen@amd.com>
Acked-by: NPavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: NWenjing Liu <wenjing.liu@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -100,6 +100,7 @@ static const struct dp_lt_fallback_entry dp_lt_fallbacks[] = {
 #endif
 
 static bool decide_fallback_link_setting(
+		struct dc_link *link,
 		struct dc_link_settings initial_link_settings,
 		struct dc_link_settings *current_link_setting,
 		enum link_training_result training_result);
@@ -2609,7 +2610,7 @@ bool perform_link_training_with_retries(
 			uint32_t req_bw;
 			uint32_t link_bw;
 
-			decide_fallback_link_setting(*link_setting, &current_setting, status);
+			decide_fallback_link_setting(link, *link_setting, &current_setting, status);
 			/* Fail link training if reduced link bandwidth no longer meets
 			 * stream requirements.
 			 */
@@ -3124,7 +3125,7 @@ bool dp_verify_link_cap(
 		 * based on the actual mode we're driving
 		 */
 		dp_disable_link_phy(link, link->connector_signal);
-	} while (!success && decide_fallback_link_setting(
+	} while (!success && decide_fallback_link_setting(link,
 			initial_link_settings, cur, status));
 
 	/* Link Training failed for all Link Settings
@@ -3376,6 +3377,7 @@ static bool decide_fallback_link_setting_max_bw_policy(
  *					and no further fallback could be done
  */
 static bool decide_fallback_link_setting(
+		struct dc_link *link,
 		struct dc_link_settings initial_link_settings,
 		struct dc_link_settings *current_link_setting,
 		enum link_training_result training_result)
@@ -3383,7 +3385,8 @@ static bool decide_fallback_link_setting(
 	if (!current_link_setting)
 		return false;
 #if defined(CONFIG_DRM_AMD_DC_DCN)
-	if (dp_get_link_encoding_format(&initial_link_settings) == DP_128b_132b_ENCODING)
+	if (dp_get_link_encoding_format(&initial_link_settings) == DP_128b_132b_ENCODING ||
+			link->dc->debug.force_dp2_lt_fallback_method)
 		return decide_fallback_link_setting_max_bw_policy(&initial_link_settings,
 				current_link_setting);
 #endif

drivers/gpu/drm/amd/display/dc/dc.h

@@ -691,6 +691,7 @@ struct dc_debug_options {
 	/* TODO - remove once tested */
 	bool legacy_dp2_lt;
 	bool set_mst_en_for_sst;
+	bool force_dp2_lt_fallback_method;
 #endif
 	union mem_low_power_enable_options enable_mem_low_power;
 	union root_clock_optimization_options root_clock_optimization;