drm/amd/display: Fix underflow issue on 175hz timing

[ Upstream commit 735688e ]

[Why]
Screen underflows happen on 175hz timing for 3 plane overlay case.

[How]
Based on dst y prefetch value clamp to equ or oto for bandwidth
calculation.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Leo Ma <hanghong.ma@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/gpu/drm/amd/display/dc/dml/dcn32/display_mode_vba_util_32.c

@@ -3454,6 +3454,7 @@ bool dml32_CalculatePrefetchSchedule(
 	double TimeForFetchingMetaPTE = 0;
 	double TimeForFetchingRowInVBlank = 0;
 	double LinesToRequestPrefetchPixelData = 0;
+	double LinesForPrefetchBandwidth = 0;
 	unsigned int HostVMDynamicLevelsTrips;
 	double  trip_to_mem;
 	double  Tvm_trips;
@@ -3883,19 +3884,23 @@ bool dml32_CalculatePrefetchSchedule(
 			TimeForFetchingMetaPTE = Tvm_oto;
 			TimeForFetchingRowInVBlank = Tr0_oto;
 			*PrefetchBandwidth = prefetch_bw_oto;
+			/* Clamp to oto for bandwidth calculation */
+			LinesForPrefetchBandwidth = dst_y_prefetch_oto;
 		} else {
 			*DestinationLinesForPrefetch = dst_y_prefetch_equ;
 			TimeForFetchingMetaPTE = Tvm_equ;
 			TimeForFetchingRowInVBlank = Tr0_equ;
 			*PrefetchBandwidth = prefetch_bw_equ;
+			/* Clamp to equ for bandwidth calculation */
+			LinesForPrefetchBandwidth = dst_y_prefetch_equ;
 		}
 
 		*DestinationLinesToRequestVMInVBlank = dml_ceil(4.0 * TimeForFetchingMetaPTE / LineTime, 1.0) / 4.0;
 
 		*DestinationLinesToRequestRowInVBlank =
 				dml_ceil(4.0 * TimeForFetchingRowInVBlank / LineTime, 1.0) / 4.0;
 
-		LinesToRequestPrefetchPixelData = *DestinationLinesForPrefetch -
+		LinesToRequestPrefetchPixelData = LinesForPrefetchBandwidth -
 				*DestinationLinesToRequestVMInVBlank - 2 * *DestinationLinesToRequestRowInVBlank;
 
 #ifdef __DML_VBA_DEBUG__