drm/amd/display: Reset steer fifo before unblanking the stream

[why]
During mode transition steer fifo could overflow. Quite often it
recovers by itself, but sometimes it doesn't.

[how]
Add steer fifo reset before unblanking the stream. Also add a short
delay when resetting dig resync fifo to make sure register writes
don't end up back-to-back, in which case the HW might miss the reset
request.
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_stream_encoder.c

@@ -492,15 +492,23 @@ void enc2_stream_encoder_dp_unblank(
 				DP_VID_N_MUL, n_multiply);
 	}
 
-	/* set DIG_START to 0x1 to reset FIFO */
+	/* make sure stream is disabled before resetting steer fifo */
+	REG_UPDATE(DP_VID_STREAM_CNTL, DP_VID_STREAM_ENABLE, false);
+	REG_WAIT(DP_VID_STREAM_CNTL, DP_VID_STREAM_STATUS, 0, 10, 5000);
 
+	/* set DIG_START to 0x1 to reset FIFO */
 	REG_UPDATE(DIG_FE_CNTL, DIG_START, 1);
+	udelay(1);
 
 	/* write 0 to take the FIFO out of reset */
 
 	REG_UPDATE(DIG_FE_CNTL, DIG_START, 0);
 
-	/* switch DP encoder to CRTC data */
+	/* switch DP encoder to CRTC data, but reset it the fifo first. It may happen
+	 * that it overflows during mode transition, and sometimes doesn't recover.
+	 */
+	REG_UPDATE(DP_STEER_FIFO, DP_STEER_FIFO_RESET, 1);
+	udelay(10);
 
 	REG_UPDATE(DP_STEER_FIFO, DP_STEER_FIFO_RESET, 0);