drm/amd/display: fail instead of div by zero/bugcheck

[why]
If pbn_per_slot is 0, fail instead of dividing by zero and
bugchecking.

[how]
Check for zero divisor before division operation.

Signed-off-by: Ashley Thomas <Ashley.Thomas2@amd.com>
Reviewed-by: Wyatt Wood <Wyatt.Wood@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -420,6 +420,8 @@ char *dc_status_to_str(enum dc_status status)
 		return "Fail clk below required CFG (hard_min in PPLIB)";
 	case DC_NOT_SUPPORTED:
 		return "The operation is not supported.";
+	case DC_UNSUPPORTED_VALUE:
+		return "The value specified is not supported.";
 	case DC_ERROR_UNEXPECTED:
 		return "Unexpected error";
 	}

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

@@ -2952,6 +2952,10 @@ enum dc_status dc_link_allocate_mst_payload(struct pipe_ctx *pipe_ctx)
 
 	/* slot X.Y for only current stream */
 	pbn_per_slot = get_pbn_per_slot(stream);
+	if (pbn_per_slot.value == 0) {
+		DC_LOG_ERROR("Failure: pbn_per_slot==0 not allowed. Cannot continue, returning DC_UNSUPPORTED_VALUE.\n");
+		return DC_UNSUPPORTED_VALUE;
+	}
 	pbn = get_pbn_from_timing(pipe_ctx);
 	avg_time_slots_per_mtp = dc_fixpt_div(pbn, pbn_per_slot);
 

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

@@ -51,6 +51,7 @@ enum dc_status {
 	DC_FAIL_CLK_BELOW_CFG_REQUIRED = 23, /*THIS IS hard_min in PPLIB*/
 
 	DC_NOT_SUPPORTED = 24,
+	DC_UNSUPPORTED_VALUE = 25,
 
 	DC_ERROR_UNEXPECTED = -1
 };