diff --git a/hw/npu.c b/hw/npu.c
index f00e9c5c7bf7..b113800b9afd 100644
--- a/hw/npu.c
+++ b/hw/npu.c
@@ -1152,6 +1152,7 @@ static void npu_probe_phb(struct dt_node *dn)
 	dt_add_property_cells(np, "ibm,npcq", dn->phandle);
 	dt_add_property_cells(np, "ibm,links", links);
 	dt_add_property(np, "ibm,mmio-window", mm_win, sizeof(mm_win));
+	dt_add_property_cells(np, "ibm,phb-diag-data-size", 0);
 }
 
 static void npu_dev_populate_vendor_cap(struct npu_dev_cap *cap)
diff --git a/hw/npu2.c b/hw/npu2.c
index 31726628329c..a8560cf15ce6 100644
--- a/hw/npu2.c
+++ b/hw/npu2.c
@@ -1209,6 +1209,7 @@ static void npu2_probe_phb(struct dt_node *dn)
 	dt_add_property_cells(np, "ibm,npcq", dn->phandle);
 	dt_add_property_cells(np, "ibm,links", links);
 	dt_add_property(np, "ibm,mmio-window", mm_win, sizeof(mm_win));
+	dt_add_property_cells(np, "ibm,phb-diag-data-size", 0);
 }
 
 static uint32_t npu2_populate_pcie_cap(struct npu2_dev *dev,