diff --git a/drivers/ntb/hw/epf/ntb_hw_epf.c b/drivers/ntb/hw/epf/ntb_hw_epf.c
index 8925c688930c..21d942824983 100644
--- a/drivers/ntb/hw/epf/ntb_hw_epf.c
+++ b/drivers/ntb/hw/epf/ntb_hw_epf.c
@@ -43,6 +43,18 @@
 #define NTB_EPF_DB_DATA(n)	(0x34 + (n) * 4)
 #define NTB_EPF_DB_OFFSET(n)	(0xB4 + (n) * 4)
 
+/*
+ * Legacy doorbell slot layout when paired with pci-epf-*ntb:
+ *
+ *   slot 0 : reserved for link events
+ *   slot 1 : unused (historical extra offset)
+ *   slot 2 : DB#0
+ *   slot 3 : DB#1
+ *   ...
+ *
+ * Thus, NTB_EPF_MIN_DB_COUNT=3 means that we at least create vectors for
+ * doorbells DB#0 and DB#1.
+ */
 #define NTB_EPF_MIN_DB_COUNT	3
 #define NTB_EPF_MAX_DB_COUNT	31
 
@@ -473,6 +485,14 @@ static int ntb_epf_peer_mw_get_addr(struct ntb_dev *ntb, int idx,
 static int ntb_epf_peer_db_set(struct ntb_dev *ntb, u64 db_bits)
 {
 	struct ntb_epf_dev *ndev = ntb_ndev(ntb);
+	/*
+	 * ffs() returns a 1-based bit index (bit 0 -> 1).
+	 *
+	 * With slot 0 reserved for link events, DB#0 would naturally map to
+	 * slot 1. Historically an extra +1 offset was added, so DB#0 maps to
+	 * slot 2 and slot 1 remains unused. Keep this mapping for
+	 * backward-compatibility.
+	 */
 	u32 interrupt_num = ffs(db_bits) + 1;
 	struct device *dev = ndev->dev;
 	u32 db_entry_size;