diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
index d59870fd3430..668d25abc7f2 100644
--- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
+++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
@@ -1419,6 +1419,25 @@ static int vntb_epf_peer_db_set(struct ntb_dev *ndev, u64 db_bits)
 	func_no = ntb->epf->func_no;
 	vfunc_no = ntb->epf->vfunc_no;
 
+	/*
+	 * pci_epc_raise_irq() for MSI expects a 1-based interrupt number.
+	 * ffs() returns a 1-based index (bit 0 -> 1). interrupt_num has already
+	 * been computed as ffs(db_bits) + 1 above. Adding one more +1 when
+	 * calling pci_epc_raise_irq() therefore results in:
+	 *
+	 *   doorbell bit 0 -> MSI #3
+	 *
+	 * Legacy mapping (kept for compatibility):
+	 *
+	 *   MSI #1 : link event (reserved)
+	 *   MSI #2 : unused (historical offset)
+	 *   MSI #3 : doorbell bit 0 (DB#0)
+	 *   MSI #4 : doorbell bit 1 (DB#1)
+	 *   ...
+	 *
+	 * Do not change this mapping to avoid breaking interoperability with
+	 * older peers.
+	 */
 	ret = pci_epc_raise_irq(ntb->epf->epc, func_no, vfunc_no,
 				PCI_IRQ_MSI, interrupt_num + 1);
 	if (ret)