[PATCH v4 06/12] PCI: endpoint: pci-epf-vntb: Exclude reserved slots from db_valid_mask

In pci-epf-vntb, db_count represents the total number of doorbell slots
exposed to the peer, including:
  - slot #0 reserved for link events, and
  - slot #1 historically unused (kept for compatibility).

Only the remaining slots correspond to actual doorbell bits. The current
db_valid_mask() exposes all slots as valid doorbells.

Limit db_valid_mask() to the real doorbell bits by returning
BIT_ULL(db_count - 2) - 1, and guard against db_count < 2.

Fixes: e35f56bb0330 ("PCI: endpoint: Support NTB transfer between RC and EP")
Reviewed-by: Frank Li <[email protected]>
Signed-off-by: Koichiro Den <[email protected]>
---
Changes since v3:
  - None.

 drivers/pci/endpoint/functions/pci-epf-vntb.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
index 524355a8b4be..58e41d95d029 100644
--- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
+++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
@@ -1364,7 +1364,10 @@ static int vntb_epf_peer_mw_count(struct ntb_dev *ntb)
 
 static u64 vntb_epf_db_valid_mask(struct ntb_dev *ntb)
 {
-	return BIT_ULL(ntb_ndev(ntb)->db_count) - 1;
+	if (ntb_ndev(ntb)->db_count < EPF_IRQ_DB_START)
+		return 0;
+
+	return BIT_ULL(ntb_ndev(ntb)->db_count - EPF_IRQ_DB_START) - 1;
 }
 
 static int vntb_epf_db_set_mask(struct ntb_dev *ntb, u64 db_bits)
-- 
2.51.0