ndev->db_count includes an unused doorbell slot due to the legacy extra
offset in the peer doorbell path. db_valid_mask must cover only the real
doorbell bits and exclude the unused slot.

Set db_valid_mask to BIT_ULL(db_count - 1) - 1.

Fixes: 812ce2f8d14e ("NTB: Add support for EPF PCI Non-Transparent Bridge")
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Koichiro Den <den@valinux.co.jp>
---
 drivers/ntb/hw/epf/ntb_hw_epf.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/ntb/hw/epf/ntb_hw_epf.c b/drivers/ntb/hw/epf/ntb_hw_epf.c
index bce7130fec39..07dc97d3270b 100644
--- a/drivers/ntb/hw/epf/ntb_hw_epf.c
+++ b/drivers/ntb/hw/epf/ntb_hw_epf.c
@@ -580,7 +580,17 @@ static int ntb_epf_init_dev(struct ntb_epf_dev *ndev)
 		return ret;
 	}
 
-	ndev->db_valid_mask = BIT_ULL(ndev->db_count) - 1;
+	if (ndev->db_count < NTB_EPF_MIN_DB_COUNT) {
+		dev_err(dev, "db_count %u is less than %u\n", ndev->db_count,
+			NTB_EPF_MIN_DB_COUNT);
+		return -EINVAL;
+	}
+
+	/*
+	 * ndev->db_count includes an extra skipped slot due to the legacy
+	 * doorbell layout, hence -1.
+	 */
+	ndev->db_valid_mask = BIT_ULL(ndev->db_count - 1) - 1;
 	ndev->mw_count = readl(ndev->ctrl_reg + NTB_EPF_MW_COUNT);
 	ndev->spad_count = readl(ndev->ctrl_reg + NTB_EPF_SPAD_COUNT);
 
-- 
2.51.0