Implement .db_vector_count and .db_vector_mask so ntb core/clients can map
doorbell events to per-vector work and avoid the thundering-herd behavior.

pci-epf-vntb reserves two slots in db_count: slot 0 for link events and
slot 1 which is historically unused. Therefore the number of doorbell
vectors is (db_count - 2).

Report vectors as 0..N-1 and return BIT_ULL(db_vector) for the
corresponding doorbell bit. While at it, use vntb_epf_db_vector_mask()
to simplify vntb_epf_db_valid_mask().

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Koichiro Den <den@valinux.co.jp>
---
 drivers/pci/endpoint/functions/pci-epf-vntb.c | 36 +++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
index d91033ab8e03..d83a9be1113f 100644
--- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
+++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
@@ -1266,12 +1266,42 @@ static int vntb_epf_peer_mw_count(struct ntb_dev *ntb)
 	return ntb_ndev(ntb)->num_mws;
 }
 
+static int vntb_epf_db_vector_count(struct ntb_dev *ntb)
+{
+	struct epf_ntb *ndev = ntb_ndev(ntb);
+
+	/*
+	 * ndev->db_count is the total number of doorbell slots exposed to
+	 * the peer, including:
+	 *   - slot #0 reserved for link events
+	 *   - slot #1 historically unused (kept for protocol compatibility)
+	 *
+	 * Report only usable per-vector doorbell interrupts.
+	 */
+	if (ndev->db_count < EPF_IRQ_DB_START)
+		return 0;
+
+	return ndev->db_count - EPF_IRQ_DB_START;
+}
+
 static u64 vntb_epf_db_valid_mask(struct ntb_dev *ntb)
 {
-	if (ntb_ndev(ntb)->db_count < EPF_IRQ_DB_START)
+	return BIT_ULL(vntb_epf_db_vector_count(ntb)) - 1;
+}
+
+static u64 vntb_epf_db_vector_mask(struct ntb_dev *ntb, int db_vector)
+{
+	int nr_vec;
+
+	/*
+	 * Doorbell vectors are numbered [0 .. nr_vec - 1], where nr_vec
+	 * excludes the two reserved slots described above.
+	 */
+	nr_vec = vntb_epf_db_vector_count(ntb);
+	if (db_vector < 0 || db_vector >= nr_vec)
 		return 0;
 
-	return BIT_ULL(ntb_ndev(ntb)->db_count - EPF_IRQ_DB_START) - 1;
+	return BIT_ULL(db_vector);
 }
 
 static int vntb_epf_db_set_mask(struct ntb_dev *ntb, u64 db_bits)
@@ -1508,6 +1538,8 @@ static const struct ntb_dev_ops vntb_epf_ops = {
 	.spad_count		= vntb_epf_spad_count,
 	.peer_mw_count		= vntb_epf_peer_mw_count,
 	.db_valid_mask		= vntb_epf_db_valid_mask,
+	.db_vector_count	= vntb_epf_db_vector_count,
+	.db_vector_mask		= vntb_epf_db_vector_mask,
 	.db_set_mask		= vntb_epf_db_set_mask,
 	.mw_set_trans		= vntb_epf_mw_set_trans,
 	.mw_clear_trans		= vntb_epf_mw_clear_trans,
-- 
2.51.0