[PATCH v4 03/12] PCI: endpoint: pci-epf-vntb: Report 0-based doorbell vector via ntb_db_event()

ntb_db_event() expects the vector number to be relative to the first
doorbell vector starting at 0.

pci-epf-vntb reserves vector 0 for link events and uses higher vector
indices for doorbells. By passing the raw slot index to ntb_db_event(),
it effectively assumes that doorbell 0 maps to vector 1.

However, because the host uses a legacy slot layout and writes doorbell
0 into the third slot, doorbell 0 ultimately appears as vector 2 from
the NTB core perspective.

Adjust pci-epf-vntb to:
  - skip the unused second slot, and
  - report doorbells as 0-based vectors (DB#0 -> vector 0).

This change does not introduce a behavioral difference until
.db_vector_count()/.db_vector_mask() are implemented, because without
those callbacks NTB clients effectively ignore the vector number.

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:
  - Use EPF_IRQ_DB_START instead of a literal offset in deferred MSI handling.

 drivers/pci/endpoint/functions/pci-epf-vntb.c | 25 ++++++++++++-------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
index cc0b356973f3..d31e2eee0869 100644
--- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
+++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
@@ -83,6 +83,12 @@ enum epf_ntb_bar {
 	VNTB_BAR_NUM,
 };
 
+enum epf_irq_slot {
+	EPF_IRQ_LINK = 0,
+	EPF_IRQ_RESERVED_DB, /* Historically skipped slot */
+	EPF_IRQ_DB_START,
+};
+
 /*
  * +--------------------------------------------------+ Base
  * |                                                  |
@@ -272,10 +278,11 @@ static void epf_ntb_cmd_handler(struct work_struct *work)
 
 	ntb = container_of(work, struct epf_ntb, cmd_handler.work);
 
-	for (i = 1; i < ntb->db_count && !ntb->msi_doorbell; i++) {
+	for (i = EPF_IRQ_DB_START; i < ntb->db_count && !ntb->msi_doorbell;
+	     i++) {
 		if (ntb->epf_db[i]) {
-			atomic64_or(1 << (i - 1), &ntb->db);
-			ntb_db_event(&ntb->ntb, i);
+			atomic64_or(1 << (i - EPF_IRQ_DB_START), &ntb->db);
+			ntb_db_event(&ntb->ntb, i - EPF_IRQ_DB_START);
 			ntb->epf_db[i] = 0;
 		}
 	}
@@ -341,10 +348,10 @@ static irqreturn_t epf_ntb_doorbell_handler(int irq, void *data)
 	struct epf_ntb *ntb = data;
 	int i;
 
-	for (i = 1; i < ntb->db_count; i++)
+	for (i = EPF_IRQ_DB_START; i < ntb->db_count; i++)
 		if (irq == ntb->epf->db_msg[i].virq) {
-			atomic64_or(1 << (i - 1), &ntb->db);
-			ntb_db_event(&ntb->ntb, i);
+			atomic64_or(1 << (i - EPF_IRQ_DB_START), &ntb->db);
+			ntb_db_event(&ntb->ntb, i - EPF_IRQ_DB_START);
 		}
 
 	return IRQ_HANDLED;
@@ -1450,8 +1457,8 @@ static void vntb_epf_peer_db_work(struct work_struct *work)
 		while (db_bits) {
 			/*
 			 * pci_epc_raise_irq() for MSI expects a 1-based
-			 * interrupt number. db_bit is zero-based, so add 3 to
-			 * preserve the historical slot offset.
+			 * interrupt number. The first usable doorbell starts
+			 * at EPF_IRQ_DB_START in the legacy slot layout.
 			 *
 			 * Legacy mapping (kept for compatibility):
 			 *
@@ -1465,7 +1472,7 @@ static void vntb_epf_peer_db_work(struct work_struct *work)
 			 * interoperability with older peers.
 			 */
 			db_bit = __ffs64(db_bits);
-			interrupt_num = db_bit + 3;
+			interrupt_num = db_bit + EPF_IRQ_DB_START + 1;
 			db_bits &= ~BIT_ULL(db_bit);
 
 			ret = pci_epc_raise_irq(epf->epc, func_no, vfunc_no,
-- 
2.51.0