NTB: epf: Document legacy doorbell slot offset in ntb_epf_peer_db_set()

ntb_epf_peer_db_set() uses ffs(db_bits) to select a doorbell to ring.
ffs() returns a 1-based bit index (bit 0 -> 1).

Entry 0 is reserved for link events, so doorbell bit 0 must map to entry
1. However, since the initial commit 812ce2f8d14e ("NTB: Add support for
EPF PCI Non-Transparent Bridge"), the implementation has been adding an
extra +1, ending up using entry 2 for bit 0. Fixing the extra increment
would break interoperability with peers running older kernels.

Keep the legacy behavior and document the offset and the resulting slot
layout to avoid confusion when enabling per-db-vector handling.

Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260513024923.451765-9-den@valinux.co.jp

diff --git a/drivers/ntb/hw/epf/ntb_hw_epf.c b/drivers/ntb/hw/epf/ntb_hw_epf.c
index 8925c688930cfedf929a81738a9766e0351938d7..21d9428249835a381b8486c1cbb25c42ad5406bd 100644 (file)
--- a/drivers/ntb/hw/epf/ntb_hw_epf.c
+++ b/drivers/ntb/hw/epf/ntb_hw_epf.c
@@ -43,6 +43,18 @@
 #define NTB_EPF_DB_DATA(n)     (0x34 + (n) * 4)
 #define NTB_EPF_DB_OFFSET(n)   (0xB4 + (n) * 4)
 
+/*
+ * Legacy doorbell slot layout when paired with pci-epf-*ntb:
+ *
+ *   slot 0 : reserved for link events
+ *   slot 1 : unused (historical extra offset)
+ *   slot 2 : DB#0
+ *   slot 3 : DB#1
+ *   ...
+ *
+ * Thus, NTB_EPF_MIN_DB_COUNT=3 means that we at least create vectors for
+ * doorbells DB#0 and DB#1.
+ */
 #define NTB_EPF_MIN_DB_COUNT   3
 #define NTB_EPF_MAX_DB_COUNT   31
 
@@ -473,6 +485,14 @@ static int ntb_epf_peer_mw_get_addr(struct ntb_dev *ntb, int idx,
 static int ntb_epf_peer_db_set(struct ntb_dev *ntb, u64 db_bits)
 {
        struct ntb_epf_dev *ndev = ntb_ndev(ntb);
+       /*
+        * ffs() returns a 1-based bit index (bit 0 -> 1).
+        *
+        * With slot 0 reserved for link events, DB#0 would naturally map to
+        * slot 1. Historically an extra +1 offset was added, so DB#0 maps to
+        * slot 2 and slot 1 remains unused. Keep this mapping for
+        * backward-compatibility.
+        */
        u32 interrupt_num = ffs(db_bits) + 1;
        struct device *dev = ndev->dev;
        u32 db_entry_size;