From: Koichiro Den <den@valinux.co.jp>
To: "Jon Mason" <jdmason@kudzu.us>,
"Dave Jiang" <dave.jiang@intel.com>,
"Allen Hubbe" <allenbh@gmail.com>,
"Manivannan Sadhasivam" <mani@kernel.org>,
"Krzysztof Wilczyński" <kwilczynski@kernel.org>,
"Kishon Vijay Abraham I" <kishon@kernel.org>,
"Bjorn Helgaas" <bhelgaas@google.com>,
"Frank Li" <Frank.Li@nxp.com>,
"Jerome Brunet" <jbrunet@baylibre.com>,
"Lorenzo Pieralisi" <lpieralisi@kernel.org>,
"Niklas Cassel" <cassel@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
ntb@lists.linux.dev
Subject: [PATCH v3 06/10] NTB: epf: Document legacy doorbell slot offset in ntb_epf_peer_db_set()
Date: Mon, 23 Mar 2026 12:15:40 +0900 [thread overview]
Message-ID: <20260323031544.2598111-7-den@valinux.co.jp> (raw)
In-Reply-To: <20260323031544.2598111-1-den@valinux.co.jp>
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.
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
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 | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
diff --git a/drivers/ntb/hw/epf/ntb_hw_epf.c b/drivers/ntb/hw/epf/ntb_hw_epf.c
index d3ecf25a5162..bce7130fec39 100644
--- 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;
--
2.51.0
next prev parent reply other threads:[~2026-03-23 3:16 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-23 3:15 [PATCH v3 00/10] PCI: endpoint: pci-epf-vntb / NTB: epf: Enable per-doorbell bit handling Koichiro Den
2026-03-23 3:15 ` [PATCH v3 01/10] PCI: endpoint: pci-epf-vntb: Document legacy MSI doorbell offset Koichiro Den
2026-03-23 3:15 ` [PATCH v3 02/10] PCI: endpoint: pci-epf-vntb: Defer pci_epc_raise_irq() out of atomic context Koichiro Den
2026-03-23 18:27 ` Frank Li
2026-05-01 16:26 ` Manivannan Sadhasivam
2026-05-11 7:54 ` Koichiro Den
2026-03-23 3:15 ` [PATCH v3 03/10] PCI: endpoint: pci-epf-vntb: Report 0-based doorbell vector via ntb_db_event() Koichiro Den
2026-03-23 3:15 ` [PATCH v3 04/10] PCI: endpoint: pci-epf-vntb: Exclude reserved slots from db_valid_mask Koichiro Den
2026-03-23 3:15 ` [PATCH v3 05/10] PCI: endpoint: pci-epf-vntb: Implement db_vector_count/mask for doorbells Koichiro Den
2026-03-23 3:15 ` Koichiro Den [this message]
2026-03-23 3:15 ` [PATCH v3 07/10] NTB: epf: Make db_valid_mask cover only real doorbell bits Koichiro Den
2026-03-23 3:15 ` [PATCH v3 08/10] NTB: epf: Report 0-based doorbell vector via ntb_db_event() Koichiro Den
2026-03-23 3:15 ` [PATCH v3 09/10] NTB: epf: Fix doorbell bitmask handling in db_read/db_clear Koichiro Den
2026-05-01 16:29 ` Manivannan Sadhasivam
2026-05-11 9:15 ` Koichiro Den
2026-03-23 3:15 ` [PATCH v3 10/10] NTB: epf: Implement db_vector_count/mask for doorbells Koichiro Den
2026-03-23 15:43 ` [PATCH v3 00/10] PCI: endpoint: pci-epf-vntb / NTB: epf: Enable per-doorbell bit handling Koichiro Den
2026-03-25 6:23 ` Niklas Cassel
2026-03-25 8:44 ` Koichiro Den
2026-04-07 15:21 ` Koichiro Den
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