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

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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 812ce2f8d1 ("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
This commit is contained in:
Koichiro Den 2026-05-13 11:49:19 +09:00 committed by Bjorn Helgaas
parent 2579f3f7f5
commit 84af8a5f5e
1 changed files with 20 additions and 0 deletions
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20
drivers/ntb/hw/epf/ntb_hw_epf.c
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@ -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;