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

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Koichiro Den 2026-05-13 11:49:19 +09:00 • committed by Bjorn Helgaas

parent 2579f3f7f5

commit 84af8a5f5e

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drivers/ntb/hw/epf/ntb_hw_epf.c

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