epf_ntb_configure_msi(9)

💅 epf_ntb_configure_msi — a Kernel API Manual moment

okay bestie, buckle up, because we're diving into epf_ntb_configure_msi(9) and honestly? it's giving "plumbing but make it PCIe" 🎀 this one lives in the Kernel API Manual, so you know it's official.

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✨ NAME ✨

epf_ntb_configure_msi — maps the OB address space over to the MSI address, periodt 💅

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🦋 SYNOPSIS 🦋

int epf_ntb_configure_msi (struct epf_ntb *ntb , enum
pci_epc_interface_type type , u16 db_count );

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🌸 ARGUMENTS 🌸

• ntb — the NTB device, aka the little matchmaker that lets HOST1 and HOST2 actually talk to each other 🫶
• type — which interface you're on: the PRIMARY interface or the SECONDARY interface
• db_count — how many doorbell interrupts you wanna map, no more no less

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💖 DESCRIPTION 💖

okay so first, the vibe check diagram, because a picture is worth a thousand words and this one is worth at least a thousand *nano*words 😭

┌──────────────────────────────────────────────────────────────────────────┐
│ BAR0                            Doorbell 1   +---+------->   MSI ADDRESS │
│ BAR1                            Doorbell 2   +---+                       │
│ BAR2       Doorbell 3   +---+                                            │
│ BAR3                            Doorbell 4   +---+                       │
│ BAR4                                                                     │
│ BAR5                                                                     │
└──────────────────────────────────────────────────────────────────────────┘
EP CONTROLLER 1             |                |           |
| |                |           |                 | (A)       EP
CONTROLLER 2             |                 | (OB SPACE)                |
| |      MW1        | |                 | |                 | (B)
+-----------------+ |                 | |                 | |
| |                 | |                 | PCI Address Space (Managed by
HOST2)

so what's actually happening here? this function does stage (B) in that diagram up there — you know, the one with Doorbell 1, Doorbell 2, Doorbell 3 and Doorbell 4? yeah. it maps the OB address space that corresponds to the doorbell over to the MSI address in the PCI address space. literally the glow-up from "outbound" to "MSI" ✨

this operation needs 3 parameters, and we're doing them as a numbered list because they deserve it:

1. the address that's reserved for the doorbell in the outbound address space
2. the MSI-X address living in the PCIe Address space
3. the number of MSI-X interrupts that have to be configured

the address in the outbound address space (for the Doorbell, bestie, keep up 💀) is stored in the epf_bar corresponding to BAR_DB_MW1 of epf_ntb_epc, and that one is connected to HOST1. it gets populated in epf_ntb_alloc_peer_mem in this driver, right alongside the address for MW1. two birds, one function, iconic.

then pci_epc_map_msi_irq comes in and does its thing: it takes the MSI address from the MSI capability register and maps the OB address (the one we got from epf_ntb_alloc_peer_mem) onto that MSI address. chef's kiss 🤌

and epf_ntb_configure_msi isn't done, because it also stores the MSI data needed to raise each interrupt in the db_data of the peer's control region. why? so the peer can raise the doorbell of the other host by writing db_data to the BAR corresponding to BAR_DB_MW1. that's the whole point, that's main character energy 🔥

ngl there's no return-value drama spelled on the page, but the signature says int — so, you know, read your return codes, bestie 💜

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🧚 SEE ALSO 🧚

the kernel file in question, before we get to the friend group:

• Kernel file ./drivers/pci/endpoint/functions/pci-epf-ntb.c

and now the full squad, nobody gets left out, this is a group chat invite list 🫶

• epf_ntb_link_up(9)
• epf_ntb_configure_mw(9)
• epf_ntb_teardown_mw(9)
• epf_ntb_configure_msix(9)
• epf_ntb_configure_db(9)
• epf_ntb_teardown_db(9)
• epf_ntb_cmd_handler(9)
• epf_ntb_peer_spad_bar_clear(9)
• epf_ntb_peer_spad_bar_set(9)
• epf_ntb_config_sspad_bar_clear(9)
• epf_ntb_config_sspad_bar_set(9)
• epf_ntb_config_spad_bar_free(9)
• epf_ntb_config_spad_bar_alloc(9)
• epf_ntb_config_spad_bar_alloc_interface(9)
• epf_ntb_free_peer_mem(9)
• epf_ntb_db_mw_bar_clear(9)
• epf_ntb_db_mw_bar_cleanup(9)
• epf_ntb_configure_interrupt(9)
• epf_ntb_alloc_peer_mem(9)
• epf_ntb_db_mw_bar_init(9)
• epf_ntb_epc_create_interface(9)
• epf_ntb_epc_create(9)
• epf_ntb_init_epc_bar_interface(9)
• epf_ntb_init_epc_bar(9)
• epf_ntb_epc_init_interface(9)
• epf_ntb_epc_cleanup_interface(9)
• epf_ntb_epc_cleanup(9)
• epf_ntb_epc_init(9)
• epf_ntb_bind(9)
• epf_ntb_unbind(9)
• epf_ntb_add_cfs(9)
• epf_ntb_probe(9)

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and that's the page, bestie ✨ sourced straight from ./drivers/pci/endpoint/functions, dated October 2026 — go map those MSI addresses, you absolute icon 👑💅