Re: [PATCH 01/10] PCI: endpoint: pci-epf-vntb: Document legacy MSI doorbell offset

[Frank Li]

On Tue, Feb 24, 2026 at 10:34:50PM +0900, Koichiro Den wrote:
> vntb_epf_peer_db_set() raises an MSI interrupt to notify the RC side of
> a doorbell event. pci_epc_raise_irq(..., PCI_IRQ_MSI, interrupt_num)
> takes a 1-based MSI interrupt number.
>
> The ntb_hw_epf driver reserves MSI #1 for link events, so doorbells
> would naturally start at MSI #2 (doorbell bit 0 -> MSI #2). However,
> pci-epf-vntb has historically applied an extra offset and maps doorbell
> bit 0 to MSI #3. This matches the legacy behavior of ntb_hw_epf and has
> been preserved since commit e35f56bb0330 ("PCI: endpoint: Support NTB
> transfer between RC and EP").
>
> This offset has not surfaced as a functional issue because:
> - ntb_hw_epf typically allocates enough MSI vectors, so the off-by-one
>   still hits a valid MSI vector, and
> - ntb_hw_epf does not implement .db_vector_count()/.db_vector_mask(), so
>   client drivers such as ntb_transport effectively ignore the vector
>   number and schedule all QPs.
>
> Correcting the MSI number would break interoperability with peers
> running older kernels.
>
> Document the legacy offset to avoid confusion when enabling
> per-db-vector handling.
>
> Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
> ---

Thanks,
Reviewed-by: Frank Li <Frank.Li@xxxxxxx>

>  drivers/pci/endpoint/functions/pci-epf-vntb.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>
> diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> index 20a400e83439..4328f05acfe6 100644
> --- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
> +++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> @@ -1375,6 +1375,25 @@ static int vntb_epf_peer_db_set(struct ntb_dev *ndev, u64 db_bits)
>  	func_no = ntb->epf->func_no;
>  	vfunc_no = ntb->epf->vfunc_no;
>
> +	/*
> +	 * pci_epc_raise_irq() for MSI expects a 1-based interrupt number.
> +	 * ffs() returns a 1-based index (bit 0 -> 1). interrupt_num has already
> +	 * been computed as ffs(db_bits) + 1 above. Adding one more +1 when
> +	 * calling pci_epc_raise_irq() therefore results in:
> +	 *
> +	 *   doorbell bit 0 -> MSI #3
> +	 *
> +	 * Legacy mapping (kept for compatibility):
> +	 *
> +	 *   MSI #1 : link event (reserved)
> +	 *   MSI #2 : unused (historical offset)
> +	 *   MSI #3 : doorbell bit 0 (DB#0)
> +	 *   MSI #4 : doorbell bit 1 (DB#1)
> +	 *   ...
> +	 *
> +	 * Do not change this mapping to avoid breaking interoperability with
> +	 * older peers.
> +	 */
>  	ret = pci_epc_raise_irq(ntb->epf->epc, func_no, vfunc_no,
>  				PCI_IRQ_MSI, interrupt_num + 1);
>  	if (ret)
> --
> 2.51.0
>