Re: [PATCH v10 7/7] PCI: endpoint: pci-ep-msi: Add embedded doorbell fallback

On Mon, Mar 02, 2026 at 04:14:27PM +0900, Koichiro Den wrote:
> Some endpoint platforms cannot use platform MSI / GIC ITS to implement
> EP-side doorbells. In those cases, EPF drivers cannot provide an
> interrupt-driven doorbell and often fall back to polling.
>
> Add an "embedded" doorbell backend that uses a controller-integrated
> doorbell target (e.g. DesignWare integrated eDMA interrupt-emulation
> doorbell).
>
> The backend locates the doorbell register and a corresponding Linux IRQ
> via the EPC aux-resource API. If the doorbell register is already
> exposed via a fixed BAR mapping, provide BAR+offset. Otherwise provide
> the DMA address returned by dma_map_resource() (which may be an IOVA
> when an IOMMU is enabled) so EPF drivers can map it into BAR space.
>
> When MSI doorbell allocation fails with -ENODEV,
> pci_epf_alloc_doorbell() falls back to this embedded backend.
>
> Signed-off-by: Koichiro Den <[email protected]>
> ---
...
> +	 */
> +	if (doorbell->bar == NO_BAR) {
> +		phys_base = addr & PAGE_MASK;
> +		off = addr - phys_base;
> +		map_size = PAGE_ALIGN(off + sizeof(u32));
> +
> +		iova_base = dma_map_resource(epc->dev.parent, phys_base,
> +					     map_size, DMA_FROM_DEVICE, 0);

DB should write. Is it DMA_TO_DEVICE?

Frank
>
>  /**
> --
> 2.51.0
>