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

[Niklas Cassel <cassel@kernel.org>]

On Wed, Mar 25, 2026 at 05:43:58PM +0900, Koichiro Den wrote:
> > > > ...
> > > > > +	 */
> > > > > +	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?
> > > 
> > > Thanks for reviewing.
> > > 
> > > The write is initiated by the peer (RC) and lands in this EP-side doorbell
> > > target, so from the EP side this is a DMA_FROM_DEVICE mapping.
> > 
> > I don't understand.
> > 
> > If the RC side does a PCI TLP write, the iATU on the EP side should translate
> > that to an AXI write, no?
> 
> Yes.
> 
> > 
> > If the target address (doorbell register in this case) is only mapped as a
> > DMA_FROM_DEVICE (read), I would have expected that AXI write to result in an
> > IOMMU fault.
> 
> My understanding is that the inbound MWr becomes a local write on the EP side.
> Here, the EP controller needs write permission to the doorbell target, so
> DMA_FROM_DEVICE is intentional.
> 
> As noted in the cover letter, IOMMU coverage was tested.
> 
> > 
> > Is the IOMMU really running in strict mode and not in passthrough mode on
> > your platform?
> > 
> > Do you get an IOMMU fault if you change this code to DMA_TO_DEVICE ?
> 
> Yes, changing it to DMA_TO_DEVICE triggers an IOMMU fault. I've confirmed this
> experimentally. This also indicates that it's not passthrough for this mapping
> on my setup.

I still think it sounds wrong as per:
https://docs.kernel.org/core-api/dma-api-howto.html#dma-direction

The transaction is a write from
PCIe bus -> PCIe controller iATU -> internal bus -> IOMMU -> PCIe controller
(the same controller as initiated the transaction).

But I guess I'm just an idiot :)


Would be interesting why this is not working like normal (when using buffers):
"For Networking drivers, it’s a rather simple affair.
For transmit packets, map/unmap them with the DMA_TO_DEVICE direction specifier.
For receive packets, just the opposite, map/unmap them with the DMA_FROM_DEVICE
direction specifier."

Pehaps it is because it is the same device that does a write, that ends up
writing to it a self? so source an destination is the same?


Kind regards,
Niklas