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

[Niklas Cassel <cassel@kernel.org>]

On Thu, Mar 26, 2026 at 05:49:13PM +0900, Koichiro Den wrote:
> > 
> > The transaction is a write from
> > PCIe bus -> PCIe controller iATU -> internal bus -> IOMMU -> PCIe controller
> > (the same controller as initiated the transaction).
> 
> Yes, I think we're on the same page about this path itself.
> 
> On my R-Car S4 setup, changing this to DMA_TO_DEVICE consistently triggers an
> IOMMU fault, so at least on this platform the local path used for the doorbell
> mapping is IOMMU-visible. That is the case this dma_map_resource() is intended
> to cover.
> 
> For that path, my understanding is that the doorbell access ends up as a local
> write on the EP side, so it needs write permission, hence DMA_FROM_DEVICE.
> 
> > 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."
> 
> I think the closer analogy is RX: the data comes from outside, but the device
> writes to the target, so it needs write permission.

I think it is from the PoV from the IOMMU, is the transaction a Read by a device
or a Write by a device?

For a NIC driver:
For a RX packet, the data is coming from the device to the memory.
device is doing a transaction to memory.
For a TX packet, the data is going from the memory to the device.

In our case, the data is coming from the device, to a device.

Almost like a P2P DMA, but in our case, both devices are the same, so
using the P2P DMA API like pci_p2pdma_add_resource() seems unnecessary.

So should it be DMA_BIDIRECTIONAL ? :)

I understand that for the R-Car S4 Spider IOMMU, it is sufficient to map
it as DMA_FROM_DEVICE. I just want to be sure that on some other IOMMU,
they might consider it sufficient to map this as DMA_TO_DEVICE (because
it is also a transaction going to a device).

I just want to make sure that the code works on more than one IOMMU.

Perhaps some IOMMU experts could help chime in.


Note that I am happy to merge the code as is, as it obviously works on the
only platform that this has been tested on (R-Car S4 Spider), and if other
platform tries to run this test case, if their IOMMU works differently, it
will scream and they will report it to the list. So all good.

I'm mostly want to know how the DMA-API is supposed to be used in this
specific scenario (device doing a write transaction to the same device).


Kind regards,
Niklas