Re: [PATCH 0/2] NTB: Allow drivers to provide DMA mapping device

On Mon, Mar 02, 2026 at 09:52:08AM -0700, Dave Jiang wrote:
> 
> 
> On 3/2/26 7:45 AM, Koichiro Den wrote:
> > Some NTB implementations are backed by a "virtual" PCI device, while the
> > actual DMA mapping context (IOMMU domain) belongs to a different device.
> > 
> > One example is vNTB, where the NTB device is represented as a virtual
> > PCI endpoint function, but DMA operations must be performed against the
> > EPC parent device, which owns the IOMMU context.
> > 
> > Today, ntb_transport implicitly relies on the NTB device's parent device
> > as the DMA mapping device. This works for most PCIe NTB hardware, but
> > breaks implementations where the NTB PCI function is not the correct
> > device to use for DMA API operations.
> 
> Actually it doesn't quite work. This resulted in 061a785a114f ("ntb: Force
> physically contiguous allocation of rx ring buffers"). As you can see it
> tries to get around the issue as a temp measure. The main issue is the
> memory window buffer is allocated before the dmaengine devices are allocated.
> So the buffer is mapped against the NTB device rather than the DMA device.
> So I think we may need to come up with a better scheme to clean up this
> issue as some of the current NTBs can utilize this change as well.

Thanks for the feedback.

I think there are two issues which are related but separable:

- 1). Ensuring the correct DMA-mapping device is used for the MW translation
      (i.e. inbound accesses from the peer).
- 2). RX-side DMA memcpy re-maps the MW source buffer against the dmaengine
      device ("double mapping").

(1) is what this series is addressing. I think this series does not worsen (2).
I agree that (2) should be improved eventually.

(Note that in some setups such as vNTB, the device returned by ntb_get_dma_dev()
can be the same as chan->device->dev, in that case the double mapping could be
optimized away. However, I undersntand that you are talking about a more
fundamental improvement.)

> 
> The per queue DMA device presents an initialization hierarchy challenge with the
> memory window context. I'm open to suggestions.  

In my view, what is written in 061a785a114f looks like the most viable long-term
direction:

    A potential future solution may be having the DMA mapping API providing a
    way to alias an existing IOVA mapping to a new device perhaps.

I do not immediately see a more practical alternative. E.g., deferring MW
inbound mapping until ntb_transport_create_queue() would require a substantial
rework, since dma_chan is determined per-QP at that stage and the mapping would
become dynamic per subrange. I doubt it would be worth doing or acceptable.
Pre-allocating dma_chans only for this purpose also seems excessive.

So I agree that (2) needs a clean-up eventually. However, in my opinion the
problem this series tries to solve is independent, and the approach here does
not interfere with that direction.

Best regards,
Koichiro

> 
> DJ
> 
> > 
> > This small series introduces an optional .get_dma_dev() callback in
> > struct ntb_dev_ops, together with a helper ntb_get_dma_dev(). If the
> > callback is not implemented, the helper falls back to the existing
> > default behavior. Drivers that implement .get_dma_dev() must return a
> > non-NULL struct device.
> > 
> > - Patch 1/2: Add .get_dma_dev() to struct ntb_dev_ops and provide
> >              ntb_get_dma_dev().
> > 
> > - Patch 2/2: Switch ntb_transport coherent allocations and frees to use
> >              ntb_get_dma_dev().
> > 
> > No functional changes are intended by this series itself.
> > 
> > A follow-up patch implementing .get_dma_dev() for the vNTB EPF driver
> > (drivers/pci/endpoint/functions/pci-epf-vntb.c) will be submitted
> > separately to the PCI Endpoint subsystem tree. That will enable
> > ntb_transport to work correctly in IOMMU-backed EPC setups.
> > 
> > Best regards,
> > Koichiro
> > 
> > 
> > Koichiro Den (2):
> >   NTB: core: Add .get_dma_dev() callback to ntb_dev_ops
> >   NTB: ntb_transport: Use ntb_get_dma_dev() for DMA buffers
> > 
> >  drivers/ntb/ntb_transport.c | 14 +++++++-------
> >  include/linux/ntb.h         | 23 +++++++++++++++++++++++
> >  2 files changed, 30 insertions(+), 7 deletions(-)
> > 
>