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

[Koichiro Den]

On Tue, Mar 03, 2026 at 08:42:53AM -0700, Dave Jiang wrote:
> 
> 
> On 3/2/26 9:56 PM, Koichiro Den wrote:
> > 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.
> 
> Fair assessment. For the series:
> Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>

Thanks for the review.

Once this looks good to Jon as well and gets queued in the NTB tree, I'll submit
a small patch to PCI EP for vNTB (the real user of the interface), something
like the following:


diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
index be6c03f4516e..8aeacbae8b77 100644
--- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
+++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
@@ -1501,6 +1501,15 @@ static int vntb_epf_link_disable(struct ntb_dev *ntb)
        return 0;
 }

+static struct device *vntb_epf_get_dma_dev(struct ntb_dev *ndev)
+{
+       struct epf_ntb *ntb = ntb_ndev(ndev);
+
+       if (!ntb || !ntb->epf)
+               return NULL;
+       return ntb->epf->epc->dev.parent;
+}
+
 static const struct ntb_dev_ops vntb_epf_ops = {
        .mw_count               = vntb_epf_mw_count,
        .spad_count             = vntb_epf_spad_count,
@@ -1522,6 +1531,7 @@ static const struct ntb_dev_ops vntb_epf_ops = {
        .db_clear_mask          = vntb_epf_db_clear_mask,
        .db_clear               = vntb_epf_db_clear,
        .link_disable           = vntb_epf_link_disable,
+       .get_dma_dev            = vntb_epf_get_dma_dev,
 };

 static int pci_vntb_probe(struct pci_dev *pdev, const struct pci_device_id *id)


Best regards,
Koichiro

> 
> > 
> > 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(-)
> >>>
> >>
> > 
>