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

[Dave Jiang]

On 3/4/26 8:56 AM, Koichiro Den wrote:
> 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)
> 
> 

Probably should include it with this series if it's small. Having the user with new code is usually preferred.