Re: [PATCH v7 00/10] PCI: endpoint: Add PCI DMA endpoint function

On Thu, Aug 13, 2026 at 02:50:59PM +0200, Manivannan Sadhasivam wrote:
> On Thu, Aug 13, 2026 at 01:46:07PM +0200, Niklas Cassel wrote:
> > Hello Koichiro,
> > 
> > On Thu, Aug 13, 2026 at 03:37:47PM +0900, Koichiro Den wrote:
> > > This is v7, the remaining patch set for PCI endpoint DMA.
> > > Parts 2 and 3 were merged per Frank's suggestion.
> > 
> > (snip)
> > 
> > > One open question is how to support endpoint controllers with only one
> > > PF. Keeping DMA in a separate EPF requires multi-function endpoint
> > > support. Folding it into vNTB would work on single-function
> > > controllers, but would also couple the two implementations. This series
> > > keeps the separate EPF model.
> > 
> > I see all the work you are putting in and I admire the effort.
> > 
> > This is now v7. I think it is time that we close the open question by
> > waiting for a reply from the PCI endpoint maintainers' opinion on the
> > design before continuing. (I am not a PCI endpoint maintainer.)
> > 
> > I understand that you want a common DMA abstraction, that can represent
> > different (embedded) DMA controllers on the endpoint side.
> > 
> > But if vNTB is the only consumer of this, then why not simply embed this
> > DMA abstraction in some BAR exposed by the vNTB EPF?
> > 
> > Looking at the host side driver that goes with the (v)NTB driver:
> > drivers/ntb/hw/epf/ntb_hw_epf.c
> > 
> > The BAR layouts are hard coded, and it only supports three different
> > layouts. Would it not be possible to add a fourth layout that has the
> > DMA abstraction somewhere in one of the BARs? ('BAR_DMA' ?)
> > 
> > 
> > Right now, I wonder if it is not a bit premature optimization to create a
> > DMA EPF, if vNTB will be the only (ever?) user.
> > 
> 
> Yeah, I feel the same. I haven't seen an usecase to program the DMA controller
> from the host outside of vNTB. This feature is supported mostly because it
> exists in hardware and someone wants to tick the checkbox.
> 
> Though, I'm not against doing it within vNTB as Niklas suggested, but
> generalising it in the form of a new EPF driver just for the sake of a single
> driver sounds like an overkill and maintenance burden.
> 
> Sorry for saying this in v7. I've been meaning to say it, but somehow ended up
> procrastinating too much.

No bother. Since Frank has given a lot of feedback on this series, I'd also like
to hear his view.

I'm fine with either direction, and can revisit the earlier vNTB-embedded
approach:
https://lore.kernel.org/r/sn67hi7kljh7cgmgodatb3naz2astlaklqfobdbxyyzgoohxqb@4nnetbhqwba4/

Best regards,
Koichiro

> 
> - Mani
> 
> -- 
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