On Thu, Nov 20, 2025 at 08:08:27AM +0100, Christian König wrote:
> On 11/19/25 20:31, Jason Gunthorpe wrote:
> > On Wed, Nov 19, 2025 at 02:42:18PM +0100, Christian König wrote:
> >
> >>>>> + case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> >>>>> + dma->state = kzalloc(sizeof(*dma->state), GFP_KERNEL);
> >>>>> + if (!dma->state) {
> >>>>> + ret = -ENOMEM;
> >>>>> + goto err_free_dma;
> >>>>> + }
> >>>>> +
> >>>>> + dma_iova_try_alloc(attach->dev, dma->state, 0, size);
> >>>>
> >>>> Oh, that is a clear no-go for the core DMA-buf code.
> >>>>
> >>>> It's intentionally up to the exporter how to create the DMA
> >>>> addresses the importer can work with.
> >>>
> >>> I can't fully understand this remark?
> >>
> >> The exporter should be able to decide if it actually wants to use
> >> P2P when the transfer has to go through the host bridge (e.g. when
> >> IOMMU/bridge routing bits are enabled).
> >
> > Sure, but this is a simplified helper for exporters that don't have
> > choices where the memory comes from.
>
> That is extremely questionable as justification to put that in common DMA-buf
> code.
>
> > I fully expet to see changes to this to support more use cases,
> > including the one above. We should do those changes along with users
> > making use of them so we can evaluate what works best.
>
> Yeah, exactly that's my concern.
>
> >> But only take that as Acked-by, I would need at least a day (or
> >> week) of free time to wrap my head around all the technical details
> >> again. And that is something I won't have before January or even
> >> later.
> >
> > Sure, it is alot, and I think DRM community in general should come up
> > to speed on the new DMA API and how we are pushing to see P2P work
> > within Linux.
> >
> > So thanks, we can take the Acked-by and progress here. Interested
> > parties can pick it up from this point when time allows.
>
> Wait a second. After sleeping a night over it I think my initial take that we
> really should not put that into common DMA-buf code seems to hold true.
>
> This is the use case for VFIO, but I absolutely want to avoid other drivers
> from re-using this code until be have more experience with that.
>
> So to move forward I now strongly think we should keep that in VFIO until
> somebody else comes along and needs that helper.
It was put in VFIO at the beginning, but Christoph objected to it,
because that will require exporting symbol for pci_p2pdma_map_type().
which was universally agreed as not good idea.
https://lore.kernel.org/all/[email protected]/
Thanks
>
> Regards,
> Christian.
>
> >
> > We can also have a mini-community call to give a summary/etc on these
> > topics.
> >
> > Thanks,
> > Jason
>
