Re: [Linaro-mm-sig] [PATCH v8 06/11] dma-buf: provide phys_vec to scatter-gather mapping routine

[Leon Romanovsky <leon@kernel.org>]

On Thu, Nov 20, 2025 at 08:54:37AM +0100, Christian König wrote:
> On 11/20/25 08:41, Leon Romanovsky wrote:
> > 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.
> 
> Yeah, that is exactly what I object here :)
> 
> We can have the helper in DMA-buf *if* pci_p2pdma_map_type() is called by drivers or at least accessible. That's what I pointed out in the other mail before as well.
> 
> The exporter must be able to make decisions based on if the transaction would go over the host bridge or not.
> 
> Background is that in a lot of use cases you rather want to move the backing store into system memory instead of keeping it in local memory if the driver doesn't have direct access over a common upstream bridge.
> 
> Currently drivers decide that based on if IOMMU is enabled or not (and a few other quirks), but essentially you absolutely want a function which gives this information to exporters. For the VFIO use case it doesn't matter because you can't switch the BAR for system memory.
> 
> To unblock you, please add a big fat comment in the kerneldoc of the mapping explaining this and that it might be necessary for exporters to call pci_p2pdma_map_type() as well.

Thanks,

What do you think about it?

diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
index a69bb73db86d..05ec84a0157b 100644
--- a/drivers/dma-buf/dma-buf-mapping.c
+++ b/drivers/dma-buf/dma-buf-mapping.c
@@ -84,6 +84,11 @@ struct dma_buf_dma {
  * PAGE_SIZE aligned.
  *
  * A mapping must be unmapped by using dma_buf_free_sgt().
+ *
+ * NOTE: While this function is intended for DMA-buf importers, it is critical
+ * that the DMA-buf exporter is capable of performing peer-to-peer (P2P) DMA
+ * directly between PCI devices, without routing transactions through the host
+ * bridge.
  */
 struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach,
                                         struct p2pdma_provider *provider,
(END)


> 
> Regards,
> Christian.
> 
> > 
> > https://lore.kernel.org/all/aPYrEroyWVOvAu-5@infradead.org/
> > 
> > Thanks
> > 
> >>
> >> Regards,
> >> Christian.
> >>
> >>>
> >>> We can also have a mini-community call to give a summary/etc on these
> >>> topics.
> >>>
> >>> Thanks,
> >>> Jason
> >>
>