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

Christian König
2025-11-20T07:08:39Z

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.

Regards,
Christian.

>
> We can also have a mini-community call to give a summary/etc on these
> topics.
>
> Thanks,
> Jason