Re: [PATCH 05/10] PCI/P2PDMA: Export pci_p2pdma_map_type() function

[Logan Gunthorpe <logang@deltatee.com>]

On 2025-07-28 10:41, Leon Romanovsky wrote:
> On Mon, Jul 28, 2025 at 10:12:31AM -0600, Logan Gunthorpe wrote:
>>
>>
>> On 2025-07-27 13:05, Jason Gunthorpe wrote:
>>> On Fri, Jul 25, 2025 at 10:30:46AM -0600, Logan Gunthorpe wrote:
>>>>
>>>>
>>>> On 2025-07-24 02:13, Leon Romanovsky wrote:
>>>>> On Thu, Jul 24, 2025 at 10:03:13AM +0200, Christoph Hellwig wrote:
>>>>>> On Wed, Jul 23, 2025 at 04:00:06PM +0300, Leon Romanovsky wrote:
>>>>>>> From: Leon Romanovsky <leonro@nvidia.com>
>>>>>>>
>>>>>>> Export the pci_p2pdma_map_type() function to allow external modules
>>>>>>> and subsystems to determine the appropriate mapping type for P2PDMA
>>>>>>> transfers between a provider and target device.
>>>>>>
>>>>>> External modules have no business doing this.
>>>>>
>>>>> VFIO PCI code is built as module. There is no way to access PCI p2p code
>>>>> without exporting functions in it.
>>>>
>>>> The solution that would make more sense to me would be for either
>>>> dma_iova_try_alloc() or another helper in dma-iommu.c to handle the
>>>> P2PDMA case.
>>>
>>> This has nothing to do with dma-iommu.c, the decisions here still need
>>> to be made even if dma-iommu.c is not compiled in.
>>
>> Doesn't it though? Every single call in patch 10 to the newly exported
>> PCI functions calls into the the dma-iommu functions. If there were
>> non-iommu paths then I would expect the code would use the regular DMA
>> api directly which would then call in to dma-iommu.
> 
> If p2p type is PCI_P2PDMA_MAP_BUS_ADDR, there will no dma-iommu and DMA
> at all.

I understand that and it is completely beside my point.

If the dma mapping for P2P memory doesn't need to create an iommu
mapping then that's fine. But it should be the dma-iommu layer to decide
that. It's not a decision that should be made by every driver doing this
kind of thing.

With P2PDMA memory we are still creating a DMA mapping. It's just the
dma address will be a PCI bus address instead of an IOVA. My opinion
remains: none of these details should be exposed to the drivers.

Logan