Re: [PATCH v8 05/11] PCI/P2PDMA: Document DMABUF model

Leon Romanovsky
2025-11-19T13:13:19Z

On Wed, Nov 19, 2025 at 10:18:08AM +0100, Christian König wrote:
>
>
> On 11/11/25 10:57, Leon Romanovsky wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> >
> > Reflect latest changes in p2p implementation to support DMABUF lifecycle.
> >
> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> > ---
> > Documentation/driver-api/pci/p2pdma.rst | 95 +++++++++++++++++++++++++--------
> > 1 file changed, 72 insertions(+), 23 deletions(-)

<...>

> > These MMIO pages have no struct page, and
>
> Well please drop "pages" here. Just say MMIO addresses.
>
> > +if used with mmap() must create special PTEs. As such there are very few
> > +kernel uAPIs that can accept pointers to them; in particular they cannot be used
> > +with read()/write(), including O_DIRECT.

<...>

> > +DMABUF provides an alternative to the above struct page-based
> > +client/provider/orchestrator system. In this mode the exporting driver will wrap
> > +some of its MMIO in a DMABUF and give the DMABUF FD to userspace.
> > +
> > +Userspace can then pass the FD to an importing driver which will ask the
> > +exporting driver to map it.
>
> "to map it to the importer".

No problem, changed.

>
> Regards,
> Christian.
>
> > +
> > +In this case the initiator and target pci_devices are known and the P2P subsystem
> > +is used to determine the mapping type. The phys_addr_t-based DMA API is used to
> > +establish the dma_addr_t.
> > +
> > +Lifecycle is controlled by DMABUF move_notify(). When the exporting driver wants
> > +to remove() it must deliver an invalidation shutdown to all DMABUF importing
> > +drivers through move_notify() and synchronously DMA unmap all the MMIO.
> > +
> > +No importing driver can continue to have a DMA map to the MMIO after the
> > +exporting driver has destroyed its p2p_provider.
> >
> >
> > P2P DMA Support Library
> >
>