[PATCH 4/6] drm/amdgpu: add checks if DMA-buf P2P is supported

[Jason Gunthorpe]

On Wed, Mar 11, 2020 at 03:43:03PM +0100, Christian König wrote:
> > > > int 'p2pdma map bar'(
> > > >      struct pci_device *source,
> > > >      unsigned int source_bar_number,
> > > >      struct pci_device *dest,
> > > >      physaddr&len *array_of_offsets & length pairs into source bar,
> > > >      struct scatterlist *output_sgl)
> > > Well that's exactly what I have to avoid since I don't have the array of
> > > offsets around and want to avoid constructing it.
> > Maybe it doesn't need an array of offsets - just a single offset and
> > callers can iterate the API?
> 
> Yes, that would of course work as well.
> 
> But I was assuming that p2pdma_map_bar() needs some state between those
> calls.

It might be able to run faster if some state is held. We've had APIs
before where the caller can provide a cache for expensive state for
APIs. Maybe that would be an appropriate pattern here?

IIRC the distance calculation is the expensive bit, that would be easy
enough to cache.

Jason