[Intel-gfx] [PATCH] drm/i915/gem: Allow importing of shmemfs objects into any device

[Chris Wilson]

Quoting Tvrtko Ursulin (2021-01-21 11:00:25)
> 
> On 20/01/2021 20:59, Chris Wilson wrote:
> > Quoting Chris Wilson (2021-01-20 18:06:08)
> >> Quoting Matthew Auld (2021-01-20 17:46:10)
> >>> On Wed, 20 Jan 2021 at 15:40, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> >>>>
> >>>> If we import a shmemfs object between devices, for example from
> >>>> Tigerlake to DG1, we can simply reuse the native object and its backing
> >>>> store.
> >>>
> >>> Hmmm interesting, so does that include re-using the actual sg mapping
> >>> for the backing pages? Does that work out-of-the-box between different
> >>> devices assuming we have iommu enabled?
> >>
> >> Indeed interesting; the dma_addr_t are supposed to be local to a device.
> > 
> > On reflection, we are expected to use cross-device dma_addr_t with
> > dma-buf. It's the exporter who assigns the dma_addr_t for the importer
> > to use, and they are always given from the original device.
> > 
> > Maybe not so bad. Definitely needs testing to see what happens in
> > practice.
> 
> What about object migration? I did not spot anything preventing it once 
> object was exported like this so owning device could move it to device 
> memory afterwards which would probably be bad.

Depends on how you do your migration, but your migration should be
checking that it is allowed to migrate the object first.
-Chris