Re: [PATCH v2 1/2] drm/shmem: add support for per object caching attributes

From: Gerd Hoffmann
Date: Wed Dec 11 2019 - 08:18:38 EST


> btw on why udl does this: Imported bo are usually rendered by real hw, and
> reading it uncached/wc is the more defensive setting. It would be kinda
> nice if dma-buf would expose this, but I fear dma-api maintainers would
> murder us if we even just propose that ... so it's a mess right now.

I suspect for imported dma-bufs we should leave the mmap() to the
exporter instead of pulling the pages out of the sgt and map them

> btw the issue extends to dma access by devices too, e.g. both i915 and
> amdgpu can select the coherency mode at runtime (using e.g. the pcie
> no-snoop transaction mode), and we have similar uncoordinated hacks in
> there too, like in udl.

Hmm. Ok. I guess I'm not going to try solve all that properly just for
the little virtio fix.

Just curious: How do you tell your hardware? Are there bits for that
in the gtt, simliar to the caching bits in the x86 page tables?