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

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Message-ID: <20191211131830.iz3a2o4xzmmkjsp7@sirius.home.kraxel.org>
Date:   Wed, 11 Dec 2019 14:18:30 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        gurchetansingh@...omium.org
Subject: Re: [PATCH v2 1/2] drm/shmem: add support for per object caching
 attributes

  Hi,

> 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
ourself.

> 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?

cheers,
  Gerd