linux-kernel - Re: [PATCH] drm/virtio: fix mmap page attributes

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Message-ID: <20191211081541.ejofg4uxo6poss6v@sirius.home.kraxel.org>
Date:   Wed, 11 Dec 2019 09:15:41 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        gurchetansingh@...omium.org
Subject: Re: [PATCH] drm/virtio: fix mmap page attributes

  Hi,

> There's similar code in udl, [1] which still uses writecombine for
> imported buffers. Virtio does not need this?

virtio doesn't support dma-buf imports (yet).
So no worries for now.

Why pick writecombine for the imported buffer btw?
It'll probably be correct for the majority of imports, but it still
looks like a educated guess to me.  What if you import from vgem?

I guess we should either ...
  (1) Ask the exporting driver to handle things, simliar to how it is
      done for vmaps already, probably by calling dma_buf_mmap(), or
  (2) Refuse to mmap imported objects via drm api.

> Aside from this, do you think we could handle all special cases within
> shmem?

Probably makes sense to teach shmem helpers about caching.

cheers,
  Gerd