linux-kernel - Re: [Spice-devel] [PATCH] Revert "drm/qxl: drop prime import/export callbacks"

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Date:   Mon, 29 Apr 2019 09:54:13 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:DRM DRIVER FOR QXL VIRTUAL GPU" 
        <virtualization@...ts.linux-foundation.org>,
        "open list:DRM DRIVER FOR QXL VIRTUAL GPU" 
        <spice-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...hat.com>
Subject: Re: [Spice-devel] [PATCH] Revert "drm/qxl: drop prime import/export
 callbacks"

On Fri, Apr 26, 2019 at 04:21:37PM +0200, Daniel Vetter wrote:
> On Fri, Apr 26, 2019 at 7:33 AM Gerd Hoffmann <kraxel@...hat.com> wrote:
> >
> > This reverts commit f4c34b1e2a37d5676180901fa6ff188bcb6371f8.
> >
> > Simliar to commit a0cecc23cfcb Revert "drm/virtio: drop prime
> > import/export callbacks".  We have to do the same with qxl,
> > for the same reasons (it breaks DRI3).
> >
> > Drop the WARN_ON_ONCE().
> >
> > Fixes: f4c34b1e2a37d5676180901fa6ff188bcb6371f8
> > Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
> 
> Maybe we need some helpers for virtual drivers which only allow
> self-reimport and nothing else at all?

The current gem prime helpers handle this reasonable well,
I don't see a need for that.

More useful would be some way to signal this self-reimport capability
to userspace somehow.  See DRM_PRIME_CAP_LOCAL patch.

Right now I have the choice to set DRM_PRIME_CAP_{IMPORT,EXPORT}, in
which case some userspace assumes it can do cross-driver export/import
and trips over that not working.  Or I do not set
DRM_PRIME_CAP_{IMPORT,EXPORT}, which breaks DRI3 ...

> I think there's qxl, virgl,

export is implemented for virgl, and import should be possible too.

qxl/bochs is simliar to vmgfx, it could be done but dma-bufs would
basically be bounce buffers (need copy from/to vram) so probably not
worth the effort.

cheers,
  Gerd