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From:   Rob Clark <robdclark@gmail.com>
Date:   Mon, 27 Feb 2023 07:40:11 -0800
Message-ID: <CAF6AEGsv1G7CPSkCPe3iHGB9JEO4iy+bTbkFLoitmx64U78RJw@mail.gmail.com>
Subject: Re: [PATCH] drm/virtio: Add option to disable KMS support
To:     Gerd Hoffmann <kraxel@redhat.com>
Cc:     dri-devel@lists.freedesktop.org, Chia-I Wu <olvaffe@gmail.com>,
        Ryan Neph <ryanneph@chromium.org>,
        Dmitry Osipenko <dmitry.osipenko@collabora.com>,
        Rob Clark <robdclark@chromium.org>,
        David Airlie <airlied@redhat.com>,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@lists.linux-foundation.org>,
        open list <linux-kernel@vger.kernel.org>
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On Sun, Feb 26, 2023 at 10:38 PM Gerd Hoffmann <kraxel@redhat.com> wrote:
>
> On Fri, Feb 24, 2023 at 10:02:24AM -0800, Rob Clark wrote:
> > From: Rob Clark <robdclark@chromium.org>
> >
> > Add a build option to disable modesetting support.  This is useful in
> > cases where the guest only needs to use the GPU in a headless mode, or
> > (such as in the CrOS usage) window surfaces are proxied to a host
> > compositor.
>
> Why make that a compile time option?  There is a config option for the
> number of scanouts (aka virtual displays) a device has.  Just set that
> to zero (and fix the driver to not consider that configuration an
> error).

The goal is to not advertise DRIVER_MODESET (and DRIVER_ATOMIC).. I
guess that could be done based on whether there are any scanouts, but
it would mean making the drm_driver struct non-const.  And I think it
is legitimate to allow the guest to make this choice, regardless of
what the host decides to expose, since it is about the ioctl surface
area that the guest kernel exposes to guest userspace.

BR,
-R