linux-kernel - Re: [PATCH] drm/virtio: Add option to disable KMS support

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Date:   Tue, 28 Feb 2023 07:28:09 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, Chia-I Wu <olvaffe@...il.com>,
        Ryan Neph <ryanneph@...omium.org>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        Rob Clark <robdclark@...omium.org>,
        David Airlie <airlied@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/virtio: Add option to disable KMS support

On Mon, Feb 27, 2023 at 07:40:11AM -0800, Rob Clark wrote:
> On Sun, Feb 26, 2023 at 10:38 PM Gerd Hoffmann <kraxel@...hat.com> wrote:
> >
> > On Fri, Feb 24, 2023 at 10:02:24AM -0800, Rob Clark wrote:
> > > From: Rob Clark <robdclark@...omium.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.

Apparently there is a drm_device->driver_features override,
(amdgpu uses that).  The driver could simply drop the DRIVER_MODESET and
DRIVER_ATOMIC bits in case no scanout is present instead of throwing an
error.

> 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.

I think it is a bad idea to make that a compile time option, I'd suggest
a runtime switch instead, for example a module parameter to ask the
driver to ignore any scanouts.

take care,
  Gerd