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

[Gerd Hoffmann]

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@xxxxxxxxxx> wrote:
> >
> > On Fri, Feb 24, 2023 at 10:02:24AM -0800, Rob Clark wrote:
> > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > >
> > > 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