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

[Javier Martinez Canillas]

Gerd Hoffmann <kraxel@xxxxxxxxxx> writes:

Hello Gerd,

> 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.
>

I don't think there's a need for a new module parameter, there's already
the virtio-gpu 'modeset' module parameter to enable/disable modsetting
and the global 'nomodeset' kernel cmdline parameter to do it for all DRM
drivers.

Currently, many drivers just fail to probe when 'nomodeset' is present,
but others only disable modsetting but keep the rendering part. In fact,
most DRM only drivers just ignore the 'nomodeset' parameter.

We could make virtio-gpu driver to only disable KMS with these params,
something like the following (untested) patch: