On Mon, Feb 27, 2023 at 07:40:11AM -0800, Rob Clark wrote:

On Sun, Feb 26, 2023 at 10:38 PM Gerd Hoffmann [off-list ref] wrote:

quoted

On Fri, Feb 24, 2023 at 10:02:24AM -0800, Rob Clark wrote:

quoted

From: Rob Clark <redacted>

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

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