Render only DRM devices

[Simon Ser]

On Tuesday, October 11th, 2022 at 13:04, Christian König <ckoenig.leichtzumerken at gmail.com> wrote:

> we already have quite a bunch of devices which are essentially render
> only and don't expose any connectors or more general display functionality.
> 
> Just recently I ran into a case where an older X/DDX combination
> caused problems for such a device so I looked a bit into the
> possibility to allow drivers to disable the primary node and only
> expose the render node.
> 
> It turned out that this effectively hides the device from X, but
> OpenGL and Vulkan can still use it perfectly fine.
> 
> The only crux is that this is checked so early in the initialization
> that drivers don't have an opportunity to update their
> dev->driver_features. So we will always need a separate drm_driver
> structure for render only devices.

Typically render-only devices still expose a primary node, but don't
expose any KMS resources on it. See drmIsKMS() in libdrm.

Primary nodes could still be used by older user-space for rendering with
legacy DRM authentication.