[PATCH] drm/vkms: Add a DRM render node to vkms

[Yi Xie]

> This doesn't sound like a good idea to me. Devices without render
> capabilities should not fake it.
>
> User-space (e.g. wlroots) relies on "no render node" to enable
> software rendering (Pixman instead of GL).

We have virtgpu driver that exports a render node even when virgl is
not supported.
Mesa has special code path to enable software rendering on it:
https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/egl/drivers/dri2/platform_device.c#L296
We can do the same for vkms to force software rendering.

On Thu, Jan 5, 2023 at 8:36 PM Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Thu, Jan 05, 2023 at 02:23:25PM +0900, Yi Xie wrote:
> > Some libraries including Mesa and virglrenderer require a render node to
> > fully function. By adding a render node to vkms those libraries will
> > work properly, supporting use cases like running crosvm with virgl GPU
> > support via llvmpipe on a headless virtual machine.
>
> This is what vgem exists for. More or less at least ... I'm honestly not
> really understanding what you're trying to fix here, it sounds a bit like
> userspace being stupid.
> -Daniel
The problem with vgem is that it crashes llvmpipe while working with vkms.
Looks like it's due to the same reason as described in this thread in Mesa:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5830

Importing buffers allocated by vgem to vkms seems to be unexpected and
causes the crash. If we create a render node on vkms then llvmpipe will use
vkms to allocate buffers and it no longer crashes.

> >
> > Signed-off-by: Yi Xie <yixie at google.com>
> > ---
> >  drivers/gpu/drm/vkms/vkms_drv.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> > index 293dbca50c31..8eea5d4dece8 100644
> > --- a/drivers/gpu/drm/vkms/vkms_drv.c
> > +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> > @@ -113,7 +113,7 @@ static void vkms_config_debugfs_init(struct drm_minor *minor)
> >  }
> >
> >  static const struct drm_driver vkms_driver = {
> > -     .driver_features        = DRIVER_MODESET | DRIVER_ATOMIC | DRIVER_GEM,
> > +     .driver_features        = DRIVER_MODESET | DRIVER_ATOMIC | DRIVER_GEM | DRIVER_RENDER,
> >       .release                = vkms_release,
> >       .fops                   = &vkms_driver_fops,
> >       DRM_GEM_SHMEM_DRIVER_OPS,
> > --
> > 2.39.0.314.g84b9a713c41-goog
> >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch