[Bug 1882851] Re: [PATCH] drm/virtio: fix unblank

[Gerd Hoffmann <1882851@bugs.launchpad.net>]

On Mon, Aug 17, 2020 at 11:03:42AM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > > --- a/drivers/gpu/drm/virtio/virtgpu_display.c
> > > +++ b/drivers/gpu/drm/virtio/virtgpu_display.c
> > > @@ -100,6 +100,7 @@ static void virtio_gpu_crtc_atomic_enable(struct drm_crtc *crtc,
> > >  	struct virtio_gpu_output *output = drm_crtc_to_virtio_gpu_output(crtc);
> > >  
> > >  	output->enabled = true;
> > > +	output->need_update = true;
> 
> > > --- a/drivers/gpu/drm/virtio/virtgpu_plane.c
> > > +++ b/drivers/gpu/drm/virtio/virtgpu_plane.c
> > > @@ -163,7 +163,8 @@ static void virtio_gpu_primary_plane_update(struct drm_plane *plane,
> > >  	    plane->state->src_w != old_state->src_w ||
> > >  	    plane->state->src_h != old_state->src_h ||
> > >  	    plane->state->src_x != old_state->src_x ||
> > > -	    plane->state->src_y != old_state->src_y) {
> > > +	    plane->state->src_y != old_state->src_y ||
> > > +	    output->need_update) {
> > 
> > Uh instead of hand-rolling what's essentially a drm_crtc_needs_modeset
> > check, why not use that one? atomic helpers try to keep the usual suspects
> > for state transitions already handy, to avoid every driver rolling their
> > own. Or do I miss something here?
> 
> Well, the virtio-gpu virtual hardware can't do plane updates and crtc
> updates independant from each other.  So the crtc callbacks handle
> disable only (we don't need a fb for that) and leave the enable to the
> plane update.
> 
> I suspect calling drm_atomic_crtc_needs_modeset() in plane update isn't
> going to fly ...

Digged a bit more, seems crtc_state->*_changed is cleared after modeset
so the following plane update wouldn't see it.  Which I think means
there is no way around tracking that in need_update.

output->enabled is probably not needed though, seems I can replace that
by either output->crtc.state->enable or ->active.  Not fully sure which
one, probably ->active.

take care,
  Gerd

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https://bugs.launchpad.net/bugs/1882851

Title:
  QEMU video freezes with "Guest disabled display" (virtio driver)

Status in QEMU:
  New

Bug description:
  I am using Arch Linux as my Guest and Host OS, after starting qemu
  with the following command:

    $ qemu-system-x86_64 -enable-kvm -hda arch-zoom.qcow2 -m 4G -vga
  virtio

  and waiting for a screen blank, I get this message:

    Guest disabled display

  And nothing happens after that, I can move the mouse or hit any key,
  and the message is still there.

  I can still reboot the VM but that's not optimal.

  I can reproduce this with the latest QEMU release (5.0.0) or git master, 
  I also tried this with older releases (4.0.0, 3.0.0) and the issue is still there.

  I can't reproduce this with other video drivers (std, qxl).

  With std/qxl the screen will blank a bit and then continue as normal.

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