[PATCH v3 7/7] [wip] virtio-gpu: add page flip support

[Daniel Vetter]

On Mon, May 30, 2016 at 02:06:50PM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > > But I'll take you up on the implied offer to help out and test ;-)
> > 
> > git://people.freedesktop.org/~danvet/drm stuff
> 
> Tried that branch.
> 
> > Would be really awesome if you could test this on virtio. Note that the
> > new nonblocking helpers require that your atomic backend gets the drm
> > event handling right. So if there's a bug in that logic then you'll see
> > lots of dmesg noise about waits timing out (after 10s of waiting). From a
> > quick inspection it should work though.
> 
> No timeouts.  Yay!
> 
> But it seems crtcs can be (temporarely) disabled now, so we might have
> to pick up the crtc from old_state in virtio_gpu_plane_atomic_update to
> figure which virtual output needs to be turned off.  Ran into this last
> week already.  Happened with multihead setups only, but the same patch
> fixes this one too ;)
> 
> https://lists.freedesktop.org/archives/dri-devel/2016-May/108772.html

Hm, smells more like virtio isn't too happy with the default ordering of
the commit operation. The default is:

- Disable any crtc/encoders that need to be disabled/change.
- Bash new plane setup into hw.
- Enable all crtcs/encoders that need to be enabled/have changed.

There's two problems:
- some hw gets real grumpy if you bash in plane state without the crtc
  state yet matching.
- if you do runtime pm nothing is enabled and the writes get lost at best,
  or hang your interconnect at worst.

That's why you can overwrite atomic_commit_tail, and use something more
sensible. See for example what it looks like for rockchip. I have a gut
feeling that should also take care of your troubles. Using the old crtc is
definitely not what you want.

Another option is that virtio isn't happy about bashing in plane state for
disabled crtc. Again helpers have you covered, look at the active_only
parameter for drm_atomic_helper_commit_planes().

Aside, if you wonder why these defaults: They match what the crtc helpers
are doing, but they are a bit surprising indeed.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch