[RFC v1 0/4] drm: Add support for DRM_CAP_DEFERRED_OUT_FENCE capability

[Daniel Vetter]

On Fri, Jul 30, 2021 at 03:38:50PM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > - We fix virtio to send out the completion event at the end of this entire
> >   pipeline, i.e. virtio code needs to take care of sending out the
> >   crtc_state->event correctly.
> 
> That sounds sensible to me.  Fence the virtio commands, make sure (on
> the host side) the command completes only when the work is actually done
> not only submitted.  Has recently been added to qemu for RESOURCE_FLUSH
> (aka frontbuffer rendering) and doing the same for SET_SCANOUT (aka
> pageflipping), then send vblank events to userspace on command
> completion certainly makes sense.

Hm how does this all work? At least drm/virtio uses
drm_atomic_helper_dirtyfb, so both DIRTYFB ioctl and atomic flips all end
up in the same driver path for everything. Or do you just combine the
resource_flush with the flip as needed and let the host side figure it all
out? From a quick read of virtgpu_plane.c that seems to be the case ...

Also to make this work we don't just need the fence, we need the timestamp
(in a clock domain the guest can correct for ofc) of the host side kms
driver flip completion. If you just have the fence then the jitter from
going through all the layers will most likely make it unusable.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch