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

[Kasireddy, Vivek]

Hi Gerd,

> 
>   Hi,
> 
> > > 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 ...
> 
> virtio_gpu_primary_plane_update() will send RESOURCE_FLUSH only for
> DIRTYFB and both SET_SCANOUT + RESOURCE_FLUSH for page-flip, and I
> think for the page-flip case the host (aka qemu) doesn't get the
> "wait until old framebuffer is not in use any more" right yet.
[Kasireddy, Vivek] As you know, with the GTK UI backend and this patch series: 
https://lists.nongnu.org/archive/html/qemu-devel/2021-06/msg06745.html
we do create a sync file fd -- after the Blit -- and wait (adding it to Qemu's main
event loop) for it to ensure that the Guest scanout FB is longer in use on the Host.
This mechanism works in a similarly way for both frontbuffer DIRTYFB case and
also the double-buffer case. 

The out_fence work is only relevant for the future Wayland UI backend though.

> 
> So we'll need a host-side fix for that and a guest-side fix to switch
> from a blocking wait on the fence to vblank events.
[Kasireddy, Vivek] Do you see any concerns with the blocking wait? And, are you
suggesting that we use a vblank timer? Not sure if that would be needed because it
would not align with the render/draw signals used with GTK. And, the DRM core
does send out an event -- immediately after the blocking wait -- to Guest compositor
as no_vblank=true.

> 
> > 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.
> 
> Well, there are no timestamps in the virtio-gpu protocol ...
> 
> Also I'm not sure they would be that helpful, any timing is *much* less
> predictable in a virtual machine, especially in case the host machine is
> loaded.
[Kasireddy, Vivek] I agree; I think sharing the Host timestamps with the Guest or 
vice-versa may not be useful. We have not run into any problems without these so far.

Thanks,
Vivek

> 
> take care,
>   Gerd