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

[Daniel Vetter]

On Mon, Aug 2, 2021 at 2:51 PM Gerd Hoffmann <kraxel at redhat.com> wrote:
>
>   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.

Hm reading the code I think you simply elide the set_scanout if it's
still the same buffer. There's no difference betweeen dirtyfb and an
atomic commit that just hands the damage rects to the driver. At least
if you use the helpers.

> 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.
>
> > 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.

Hm yeah if the output is currently not displaying, then the timestamp
is very fake. But if you display you should be able to pass it all
around in both direction. So target vblank (or whatever it's called)
would go from guest to host to host-compositor (over wayland protocol)
to host-side kms, and the timestamp could travel all the way back.

But yeah making this all work correctly is going to be a pile of work.
Also I have no idea how well compositors take it when a kms driver
switches between high-precision timestamps and frame scheduling to the
entirely virtual/vblank-less approach on the fly.
-Daniel

> take care,
>   Gerd
>


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