[RFC v1 2/4] virtio-gpu uapi: Add VIRTIO_GPU_F_OUT_FENCE feature

[Kasireddy, Vivek]

Hi Gerd,

> 
> On Thu, Jul 29, 2021 at 01:16:57AM -0700, Vivek Kasireddy wrote:
> > This feature enables the Guest to wait to know when a resource
> > is completely consumed by the Host.
> 
> virtio spec update?
> 
> What are the exact semantics?
[Kasireddy, Vivek] As of now, this is still a RFC version. If everyone (Weston
Upstream, drm upstream and you) agree that this is a reasonable way to
solve https://gitlab.freedesktop.org/wayland/weston/-/issues/514 then I'd go 
ahead and send out the spec updates and cleaner versions of these patches --
with more documentation.

> 
> Why a new command?  Can't you simply fence one of the commands sent
> anyway (set_scanout probably for page-flip updates)?
[Kasireddy, Vivek] Yes, I think I could add a fence (and an out_fence) to set-scanout-blob. 

> 
> (feature flag is probably needed even in case we don't need a new
> command to make sure the host sends the completion when processing
> the command is actually done, i.e. in case of qemu the recently added
> fence support is there).
[Kasireddy, Vivek] The recently added fence support was for resource_flush and
specifically for GTK-UI or similar backends. I tried using the same mechanism for
Wayland-UI backend but ran into the above Weston issue. This feature (OUT_FENCE)
is a potential solution for this issue.

Thanks,
Vivek
> 
> take care,
>   Gerd