Re: [PATCH] virtio-gpu: Respect graphics update interval for EDID

[Akihiko Odaki <akihiko.odaki@gmail.com>]

2021年2月24日(水) 20:17 Gerd Hoffmann <kraxel@redhat.com>:
>
> On Tue, Feb 23, 2021 at 01:50:51PM +0900, Akihiko Odaki wrote:
> > 2021年2月22日(月) 19:57 Gerd Hoffmann <kraxel@redhat.com>:
> > >
> > > On Sun, Feb 21, 2021 at 10:34:14PM +0900, Akihiko Odaki wrote:
> > > > This change introduces an additional member, refresh_rate to
> > > > qemu_edid_info in include/hw/display/edid.h.
> > > >
> > > > This change also isolates the graphics update interval from the
> > > > display update interval. The guest will update the frame buffer
> > > > in the graphics update interval, but displays can be updated in a
> > > > dynamic interval, for example to save update costs aggresively
> > > > (vnc) or to respond to user-generated events (sdl).
> > > > It stabilizes the graphics update interval and prevents the guest
> > > > from being confused.
> > >
> > > Hmm.  What problem you are trying to solve here?
> > >
> > > The update throttle being visible by the guest was done intentionally,
> > > so the guest can throttle the display updates too in case nobody is
> > > watching those display updated anyway.
> >
> > Indeed, we are throttling the update for vnc to avoid some worthless
> > work. But typically a guest cannot respond to update interval changes
> > so often because real display devices the guest is designed for does
> > not change the update interval in that way.
>
> What is the problem you are seeing?
>
> Some guest software raising timeout errors when they see only
> one vblank irq every 3 seconds?  If so: which software is this?
> Any chance we can fix this on the guest side?
>
> > That is why we have to
> > tell the guest a stable update interval even if it results in wasted
> > frames.
>
> Because of the wasted frames I'd like this to be an option you can
> enable when needed.  For the majority of use cases this seems to be
> no problem ...

I see blinks with GNOME on Wayland on Ubuntu 20.04 and virtio-gpu with
the EDID change included in this patch. The only devices inspecting
the variable, xenfb and modified virtio-gpu, do not yield vblank irq,
but they report the refresh rate to the guest, and the guest
proactively requests them to switch the surface.

I suspect Linux's kernel mode setting causes blinks and other guests
have similar problems.

>
> Also: the EDID changes should go to a separate patch.

That makes sense. I'll isolate it to a seperate patch in a series.

Regards,
Akihiko Odaki

>
> take care,
>   Gerd
>