Re: [PATCH] virtio-gpu: do not replace surface when scanout is disabled

[Kim, Dongwon]

On 7/4/2023 8:12 AM, Marc-André Lureau wrote:
> Hi
>
> On Wed, Jun 28, 2023 at 12:32 AM Dongwon Kim <dongwon.kim@intel.com> 
> wrote:
>
>     Surface is replaced with a place holder whenever the surface res
>     is unreferenced by the guest message. With this logic, there is
>     very frequent switching between guest display and the place holder
>     image, which is looking like a flickering display if the guest driver
>     is designed to unref the current scanout resource before sending out
>     a new scanout resource. So it is better to leave the current scanout
>     image until there is a new one flushed by the guest.
>
>     Cc: Gerd Hoffmann <kraxel@redhat.com>
>     Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
>     Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
>     Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
>
>
> Why is the driver not setting a different scanout before destroying 
> the resource?
>
> I think it's wrong to not replace the surface, as the associated 
> scanout resource may be destroyed or explicitly disabled for various 
> purposes, and we don't want to display garbage either.

Yeah..I got your point. This is to address very specific to our use-case 
with windows guest that runs virtio-gpu like driver that does unref 
before the next framebuffer is set. And I agree that this sequence 
doesn't look right. Let me check if we could change the sequence in the 
guest driver.

>     ---
>      hw/display/virtio-gpu.c | 1 -
>      1 file changed, 1 deletion(-)
>
>     diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
>     index 66cddd94d9..9d3e922c8f 100644
>     --- a/hw/display/virtio-gpu.c
>     +++ b/hw/display/virtio-gpu.c
>     @@ -387,7 +387,6 @@ static void
>     virtio_gpu_disable_scanout(VirtIOGPU *g, int scanout_id)
>              res->scanout_bitmask &= ~(1 << scanout_id);
>          }
>
>     -    dpy_gfx_replace_surface(scanout->con, NULL);
>          scanout->resource_id = 0;
>          scanout->ds = NULL;
>          scanout->width = 0;
>     -- 
>     2.34.1
>
>
>
>
> --
> Marc-André Lureau