[PATCH weston 2/2] compositor: Skip repaint delay for nested compositors

[Pekka Paalanen]

On Wed,  1 Apr 2015 12:37:34 -0500
Derek Foreman <derekf at osg.samsung.com> wrote:

> The repaint delay becomes cumulative if it's used in both a parent
> compositor and a nested compositor.  This can lead to dropped frames
> and visible latency.
> 
> Signed-off-by: Derek Foreman <derekf at osg.samsung.com>
> ---
>  src/compositor.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/src/compositor.c b/src/compositor.c
> index 4de8fbf..433f6d2 100644
> --- a/src/compositor.c
> +++ b/src/compositor.c
> @@ -2412,6 +2412,14 @@ weston_output_finish_frame(struct weston_output *output,
>  	msec = (refresh_nsec - timespec_to_nsec(&gone)) / 1000000; /* floor */
>  	msec -= compositor->repaint_msec;
>  
> +	/* If nested outputs try to use a repaint window the delay sums
> +	 * with the parent's delay and we start dropping frames...
> +	 */
> +	if (compositor->nested) {
> +		output_repaint_timer_handler(output);
> +		return;
> +	}
> +
>  	if (msec < -1000 || msec > 1000) {
>  		static bool warned;
>  

Yeah... except the "dropping frames" part does not make sense to me.

When the parent compositor sends a frame callback, it is already too
late for the child compositor to hit the very next vblank in absolute
time. Instead, the child compositor now has almost a whole frame
period's time until the deadline set by the parent compositor for the
following vblank. So, it might just as well wait that 9 ms or whatever,
it should still hit the very same vblank as it would by rendering
immediately.

There must be something else going on here.

I can definitely reproduce problems with Weston/wayland nested in
Weston/DRM.

I have a hunch the problem lies in the Wayland backend's finish-frame
timestamps, which have no relation to the presentation clock used in
the wayland backend... will investigate.

A proper solution would be to make the Wayland backend use the
Presentation extension, but we also do need a way for it to work fine
if there is not Presentation available.


Thanks,
pq