[PATCH weston v2 10/11] [RFC] Account for very large repaint window misses

[Pekka Paalanen]

On Wed, 29 Mar 2017 22:35:46 +0200
Mario Kleiner <mario.kleiner.de at gmail.com> wrote:

> On 03/29/2017 12:30 PM, Pekka Paalanen wrote:
> > On Tue, 28 Mar 2017 20:53:53 +0200
> > Mario Kleiner <mario.kleiner.de at gmail.com> wrote:
> >  
> ...
> >>
> >> Agreed. The adjustment can never be more than 1 refresh period, ergo
> >> doesn't need a loop, unless we'd get the very unlikely case of Weston
> >> getting preempted for a long time exactly during the few instructions
> >> leading from output->start_repaint_loop to weston_output_finish_frame
> >> reading the compositor clock. In which case we'd glitch badly in any
> >> case. I can't think of a way, apart from kms driver bug, that the
> >> pageflip event could carry a bad timestamp, and for the pageflip driven
> >> path we don't hit that one-shot adjustment (or potential loop) anyway,
> >> as the flags aren't WP_PRESENTATION_FEEDBACK_INVALID.  
> >
> > Oh, that is true about the pageflip fallback path of
> > start_repaint_loop. Makes me wonder if I should "fix" that, since
> > start_repaint_loop should never result in sending out presentation
> > feedback events, and can't assert on that if the flags are not
> > FEEDBACK_INVALID. Oh well, I've been meaning to get rid of the INVALID
> > flag by introducing a different function to call in that case.
> >  
> 
> I think that can't happen, because we only copy the pending feedback 
> events for a surface into the list for an output in the call sequence:
> 
> weston_output_maybe_repaint -> weston_output_repaint -> 
> weston_output_take_feedback_list
> 
> That in turn only happens if output->repaint_status == 
> REPAINT_SCHEDULED, which only gets set at the end of 
> weston_output_finish_frame().
> 
> Iow. If the repaint loop is restarted in any way, that first 
> weston_output_finish_frame() call from the restart code will call 
> weston_presentation_feedback_present_list() with an empty 
> output->feedback_list - a no-op. Only then we switch to 
> REPAINT_SCHEDULED, sample, take pending feedback events for dispatch in 
> the next call to weston_output_finish_frame().
> 
> So it doesn't matter if we get called by the pageflip fallback during 
> restart with flags other than WP_PRESENTATION_FEEDBACK_INVALID.
> 
> But it confused me initially as well when i reread the code.

Assert()s generally are for things that "can't happen". ;-)


Thanks,
pq
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