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

[Mario Kleiner]

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.
-mario