[PATCH weston v2 09/11] Switch to global output repaint timer

Pekka Paalanen ppaalanen at gmail.com
Mon Mar 13 12:33:09 UTC 2017

On Fri, 10 Mar 2017 14:37:14 +0000
Daniel Stone <daniel at fooishbar.org> wrote:

> Hi,
> On 8 March 2017 at 14:05, Pekka Paalanen <pekka.paalanen at collabora.co.uk> wrote:
> > On Wed,  1 Mar 2017 11:34:08 +0000 Daniel Stone <daniels at collabora.com> wrote:  
> >> +     /* XXX: This can keep postponing forever, maybe? */
> >> +     if (msec_to_next < 1)
> >> +             msec_to_next = 1;  
> >
> > I think there is no longer a risk of postponing indefinitely, because
> > output_repaint_timer_arm() is no longer called from
> > weston_output_schedule_repaint(). The other call sites are well
> > throttled.  
> Yeah, as detailed; that was just a placeholder until you'd fully
> reviewed the new code. If you're happy to merge, then do take it out.
> > I'm curious about the reason to have at least 1 ms delay, even if
> > weston_output_finish_frame() deemed that we should paint ASAP. Is it to
> > allow coalescing several outputs' finish_frame needing immediate
> > repaint into single call through output_repaint_timer_handler()?
> >
> > Would be really nice to see some explanation in a comment here. Not
> > that it actually matters much, since we are talking about the time
> > instant to start painting, which is not that crucial to begin with.  
> Well, wl_event_source_timer_update() only takes milliseconds, and a
> value of 0 milliseconds disables the timer entirely (cf. man
> timerfd_settime). So, it's either a minimum 1ms delay, or bypassing
> the timer entirely. Calling repaint directly means we lose the ability
> to coalesce all the output repaints together, which was the entire
> point of this series! So, our two options are to take the 1ms delay as
> a given, or to have two timers: a delay timer armed for >= 1ms, or an
> idle timer for < 1ms. Which would be nice, but seems like the kind of
> thing that wl_event_source's timer should be doing for us, rather than
> having to juggle these through the repaint loop.
> > Although, I suppose if one constructed a 16-head monster, each head
> > adding another 1 ms delay could ruin everything depending on how
> > finish_frame calls get timed.  
> It'd have to be pretty special, in terms of very slightly staggered
> clock offsets.

Ok, I'll remove the XXX comment and add some words about the coalescing
of repaints together being the reason for the 1 ms delay.

Meanwhile we have https://bugs.freedesktop.org/show_bug.cgi?id=100115
I can try to have a look at it this week, I think.

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