[RFC wayland-protocols] presentation-time: Add request to subscribe to wl_output presentation timings

Pekka Paalanen ppaalanen at gmail.com
Thu Aug 17 08:10:12 UTC 2017

On Mon,  7 Aug 2017 19:00:40 +0300
Alexandros Frantzis <alexandros.frantzis at collabora.com> wrote:

> The presentation.timing request creates an object that informs the
> client about the presentation timing of an wl_output in a precise and
> low-overhead way, without the need to submit any surface content updates.

Hi Alf,

the last time I looked at a similar feature was in 2014, the
refresh_stream interface mentioned in

However, I like your / the Chromium design much better, because it doesn't
spam events on every refresh.

This is a good proposal.

> This functionality can be useful, among others, for clients that want to know
> the output timings before submitting any frames, and also for clients that
> want to synchronize with server presentation timings using a simpler and lower
> overhead way compared to the presentation.feedback objects.

Yes, I suppose this is justification enough, and particularly as CrOS
is already using it.

> This functionality is inspired by the vsync-feedback protocol used in
> Chromium OS ([1]), and, hopefully, with such an addition, Chromium OS may
> be able to transition to the upstream protocol.
> It may be useful to add a flags argument to the presentation_timing.updated
> event, containing information that can be used to assess the reliability (and
> other aspects) of the provided information, in a manner similar to the 'kind'
> flags for the presentation_feedback.presented event.

I think adding flags would be nice indeed, even if we cannot specify
any at the moment. Flags can be added in later revisions but only if
the argument already exists, otherwise we would need to add a new event.

> A proof-of-concept implementation for Weston can be found at:
> https://git.collabora.com/cgit/user/alf/weston.git/log/?h=presentation-time-v2
> [1] https://chromium.googlesource.com/chromium/src/third_party/+/master/wayland-protocols/unstable/vsync-feedback/vsync-feedback-unstable-v1.xml
> ---
>  stable/presentation-time/presentation-time.xml | 68 ++++++++++++++++++++++++--
>  1 file changed, 65 insertions(+), 3 deletions(-)

This is written as an evolution of wp_presentation global interface
which has already been declared stable. There is the question whether
that is fine or should the new feature be an independent extension.

To me this question is more about the ability to extend the
interface(s) later and a feature being optional or mandatory. If the
request is added in wp_presentation interface, then we intend it to be
an integral part of wp_presentation, and that if a compositor wants to
expose wp_presentation, then it will also want to expose the new
feature. The rationale is that if a second new feature is added to
wp_presentation, it is not possible to support it without also
implementing the first new feature.

IOW, the question is, would anyone realistically need the option to not
implement wp_presentation_timing while they do implement wp_presentation?

My initial feeling is "no", which would favour adding this in

Also, adding to wp_presentation makes it easy to define things with the
presentation clock provided by wp_presentation.

All in all, good.

> diff --git a/stable/presentation-time/presentation-time.xml b/stable/presentation-time/presentation-time.xml
> index a46994c..56a9c9b 100644
> --- a/stable/presentation-time/presentation-time.xml
> +++ b/stable/presentation-time/presentation-time.xml
> @@ -3,7 +3,7 @@
>  <!-- wrap:70 -->
>    <copyright>
> -    Copyright © 2013-2014 Collabora, Ltd.
> +    Copyright © 2013-2017 Collabora, Ltd.

Just checking, the Chromium extension's copyrights don't need to be
added here, do they?

>      Permission is hereby granted, free of charge, to any person obtaining a
>      copy of this software and associated documentation files (the "Software"),
> @@ -25,8 +25,8 @@
>    </copyright>
> -  <interface name="wp_presentation" version="1">
> -    <description summary="timed presentation related wl_surface requests">
> +  <interface name="wp_presentation" version="2">
> +    <description summary="timed presentation related requests">
>  <!-- Introduction -->
> @@ -49,6 +49,16 @@
>        presentation time can differ from the compositor's predicted
>        display update time and the update's target time, especially
>        when the compositor misses its target vertical blanking period.
> +
> +<!-- Presentation timing -->
> +
> +      In some cases it's also useful for clients to know about the
> +      presentation timing of an output without having submitted a
> +      surface content update. For this purpose the 'timing' request
> +      creates an object that is used to inform the client about
> +      the presentation timing of an wl_output in a precise and
> +      low-overhead way.
> +
>      </description>
>      <enum name="error">
> @@ -122,6 +132,22 @@
>        <arg name="clk_id" type="uint" summary="platform clock identifier"/>
>      </event>
> +    <!-- Version 2 additions -->
> +
> +    <request name="timing" since="2">
> +      <description summary="request timing information">
> +        Create a new timing object that represents a subscription to
> +        presentation timing updates on the given wl_output object.
> +
> +        The newly created object will signal an update to notify the subscriber
> +        of initial timing parameters as soon as these become available.

If the output was already or becomes destroyed in the compositor, there
will be no event, right? More below.

> +      </description>
> +      <arg name="output" type="object" interface="wl_output"
> +           summary="the wl_output object to subscribe for timings of"/>
> +      <arg name="id" type="new_id" interface="wp_presentation_timing"
> +           summary="the new timing object"/>
> +    </request>
> +
>    </interface>
>    <interface name="wp_presentation_feedback" version="1">
> @@ -263,4 +289,40 @@
>      </event>
>    </interface>
> +  <interface name="wp_presentation_timing" version="1">

An interface can have an overhead doc as well. It might be a nice place
to document how a wp_presentation_timing object reacts to:

a) the output being destroyed in the compositor

b) the wl_output object used in wp_presentation.timing request being
   destroyed by the client

Another thing is, how will a client know which output's timings it
should be looking at for a specific wl_surface. The wl_surface enter
and leave events do not tell that when a surface is on more than one
output at the same time, the client won't know which one the compositor
is using for the timings. The client needs to use
wp_presentation_feedback to know which output it is at any time.

This brings a different idea to my mind: what if the timings were asked
for a wl_surface instead of a wl_output? The issue with that is
surfaces that are not currently shown, like ones that are not mapped
yet, are off-screen, etc. OTOH, maybe the issue is not significant: in
a multi-output setup, you cannot always know which output a surface
will be synced to anyway until it actually becomes visible.

Which design would better serve the use cases?

> +    <request name="destroy" type="destructor">
> +      <description summary="destroy timing object">
> +        Destroy this timing object.
> +      </description>
> +    </request>
> +
> +    <event name="updated">
> +      <description summary="the timing has been updated">
> +        Notifies the client that the timing of the associated wl_output
> +        has changed.

Maybe there should be some wording that the timing does not necessarily
need to change for the compositor to send this event. A compositor
could send this event at arbitrary times to keep the predictions from
drifting too far in clients. E.g. I could imagine Weston sending this
event again when the prediction error grows beyond a certain threshold
to reset the timebase.

> +
> +        Timing information consists of two data, timebase and interval.
> +        Timebase is an absolute timestamp of the event that caused the
> +        timing to change. Interval is a period of time between subsequent
> +        presentation events.

"Event that caused the timing to change" is not unambiguously a vblank,
timings might have changed slightly before the first vblank produced
with the new timings. Should this be written so that the timebase is
always the same kind of timestamp as wp_presentation_feedback.presented

> +
> +        For the interpretation of the timestamp, see presentation.clock_id
> +        event.
> +
> +        If the server cannot accurately predict the interval between
> +        subsequent presentation events for the output, or the output does
> +        not have a constant presentation rate, then the interval argument
> +        must be zero.

Yes. Should we note that such occasion could also be temporary, or is
it obvious?

> +      </description>
> +      <arg name="timebase_tv_sec_hi" type="uint"
> +           summary="high 32 bits of the seconds part of the new presentation timebase"/>
> +      <arg name="timebase_tv_sec_low" type="uint"
> +           summary="low 32 bits of the seconds part of the new presentation timebase"/>
> +      <arg name="timebase_tv_nsec" type="uint"
> +           summary="nanoseconds part of the new presentation timebase"/>
> +      <arg name="interval" type="uint"
> +           summary="new presentation interval in nanoseconds"/>

I wonder if people would find use for MSC counter in this event? Roman
Gilg for Xwayland/Present perhaps?

> +    </event>
> +  </interface>
> +
>  </protocol>

Ideas to think about in the future, probably not worth including right

This extension gives out the vblank timings, but is that enough? Should
we have provision for compositor deadlines as well, or are those too
implementation-specific to be exposed in a generic extension?

By deadline I mean the point at which the compositor starts a repaint
in the output repaint cycle, which is an arbitrary time point between
the previous and next vblank. E.g. Weston has a crude configurable
delay for starting a new repaint after a vblank, and clients that miss
the deadline get postponed vblank+2 instead of hitting vblank+1.

Details of Weston's current behaviour are here:
But that is now slightly outdated since we now have a single repaint
timer instead of per-output timers, and it causes some rounding
(jitter?) on the deadlines.

I have no idea what other compositors do, or if the concept of a
deadline like this is universal. If it's not universal, then nevermind.

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