Core protocol change; [RFC v2] Wayland presentation extension

Pekka Paalanen ppaalanen at
Mon Feb 17 00:34:45 PST 2014


there is one important thing in the below spec I really need to
highlight! See further below.

On Thu, 30 Jan 2014 17:35:17 +0200
Pekka Paalanen <ppaalanen at> wrote:

> Hi,
> it's time for a take two on the Wayland presentation extension.
> 		1. Introduction
> The v1 proposal is here:
> In v2 the basic idea is the same: you can queue frames with a
> target presentation time, and you can get accurate presentation
> feedback. All the details are new, though. The re-design started
> from the wish to handle resizing better, preferably without
> clearing the buffer queue.
> All the changed details are probably too much to describe here,
> so it is maybe better to look at this as a new proposal. It
> still does build on Frederic's work, and everyone who commented
> on it. Special thanks to Axel Davy for his counter-proposal and
> fighting with me on IRC. :-)
> Some highlights:
> - Accurate presentation feedback is possible also without
>   queueing.
> - You can queue also EGL-based rendering, and get presentation
>   feedback if you want. Also EGL can do this internally, too, as
>   long as EGL and the app do not try to use queueing at the same time.
> - More detailed presentation feedback to better allow predicting
>   future display refreshes.
> - If wl_viewport is used, neither video resolution changes nor
>   surface (window) size changes alone require clearing the queue.
>   Video can continue playing even during resizes.
> The protocol interfaces are arranged as
> 	global.method(wl_surface, ...)
> just for brewity. We could as well do the factory approach:
> 	o = global.get_presentation(wl_surface)
> 	o.method(...)
> Or if we wanted to make it a mandatory part of the Wayland core
> protocol, we could just extend wl_surface itself:
> 	wl_surface.method(...)
> and put the clock_id event in wl_compositor. That all is still
> open and fairly uninteresting, so let's concentrate on the other
> details.
> The proposal refers to wl_viewport.set_source and
> wl_viewport.destination requests, which do not yet exist in the
> scaler protocol extension. These are just the wl_viewport.set
> arguments split into separate src and dst requests.
> Here is the new proposal, some design rationale follows. Please,
> do ask why something is designed like it is if it puzzles you. I
> have a load of notes I couldn't clean up for this email. This
> does not even intend to completely solve all XWayland needs, but
> for everything native on Wayland I hope it is sufficient.
> 		2. The protocol specification
> <?xml version="1.0" encoding="UTF-8"?>
> <protocol name="presentation_timing">
>   <copyright>
>     Copyright © 2013-2014 Collabora, Ltd.
>     Permission to use, copy, modify, distribute, and sell this
>     software and its documentation for any purpose is hereby granted
>     without fee, provided that the above copyright notice appear in
>     all copies and that both that copyright notice and this permission
>     notice appear in supporting documentation, and that the name of
>     the copyright holders not be used in advertising or publicity
>     pertaining to distribution of the software without specific,
>     written prior permission.  The copyright holders make no
>     representations about the suitability of this software for any
>     purpose.  It is provided "as is" without express or implied
>     warranty.
>   </copyright>
>   <interface name="presentation" version="1">
>     <description summary="timed presentation related wl_surface requests">
>       The main features of this interface are accurate presentation
>       timing feedback, and queued wl_surface content updates to ensure
>       smooth video playback while maintaining audio/video
>       synchronization. Some features use the concept of a presentation
>       clock, which is defined in presentation.clock_id event.
>       Requests 'feedback' and 'queue' can be regarded as additional
>       wl_surface methods. They are part of the double-buffered
>       surface state update mechanism, where other requests first set
>       up the state and then wl_surface.commit atomically applies the
>       state into use. In other words, wl_surface.commit submits a
>       content update.
>       Interface wl_surface has requests to set surface related state
>       and buffer related state, because there is no separate interface
>       for buffer state alone. Queueing requires separating the surface
>       from buffer state, and buffer state can be queued while surface
>       state cannot.
>       Buffer state includes the wl_buffer from wl_surface.attach, the
>       state assigned by wl_surface requests frame,
>       set_buffer_transform and set_buffer_scale, and any
>       buffer-related state from extensions, for instance
>       wl_viewport.set_source. This state is inherent to the buffer
>       and the content update, rather than the surface.
>       Surface state includes all other state associated with
>       wl_surfaces, like the x,y arguments of wl_surface.attach, input
>       and opaque regions, damage, and extension state like
>       wl_viewport.destination. In general, anything expressed in
>       surface local coordinates is better as surface state.
>       The standard way of posting new content to a surface using the
>       wl_surface requests damage, attach, and commit is called
>       immediate content submission. This happens when a
>       presentation.queue request has not been sent since the last
>       wl_surface.commit.
>       The new way of posting a content update is a queued content
>       update submission. This happens on a wl_surface.commit when a
>       presentation.queue request has been sent since the last
>       wl_surface.commit.
>       Queued content updates do not get applied immediately in the
>       compositor but are pushed to a queue on receiving the
>       wl_surface.commit. The queue is ordered by the submission target
>       timestamp. Each item in the queue contains the wl_buffer, the
>       target timestamp, and all the buffer state as defined above. All
>       the queued state is taken from the pending wl_surface state at
>       the time of the commit, exactly like an immediate commit would
>       have taken it.
>       For instance on a queueing commit, the pending buffer is queued
>       and no buffer is pending afterwards. The stored values of the
>       x,y parameters of wl_surface.attach are reset to zero, but they
>       also are not queued; queued content updates do not carry the
>       attach offsets. All other surface state (that is not queued),
>       e.g. damage, is not applied nor reset.
>       Issuing a queueing commit without a wl_surface.attach is
>       undefined. However, queueing a commit with explicitly attached
>       NULL wl_buffer works; when and if the content update is
>       executed, the surface content is removed as defined for
>       wl_surface.attach.
>       If a queued content update has been submitted, and the wl_buffer
>       used in the update is destroyed before the wl_buffer.release
>       event, the results are undefined. The compositor may or may not
>       have executed the update, therefore the surface contents become
>       undefined as explained in wl_surface.attach. Whether any
>       presentation feedback or frame callbacks occur is undefined.
>       For each surface, the compositor maintains an association to a
>       single output that is considered as the main output for the
>       surface. Queued content updates are synchronized to the
>       surface's main output, to provide a consistent and meaningful
>       definition of the moment the update is displayed to the user.
>       When a compositor updates an output, it processes only the
>       queues of the surfaces whose main output is the one being
>       updated. The queues of other surfaces, even if they are part of
>       the redrawing, are not processed at that time.
>       When a compositor chooses to update an output, it must predict
>       the presentation clock value when the display update will occur.
>       For the definition of the moment of display update, see
>       presentation_feedback.presented. Therefore if the prediction is
>       absolutely perfect, presentation_feedback.presented will carry
>       the same clock value.
>       For each surface with queued content updates and matching main
>       output, the compositor picks the update with the highest
>       timestamp no later than a half frame period after the predicted
>       presentation time. The intent is to pick the content update
>       whose target timestamp as rounded to the output refresh period
>       granularity matches the same display update as the compositor is
>       targeting, while not displaying any content update more than a
>       half frame period too early. If all the updates in the queue are
>       already late, the highest timestamp update is taken regardless
>       of how late it is. Once an update in a queue has been chosen,
>       all remaining updates with an earlier timestamp in the queue are
>       discarded.
>       The compositor applies the chosen update to the wl_surface,
>       regardless of possible wl_subsurface.set_sync mode. This allows
>       e.g. a video to continue running in a sub-surface also during
>       window resizing. It is assumed that buffer state updates do not
>       cause visual disruption to the window like surface state updates
>       can. Support for wl_viewport is needed for glitch-free resizing
>       if the resizing involves changing the (sub-)surface size.
>       When the chosen update is applied, the associated frame
>       callbacks are sent. Damage for the whole surface is assumed,
>       as damage is not explicitly queued with buffer state.
>       When the final realized presentation time is available, e.g.
>       after a framebuffer flip completes, the requested
>       presentation_feedback.presented events are sent. The final
>       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.
>       When updates from the queue are discarded, the
>       presentation_feedback.discarded event is delivered if feedback
>       was requested. Also the associated frame callbacks are sent.
>       An immediate content update with an attach request automatically
>       discards the whole queue just before the update gets applied. If
>       wl_surface.attach has not been sent for an immediate content
>       submission, the queue is not discarded, and the content update
>       applies only the surface state, but no buffer state.

If you read the above paragraph carefully, you see that the last

The change is very subtle. It means, that without a wl_surface.attach,
the buffer state is no longer applied on commit at all! To recap, the
buffer state is:
- frame callbacks (!)
- set_buffer_transform
- set_buffer_scale
- the src_* arguments of wl_viewport.set

The reason is explained in my recent email:

An immediate commit without an attach should not apply any buffer
state, because previous queueing of frames may have left buffer state
that is incorrect for the currently showing buffer. Immediate commits
without attach are used to update surface (and shell!) state, and
applying incorrect buffer state could cause a visual glitch.

We could claim, that this change in the core protocol exists only if
the presentation extension is advertised by the server, but that would
cause a lot more work to fix clients that get bit by this change, rather
than fix the clients to always attach a wl_buffer when they want to
change buffer state, even if it is the same buffer they just attached
and committed already.

Therefore I would like to bring the concepts of surface state and
buffer state to the core protocol, and have the core procotol define
that buffer state is applied only if there is an attach.

In the past, we already changed the wl_surface.attach semantics to not
re-attach the "current" buffer again, when there is a wl_surface.commit.
The practical consequence of that was that a commit without an attach
cannot cause any wl_buffer on this surface to become reserved and
readable by the server, and hence no (new) wl.buffer.release would be
posted either.

That means that clients already need to re-attach the same wl_buffer
again, if they changed the buffer contents and want to show the new
image. I think this should mitigate the impact of the core protocol

I guess the only interesting case is the frame callback, and whether
anyone (ab)uses it without an attach.

Would this proposition be acceptable?


>       If a wl_surface has queued content updates when it is destroyed,
>       the whole queue is implicitly discarded as if
>       presentation.discard_queue was sent immediately prior to the
>       destroy request.
>     </description>
>     <request name="destroy" type="destructor">
>       <description summary="unbind from the presentation interface">
>         Informs the server that the client will not be using this
>         protocol object anymore. This does not affect any content
>         update queues nor existing objects created by this interface.
>       </description>
>     </request>
>     <request name="feedback">
>       <description summary="request presentation feedback information">
>         With this request, presentation feedback will be provided for
>         the current content submission of the given surface. A new
>         presentation_feedback object is created, and that object will
>         deliver the information once. The object is tied to this
>         content submission only. Multiple presentation_feedback objects
>         may be created for the same submission, and they will all
>         return the same information.
>         For details on what information is returned, see
>         presentation_feedback interface.
>       </description>
>       <arg name="surface" type="object" interface="wl_surface"
>            summary="target surface"/>
>       <arg name="callback" type="new_id" interface="presentation_feedback"
>            summary="new feedback object"/>
>     </request>
>     <request name="queue">
>       <description summary="queue the buffer instead of immediate presentation">
>         This request changes the behaviour of the very next
>         wl_surface.commit of the given wl_surface and that commit
>         only. Instead of immediately applying the pending wl_surface
>         state as defined in wl_surface.commit, the commit will queue a
>         new content update, using the pending buffer state only.
>         For a more detailed description and what is buffer state, see
>         the documentation for presentation interface.
>         The value of the target timestamp is in the presentation clock
>         domain, see presentation.clock_id.
>         If queue request has already been sent for the unfinished content
>         update submission on the given wl_surface, a new queue request
>         will override the previous one.
>       </description>
>       <arg name="surface" type="object" interface="wl_surface"
>            summary="target surface"/>
>       <arg name="tv_sec" type="uint"
>            summary="seconds part of the target timestamp"/>
>       <arg name="tv_nsec" type="uint"
>            summary="nanoseconds part of the target timestamp"/>
>     </request>

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