weston/protocol/presentation_timing.xml

<?xml version="1.0" encoding="UTF-8"?>
<protocol name="presentation_timing">
<!-- wrap:70 -->

  <copyright>
    Copyright © 2013-2014 Collabora, Ltd.

    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
    to deal in the Software without restriction, including without limitation
    the rights to use, copy, modify, merge, publish, distribute, sublicense,
    and/or sell copies of the Software, and to permit persons to whom the
    Software is furnished to do so, subject to the following conditions:

    The above copyright notice and this permission notice (including the next
    paragraph) shall be included in all copies or substantial portions of the
    Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
    DEALINGS IN THE SOFTWARE.
  </copyright>

  <interface name="presentation" version="1">
    <description summary="timed presentation related wl_surface requests">

<!-- Introduction -->

      The main feature of this interface is accurate presentation
      timing feedback 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.

      Request 'feedback' can be regarded as an additional wl_surface
      method. It is 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.

<!-- Completing presentation -->

      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.
    </description>

    <enum name="error">
      <description summary="fatal presentation errors">
        These fatal protocol errors may be emitted in response to
        illegal presentation requests.
      </description>
      <entry name="invalid_timestamp" value="0"
             summary="invalid value in tv_nsec"/>
      <entry name="invalid_flag" value="1"
             summary="invalid flag"/>
    </enum>

    <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 existing
        objects created by this interface.
      </description>
    </request>

    <request name="feedback">
      <description summary="request presentation feedback information">
        Request presentation feedback for the current content submission
        on the given surface. This creates a new presentation_feedback
        object, which will deliver the feedback information once. If
        multiple presentation_feedback objects are created for the same
        submission, they will all deliver 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>

    <event name="clock_id">
      <description summary="clock ID for timestamps">
        This event tells the client in which clock domain the
        compositor interprets the timestamps used by the presentation
        extension. This clock is called the presentation clock.

        The compositor sends this event when the client binds to the
        presentation interface. The presentation clock does not change
        during the lifetime of the client connection.

        The clock identifier is platform dependent. Clients must be
        able to query the current clock value directly, not by asking
        the compositor.

        On Linux/glibc, the identifier value is one of the clockid_t
        values accepted by clock_gettime(). clock_gettime() is defined
        by POSIX.1-2001.

        Compositors should prefer a clock which does not jump and is
        not slewed e.g. by NTP. The absolute value of the clock is
        irrelevant. Precision of one millisecond or better is
        recommended.

        Timestamps in this clock domain are expressed as tv_sec_hi,
        tv_sec_lo, tv_nsec triples, each component being an unsigned
        32-bit value. Whole seconds are in tv_sec which is a 64-bit
        value combined from tv_sec_hi and tv_sec_lo, and the
        additional fractional part in tv_nsec as nanoseconds. Hence,
        for valid timestamps tv_nsec must be in [0, 999999999].

        Note that clock_id applies only to the presentation clock,
        and implies nothing about e.g. the timestamps used in the
        Wayland core protocol input events.
      </description>

      <arg name="clk_id" type="uint" summary="platform clock identifier"/>
    </event>

  </interface>

  <interface name="presentation_feedback" version="1">
    <description summary="presentation time feedback event">
      A presentation_feedback object returns an indication that a
      wl_surface content update has become visible to the user.
      One object corresponds to one content update submission
      (wl_surface.commit). There are two possible outcomes: the
      content update is presented to the user, and a presentation
      timestamp delivered; or, the user did not see the content
      update because it was superseded or its surface destroyed,
      and the content update is discarded.

      Once a presentation_feedback object has delivered an 'presented'
      or 'discarded' event it is automatically destroyed.
    </description>

    <event name="sync_output">
      <description summary="presentation synchronized to this output">
        As presentation can be synchronized to only one output at a
        time, this event tells which output it was. This event is only
        sent prior to the presented event.

        As clients may bind to the same global wl_output multiple
        times, this event is sent for each bound instance that matches
        the synchronized output. If a client has not bound to the
        right wl_output global at all, this event is not sent.
      </description>

      <arg name="output" type="object" interface="wl_output"
           summary="presentation output"/>
    </event>

    <enum name="kind">
      <description summary="bitmask of flags in presented event">
        These flags provide information about how the presentation of
        the related content update was done. The intent is to help
        clients assess the reliability of the feedback and the visual
        quality with respect to possible tearing and timings. The
        flags are:

        VSYNC:
        The presentation was synchronized to the "vertical retrace" by
        the display hardware such that tearing does not happen.
        Relying on user space scheduling is not acceptable for this
        flag. If presentation is done by a copy to the active
        frontbuffer, then it must guarantee that tearing cannot
        happen.

        HW_CLOCK:
        The display hardware provided measurements that the hardware
        driver converted into a presentation timestamp. Sampling a
        clock in user space is not acceptable for this flag.

        HW_COMPLETION:
        The display hardware signalled that it started using the new
        image content. The opposite of this is e.g. a timer being used
        to guess when the display hardware has switched to the new
        image content.

        ZERO_COPY:
        The presentation of this update was done zero-copy. This means
        the buffer from the client was given to display hardware as
        is, without copying it. Compositing with OpenGL counts as
        copying, even if textured directly from the client buffer.
        Possible zero-copy cases include direct scanout of a
        fullscreen surface and a surface on a hardware overlay.
      </description>

      <entry name="vsync" value="0x1" summary="presentation was vsync'd"/>
      <entry name="hw_clock" value="0x2"
             summary="hardware provided the presentation timestamp"/>
      <entry name="hw_completion" value="0x4"
             summary="hardware signalled the start of the presentation"/>
      <entry name="zero_copy" value="0x8"
             summary="presentation was done zero-copy"/>
    </enum>

    <event name="presented">
      <description summary="the content update was displayed">
        The associated content update was displayed to the user at the
        indicated time (tv_sec_hi/lo, tv_nsec). For the interpretation of
        the timestamp, see presentation.clock_id event.

        The timestamp corresponds to the time when the content update
        turned into light the first time on the surface's main output.
        Compositors may approximate this from the framebuffer flip
        completion events from the system, and the latency of the
        physical display path if known.

        This event is preceded by all related sync_output events
        telling which output's refresh cycle the feedback corresponds
        to, i.e. the main output for the surface. Compositors are
        recommended to choose the output containing the largest part
        of the wl_surface, or keeping the output they previously
        chose. Having a stable presentation output association helps
        clients predict future output refreshes (vblank).

        Argument 'refresh' gives the compositor's prediction of how
        many nanoseconds after tv_sec, tv_nsec the very next output
        refresh may occur. This is to further aid clients in
        predicting future refreshes, i.e., estimating the timestamps
        targeting the next few vblanks. If such prediction cannot
        usefully be done, the argument is zero.

        The 64-bit value combined from seq_hi and seq_lo is the value
        of the output's vertical retrace counter when the content
        update was first scanned out to the display. This value must
        be compatible with the definition of MSC in
        GLX_OML_sync_control specification. Note, that if the display
        path has a non-zero latency, the time instant specified by
        this counter may differ from the timestamp's.

        If the output does not have a constant refresh rate, explicit
        video mode switches excluded, then the refresh argument must
        be zero.

        If the output does not have a concept of vertical retrace or a
        refresh cycle, or the output device is self-refreshing without
        a way to query the refresh count, then the arguments seq_hi
        and seq_lo must be zero.
      </description>

      <arg name="tv_sec_hi" type="uint"
           summary="high 32 bits of the seconds part of the presentation timestamp"/>
      <arg name="tv_sec_lo" type="uint"
           summary="low 32 bits of the seconds part of the presentation timestamp"/>
      <arg name="tv_nsec" type="uint"
           summary="nanoseconds part of the presentation timestamp"/>
      <arg name="refresh" type="uint" summary="nanoseconds till next refresh"/>
      <arg name="seq_hi" type="uint"
           summary="high 32 bits of refresh counter"/>
      <arg name="seq_lo" type="uint"
           summary="low 32 bits of refresh counter"/>
      <arg name="flags" type="uint" summary="combination of 'kind' values"/>
    </event>

    <event name="discarded">
      <description summary="the content update was not displayed">
        The content update was never displayed to the user.
      </description>
    </event>
  </interface>

</protocol>