[PATCH 1/8] protocol: add presentation extension v4

[Bryce Harrington]

Mostly just some minor grammar improvement suggestions...

On Mon, Sep 15, 2014 at 04:16:40PM -0400, Louis-Francis Ratté-Boulianne wrote:
<snip>
> diff --git a/Makefile.am b/Makefile.am
> diff --git a/protocol/presentation_timing.xml b/protocol/presentation_timing.xml
> new file mode 100644
> index 0000000..8aadff0
> --- /dev/null
> +++ b/protocol/presentation_timing.xml
> @@ -0,0 +1,247 @@
> +<?xml version="1.0" encoding="UTF-8"?>
> +<protocol name="presentation_timing">
> +<!-- wrap:70 -->
> +
> +
> +    <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.

The language here is a bit clunky.  Here's my try:

"In response to this request, a new presentation_feedback object is
created for the given surface's current content submission.  The object
contains a static snapshot of the information.  Each
request against the same submission will create a new
presentation_feedback object, but they will all have the same
information."

Not sure that's any better!  But maybe gives some wording ideas.

Also, it sounds like the client will need to free this object itself?
Either way, it might be worth stating explicitly here.

> +        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

Comma is incorrect here.

"...tells the client which clock domain..."

> +        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 identifer 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

Comma also incorrect here.

"Compositors should prefer a clock that does not jump and..."

> +        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,

No comma needed after Note.  You can either do "Note that clock_id..."
or "Note: clock_id ..."

> +        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 the feedback information
> +      about a wl_surface content update becoming visible to the user.

"A presentation_feedback object returns an indication that a wl_surface
content update has become visible to the user."

Otherwise the sentence sounds a bit like, "An apple tastes like an
apple."  ;-)

> +      One object corresponds to one content update submission
> +      (wl_surface.commit). There are two possible outcomes: the
> +      content update may be presented to the user, in which case the
> +      presentation timestamp is delivered. Otherwise, the content
> +      update is discarded, and the user never had a chance to see it
> +      before it was superseded or the surface was destroyed.

"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 event, it
> +      becomes inert, and should be destroyed by the client.

The second comma is incorrect; the first one is ok but I think it can be
omitted as well.

> +    </description>
> +
> +    <request name="destroy" type="destructor">
> +      <description summary="destroy presentation feedback object">
> +        The object is destroyed. If a feedback event had not been
> +        delivered yet, it is cancelled.

Comma can be omitted.

> +      </description>
> +    </request>
> +
> +    <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">
> +      </description>
> +
> +      <entry name="none" value="0"/>
> +    </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 preceeded 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 to the output containing the largest

"to choose to" sounds wrong...

> +        part of the wl_surface, or keeping the output they previously
> +        chose. Having a stable presentation output association helps
> +        clients to predict future output refreshes (vblank).

"...helps clients predict..."

> +        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>

Bryce