[RFC v.2] Extend wl_surface protocol

Pekka Paalanen ppaalanen at gmail.com
Tue Nov 12 07:25:46 PST 2013

On Mon, 11 Nov 2013 21:42:01 +0000
Daniel Stone <daniel at fooishbar.org> wrote:

> Hi,
> On 11 November 2013 15:41, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> >>     <request name="destroy" type="destructor">
> >>       <description summary="remove buffer_queue interface">
> >>         The buffer_queue interface is removed from the buffer_queue-enabled
> >>         surface.
> >
> > This could also mention, that the queue is emptied first, and release
> > and presentation feedback events are emitted as usual. Can describe it
> > as an implicit "clear" request.
> Hm, I'd prefer an explicit clear/flush request, than suggesting people
> repeatedly destroy and recreate queue objects.

There already is one. I just thought it would be good to fully describe
what destroy does. Destroy works as if you sent a "clear" request first.

> >>       Note that presentation time only tells client when the compositor presen-
> >>       ted the buffer to display hardware, not when the buffer was turned into
> >>       light (actually displayed on screen) by this hardware. As there could
> >>       be anything displaying those buffers, from very fast, low-latency
> >>       computer monitors to slow, hi-latency HDMI TV screens, it is the client's
> >>       responsibility to make sure it knows what display hardware is currently
> >>       connected and what is its latency.
> >
> > Do people agree with this definition?
> >
> > I assume it means when the gfx card starts to emit the pixels to the
> > physical wire.
> >
> > Or should we allow the compositor to factor in the possible information
> > from the monitor about its latency?
> >
> > I'm kind of thinking that we should. If a monitor does not tell its
> > latency, and the user can see it, the compositor could allow the user
> > to configure an assumed latency. Perhaps even measured by the user.
> >
> > Or is that something that should be (or even already is?) configured in
> > the drivers, so they already give out the "turns into light" time?
> >
> > Also, the presentation timestamp can only be given wrt. one output. If
> > the surface spans several outputs, the compositor chooses one to sync
> > to.
> Actually, I think the definition there is good, if perhaps a little
> unclear. The compositor should make its best effort to determine
> exactly when the buffer hits the screen, but that's it. If we specify
> that the time is kernel submission rather than display, then that
> sucks if we know when it was actually displayed. OTOH, if we specify
> it's exactly when it's displayed, systems which can't work that out
> will technically be non-conformant.

My question was, what does "display" mean? Is it when the signal exits
the computer, or when the TV turns it into light? Which way should it be

> For cases like HDMI, the CEA chunk of the EDID does actually allow it
> to specify an optional latency, so that could be good to use. (And
> hope to hell whatever's doing your HDMI audio takes note of that too.)
> Either way, if the user's observing additional latency, they'll have
> to tweak things, but I'd rather that offset be applied in the media
> player (all of which already have tweaks for this) than attempting to
> shove it in the protocol. (Having it global would ruin things for
> everyone, so it'd have to be per-object, in which case, eh, just do it
> yourself ...)

I never meant that number to go into the protocol, or to clients.

> Of course, there's nothing precluding the compositor itself from being
> able to configure a constant latency, for people shipping devices with
> a known predictable offset.

That's what I meant. The compositor can add a pre-configured value to
all page flip timestamps coming from the kernel.

And the pre-configured value could be given by the user (a person, not
a software component) via some fancy display configuration GUI, if he
has some means to determine it.

But all this is possible only, if we define the timestamp to correspond
to the time when the pixels turn into light, not when they enter the
HDMI cable. So that would need changing in the proposal wording.


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