[PATCH wayland v2] protocol: try to clarify frame callback semantics

Jason Ekstrand jason at jlekstrand.net
Sat Feb 22 05:50:01 PST 2014

On Feb 22, 2014 2:44 AM, "Axel Davy" <axel.davy at ens.fr> wrote:
> Hi,
> I like very much the rewording proposed by Pekka.
> But I dislike your proposition to send frame callbacks right away if the
attached buffer has been attached for a long time.
> Your argument seems to be that the client may manage to get to the next
pageflip if the frame callback is called right away. But with this
argument, I don't see why this behaviour would be only for buffers attached
long ago (and then we refresh at a higher frequency than the screen refresh)
> Moreover we may say we can always get the two behaviours with client side
> . If we keep the current behaviour, the client could know it has attached
a buffer for a long time (and that the frame callback it had put, was
already called), so if it wants to try to hit next pageflip, it could just
commit right away with a new attach

Yes, this is what they should be doing.  The more I think about the frame
callback, the more I think that clients should just put one with every new
draw and just track whether that one has been released or not.

Unfortunately, we have to do something reasonable in the case where the
client requests a frame without drawing.  I don't want to restrict the
server too much on what it does in that case.  It may, for instance, be
running on some sort of refresh-on-demand hardware and have no concept of
"next flip".  When the client asks for a frame callback, it is effectively
asking when is a good time to repaint.  If now is a good time, the server
should be allowed to say so.

Pekka, one thing I forgot to mention that should probably be added is that
we really should guarantee that frame callbacks get fired in the same order
they are requested (per surface order, not global order).

> . With your proposition the client could always attach (and perhaps
+damage) with a frame+commit (even with the old buffer not released), to be
sure to get current behaviour.

What do you mean by "current behavior"? And why would they want it?

> I don't think having to do an attach with the old buffer is a good idea,
and I favor Pekka's proposition.

I wasn't arguing against it. :-)

> Axel Davy
> On 22/02/2014, Jason Ekstrand wrote :
>> Pekka,
>> Sorry this e-mail took so long to send.  Not much time lately.  The
first time or two I read this suggested re-wording I didn't like it, but
now it's starting to grow on me.  I still kind of like the idea of "the
buffer you sent is now in use, go ahead and send the next one" but I don't
know that it's that much better or that it actually changes anything.
>> The big thing I'd like to leave open (and I think your change does) is
the following:  Suppose a client commits a buffer and then, several seconds
later (after the attached buffer was first used), the user does something
that causes the client to refresh.  If it does a frame+commit without an
attach, the server should be able to respond immediately without waiting
for another pageflip.  This way the client may be able to render in time
for the next flip.  Sure, the client might be too slow and miss the flip,
but that's really no worse than waiting before sending the frame callback.
>> Point is, it should be a compositor decision and I think you made that
clear enough.
>> Looks good to me.
>> --Jason Ekstrand
>> Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
>> On Fri, Feb 21, 2014 at 7:46 AM, Pekka Paalanen <ppaalanen at gmail.com>
>>> From: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
>>> "the callback event will arrive after the next output refresh" is wrong,
>>> if you interpret "output refresh" as framebuffer flip or the moment when
>>> the new pixels turn into light the first time. Weston has probably never
>>> worked this way.
>>> Weston triggers the frame callbacks when it submits repainting commands
>>> to the GPU, which is before the framebuffer flip.
>>> Strike the incorrect claim, and the rest of the paragraph which no
>>> longer offers useful information.
>>> As a replacement, expand on the "throttling and driving animations"
>>> characteristic. The main purpose is to let clients animate at the
>>> display refresh rate, while avoiding drawing frames that will never be
>>> presented.
>>> The new claim is that the server should give some time between
>>> triggering frame callbacks and repainting itself, for clients to draw
>>> and commit. This is somewhat intimate with the repaint scheduling
>>> algorithm a compositor uses, but hopefully the right intention.
>>> Another point of this update is to imply, that frame callbacks should
>>> not be used to count compositor repaint cycles nor monitor refresh
>>> cycles. It has never been guaranteed to work. Removing the mention of
>>> frame callback without an attach hopefully discourages such use.
>>> v2: don't just remove a paragraph, but add useful information about the
>>> request's intent.
>>> Signed-off-by: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
>>> Cc: Axel Davy <axel.davy at ens.fr>
>>> Cc: Jason Ekstrand <jason at jlekstrand.net>
>>> ---
>>>  protocol/wayland.xml | 26 ++++++++++++++++++--------
>>>  1 file changed, 18 insertions(+), 8 deletions(-)
>>> diff --git a/protocol/wayland.xml b/protocol/wayland.xml
>>> index e1edbe5..6e370ad 100644
>>> --- a/protocol/wayland.xml
>>> +++ b/protocol/wayland.xml
>>> @@ -1059,22 +1059,32 @@
>>>      </request>
>>>      <request name="frame">
>>> -      <description summary="request repaint feedback">
>>> -       Request notification when the next frame is displayed. Useful
>>> -       for throttling redrawing operations, and driving animations.
>>> +      <description summary="request a frame throttling hint">
>>> +       Request a notification when it is a good time start drawing a
>>> +       frame, by creating a frame callback. This is useful for
>>> +       redrawing operations, and driving animations.
>>> +
>>> +       When a client is animating on a wl_surface, it can use the
>>> +       request to get notified when it is a good time to draw and
commit the
>>> +       next frame of animation. If the client commits an update
earlier than
>>> +       that, it is likely that some updates will not make it to the
>>> +       and the client is wasting resources by drawing too often.
>>> +
>>>         The frame request will take effect on the next
>>>         The notification will only be posted for one frame unless
>>>         requested again.
>>> +       The server must send the notifications so that a client
>>> +       will not send excessive updates, while still allowing
>>> +       the highest possible update rate for clients that wait for the
>>> +       before drawing again. The server should give some time for the
>>> +       to draw and commit after sending the frame callback events to
let them
>>> +       hit the next output refresh.
>>> +
>>>         A server should avoid signalling the frame callbacks if the
>>>         surface is not visible in any way, e.g. the surface is
>>>         or completely obscured by other opaque surfaces.
>>> -       A client can request a frame callback even without an attach,
>>> -       damage, or any other state changes. wl_surface.commit triggers a
>>> -       display update, so the callback event will arrive after the next
>>> -       output refresh where the surface is visible.
>>> -
>>>         The object returned by this request will be destroyed by the
>>>         compositor after the callback is fired and as such the client
must not
>>>         attempt to use it after that point.
>>> --
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