[RFC v4] Fullscreen shell protocol

Jason Ekstrand jason at jlekstrand.net
Fri Feb 14 09:11:33 PST 2014

Hi Pekka!  Thanks for the review.  Comments follow.

On Fri, Feb 14, 2014 at 1:14 AM, Pekka Paalanen <ppaalanen at gmail.com> wrote:

> Hi Jason
> On Thu, 13 Feb 2014 22:37:53 -0600
> Jason Ekstrand <jason at jlekstrand.net> wrote:
> > The following is yet another take on the fullscreen shell protocol.
> > Previous versions more-or-less followed the approach taken in wl_shell.
> > This version completely reworks the concept.  In particular, the protocol
> > is split into two use-cases.  The first is that of a simple client that
> > wants to present a surface or set of surfaces possibly with some scaling.
> > This happens through the present_surface request which looks similar to
> > that of wl_shell only without the modesetting.
> >
> > The second use-case is of a client that wants more control over the
> > outputs.  In this case, the client uses the present_surface_for_mode
> > request to present the surface at a particular output mode.  This request
> > provides a more-or-less atomic modeset operation.  If the compositor can
> > satisfy the requested mode, then the mode is changed and the new surface
> is
> > presented.  Otherwise, the compositor harmlessly falls back to the
> > previously presented surface and the client is informed that the switch
> > failed.  This way, the surface is either displayed correctly or not at
> all.
> > Of course, a client is free to call present_surface_for_mode with the
> > currently presented surface and hope for the best.  However, this may
> > result in strange behavior and there is no reliable fallback if the mode
> > switch fails.
> >
> > In particular, I would like feedback on the modesetting portion of this
> > protocol.  This is particularly targetted at compositors that want to run
> > inside weston or some other fullscreen compositor.  In the next week or
> so,
> > I will attempt to implement all this in weston and see how well it works.
> > However, I would also like to know how well this will work for other
> > compositors such as KWin or Hawaii.
> >
> > Thanks for your feedback,
> > --Jason Ekstrand
> >
> > ===== Protocol follows: =====
> >
> > <protocol name="fullscreen_shell">
> >   <interface name="wl_fullscreen_shell" version="1">
> This interface should have a destructor request IMO. It's not stricly
> required, but I think it would be consistent (I think all global
> interfaces need an explicit destructor request) and more future-proof.

Thanks for reminding me.  I'll get one added.

> >     <description summary="Displays a single surface per output">
> >       Displays a single surface per output.
> >
> >       This interface provides a mechanism for a single client to display
> >       simple full-screen surfaces.  While there technically may be
> multiple
> >       clients bound to this interface, only one of those clients should
> be
> >       shown at a time.
> >
> >       To present a surface, the client uses either the present_surface or
> >       present_surface_for_mode requests.  Presenting a surface takes
> effect
> >       on the next wl_surface.commit.  See the individual requests for
> >       details about scaling and mode switches.
> >
> >       The client can have at most one surface per output at any time.
> >       Requesting a surface be presented on an output that already has a
> >       surface replaces the previously presented surface.  Presenting a
> null
> >       surface removes its content and effectively disables the output.
> >       Exactly what happens when an output is "disabled" is
> >       compositor-specific.  The same surface may be presented multiple
> >       outputs simultaneously.
> If the same surface is presented on multiple outputs, should the client
> have a way to say which output is to be considered the surface's main
> output, where e.g. presentation feedback is synced to?

That's a good question.  Simple clients probably don't care.  More complex
clients such as compositors probably will.  However, I'd expect them to
have one surface per output most of the time anyway.  I'll give that some

> Maybe also note explicitly, that once a surface has been presented on
> an output, it stays on that output until explicitly removed, or output
> is unplugged? So that simple attach+damage+commit can be used to update
> the content, if that is the intention.

Yes, that's a good point.  And I do intend to provide that guarantee.

> >     </description>
> >
> >     <enum name="present_method">
> >       <description summary="different method to set the surface
> fullscreen">
> >       Hints to indicate to the compositor how to deal with a conflict
> >       between the dimensions of the surface and the dimensions of the
> >       output. The compositor is free to ignore this parameter.
> >       </description>
> >       <entry name="default" value="0" summary="no preference, apply
> default policy"/>
> >       <entry name="center" value="1" summary="center the surface on the
> output"/>
> >       <entry name="zoom" value="2" summary="scale the surface,
> preserving aspect ratio, to the largest size that will fit on the output" />
> >       <entry name="zoom_crop" value="3" summary="scale the surface,
> preserving aspect ratio, to fully fill the output cropping if needed" />
> >       <entry name="stretch" value="4" summary="scale the surface to the
> size of the output ignoring aspect ratio" />
> >     </enum>
> >
> >     <request name="present_surface">
> >       <description summary="present surface for display">
> >       Present a surface on the given output.
> >
> >       If the output is null, the compositor will present the surface on
> >       whatever display (or displays) it thinks best.  In particular, this
> >       may replace any or all surfaces currently presented so it should
> >       not be used in combination with placing surfaces on specific
> >       outputs.
> >
> >       The method parameter is a hit to the compositor for how the surface
> *hint


> >       is to be presented.  In particular, it tells the compostior how to
> >       handle a size mismatch between the presented surface and the
> >       output.  The compositor is free to ignore this parameter.
> >
> >       The "zoom", "zoom_crop", and "stretch" methods imply a scaling
> >       operation on the surface.  This will override any kind of output
> >       scaling, so the buffer_scale property of the surface is effectively
> >       ignored.
> >       </description>
> >       <arg name="surface" type="object" interface="wl_surface"/>
> >       <arg name="method" type="uint"/>
> >       <arg name="output" type="object" interface="wl_output"
> allow-null="true"/>
> >     </request>
> >
> >     <request name="present_surface_for_mode">
> >       <description summary="present surface for display at a particular
> mode">
> >       Presents a surface on the given output for a particular mode.
> >
> >       If the current size of the output differs from that of the surface,
> >       the compositor will attempt to change the size of the output to
> >       match the surface.  The result of the mode-swith operation will be
> >       returned via the provided wl_fullscreen_shell_mode_feedback object.
> Is it sufficient to infer the mode from the buffer size, or could there
> be use cases for forcing a particular mode and scaling a buffer from a
> different size?
> If you had a separate "set_mode" request, you could do with only the
> "present_surface" request for setting the surface.

Hehe.  That was my original thought on the matter.  Later, I thought better
of it.  I'll have more comments on the matter at the end.

> Taking this idea further, like atomic mode setting over all outputs,
> we're getting pretty close to the DRM KMS APIs. Would it make sense to
> model these interfaces according to the KMS APIs but sanitized to offer
> all the modern features like atomic modeset and planes in a clean
> uniform way, or do you intentionally want to keep this protocol
> interface simple and not e.g. consider planes explicitly?

I'll take a look at KMS.  However, I think planes are better left to
subsurfaces.  More on that below.

> >       If the current output mode matches the one requested or if the
> >       compositor successfully switches the mode to match the surface,
> >       then the mode_successfull event will be sent and the output will
> >       contain the contents of the given surface.  If the compositor
> >       cannot match the output size to the surface size, the mode_failed
> >       will be sent and the output will contain the contents of the
> >       previously presented surface (if any).  If another surface is
> >       presented on the given output before either of these has a chance
> >       to happen, the present_canceled event will be sent.
> >
> >       If the size of the presented surface changes, the resulting output
> >       is undefined.  The compositor may attempt to change the output mode
> >       to compensate.  However, there is no guarantee that a suitable mode
> >       will be found and the client has no way to be notified of success
> >       or failure.
> The above sounds to me like you want the client be in explicit control
> of the video mode when it asks one, without the server fuzzing in
> between with scaling.
> With an explicit "set_mode" request, I think you could make the above
> cases defined.
> How much control do you want to give to the client? Apparently you want
> the two different cases: simple client, smart server; and smart client
> with explicit mode setting, simple server just obeying or refusing
> without any fuzz.

Pretty much.   Again, read below.

> >       The framerate parameter specifies the target framerate for the
> >       output.  The compositor is free to ignore this parameter.  A value
> >       of 0 indicates that the client has no preference.
> >
> >       If the surface has a buffer_scale greater than 1, the compositor
> >       may choose a mode that matches either the buffer size or the
> >       surface size.  In either case, the surface will fill the output.
> >       </description>
> >       <arg name="surface" type="object" interface="wl_surface"/>
> >       <arg name="output" type="object" interface="wl_output"/>
> >       <arg name="framerate" type="int"/>
> >       <arg name="feedback" type="new_id"
> interface="wl_fullscreen_shell_mode_feedback"/>
> >     </request>
> >
> >     <enum name="error">
> >       <description summary="wl_fullscreen_shell error values">
> >       These errors can be emitted in response to wl_fullscreen_shell
> requests
> >       </description>
> >       <entry name="invalid_method" value="0" summary="present_method is
> not known"/>
> >     </enum>
> >   </interface>
> >
> >   <interface name="wl_fullscreen_shell_mode_feedback" version="1">
> >     <event name="mode_successful">
> >       <description summary="mode switch succeeded">
> >       This event indicates that the attempted mode switch operation was
> >       successful.  A surface of the size requested in the mode switch
> >       will fill the output without scaling.
> >
> >       Upon recieving this event, the client should destroy the
> >       wl_fullscreen_shell_mode_feedback object.
> >       </description>
> >     </event>
> >     <event name="mode_failed">
> >       <description summary="mode switch succeeded">
> >       This event indicates that the attempted mode switch operation
> >       failed. This may be because the requested output mode is not
> >       possible or it may mean that the compositor does not want to allow
> >       mode switches at this time.
> >
> >       Upon recieving this event, the client should destroy the
> >       wl_fullscreen_shell_mode_feedback object.
> >       </description>
> >     </event>
> >     <event name="present_canceled">
> >       <description summary="mode switch succeeded">
> >       This event indicates that the attempted mode switch operation was
> >       canceled.  Most likely this is because the client requested a
> >       second mode switch before the first one completed.
> >
> >       Upon recieving this event, the client should destroy the
> >       wl_fullscreen_shell_mode_feedback object.
> >       </description>
> >     </event>
> This interface has no destructor protocol, so the server cannot know
> when it gets destroyed by the client. I'm always a bit wary of that,
> but here it seems ok, since this is a one-shot feedback object without
> any requests. It is short-lived, doesn't take resources in the server
> much nor bandwidth when it triggers, so there's no need to ever be able
> to cancel it.

Yeah, I thought about having a destructor but it's intentionally a one-shot
and that just adds more protocol noise for little benefit. Maybe it's a
good idea, but I don't think it's needed.

Ok, now for the promised "more detail"...

When designing this interface, I have three primary use-cases in mind:

1) Simple clients such as splash screens and terminal emulators.  These
would rather not mess with KMS and, in a world without VT's, need some way
to get to the screen.  I want to provide a simple but powerful enough
interface to serve these clients.

2) Full compositors such as KWin that would rather not deal with KMS
themselves but would prefer to let Weston or some other implementation
handle it.  In this case, we want more fine-grained control and we want
things like planes, cursors, etc.

3) Other non-KMS "backends" such as VNC/RDP servers, screen recorders, or
anything else where it may not be practical to implement full DRM/KMS.

As you surmised, the first request is to handle the first of these
use-cases and the second request is for the other two.  Breaking it into
two completely separate cases was a deliberate decision.  Maybe not the
right one, but deliberate none the less.  Allow me to explain.

First, I think that planes and the like are already pretty-well covered by
wl_subsurface and wl_viewport.  There's no reason for me to duplicate that
protocol here if it's already covered by those.  This may not map
perfectly, but I think it will cover well enough for most cases.  Axel,
this also covers cursors and the like.

Second, I wanted to ensure that things were atomic.  By having presentation
explicitly tied to mode set, we can ensure this.  There is the issue of the
fallback not being atomic.  One solution to this would be to have a
fallback mode option so if it can't find a mode it will scale or something.
 However, I would rather clients pick one interface and stick with it than
switching between the two.  I thought about splitting it into a set_mode
request and a present_surface request.  However, a) it's harder (not
impossible) to make this atomic and b) it makes the compositor's life more
difficult (see next paragraph).  If they want scaling+modeset, they can
just use a wl_viewport.

Third, I want to allow VNC/RDP-type compositors to be as simple as
possible.  If they don't provide wl_subsurface and wl_viewport then they
don't have to do any real composting.  They still have to handle the first
request but that's easily enough done with pixman or similar.  Also, it
explicitly states that they can ignore the method parameter and just always
center the surface or something.  For the present_for_mode request, it
means that they can always do a direct pixel copy from the source to the
destination (or directly flip in the case of DRM/KMS).  By requiring the
surface and output size to match, it takes a lot of the guesswork out of
the mode switches.

I hope that makes my thinking on this whole protocol more clear.  Thanks
for reviewing;  I look forward to any future comments you may have!
--Jason Ekstrand
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