[RFC v4] Fullscreen shell protocol

[Jason Ekstrand]

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">
    <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.
    </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
	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.

	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 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>
  </interface>
</protocol>