protocol: add sub-surfaces to the core

The sub-surface protocol was originally committed into Weston on May
10th, 2013, in commit 2396aec6842c709a714f3825dbad9fd88478f2e6. The
design for the protocol had started in the beginning of December 2012. I
think it is high time to move this into the core now.

This patch copies the sub-surface protocol as it was in Weston on Nov
15th, 2013, into Wayland. Weston gets a patch to remove the protocol from
there.

Sub-surface is a wl_surface role. You create a wl_surface as usual, and
assign it the sub-surface role and a parent wl_surface. Sub-surfaces are
an integral part of the parent surface, and stay glued to the parent.
For window management, a window is the union of the top-level
wl_surface and all its sub-surfaces. Sub-surfaces are not clipped to the
parent, and the union of the surface tree can be larger than the
(top-level) wl_surface at its root.

The representative use case for sub-surfaces is a video player window.
When the video content is given its own wl_surface, there is no need to
modify the video frame contents after decoding or copy them into a whole
window sized buffer before submitting it to the compositor. This allows
efficient, zero-copy video presentation paths, where video decoding
hardware produces a (YUV) buffer, which eventually ends up in a
(YUV-capable) hardware overlay and is scanned out directly.

This can also be used for zero-copy presentation of windowed OpenGL
content, where the OpenGL rendering engine does not need to draw or
avoid window decorations.

Sub-surfaces allow mixing different buffer types into the same window,
e.g. software-rendered decorations in wl_shm buffers, and live content
in EGL-based buffers.

However, the sub-surface extension does not offer clipping or scaling
facilities, or accurate presentation timing. Those are topics for
additional extensions.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

protocol/wayland.xml

@@ -4,6 +4,7 @@
   <copyright>
     Copyright © 2008-2011 Kristian Høgsberg
     Copyright © 2010-2011 Intel Corporation
+    Copyright © 2012-2013 Collabora, Ltd.
 
     Permission to use, copy, modify, distribute, and sell this
     software and its documentation for any purpose is hereby granted
@@ -1795,4 +1796,220 @@
 
   </interface>
 
+  <interface name="wl_subcompositor" version="1">
+    <description summary="sub-surface compositing">
+      The global interface exposing sub-surface compositing capabilities.
+      A wl_surface, that has sub-surfaces associated, is called the
+      parent surface. Sub-surfaces can be arbitrarily nested and create
+      a tree of sub-surfaces.
+
+      The root surface in a tree of sub-surfaces is the main
+      surface. The main surface cannot be a sub-surface, because
+      sub-surfaces must always have a parent.
+
+      A main surface with its sub-surfaces forms a (compound) window.
+      For window management purposes, this set of wl_surface objects is
+      to be considered as a single window, and it should also behave as
+      such.
+
+      The aim of sub-surfaces is to offload some of the compositing work
+      within a window from clients to the compositor. A prime example is
+      a video player with decorations and video in separate wl_surface
+      objects. This should allow the compositor to pass YUV video buffer
+      processing to dedicated overlay hardware when possible.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="unbind from the subcompositor interface">
+	Informs the server that the client will not be using this
+	protocol object anymore. This does not affect any other
+	objects, wl_subsurface objects included.
+      </description>
+    </request>
+
+    <enum name="error">
+      <entry name="bad_surface" value="0"
+             summary="the to-be sub-surface is invalid"/>
+    </enum>
+
+    <request name="get_subsurface">
+      <description summary="give a surface the role sub-surface">
+	Create a sub-surface interface for the given surface, and
+	associate it with the given parent surface. This turns a
+	plain wl_surface into a sub-surface.
+
+	The to-be sub-surface must not already have a dedicated
+	purpose, like any shell surface type, cursor image, drag icon,
+	or sub-surface. Otherwise a protocol error is raised.
+      </description>
+
+      <arg name="id" type="new_id" interface="wl_subsurface"
+           summary="the new subsurface object id"/>
+      <arg name="surface" type="object" interface="wl_surface"
+           summary="the surface to be turned into a sub-surface"/>
+      <arg name="parent" type="object" interface="wl_surface"
+           summary="the parent surface"/>
+    </request>
+  </interface>
+
+  <interface name="wl_subsurface" version="1">
+    <description summary="sub-surface interface to a wl_surface">
+      An additional interface to a wl_surface object, which has been
+      made a sub-surface. A sub-surface has one parent surface. A
+      sub-surface's size and position are not limited to that of the parent.
+      Particularly, a sub-surface is not automatically clipped to its
+      parent's area.
+
+      A sub-surface becomes mapped, when a non-NULL wl_buffer is applied
+      and the parent surface is mapped. The order of which one happens
+      first is irrelevant. A sub-surface is hidden if the parent becomes
+      hidden, or if a NULL wl_buffer is applied. These rules apply
+      recursively through the tree of surfaces.
+
+      The behaviour of wl_surface.commit request on a sub-surface
+      depends on the sub-surface's mode. The possible modes are
+      synchronized and desynchronized, see methods
+      wl_subsurface.set_sync and wl_subsurface.set_desync. Synchronized
+      mode caches the wl_surface state to be applied when the parent's
+      state gets applied, and desynchronized mode applies the pending
+      wl_surface state directly. A sub-surface is initially in the
+      synchronized mode.
+
+      Sub-surfaces have also other kind of state, which is managed by
+      wl_subsurface requests, as opposed to wl_surface requests. This
+      state includes the sub-surface position relative to the parent
+      surface (wl_subsurface.set_position), and the stacking order of
+      the parent and its sub-surfaces (wl_subsurface.place_above and
+      .place_below). This state is applied when the parent surface's
+      wl_surface state is applied, regardless of the sub-surface's mode.
+      As the exception, set_sync and set_desync are effective immediately.
+
+      The main surface can be thought to be always in desynchronized mode,
+      since it does not have a parent in the sub-surfaces sense.
+
+      Even if a sub-surface is in desynchronized mode, it will behave as
+      in synchronized mode, if its parent surface behaves as in
+      synchronized mode. This rule is applied recursively throughout the
+      tree of surfaces. This means, that one can set a sub-surface into
+      synchronized mode, and then assume that all its child and grand-child
+      sub-surfaces are synchronized, too, without explicitly setting them.
+
+      If the wl_surface associated with the wl_subsurface is destroyed, the
+      wl_subsurface object becomes inert. Note, that destroying either object
+      takes effect immediately. If you need to synchronize the removal
+      of a sub-surface to the parent surface update, unmap the sub-surface
+      first by attaching a NULL wl_buffer, update parent, and then destroy
+      the sub-surface.
+
+      If the parent wl_surface object is destroyed, the sub-surface is
+      unmapped.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="remove sub-surface interface">
+	The sub-surface interface is removed from the wl_surface object
+	that was turned into a sub-surface with
+	wl_subcompositor.get_subsurface request. The wl_surface's association
+	to the parent is deleted, and the wl_surface loses its role as
+	a sub-surface. The wl_surface is unmapped.
+      </description>
+    </request>
+
+    <enum name="error">
+      <entry name="bad_surface" value="0"
+             summary="wl_surface is not a sibling or the parent"/>
+    </enum>
+
+    <request name="set_position">
+      <description summary="reposition the sub-surface">
+	This schedules a sub-surface position change.
+	The sub-surface will be moved so, that its origin (top-left
+	corner pixel) will be at the location x, y of the parent surface
+	coordinate system. The coordinates are not restricted to the parent
+	surface area. Negative values are allowed.
+
+	The next wl_surface.commit on the parent surface will reset
+	the sub-surface's position to the scheduled coordinates.
+
+	The initial position is 0, 0.
+      </description>
+
+      <arg name="x" type="int" summary="coordinate in the parent surface"/>
+      <arg name="y" type="int" summary="coordinate in the parent surface"/>
+    </request>
+
+    <request name="place_above">
+      <description summary="restack the sub-surface">
+	This sub-surface is taken from the stack, and put back just
+	above the reference surface, changing the z-order of the sub-surfaces.
+	The reference surface must be one of the sibling surfaces, or the
+	parent surface. Using any other surface, including this sub-surface,
+	will cause a protocol error.
+
+	The z-order is double-buffered state, and will be applied on the
+	next commit of the parent surface.
+	See wl_surface.commit and wl_subcompositor.get_subsurface.
+
+	A new sub-surface is initially added as the top-most in the stack
+	of its siblings and parent.
+      </description>
+
+      <arg name="sibling" type="object" interface="wl_surface"
+           summary="the reference surface"/>
+    </request>
+
+    <request name="place_below">
+      <description summary="restack the sub-surface">
+	The sub-surface is placed just below of the reference surface.
+	See wl_subsurface.place_above.
+      </description>
+
+      <arg name="sibling" type="object" interface="wl_surface"
+           summary="the reference surface"/>
+    </request>
+
+    <request name="set_sync">
+      <description summary="set sub-surface to synchronized mode">
+	Change the commit behaviour of the sub-surface to synchronized
+	mode, also described as the parent dependant mode.
+
+	In synchronized mode, wl_surface.commit on a sub-surface will
+	accumulate the committed state in a cache, but the state will
+	not be applied and hence will not change the compositor output.
+	The cached state is applied to the sub-surface immediately after
+	the parent surface's state is applied. This ensures atomic
+	updates of the parent and all its synchronized sub-surfaces.
+	Applying the cached state will invalidate the cache, so further
+	parent surface commits do not (re-)apply old state.
+
+	See wl_subsurface for the recursive effect of this mode.
+      </description>
+    </request>
+
+    <request name="set_desync">
+      <description summary="set sub-surface to desynchronized mode">
+	Change the commit behaviour of the sub-surface to desynchronized
+	mode, also described as independent or freely running mode.
+
+	In desynchronized mode, wl_surface.commit on a sub-surface will
+	apply the pending state directly, without caching, as happens
+	normally with a wl_surface. Calling wl_surface.commit on the
+	parent surface has no effect on the sub-surface's wl_surface
+	state. This mode allows a sub-surface to be updated on its own.
+
+	If cached state exists when wl_surface.commit is called in
+	desynchronized mode, the pending state is added to the cached
+	state, and applied as whole. This invalidates the cache.
+
+	Note: even if a sub-surface is set to desynchronized, a parent
+	sub-surface may override it to behave as synchronized. For details,
+	see wl_subsurface.
+
+	If a surface's parent surface behaves as desynchronized, then
+	the cached state is applied on set_desync.
+      </description>
+    </request>
+
+  </interface>
+
 </protocol>