4 Wayland is a nano display server, relying on drm modesetting, gem
5 batchbuffer submission and hw initialization generally in the kernel.
6 Wayland puts the compositing manager and display server in the same
7 process. Window management is largely pushed to the clients, they
8 draw their own decorations and move and resize themselves, typically
9 implemented in a toolkit library. More of the core desktop could be
10 pushed into wayland, for example, stock desktop components such as the
11 panel or the desktop background.
13 The actual compositor will define a fair bit of desktop policy and it
14 is expected that different use cases (desktop environments, devices,
15 appliances) will provide their own custom compositor.
17 It is still designed with a windowed type of desktop in mind, as
18 opposed to fullscreen-all-the-time type of interface, but should be
19 useful wherever several processes contribute content to be composited.
21 Current trends goes towards less and less rendering in X server, more
22 hardware setup and management in kernel and shared libraries allow
23 code sharing without putting it all in a server. freetype,
24 fontconfig, cairo all point in this direction, as does direct
27 Client allocates DRM buffers, draws decorations, and full window
28 contents and posts entire thing to server along with dimensions.
30 Everything is direct rendered and composited. No cliprects, no
31 drawing api/protocl between server and client. No
32 pixmaps/windows/drawables, only surfaces (essentially pixmaps). No
33 gcs/fonts, no nested windows. OpenGL is already direct rendered,
34 pixman may be direct rendered which adds the cairo API, or cairo
35 may gain a GL backend.
37 Could be a "shell" for launching gdm X server, user session servers,
38 safe mode xservers, graphics text console. From gdm, we could also
39 launch a rdp session, solid ice sessions.
41 All surface commands (copy, attach, map=set quads) are buffered until
42 the client sends a commit command, which executes everything
43 atomically. The commit command includes a cookie, which will be
44 returned in an event generated by the server once the commit has been
45 executed. This allows clients to throttle themselves against the
46 server and implement smooth animations.
51 Include panel and desktop background in wayland?
53 How does clients move their surfaces? set a full tri-mesh every time?
55 How does the server apply transformations to a surface behind the
56 clients back? (wobbly, minimize, zoom) Maybe wobble is client side?
58 How do apps share the glyph cache?
60 Input handling - keyboard focus, multiple input devices, multiple
61 pointers, multi touch.
63 Drawing cursors, moving them, cursor themes, attaching surfaces to
64 cursors. How do you change cursors when you mouse over a text
65 field if you don't have subwindows?
67 synaptics, 3-button emulation, xkb, scim
69 changing screen resolution, adding monitors.
71 What to do when protocol out buffer fills up? Just block on write
72 would work I guess. Clients are supposed to throttle using the bread
73 crumb events, so we shouldn't get into this situation.
75 When a surface is the size of the screen and on top, we can set the
76 scanout buffer to that surface directly. Like compiz unredirect
77 top-level window feature. Except it won't have any protocol state
78 side-effects and the client that owns the surface won't know. We lose
79 control of updates. Should work well for X server root window under
82 Throttling/scheduling - there is currently no mechanism for scheduling
83 clients to prevent greedy clients from spamming the server and
84 starving other clients. On the other hand, now that recompositing is
85 done in the idle handler (and eventually at vertical retrace time),
86 there's nothing a client can do to hog the server. Unless we include
87 a copyregion type request, to let a client update it's surface
88 contents by asking the server to atomically copy a region from some
89 other buffer to the surface buffer.
91 Atomicity - we have the map and the attach requests which sometimes
92 will have to be executed atomically. Moving the window is done using
93 the map request and will not involve an attach requet. Updating the
94 window contents will use an attach request but no map. Resizing,
95 however, will use both and in that case must be executed atomically.
96 One way to do this is to have the server always batch up requests and
97 then introduce a kind of "commit" request, which will push the batched
98 changes into effect. This is easier than it sounds, since we only
99 have to remember the most recent map and most recent attach. The
100 commit request will generate an corresponding commit event once the
101 committed changes become visible on screen. The client can provide a
102 bread-crumb id in the commit request, which will be sent back in the
105 - is batching+commit per client or per surface? Much more convenient
106 if per-client, since a client can batch up a bunch of stuff and get
107 atomic updates to multiple windows. Also nice to only get one
108 commit event for changes to a bunch of windows. Is a little more
109 tricky server-side, since we now have to keep a list of windows
110 with pending changes in the wl_client struct.
112 - batching+commit also lets a client reuse parts of the surface
113 buffer without allocating a new full-size back buffer. For
114 scrolling, for example, the client can render just the newly
115 exposed part of the page to a smaller temporary buffer, then issue
116 a copy request to copy the preserved part of the page up, and the
117 new part of the page into the exposed area.
119 - This does let a client batch up an uncontrolled amount of copy
120 requests that the server has to execute when it gets the commit
121 request. This could potentially lock up the server for a while,
122 leading to lost frames. Should never cause tearing though, we're
123 changing the surface contents, not the server back buffer which is
124 what is scheduled for blitting at vsync time.
129 The wayland protocol is a async object oriented protocol. All
130 requests are method invocations on some object. The request include
131 an object id that uniquely identifies an object on the server. Each
132 object implements an interface and the requests include an opcode that
133 identifies which method in the interface to invoke.
135 The server sends back events to the client, each event is emitted from
136 an object. Events can be error conditions. The event includes the
137 object id and the event opcode, from which the client can determine
138 the type of event. Events are generated both in repsonse to a request
139 (in which case the request and the event constitutes a round trip) or
140 spontanously when the server state changes.
142 the get_interface method is called on an object to get an object
143 handle that implements the specified interface.