1 <?xml version='1.0' encoding='utf-8' ?> 2 <!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [ 3 <!ENTITY % BOOK_ENTITIES SYSTEM "Wayland.ent"> 4 %BOOK_ENTITIES; 5 ]> 6 <chapter id="chap-Protocol"> 7 <title>Wayland Protocol and Model of Operation</title> 8 <section id="sect-Protocol-Basic-Principles"> 9 <title>Basic Principles</title> 10 <para> 11 The Wayland protocol is an asynchronous object oriented protocol. All 12 requests are method invocations on some object. The requests include 13 an object ID that uniquely identifies an object on the server. Each 14 object implements an interface and the requests include an opcode that 15 identifies which method in the interface to invoke. 16 </para> 17 <para> 18 The protocol is message-based. A message sent by a client to the server 19 is called request. A message from the server to a client is called event. 20 A message has a number of arguments, each of which has a certain type (see 21 <xref linkend="sect-Protocol-Wire-Format"/> for a list of argument types). 22 </para> 23 <para> 24 Additionally, the protocol can specify <type>enum</type>s which associate 25 names to specific numeric enumeration values. These are primarily just 26 descriptive in nature: at the wire format level enums are just integers. 27 But they also serve a secondary purpose to enhance type safety or 28 otherwise add context for use in language bindings or other such code. 29 This latter usage is only supported so long as code written before these 30 attributes were introduced still works after; in other words, adding an 31 enum should not break API, otherwise it puts backwards compatibility at 32 risk. 33 </para> 34 <para> 35 <type>enum</type>s can be defined as just a set of integers, or as 36 bitfields. This is specified via the <type>bitfield</type> boolean 37 attribute in the <type>enum</type> definition. If this attribute is true, 38 the enum is intended to be accessed primarily using bitwise operations, 39 for example when arbitrarily many choices of the enum can be ORed 40 together; if it is false, or the attribute is omitted, then the enum 41 arguments are a just a sequence of numerical values. 42 </para> 43 <para> 44 The <type>enum</type> attribute can be used on either <type>uint</type> 45 or <type>int</type> arguments, however if the <type>enum</type> is 46 defined as a <type>bitfield</type>, it can only be used on 47 <type>uint</type> args. 48 </para> 49 <para> 50 The server sends back events to the client, each event is emitted from 51 an object. Events can be error conditions. The event includes the 52 object ID and the event opcode, from which the client can determine 53 the type of event. Events are generated both in response to requests 54 (in which case the request and the event constitutes a round trip) or 55 spontaneously when the server state changes. 56 </para> 57 <para> 58 <itemizedlist> 59 <listitem> 60 <para> 61 State is broadcast on connect, events are sent 62 out when state changes. Clients must listen for 63 these changes and cache the state. 64 There is no need (or mechanism) to query server state. 65 </para> 66 </listitem> 67 <listitem> 68 <para> 69 The server will broadcast the presence of a number of global objects, 70 which in turn will broadcast their current state. 71 </para> 72 </listitem> 73 </itemizedlist> 74 </para> 75 </section> 76 <section id="sect-Protocol-Code-Generation"> 77 <title>Code Generation</title> 78 <para> 79 The interfaces, requests and events are defined in 80 <filename>protocol/wayland.xml</filename>. 81 This xml is used to generate the function prototypes that can be used by 82 clients and compositors. 83 </para> 84 <para> 85 The protocol entry points are generated as inline functions which just 86 wrap the <function>wl_proxy_*</function> functions. The inline functions aren't 87 part of the library ABI and language bindings should generate their 88 own stubs for the protocol entry points from the xml. 89 </para> 90 </section> 91 <section id="sect-Protocol-Wire-Format"> 92 <title>Wire Format</title> 93 <para> 94 The protocol is sent over a UNIX domain stream socket, where the endpoint 95 usually is named <systemitem class="service">wayland-0</systemitem> 96 (although it can be changed via <emphasis>WAYLAND_DISPLAY</emphasis> 97 in the environment). 98 </para> 99 <para> 100 Every message is structured as 32-bit words; values are represented in the 101 host's byte-order. The message header has 2 words in it: 102 <itemizedlist> 103 <listitem> 104 <para> 105 The first word is the sender's object ID (32-bit). 106 </para> 107 </listitem> 108 <listitem> 109 <para> 110 The second has 2 parts of 16-bit. The upper 16-bits are the message 111 size in bytes, starting at the header (i.e. it has a minimum value of 8).The lower is the request/event opcode. 112 </para> 113 </listitem> 114 </itemizedlist> 115 The payload describes the request/event arguments. Every argument is always 116 aligned to 32-bits. Where padding is required, the value of padding bytes is 117 undefined. There is no prefix that describes the type, but it is 118 inferred implicitly from the xml specification. 119 </para> 120 <para> 121 122 The representation of argument types are as follows: 123 <variablelist> 124 <varlistentry> 125 <term>int</term> 126 <term>uint</term> 127 <listitem> 128 <para> 129 The value is the 32-bit value of the signed/unsigned 130 int. 131 </para> 132 </listitem> 133 </varlistentry> 134 <varlistentry> 135 <term>fixed</term> 136 <listitem> 137 <para> 138 Signed 24.8 decimal numbers. It is a signed decimal type which 139 offers a sign bit, 23 bits of integer precision and 8 bits of 140 decimal precision. This is exposed as an opaque struct with 141 conversion helpers to and from double and int on the C API side. 142 </para> 143 </listitem> 144 </varlistentry> 145 <varlistentry> 146 <term>string</term> 147 <listitem> 148 <para> 149 Starts with an unsigned 32-bit length, followed by the 150 string contents, including terminating null byte, then padding 151 to a 32-bit boundary. 152 </para> 153 </listitem> 154 </varlistentry> 155 <varlistentry> 156 <term>object</term> 157 <listitem> 158 <para> 159 32-bit object ID. 160 </para> 161 </listitem> 162 </varlistentry> 163 <varlistentry> 164 <term>new_id</term> 165 <listitem> 166 <para> 167 The 32-bit object ID. On requests, the client 168 decides the ID. The only events with <type>new_id</type> are 169 advertisements of globals, and the server will use IDs below 170 0x10000. 171 </para> 172 </listitem> 173 </varlistentry> 174 <varlistentry> 175 <term>array</term> 176 <listitem> 177 <para> 178 Starts with 32-bit array size in bytes, followed by the array 179 contents verbatim, and finally padding to a 32-bit boundary. 180 </para> 181 </listitem> 182 </varlistentry> 183 <varlistentry> 184 <term>fd</term> 185 <listitem> 186 <para> 187 The file descriptor is not stored in the message buffer, but in 188 the ancillary data of the UNIX domain socket message (msg_control). 189 </para> 190 </listitem> 191 </varlistentry> 192 </variablelist> 193 </para> 194 </section> 195 <xi:include href="ProtocolInterfaces.xml" xmlns:xi="http://www.w3.org/2001/XInclude"/> 196 <section id="sect-Protocol-Versioning"> 197 <title>Versioning</title> 198 <para> 199 Every interface is versioned and every protocol object implements a 200 particular version of its interface. For global objects, the maximum 201 version supported by the server is advertised with the global and the 202 actual version of the created protocol object is determined by the 203 version argument passed to wl_registry.bind(). For objects that are 204 not globals, their version is inferred from the object that created 205 them. 206 </para> 207 <para> 208 In order to keep things sane, this has a few implications for 209 interface versions: 210 <itemizedlist> 211 <listitem> 212 <para> 213 The object creation hierarchy must be a tree. Otherwise, 214 infering object versions from the parent object becomes a much 215 more difficult to properly track. 216 </para> 217 </listitem> 218 <listitem> 219 <para> 220 When the version of an interface increases, so does the version 221 of its parent (recursively until you get to a global interface) 222 </para> 223 </listitem> 224 <listitem> 225 <para> 226 A global interface's version number acts like a counter for all 227 of its child interfaces. Whenever a child interface gets 228 modified, the global parent's interface version number also 229 increases (see above). The child interface then takes on the 230 same version number as the new version of its parent global 231 interface. 232 </para> 233 </listitem> 234 </itemizedlist> 235 </para> 236 <para> 237 To illustrate the above, consider the wl_compositor interface. It 238 has two children, wl_surface and wl_region. As of wayland version 239 1.2, wl_surface and wl_compositor are both at version 3. If 240 something is added to the wl_region interface, both wl_region and 241 wl_compositor will get bumpped to version 4. If, afterwards, 242 wl_surface is changed, both wl_compositor and wl_surface will be at 243 version 5. In this way the global interface version is used as a 244 sort of "counter" for all of its child interfaces. This makes it 245 very simple to know the version of the child given the version of its 246 parent. The child is at the highest possible interface version that 247 is less than or equal to its parent's version. 248 </para> 249 <para> 250 It is worth noting a particular exception to the above versioning 251 scheme. The wl_display (and, by extension, wl_registry) interface 252 cannot change because it is the core protocol object and its version 253 is never advertised nor is there a mechanism to request a different 254 version. 255 </para> 256 </section> 257 <section id="sect-Protocol-Connect-Time"> 258 <title>Connect Time</title> 259 <para> 260 There is no fixed connection setup information, the server emits 261 multiple events at connect time, to indicate the presence and 262 properties of global objects: outputs, compositor, input devices. 263 </para> 264 </section> 265 <section id="sect-Protocol-Security-and-Authentication"> 266 <title>Security and Authentication</title> 267 <para> 268 <itemizedlist> 269 <listitem> 270 <para> 271 mostly about access to underlying buffers, need new drm auth 272 mechanism (the grant-to ioctl idea), need to check the cmd stream? 273 </para> 274 </listitem> 275 <listitem> 276 <para> 277 getting the server socket depends on the compositor type, could 278 be a system wide name, through fd passing on the session dbus. 279 or the client is forked by the compositor and the fd is 280 already opened. 281 </para> 282 </listitem> 283 </itemizedlist> 284 </para> 285 </section> 286 <section id="sect-Protocol-Creating-Objects"> 287 <title>Creating Objects</title> 288 <para> 289 Each object has a unique ID. The IDs are allocated by the entity 290 creating the object (either client or server). IDs allocated by the 291 client are in the range [1, 0xfeffffff] while IDs allocated by the 292 server are in the range [0xff000000, 0xffffffff]. The 0 ID is 293 reserved to represent a null or non-existant object. 294 295 For efficiency purposes, the IDs are densely packed in the sense that 296 the ID N will not be used until N-1 has been used. Any ID allocation 297 algorithm that does not maintain this property is incompatible with 298 the implementation in libwayland. 299 </para> 300 </section> 301 <section id="sect-Protocol-Compositor"> 302 <title>Compositor</title> 303 <para> 304 The compositor is a global object, advertised at connect time. 305 </para> 306 <para> 307 See <xref linkend="protocol-spec-wl_compositor"/> for the 308 protocol description. 309 </para> 310 </section> 311 <section id="sect-Protocol-Surface"> 312 <title>Surfaces</title> 313 <para> 314 A surface manages a rectangular grid of pixels that clients create 315 for displaying their content to the screen. Clients don't know 316 the global position of their surfaces, and cannot access other 317 clients' surfaces. 318 </para> 319 <para> 320 Once the client has finished writing pixels, it 'commits' the 321 buffer; this permits the compositor to access the buffer and read 322 the pixels. When the compositor is finished, it releases the 323 buffer back to the client. 324 </para> 325 <para> 326 See <xref linkend="protocol-spec-wl_surface"/> for the protocol 327 description. 328 </para> 329 </section> 330 <section id="sect-Protocol-Input"> 331 <title>Input</title> 332 <para> 333 A seat represents a group of input devices including mice, 334 keyboards and touchscreens. It has a keyboard and pointer 335 focus. Seats are global objects. Pointer events are delivered 336 in surface-local coordinates. 337 </para> 338 <para> 339 The compositor maintains an implicit grab when a button is 340 pressed, to ensure that the corresponding button release 341 event gets delivered to the same surface. But there is no way 342 for clients to take an explicit grab. Instead, surfaces can 343 be mapped as 'popup', which combines transient window semantics 344 with a pointer grab. 345 </para> 346 <para> 347 To avoid race conditions, input events that are likely to 348 trigger further requests (such as button presses, key events, 349 pointer motions) carry serial numbers, and requests such as 350 wl_surface.set_popup require that the serial number of the 351 triggering event is specified. The server maintains a 352 monotonically increasing counter for these serial numbers. 353 </para> 354 <para> 355 Input events also carry timestamps with millisecond granularity. 356 Their base is undefined, so they can't be compared against 357 system time (as obtained with clock_gettime or gettimeofday). 358 They can be compared with each other though, and for instance 359 be used to identify sequences of button presses as double 360 or triple clicks. 361 </para> 362 <para> 363 See <xref linkend="protocol-spec-wl_seat"/> for the 364 protocol description. 365 </para> 366 <para> 367 Talk about: 368 369 <itemizedlist> 370 <listitem> 371 <para> 372 keyboard map, change events 373 </para> 374 </listitem> 375 <listitem> 376 <para> 377 xkb on Wayland 378 </para> 379 </listitem> 380 <listitem> 381 <para> 382 multi pointer Wayland 383 </para> 384 </listitem> 385 </itemizedlist> 386 </para> 387 <para> 388 A surface can change the pointer image when the surface is the pointer 389 focus of the input device. Wayland doesn't automatically change the 390 pointer image when a pointer enters a surface, but expects the 391 application to set the cursor it wants in response to the pointer 392 focus and motion events. The rationale is that a client has to manage 393 changing pointer images for UI elements within the surface in response 394 to motion events anyway, so we'll make that the only mechanism for 395 setting or changing the pointer image. If the server receives a request 396 to set the pointer image after the surface loses pointer focus, the 397 request is ignored. To the client this will look like it successfully 398 set the pointer image. 399 </para> 400 <para> 401 The compositor will revert the pointer image back to a default image 402 when no surface has the pointer focus for that device. Clients can 403 revert the pointer image back to the default image by setting a NULL 404 image. 405 </para> 406 <para> 407 What if the pointer moves from one window which has set a special 408 pointer image to a surface that doesn't set an image in response to 409 the motion event? The new surface will be stuck with the special 410 pointer image. We can't just revert the pointer image on leaving a 411 surface, since if we immediately enter a surface that sets a different 412 image, the image will flicker. Broken app, I suppose. 413 </para> 414 </section> 415 <section id="sect-Protocol-Output"> 416 <title>Output</title> 417 <para> 418 An output is a global object, advertised at connect time or as it 419 comes and goes. 420 </para> 421 <para> 422 See <xref linkend="protocol-spec-wl_output"/> for the protocol 423 description. 424 </para> 425 <para> 426 </para> 427 <itemizedlist> 428 <listitem> 429 <para> 430 laid out in a big (compositor) coordinate system 431 </para> 432 </listitem> 433 <listitem> 434 <para> 435 basically xrandr over Wayland 436 </para> 437 </listitem> 438 <listitem> 439 <para> 440 geometry needs position in compositor coordinate system 441 </para> 442 </listitem> 443 <listitem> 444 <para> 445 events to advertise available modes, requests to move and change 446 modes 447 </para> 448 </listitem> 449 </itemizedlist> 450 </section> 451 <section id="sect-Protocol-data-sharing"> 452 <title>Data sharing between clients</title> 453 <para> 454 The Wayland protocol provides clients a mechanism for sharing 455 data that allows the implementation of copy-paste and 456 drag-and-drop. The client providing the data creates a 457 <function>wl_data_source</function> object and the clients 458 obtaining the data will see it as <function>wl_data_offer</function> 459 object. This interface allows the clients to agree on a mutually 460 supported mime type and transfer the data via a file descriptor 461 that is passed through the protocol. 462 </para> 463 <para> 464 The next section explains the negotiation between data source and 465 data offer objects. <xref linkend="sect-Protocol-data-sharing-devices"/> 466 explains how these objects are created and passed to different 467 clients using the <function>wl_data_device</function> interface 468 that implements copy-paste and drag-and-drop support. 469 </para> 470 <para> 471 See <xref linkend="protocol-spec-wl_data_offer"/>, 472 <xref linkend="protocol-spec-wl_data_source"/>, 473 <xref linkend="protocol-spec-wl_data_device"/> and 474 <xref linkend="protocol-spec-wl_data_device_manager"/> for 475 protocol descriptions. 476 </para> 477 <para> 478 MIME is defined in RFC's 2045-2049. A 479 <ulink url="ftp://ftp.isi.edu/in-notes/iana/assignments/media-types/"> 480 registry of MIME types</ulink> is maintained by the Internet Assigned 481 Numbers Authority (IANA). 482 </para> 483 <section> 484 <title>Data negotiation</title> 485 <para> 486 A client providing data to other clients will create a <function>wl_data_source</function> 487 object and advertise the mime types for the formats it supports for 488 that data through the <function>wl_data_source.offer</function> 489 request. On the receiving end, the data offer object will generate one 490 <function>wl_data_offer.offer</function> event for each supported mime 491 type. 492 </para> 493 <para> 494 The actual data transfer happens when the receiving client sends a 495 <function>wl_data_offer.receive</function> request. This request takes 496 a mime type and a file descriptor as arguments. This request will generate a 497 <function>wl_data_source.send</function> event on the sending client 498 with the same arguments, and the latter client is expected to write its 499 data to the given file descriptor using the chosen mime type. 500 </para> 501 </section> 502 <section id="sect-Protocol-data-sharing-devices"> 503 <title>Data devices</title> 504 <para> 505 Data devices glue data sources and offers together. A data device is 506 associated with a <function>wl_seat</function> and is obtained by the clients using the 507 <function>wl_data_device_manager</function> factory object, which is also responsible for 508 creating data sources. 509 </para> 510 <para> 511 Clients are informed of new data offers through the 512 <function>wl_data_device.data_offer</function> event. After this 513 event is generated the data offer will advertise the available mime 514 types. New data offers are introduced prior to their use for 515 copy-paste or drag-and-drop. 516 </para> 517 <section> 518 <title>Selection</title> 519 <para> 520 Each data device has a selection data source. Clients create a data 521 source object using the device manager and may set it as the 522 current selection for a given data device. Whenever the current 523 selection changes, the client with keyboard focus receives a 524 <function>wl_data_device.selection</function> event. This event is 525 also generated on a client immediately before it receives keyboard 526 focus. 527 </para> 528 <para> 529 The data offer is introduced with 530 <function>wl_data_device.data_offer</function> event before the 531 selection event. 532 </para> 533 </section> 534 <section> 535 <title>Drag and Drop</title> 536 <para> 537 A drag-and-drop operation is started using the 538 <function>wl_data_device.start_drag</function> request. This 539 requests causes a pointer grab that will generate enter, motion and 540 leave events on the data device. A data source is supplied as 541 argument to start_drag, and data offers associated with it are 542 supplied to clients surfaces under the pointer in the 543 <function>wl_data_device.enter</function> event. The data offer 544 is introduced to the client prior to the enter event with the 545 <function>wl_data_device.data_offer</function> event. 546 </para> 547 <para> 548 Clients are expected to provide feedback to the data sending client 549 by calling the <function>wl_data_offer.accept</function> request with 550 a mime type it accepts. If none of the advertised mime types is 551 supported by the receiving client, it should supply NULL to the 552 accept request. The accept request causes the sending client to 553 receive a <function>wl_data_source.target</function> event with the 554 chosen mime type. 555 </para> 556 <para> 557 When the drag ends, the receiving client receives a 558 <function>wl_data_device.drop</function> event at which it is expected 559 to transfer the data using the 560 <function>wl_data_offer.receive</function> request. 561 </para> 562 </section> 563 </section> 564 </section> 565 </chapter> 566
