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129 lines
4.6 KiB
129 lines
4.6 KiB
4 months ago
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<?xml version='1.0' encoding='utf-8' ?>
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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % BOOK_ENTITIES SYSTEM "Wayland.ent">
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%BOOK_ENTITIES;
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]>
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<chapter id="chap-Compositors">
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<title>Types of Compositors</title>
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<para>
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Compositors come in different types, depending on which
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role they play in the overall architecture of the OS.
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For instance, a
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<link linkend="sect-Compositors-System-Compositor">system compositor</link>
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can be used for booting the system, handling multiple user switching, a
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possible console terminal emulator and so forth. A different compositor, a
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<link linkend="sect-Compositors-Session-Compositor">session compositor</link>
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would provide the actual desktop environment. There are many ways for
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different types of compositors to co-exist.
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</para>
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<para>
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In this section, we introduce three types of Wayland compositors relying
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on <link linkend="sect-Library-Server">libwayland-server</link>.
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</para>
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<section id="sect-Compositors-System-Compositor">
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<title>System Compositor</title>
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<para>
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A system compositor can run from early boot until shutdown.
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It effectively replaces the kernel vt system, and can tie in
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with the systems graphical boot setup and multiseat support.
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</para>
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<para>
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A system compositor can host different types of session
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compositors, and let us switch between multiple sessions
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(fast user switching, or secure/personal desktop switching).
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</para>
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<para>
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A linux implementation of a system compositor will typically
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use libudev, egl, kms, evdev and cairo.
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</para>
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<para>
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For fullscreen clients, the system compositor can reprogram the
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video scanout address to read directly from the client provided
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buffer.
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</para>
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</section>
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<section id="sect-Compositors-Session-Compositor">
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<title>Session Compositor</title>
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<para>
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A session compositor is responsible for a single user session.
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If a system compositor is present, the session compositor will
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run nested under the system compositor. Nesting is feasible because
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the protocol is asynchronous; roundtrips would be too expensive
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when nesting is involved. If no system compositor is present, a
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session compositor can run directly on the hardware.
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</para>
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<para>
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X applications can continue working under a session compositor
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by means of a root-less X server that is activated on demand.
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</para>
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<para>
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Possible examples for session compositors include
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<itemizedlist>
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<listitem>
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<para>
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gnome-shell
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</para>
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</listitem>
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<listitem>
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<para>
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moblin
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</para>
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</listitem>
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<listitem>
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<para>
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kwin
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</para>
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</listitem>
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<listitem>
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<para>
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kmscon
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</para>
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</listitem>
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<listitem>
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<para>
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rdp session
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</para>
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</listitem>
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<listitem>
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<para>
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Weston with X11 or Wayland backend is a session compositor nested
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in another session compositor.
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</para>
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</listitem>
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<listitem>
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<para>
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fullscreen X session under Wayland
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</section>
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<section id="sect-Compositors-Embedding-Compositor">
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<title>Embedding Compositor</title>
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<para>
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X11 lets clients embed windows from other clients, or lets clients
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copy pixmap contents rendered by another client into their window.
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This is often used for applets in a panel, browser plugins and similar.
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Wayland doesn't directly allow this, but clients can communicate GEM
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buffer names out-of-band, for example, using D-Bus, or command line
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arguments when the panel launches the applet. Another option is to
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use a nested Wayland instance. For this, the Wayland server will have
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to be a library that the host application links to. The host
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application will then pass the Wayland server socket name to the
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embedded application, and will need to implement the Wayland
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compositor interface. The host application composites the client
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surfaces as part of it's window, that is, in the web page or in the
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panel. The benefit of nesting the Wayland server is that it provides
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the requests the embedded client needs to inform the host about buffer
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updates and a mechanism for forwarding input events from the host
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application.
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</para>
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<para>
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An example for this kind of setup is firefox embedding the flash
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player as a kind of special-purpose compositor.
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</para>
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</section>
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</chapter>
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