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<!ENTITY % BOOK_ENTITIES SYSTEM "Wayland.ent">
%BOOK_ENTITIES;
]>
<chapter id="chap-Wayland-Overview">
<title>Wayland Overview</title>
<para>
<orderedlist>
<listitem>
<para>
Wayland is a protocol for a new display server
</para>
</listitem>
<listitem>
<para>
Weston is the open source project implementing a wayland-based
copmositor
</para>
</listitem>
</orderedlist>
</para>
<section id="sect-Wayland-Overview-Replacing-X11">
<title>Replacing X11</title>
<para>
In Linux and other Unix-like systems, the X stack has grown to
encompass functionality arguably belonging in client libraries,
helper libraries, or the host operating system kernel. Support for
things like PCI resource management, display configuration management,
direct rendering, and memory management has been integrated into the X
stack, imposing limitations like limited support for standalone
applications, duplication in other projects (e.g. the Linux fb layer
or the DirectFB project), and high levels of complexity for systems
combining multiple elements (for example radeon memory map handling
between the fb driver and X driver, or VT switching).
</para>
<para>
Moreover, X has grown to incorporate modern features like offscreen
rendering and scene composition, but subject to the limitations of the
X architecture. For example, the X implementation of composition adds
additional context switches and makes things like input redirection
difficult.
</para>
<mediaobject>
<imageobject>
<imagedata fileref="images/x-architecture.png" format="PNG" />
</imageobject>
</mediaobject>
<para>
The diagram above illustrates the central role of the X server and
compositor in operations, and the steps required to get contents on to
the screen.
</para>
<para>
Over time, X developers came to understand the shortcomings of this
approach and worked to split things up. Over the past several years,
a lot of functionality has moved out of the X server and into
client-side libraries or kernel drivers. One of the first components
to move out was font rendering, with freetype and fontconfig providing
an alternative to the core X fonts. Direct rendering OpenGL as a
graphics driver in a client side library went through some iterations,
ending up as DRI2, which abstracted most of the direct rendering
buffer management from client code. Then cairo came along and provided
a modern 2D rendering library independent of X, and compositing
managers took over control of the rendering of the desktop as toolkits
like GTK+ and Qt moved away from using X APIs for rendering. Recently,
memory and display management have moved to the Linux kernel, further
reducing the scope of X and its driver stack. The end result is a
highly modular graphics stack.
</para>
</section>
<section id="sect-Wayland-Overview-Make-compositing-manager-display-server">
<title>Make the compositing manager the display server</title>
<para>
Wayland is a new display server and compositing protocol, and Weston
is the implementation of this protocol which builds on top of all the
components above. We are trying to distill out the functionality in
the X server that is still used by the modern Linux desktop. This
turns out to be not a whole lot. Applications can allocate their own
off-screen buffers and render their window contents directly, using
hardware accelerated libraries like libGL, or high quality software
implementations like those found in Cairo. In the end, what’s needed
is a way to present the resulting window surface for display, and a
way to receive and arbitrate input among multiple clients. This is
what Wayland provides, by piecing together the components already in
the eco-system in a slightly different way.
</para>
<para>
X will always be relevant, in the same way Fortran compilers and VRML
browsers are, but it’s time that we think about moving it out of the
critical path and provide it as an optional component for legacy
applications.
</para>
<para>
Overall, the philosophy of Wayland is to provide clients with a way to
manage windows and how their contents is displayed. Rendering is left
to clients, and system wide memory management interfaces are used to
pass buffer handles between clients and the compositing manager.
</para>
<mediaobject>
<imageobject>
<imagedata fileref="images/wayland-architecture.png" format="PNG" />
</imageobject>
</mediaobject>
<para>
The figure above illustrates how Wayland clients interact with a
Wayland server. Note that window management and composition are
handled entirely in the server, significantly reducing complexity
while marginally improving performance through reduced context
switching. The resulting system is easier to build and extend than a
similar X system, because often changes need only be made in one
place. Or in the case of protocol extensions, two (rather than 3 or 4
in the X case where window management and/or composition handling may
also need to be updated).
</para>
</section>
</chapter>
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