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<?xml version='1.0' encoding='utf-8' ?>
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY % BOOK_ENTITIES SYSTEM "Wayland.ent">
%BOOK_ENTITIES;
]>
<chapter id="chap-Wayland-Architecture">
<title>Wayland Architecture</title>
<section id="sect-Wayland-Architecture-wayland_architecture">
<title>X vs. Wayland Architecture</title>
<para>
A good way to understand the Wayland architecture
and how it is different from X is to follow an event
from the input device to the point where the change
it affects appears on screen.
</para>
<para>
This is where we are now with X:
</para>
<figure>
<title>X architecture diagram</title>
<mediaobjectco>
<imageobjectco>
<areaspec id="map1" units="other" otherunits="imagemap">
<area id="area1_1" linkends="x_flow_1" x_steal="#step_1"/>
<area id="area1_2" linkends="x_flow_2" x_steal="#step_2"/>
<area id="area1_3" linkends="x_flow_3" x_steal="#step_3"/>
<area id="area1_4" linkends="x_flow_4" x_steal="#step_4"/>
<area id="area1_5" linkends="x_flow_5" x_steal="#step_5"/>
<area id="area1_6" linkends="x_flow_6" x_steal="#step_6"/>
</areaspec>
<imageobject>
<imagedata fileref="images/x-architecture.png" format="PNG" />
</imageobject>
</imageobjectco>
</mediaobjectco>
</figure>
<para>
<orderedlist>
<listitem id="x_flow_1">
<para>
The kernel gets an event from an input
device and sends it to X through the evdev
input driver. The kernel does all the hard
work here by driving the device and
translating the different device specific
event protocols to the linux evdev input
event standard.
</para>
</listitem>
<listitem id="x_flow_2">
<para>
The X server determines which window the
event affects and sends it to the clients
that have selected for the event in question
on that window. The X server doesn't
actually know how to do this right, since
the window location on screen is controlled
by the compositor and may be transformed in
a number of ways that the X server doesn't
understand (scaled down, rotated, wobbling,
etc).
</para>
</listitem>
<listitem id="x_flow_3">
<para>
The client looks at the event and decides
what to do. Often the UI will have to change
in response to the event - perhaps a check
box was clicked or the pointer entered a
button that must be highlighted. Thus the
client sends a rendering request back to the
X server.
</para>
</listitem>
<listitem id="x_flow_4">
<para>
When the X server receives the rendering
request, it sends it to the driver to let it
program the hardware to do the rendering.
The X server also calculates the bounding
region of the rendering, and sends that to
the compositor as a damage event.
</para>
</listitem>
<listitem id="x_flow_5">
<para>
The damage event tells the compositor that
something changed in the window and that it
has to recomposite the part of the screen
where that window is visible. The compositor
is responsible for rendering the entire
screen contents based on its scenegraph and
the contents of the X windows. Yet, it has
to go through the X server to render this.
</para>
</listitem>
<listitem id="x_flow_6">
<para>
The X server receives the rendering requests
from the compositor and either copies the
compositor back buffer to the front buffer
or does a pageflip. In the general case, the
X server has to do this step so it can
account for overlapping windows, which may
require clipping and determine whether or
not it can page flip. However, for a
compositor, which is always fullscreen, this
is another unnecessary context switch.
</para>
</listitem>
</orderedlist>
</para>
<para>
As suggested above, there are a few problems with this
approach. The X server doesn't have the information to
decide which window should receive the event, nor can it
transform the screen coordinates to window-local
coordinates. And even though X has handed responsibility for
the final painting of the screen to the compositing manager,
X still controls the front buffer and modesetting. Most of
the complexity that the X server used to handle is now
available in the kernel or self contained libraries (KMS,
evdev, mesa, fontconfig, freetype, cairo, Qt etc). In
general, the X server is now just a middle man that
introduces an extra step between applications and the
compositor and an extra step between the compositor and the
hardware.
</para>
<para>
In Wayland the compositor is the display server. We transfer
the control of KMS and evdev to the compositor. The Wayland
protocol lets the compositor send the input events directly
to the clients and lets the client send the damage event
directly to the compositor:
</para>
<figure>
<title>Wayland architecture diagram</title>
<mediaobjectco>
<imageobjectco>
<areaspec id="mapB" units="other" otherunits="imagemap">
<area id="areaB_1" linkends="wayland_flow_1" x_steal="#step_1"/>
<area id="areaB_2" linkends="wayland_flow_2" x_steal="#step_2"/>
<area id="areaB_3" linkends="wayland_flow_3" x_steal="#step_3"/>
<area id="areaB_4" linkends="wayland_flow_4" x_steal="#step_4"/>
</areaspec>
<imageobject>
<imagedata fileref="images/wayland-architecture.png" format="PNG" />
</imageobject>
</imageobjectco>
</mediaobjectco>
</figure>
<para>
<orderedlist>
<listitem id="wayland_flow_1">
<para>
The kernel gets an event and sends
it to the compositor. This
is similar to the X case, which is
great, since we get to reuse all the
input drivers in the kernel.
</para>
</listitem>
<listitem id="wayland_flow_2">
<para>
The compositor looks through its
scenegraph to determine which window
should receive the event. The
scenegraph corresponds to what's on
screen and the compositor
understands the transformations that
it may have applied to the elements
in the scenegraph. Thus, the
compositor can pick the right window
and transform the screen coordinates
to window-local coordinates, by
applying the inverse
transformations. The types of
transformation that can be applied
to a window is only restricted to
what the compositor can do, as long
as it can compute the inverse
transformation for the input events.
</para>
</listitem>
<listitem id="wayland_flow_3">
<para>
As in the X case, when the client
receives the event, it updates the
UI in response. But in the Wayland
case, the rendering happens in the
client, and the client just sends a
request to the compositor to
indicate the region that was
updated.
</para>
</listitem>
<listitem id="wayland_flow_4">
<para>
The compositor collects damage
requests from its clients and then
recomposites the screen. The
compositor can then directly issue
an ioctl to schedule a pageflip with
KMS.
</para>
</listitem>
</orderedlist>
</para>
</section>
<section id="sect-Wayland-Architecture-wayland_rendering">
<title>Wayland Rendering</title>
<para>
One of the details I left out in the above overview
is how clients actually render under Wayland. By
removing the X server from the picture we also
removed the mechanism by which X clients typically
render. But there's another mechanism that we're
already using with DRI2 under X: direct rendering.
With direct rendering, the client and the server
share a video memory buffer. The client links to a
rendering library such as OpenGL that knows how to
program the hardware and renders directly into the
buffer. The compositor in turn can take the buffer
and use it as a texture when it composites the
desktop. After the initial setup, the client only
needs to tell the compositor which buffer to use and
when and where it has rendered new content into it.
</para>
<para>
This leaves an application with two ways to update its window contents:
</para>
<para>
<orderedlist>
<listitem>
<para>
Render the new content into a new buffer and tell the compositor
to use that instead of the old buffer. The application can
allocate a new buffer every time it needs to update the window
contents or it can keep two (or more) buffers around and cycle
between them. The buffer management is entirely under
application control.
</para>
</listitem>
<listitem>
<para>
Render the new content into the buffer that it previously
told the compositor to to use. While it's possible to just
render directly into the buffer shared with the compositor,
this might race with the compositor. What can happen is that
repainting the window contents could be interrupted by the
compositor repainting the desktop. If the application gets
interrupted just after clearing the window but before
rendering the contents, the compositor will texture from a
blank buffer. The result is that the application window will
flicker between a blank window or half-rendered content. The
traditional way to avoid this is to render the new content
into a back buffer and then copy from there into the
compositor surface. The back buffer can be allocated on the
fly and just big enough to hold the new content, or the
application can keep a buffer around. Again, this is under
application control.
</para>
</listitem>
</orderedlist>
</para>
<para>
In either case, the application must tell the compositor
which area of the surface holds new contents. When the
application renders directly to the shared buffer, the
compositor needs to be noticed that there is new content.
But also when exchanging buffers, the compositor doesn't
assume anything changed, and needs a request from the
application before it will repaint the desktop. The idea
that even if an application passes a new buffer to the
compositor, only a small part of the buffer may be
different, like a blinking cursor or a spinner.
</para>
</section>
<section id="sect-Wayland-Architecture-wayland_hw_enabling">
<title>Accelerated GPU Buffer Exchange</title>
<para>
Clients
<ulink url="https://docs.kernel.org/userspace-api/dma-buf-alloc-exchange.html">exchange</ulink>
GPU buffers with the compositor as dma-buf file descriptors, which are universal handles
that are independent of any particular rendering API or memory allocator. The
<ulink url="https://gitlab.freedesktop.org/wayland/wayland-protocols/-/blob/main/stable/linux-dmabuf/linux-dmabuf-v1.xml">linux-dmabuf-v1</ulink>
protocol is used to turn one or more dma-buf FDs into a
<link linkend="protocol-spec-wl_buffer">wl_buffer</link>.
</para>
<para>
If the client uses the
<ulink url="https://docs.vulkan.org/spec/latest/chapters/VK_KHR_surface/wsi.html">Vulkan</ulink>
or
<ulink url="https://registry.khronos.org/EGL/extensions/EXT/EGL_EXT_platform_wayland.txt">EGL</ulink>
(via
<ulink url="https://gitlab.freedesktop.org/wayland/wayland/-/tree/main/egl">wayland-egl</ulink>)
window-system integration
(WSI), this is done transparently by the WSI.
</para>
<para>
Clients can alternatively allocate and import dma-bufs themselves
using the GBM library, Vulkan, udmabuf, or dma-buf heaps.
</para>
<itemizedlist>
<listitem>
<para>
Using GBM, the client can allocate a gbm_bo and export one or more
dma-buf FDs from it.
</para>
</listitem>
<listitem>
<para>
Using Vulkan, the client can create a VkDeviceMemory object and use
<ulink url="https://docs.vulkan.org/refpages/latest/refpages/source/VK_EXT_external_memory_dma_buf.html">VK_EXT_external_memory_dma_buf</ulink>
and
<ulink url="https://docs.vulkan.org/refpages/latest/refpages/source/VK_EXT_image_drm_format_modifier.html">VK_EXT_image_drm_format_modifier</ulink>
to export a dma-buf FD from it.
</para>
</listitem>
<listitem>
<para>
<ulink url="https://lwn.net/Articles/749206/">udmabuf</ulink>
can be used to create dma-buf FDs from linear host memory.
</para>
</listitem>
<listitem>
<para>
<ulink url="https://docs.kernel.org/userspace-api/dma-buf-heaps.html">Dma-buf heaps</ulink>
can be used by privileged applications to create dma-buf FDs on embedded
devices.
</para>
</listitem>
</itemizedlist>
<para>
Compositors use
<ulink url="https://docs.vulkan.org/refpages/latest/refpages/source/VK_EXT_external_memory_dma_buf.html">VK_EXT_external_memory_dma_buf</ulink>
and
<ulink url="https://docs.vulkan.org/refpages/latest/refpages/source/VK_EXT_image_drm_format_modifier.html">VK_EXT_image_drm_format_modifier</ulink>
or
<ulink url="https://registry.khronos.org/EGL/extensions/EXT/EGL_EXT_image_dma_buf_import.txt">EGL_EXT_image_dma_buf_import</ulink>
and
<ulink url="https://registry.khronos.org/EGL/extensions/EXT/EGL_EXT_image_dma_buf_import_modifiers.txt">EGL_EXT_image_dma_buf_import_modifiers</ulink>
to import the dma-bufs provided by the client into their own Vulkan or
EGL renderers.
</para>
<para>
Clients do not need to wait for the GPU to finish rendering before submitting
dma-bufs to the compositor. Clients can use the
<ulink url="https://gitlab.freedesktop.org/wayland/wayland-protocols/-/blob/main/staging/linux-drm-syncobj/linux-drm-syncobj-v1.xml">linux-drm-syncobj-v1</ulink>
protocol to exchange DRM synchronization objects with the compositor. These objects
are used to asynchronously signal ownership transfer of buffers from clients to the
compositor and vice versa. The WSIs do this transparently.
</para>
<para>
If the linux-drm-syncobj-v1 protocol is not supported by the compositor, clients
and compositors can use the
<ulink url="https://docs.kernel.org/driver-api/dma-buf.html#c.dma_buf_export_sync_file">DMA_BUF_IOCTL_EXPORT_SYNC_FILE</ulink>
and
<ulink url="https://docs.kernel.org/driver-api/dma-buf.html#c.dma_buf_import_sync_file">DMA_BUF_IOCTL_IMPORT_SYNC_FILE</ulink>
ioctls to access and create implicit synchronization barriers.
</para>
</section>
<section id="sect-Wayland-Architecture-kms">
<title>Display Programming</title>
<para>
Compositors enumerate DRM KMS devices using
<ulink url="https://en.wikipedia.org/wiki/Udev">udev</ulink>.
Udev also notifies compositors of KMS device and display hotplug events.
</para>
<para>
Access to DRM KMS device ioctls is privileged. Since compositors usually run as
unprivileged applications, they typically gain access to a privileged file
descriptor using the
<ulink url="https://www.freedesktop.org/software/systemd/man/latest/org.freedesktop.login1.html#Session%20Objects">TakeDevice</ulink>
method provided by logind.
</para>
<para>
Using the file descriptor, compositors use KMS
<ulink url="https://docs.kernel.org/gpu/drm-kms.html">ioctls</ulink>
to enumerate the available displays.
</para>
<para>
Compositors use
<ulink url="https://docs.kernel.org/gpu/drm-kms.html#atomic-mode-setting">atomic mode setting</ulink>
to change the buffer shown by the display, to change the display's resolution, to
enable or disable HDR, and so on.
</para>
</section>
</chapter>