Commit a6f6999e authored by Kristian Høgsberg's avatar Kristian Høgsberg
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Consolidate NOTES and README

parent a46dc06d
Wayland is a nano display server, relying on drm modesetting, gem
batchbuffer submission and hw initialization generally in the kernel.
Wayland puts the compositing manager and display server in the same
process. Window management is largely pushed to the clients, they
draw their own decorations and move and resize themselves, typically
implemented in a toolkit library. More of the core desktop could be
pushed into wayland, for example, stock desktop components such as the
panel or the desktop background.
The actual compositor will define a fair bit of desktop policy and it
is expected that different use cases (desktop environments, devices,
appliances) will provide their own custom compositor.
It is still designed with a windowed type of desktop in mind, as
opposed to fullscreen-all-the-time type of interface, but should be
useful wherever several processes contribute content to be composited.
Current trends goes towards less and less rendering in X server, more
hardware setup and management in kernel and shared libraries allow
code sharing without putting it all in a server. freetype,
fontconfig, cairo all point in this direction, as does direct
rendering mesa.
Client allocates DRM buffers, draws decorations, and full window
contents and posts entire thing to server along with dimensions.
Everything is direct rendered and composited. No cliprects, no
drawing api/protocl between server and client. No
pixmaps/windows/drawables, only surfaces (essentially pixmaps). No
gcs/fonts, no nested windows. OpenGL is already direct rendered,
pixman may be direct rendered which adds the cairo API, or cairo
may gain a GL backend.
Could be a "shell" for launching gdm X server, user session servers,
safe mode xservers, graphics text console. From gdm, we could also
launch a rdp session, solid ice sessions.
All surface commands (copy, attach, map=set quads) are buffered until
the client sends a commit command, which executes everything
atomically. The commit command includes a cookie, which will be
returned in an event generated by the server once the commit has been
executed. This allows clients to throttle themselves against the
server and implement smooth animations.
Throttling/scheduling - there is currently no mechanism for scheduling
clients to prevent greedy clients from spamming the server and
starving other clients. On the other hand, now that recompositing is
done in the idle handler (and eventually at vertical retrace time),
there's nothing a client can do to hog the server. Unless we include
a copyregion type request, to let a client update it's surface
contents by asking the server to atomically copy a region from some
other buffer to the surface buffer.
Atomicity - we have the map and the attach requests which sometimes
will have to be executed atomically. Moving the window is done using
the map request and will not involve an attach requet. Updating the
window contents will use an attach request but no map. Resizing,
however, will use both and in that case must be executed atomically.
One way to do this is to have the server always batch up requests and
then introduce a kind of "commit" request, which will push the batched
changes into effect. This is easier than it sounds, since we only
have to remember the most recent map and most recent attach. The
commit request will generate an corresponding commit event once the
committed changes become visible on screen. The client can provide a
bread-crumb id in the commit request, which will be sent back in the
commit event.
- is batching+commit per client or per surface? Much more convenient
if per-client, since a client can batch up a bunch of stuff and get
atomic updates to multiple windows. Also nice to only get one
commit event for changes to a bunch of windows. Is a little more
tricky server-side, since we now have to keep a list of windows
with pending changes in the wl_client struct.
- batching+commit also lets a client reuse parts of the surface
buffer without allocating a new full-size back buffer. For
scrolling, for example, the client can render just the newly
exposed part of the page to a smaller temporary buffer, then issue
a copy request to copy the preserved part of the page up, and the
new part of the page into the exposed area.
- This does let a client batch up an uncontrolled amount of copy
requests that the server has to execute when it gets the commit
request. This could potentially lock up the server for a while,
leading to lost frames. Should never cause tearing though, we're
changing the surface contents, not the server back buffer which is
what is scheduled for blitting at vsync time.
These instructions assume some familiarity with git and building and
running experimental software. And be prepared that this project
What is Wayland
Wayland is a project to define a protocol for a compositor to talk to
its clients as well as a library implementation of the protocol. The
compositor can be a standalone display server running on Linux kernel
modesetting and evdev input devices, an X applications, or a wayland
client itself. The clients can be traditional appliactions, X servers
(rootless or fullscreen) or other display servers.
The wayland protocol is essentially only about input handling and
buffer management. The compositor receives input events and forwards
them to the relevant client. The clients creates buffers and renders
into them and notifies the compositor when it needs to redraw. The
protocol also handles drag and drop, selections, window management and
other interactions that must go throught the compositor. However, the
protocol does not handle rendering, which is one of the features that
makes wayland so simple. All clients are expected to handle rendering
themselves, typically through cairo or OpenGL.
The wayland repository includes a compositor and a few clients, but
both the compositor and clients are essentially test cases.
Building Instructions
The instructions below assume some familiarity with git and building
and running experimental software. And be prepared that this project
isn't at all useful right now, it's still very much a prototype. When
the instructions suggest to clone a git repo, you can of course just
add a remote and fetch instead, if you have a clone of that repo
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