This is an introductory overview post for the Linux Graphics Stack,
and how it currently all fits together. I initially wrote it for myself
after having conversations with people like Owen Taylor, Ray Strode and
Adam Jackson about this stack. I had to go back to them every month or
so and learn the stuff from the ground up all over again, as I had
forgotten every single piece. I asked them for a good high-level
overview document so I could stop bothering them. They didn’t know of
any. I started this one. It has been reviewed by Adam Jackson and David
Airlie, both of whom work on this exact stack.
Also, I want to point out that a large amount of this stack applies
only to the free software drivers. That means that a lot of what you
read here may not apply for the AMD Catalyst and NVidia proprietary
drivers. They may have their own implementations of OpenGL, or an
internal fork of mesa. I’m describing the stack that comes with the free
radeon, nouveau and Intel drivers.
If you have any questions, or if at any point things were unclear, or
if I’m horribly, horribly wrong about something, or if I just botched a
sentence so that it’s incomprehensible, please ask me or let me know in
the comments section below.
To start us off, I’m going to paste the entire big stack right here,
to let you get a broad overview of where every piece fits in the stack.
If you do not understand this right away, don’t be scared. Feel free to
refer to this throughout the post.
Here’s a handy link.
So, to be precise, there are two different paths, depending on the type of rendering you’re doing.
- 3D rendering with OpenGL
-
- Your program starts up, using “OpenGL” to draw.
- A library, “mesa”, implements the OpenGL API. It uses card-specific
drivers to translate the API into a hardware-specific form. If the
driver uses Gallium internally, there’s a shared component that turns
the OpenGL API into a common intermediate representation, TGSI. The API
is passed through Gallium, and all the device-specific driver does is
translate from TGSI into hardware commands,
- libdrm uses special secret card-specific
ioctls to talk to the Linux kernel
- The Linux kernel, having special permissions, can allocate memory on and for the card.
- Back out at the mesa level, mesa uses DRI2 to talk to Xorg to make
sure that buffer flips and window positions, etc. are synchronized.
- 2D rendering with cairo
-
- Your program starts up, using cairo to draw.
- You draw some circles using a gradient. cairo decomposes the circles
into trapezoids, and sends these trapezoids and gradients to the X
server using the XRender extension. In the case where the X server
doesn’t support the XRender extension, cairo draws locally using
libpixman, and uses some other method to send the rendered pixmap to the
X server.
- The X server acknowledges the XRender request. Xorg can use multiple specialized drivers to the drawing.
- In a software fallback case, or in the case the graphics driver
isn’t up to the task, Xorg will use pixman to do the actual drawing,
similar to how cairo does it in its case.
- In a hardware-accelerated case, the Xorg driver will speak libdrm to
the kernel, and send textures and commands to the card in the same way.
As to how Xorg gets things on the screen, Xorg itself will set up a
framebuffer to draw into using KMS and card-specific drivers.
X Window System, X11, Xlib, Xorg
X11 isn’t just related to graphics; it has an event delivery system,
the concept of properties attached to windows, and more. Lots of other
non-graphical things are built on top of it (clipboard, drag and drop).
Only listed in here for completeness, and as an introduction. I’ll try
and post about the entire X Window System, X11 and all its strange
design decisions later.
- X11
- The wire protocol used by the X Window System
- Xlib
- The reference implementation of the client side of the system, and a
host of tons of other utilities to manage windows on the X Window
System. Used by toolkits with support for X, like GTK+ and Qt.
Vanishingly rare to see in applications today.
- XCB
- Sometimes quoted as an alternative to Xlib. It implements a large
amount of the X11 protocol. Its API is at a much lower level than Xlib,
such that Xlib is actually built on XCB nowadays. Only mentioning it
because it’s another acronym.
- Xorg
- The reference implementation of the server side of the system.
As
I’ll try to be very careful to label something correctly. If I say “the
X server”, I’m talking about a generic X server: it may be Xorg, it may
be Apple’s X server implementation, it may be Kdrive. We don’t know. If
I say “X11″, or the “X Window System”, I’m talking about the design of
the protocol or system overall. If I say “Xorg”, that means that it’s an
implementation detail of Xorg, which is the most used X server, and may
not apply to any other X servers at all. If I ever say “X” by itself,
it’s a bug.
X11, the protocol, was designed to be extensible, which means that
support for new features can be added without creating a new protocol
and breaking existing old clients. As an example,
xeyes and
oclock get their fancy shapes because of the
Shape Extension,
which provide support for non-rectangular windows. If you’re curious
how this magic functionality can just appear out of nowhere, the answer
is that it doesn’t: support for the extension has to be added to both
the server and client before it can be used. There is functionality in
the core protocol itself so that clients can ask the server what
extensions it has support for, so it knows what functionality it can or
cannot use.
X11 was also designed to be “network transparent”. Most importantly
it means that we cannot rely on the X server and any X client being on
the same machine, so any talking between the two must go over the
network. In actuality, a modern desktop environment does not work out of
the box in this scenario, as a lot of inter-process communication goes
through systems other than X11, like DBus. Over the network, it’s quite
chatty, and results in a lot of traffic. When the server and client are
on the same machine, instead of going over the network, they’ll go over a
UNIX socket, and the kernel doesn’t have to copy data around.
We’ll get back to the X Window System and its numerous extensions in a little bit.
cairo
cairo is a drawing library used either by applications like
Firefox directly, or through libraries like GTK+, to draw vector shapes.
GTK+3′s drawing model is built entirely on cairo. If you’ve ever used
an HTML5
<canvas>, cairo implements pratically the same API. Although
<canvas>
was originally developed by Apple, the vector drawing model is
well-known, as it’s the PostScript vector drawing model, which has found
support in other vector graphics technologies and standards such as
PDF, Flash, SVG, Direct2D, Quartz 2D, OpenVG, and lots more than I can
possibly give an exhaustive list for.
cairo has support to draw to X11 surfaces through the
Xlib backend.
cairo has been used in toolkits like GTK+. Functionality was added in
GTK+ 2 to optionally use cairo in GTK+ 2.8. The drawing model in GTK+3
requires cairo.
XRender Extension
X11 has a special extension,
XRender, which adds support for
anti-aliased drawing primitives (X11′s existing graphics were aliased),
gradients, matrix transforms and more. The original intention was that
drivers could have specialized accelerated code paths for doing specific
drawing. Unfortunately, it seems to be the case that
software rasterization is just as fast,
for unintuitive reasons. Oh well. XRender deals in aligned trapezoids –
rectangles with an optional slant to the left and right edges. Carl
Worth and Keith Packard came up with a “fast” software rasterization
method for trapezoids. Trapezoids are also super easy to decompose into
two triangles, to align ourselves with fast hardware rendering. Cairo
includes a wonderful
show-traps utility that might give you a bit of insight into the tesselation into trapezoids it does.
Here’s a simple red circle we drew. This is decomposed into two sets
of trapezoids – one for the stroke, and one for the fill. Since the
diagrams that
show-traps gives you by default aren’t very
enlightening, I hacked up the utility to give each trapezoid a unique
color. Here’s the set of trapezoids for the black stroke.
Psychedelic.
pixman
Both the X server and cairo need to do pixel level manipulation at
some point. cairo and Xorg had separate implementations of things like
basic rasterization algorithms, pixel-level access for certain kinds of
buffers (ARGB32, RGB24, RGB565), gradients, matrices, and a lot more.
Now, both the X server and cairo share a low-level library called
pixman,
which handles these things for it. pixman is not supposed to be a
public API, nor is it a drawing API. It’s not really any API at all;
it’s just a solution to some code duplication between various parts.
OpenGL, mesa, gallium
Now comes the fun part: modern hardware acceleration. I assume
everybody already knows what OpenGL is. It’s not a library, there will
never be one set of sources to a
libGL.so. Each vendor is supposed to provide its own
libGL.so. nVidia provides its own implementation of OpenGL and ships its own
libGL.so, based on its implementations for Windows and OS X.
If you are running open-source drivers, your
libGL.so implementation probably comes from
mesa.
mesa
is many things, but one of the major things it provides that it is most
famous for is its OpenGL implementation. It is an open-source
implementation of the OpenGL API. mesa itself has multiple backends for
which it provides support. It has three CPU-based implementations:
swrast (outdated and old, do not use it), softpipe (slow), llvmpipe
(potentially fast).
mesa also has hardware-specific
drivers. Intel supports mesa and has built a number of drivers for their
chipsets which are shipped inside mesa. The radeon and nouveau drivers
are also supported in mesa, but are built on a different architecture:
gallium.
gallium isn’t really anything magical. It’s a set of
components to make implementing drivers a lot easier. The idea is that
there are state trackers that implement some form of API (OpenGL, GLSL,
Direct3D), transform that state to an intermediate representation (known
as
Tungsten Graphics Shader Infrastructure, or TGSI),
and then the backends take that intermediate representation and convert
it to the operations that would be consumed by the hardware itself.
Sadly, the Intel drivers don’t use Gallium. My coworkers tell me it’s
because the Intel driver developers do not like having a layer between
Mesa and their driver.
A quick aside: more acronyms
Since there’s a lot of confusing acronyms to cover, and I don’t want
to give them each an H3 and write a paragraph for each, I’m just going
to list them here. Most of them don’t really matter in today’s world,
they’re just an easy reference so you don’t get confused.
- GLES
- OpenGL has several profiles for different form factors. GLES
is one of them, standing for “GL Embedded System” or “GL Embedded
Subset”, depending on who you ask. It’s the latest approach to target
the embedded market. The iPhone supports GLES 2.0.
- GLX
- OpenGL has no concept of platform and window systems on its own.
Thus, bindings are needed to translate between the differences of OpenGL
and something like X11. For instance, putting an OpenGL scene inside an
X11 window. GLX is this glue.
- WGL
- See above, but replace “X11″ with “Windows”, that is, that one Microsoft Operating System.
- EGL
- EGL and GLES are often confused. EGL is a new
platform-agnostic API, developed by Khronos Group (the same group that
develops and standardizes OpenGL) that provides facilities to get an
OpenGL scene up and running on a platform. Like OpenGL, this is
vendor-implemented; it’s an alternative to bindings like WGL/GLX and not
just a library on top of them, like GLUT.
- fglrx
- fglrx is former name for AMD’s proprietary Xorg OpenGL
driver, now known as “Catalyst”. It stands for “FireGL and Radeon for
X”. Since it is a proprietary driver, it has its own implemnetation of
libGL.so.
I do not know if it is based on mesa. I’m only mentioning it because
it’s sometimes confused with generic technology like AIGLX or GLX, due
to the appearance of the letters “GL” and “X”.
- DIX, DDX
- The X graphics parts of Xorg is made up of two major parts,
DIX, the “Driver Independent X” subsystem, and DDX, the “Driver Dependent X” subsystem. When we talk about an Xorg driver, the more technically accurate term is a DDX driver.
Xorg Drivers, DRM, DRI
A little while back I mentioned that Xorg has the ability to do
accelerated rendering, based on specific pieces of hardware. I’ll also
say that this is not implemented by translating from X11 drawing
commands into OpenGL calls. If the drivers are implemented in mesa land,
how can this work without making Xorg dependent on mesa?
The answer was to create a new piece of infrastructure to be shared
between mesa and Xorg. mesa implements the OpenGL parts, Xorg implements
the X11 drawing parts, and they both convert to a set of card-specific
commands. These commands are then uploaded to the kernel, using
something called the “Direct Rendering Manager”, or
DRM. libdrm uses a set of generic, private
ioctls with the kernel to allocate things on the card, and stuffs the commands and textures and things it needs in there. This
ioctl interface comes in two forms: Intel’s
GEM, and Tungsten Graphics’s
TTM.
There is no good distinction between them; they both do the same thing,
they’re just different competing implementation. Historically, GEM was
designed and proudly announced to be a simpler alternative to TTM, but
over time, it has quietly grown to about the same complexity as TTM.
Welp.
This means that when you run something like
glxgears, it loads mesa. mesa itself loads libdrm, and that talks to the kernel driver directly using GEM/TTM. Yes,
glxgears
talks to the kernel driver directly to show you some spinning gears,
and sternly remind you of the benchmarking contents of said utility.
If you poke in
ls /usr/lib64/libdrm_*, you’ll note that
there are hardware-specific drivers. For cases when GEM/TTM aren’t
enough, the mesa and X server drivers will have a set of private
ioctls to talk to the kernel, which are encapsulated in here. libdrm itself doesn’t actually load these.
The X server needs to know what’s happening here, though, so it can
do things like synchronization. This synchronization between your
glxgears, the kernel, and the X server is called
DRI,
or more accurately, DRI2. “DRI” stands for “Direct Rendering
Infrastructure”, but it’s sort of a strange acronym. “DRI” refers to
both the project that glued mesa and Xorg together (introducing DRM and a
bunch of the things I talk about in this article), as well as the DRI
protocol and library. DRI 1 wasn’t really that good, so we threw it out
and replaced it with DRI 2.
KMS
As a sort of aside, let’s say you’re working on a new X server, or
maybe you want to show graphics on a VT without using an X server. How
do you do it? You have to configure the actual hardware to be able to
put up graphics. Inside of libdrm and the kernel, there’s a special
subsystem that does exactly that, called
KMS, which stands for “Kernel Mode Setting”. Again, through a set of
ioctls,
it’s possible to set up a graphics mode, map a framebuffer, and so on,
to display directly on a TTY. Before, there were (and still are)
hardware-specific
ioctls, so a shared library called libkms
was created to give a shared API. Eventually, there was a new API at
the kernel level, literally called the “dumb
ioctls”. With the new dumb
ioctls in place, it is recommended to use those and not libkms.
While it’s very low-level, it’s entirely possible to do. Plymouth,
the boot splash screen integrated into modern distributions, is a good
example of a very simple application that does this to set up graphics
without relying on an X server.
The “Expose” model, Redirection, TFP, Compositing, AIGLX
I’ve used the term “compositing window manager” without really
describing what it means to composite, or what a window manager does.
You see, back in the 80s when the X Window System was designed on UNIX
systems, a lot of random other companies like HP, Digital Equipment
Corp., Sun Microsystems, SGI, were also developing products based on the
X Window System. X11 intentionally didn’t mandate any basic policy on
how windows were to be controlled, and delegated that responsibility to a
separate process called the “window manager”.
As an example, the popular environment at the time, CDE, had a system
called “focus follows mouse”, which focused windows when the user moved
their mouse over the window itself. This is different from the more
popular model that Windows and Mac OS X use by default, “click to
focus”.
As window managers started to become more and more complex,
documents started to
appear
describing interoperability between the many different environments.
It, too, wouldn’t really mandate any policy either like “Click to Focus”
either.
Additionally, back in the 80s, many systems did not have much memory.
They could not store the entire pixel contents of all window contents.
Windows and X11 solve this issue in the same way: each X11 window should
be lossy. Namely, a program will be notified that a window has been
“exposed”.
Imagine a set of windows like this. Now let’s say the user drags GIMP away:
The area in the dark gray has been exposed. An
ExposeEvent
will get sent to the program that owns the window, and it will have to
redraw contents. This is why hung programs in versions of Windows or
Linux would go blank after you dragged a window over them. Consider the
fact that in Windows, the desktop itself is just another program without
any special privileges, and can hang like all the others, and you have
one hell of a bug report.
So, now that machines have as much memory as they do, we have the opportunity to make X11 windows
lossless, by a mechanism called
redirection.
When we redirect a window, the X server will create backing pixmaps for
each window, instead of drawing directly to the backbuffer. This means
that the window will be hidden entirely. Something
else has to take the opportunity to display the pixels in the memory buffer.
The
composite extension allows a compositing window manager, or “compositor”, to set up something called the
Composite Overlay Window, or
COW.
The compositor owns the COW, and can paint to it. When you run Compiz
or GNOME Shell, these programs are using OpenGL to display the
redirected windows on the screen. The X server can give them the window
contents by a GL extension, “
Texture from Pixmap“, or
TFP. It lets an OpenGL program use an X11 Pixmap as if it were an OpenGL Texture.
Compositing window managers don’t
have to use TFP or OpenGL,
per se, it’s just the easiest way to do so. They could, if they wanted
to, draw the window pixmaps onto the COW normally. I’ve been told that
kwin4 uses Qt directly to composite windows.
Compositing window managers grab the pixmap from the X server using
TFP, and then render it on to the OpenGL scene at just the right place,
giving the illusion that the thing that you think you’re clicking on is
the actual X11 window. It may sound silly to describe this as an
illusion, but play around with GNOME Shell and adjust the size or
position of the window actors (Enter
global.get_window_actors().forEach(function(w) { w.scale_x = w.scale_y = 0.5; })
in the looking glass), and you’ll quickly see the illusion break down
in front of you, as you realize that when you click, you’re poking
straight through the video player, and to the actual window underneath
(Change
0.5 to
1.0 in the above snippet to revert the behavior).
With this information, let’s explain one more acronym:
AIGLX.
AIGLX stands for “Accelerated Indirect GLX”. As X11 is a networked
protocol, this means that OpenGL should have to work over the network.
When OpenGL is being used over the network, this is called an “indirect
context”, versus a “direct context” where things are on the same
machine. The network protocol used in an “indirect context’ is fairly
incomplete and unstable.
To understand the design decision behind AIGLX, you have to
understand the problem that it was trying to solve: making compositing
window managers like Compiz fast. While nvidia’s proprietary driver had
kernel-level memory manangement through a custom interface, the
open-source stack at this point hadn’t achieved that yet. Pulling a
window texture from the X server into the graphics hardware would have
meant that it would have to be copied every time the window was updated.
Slow. As such, AIGLX was a temporary hack to implement OpenGL in
software, preventing the copy into hardware acceleration. As the scene
that compositors like Compiz used wasn’t very complex, it worked well
enough.
Despite all the fanfare and Phoronix articles, AIGLX hasn’t been used
realistically for a while, as we now have the entire DRI stack which
can be used to implement TFP without a copy.
As you can imagine, copying (or more accurately, sampling) the window
texture contents so that it can be painted as an OpenGL texture
requires copying data. As such, most window managers have a feature to
turn redirection off for a window that’s full-screen. It may sound a bit
silly to describe this as
unredirection, as that’s the initial
state a window is in. But while it may be the initial state for a
window, with our modern Linux desktops, it’s hardly the usual state. The
logic here is that if a window would be covering the COW anyway and
compositing features aren’t necessary, it can safely be unredirected.
This feature is designed to give high performance to programs like
games, which need to run with high performance at 60 frames per second.
Wayland
As you can see, we’ve split out quite a large bit of the original
infrastructure from X’s initial monolithic behavior. This isn’t the only
place where we’ve tore down the monolithic parts of X: a lot of the
input device handling has moved into the kernel with evdev, and things
like device hotplug support has been moved back into udev.
A large reason that the X Window System stuck around for now was just
because it was a lot of effort to replace it. With Xorg stripped down
from what it initially was, and with a large amount of functionality
required for a modern desktop environment provided solely by extensions,
well, how can I say this,
X is overdue.
Enter Wayland. Wayland reuses a lot of existing infrastructure that
we’ve built up. One of the most controversial things about it is that it
lacks any sort of network transparency or drawing protocol. X’s network
transparency falls flat in modern times. A large amount of Linux
features are hosted in places like DBus, not X, and it’s a shame to see
things like Drag and Drop and Clipboard support being by and large hacks
with the X Window System solely for network support.
Wayland can use almost the entire stack as detailed above to get a
framebuffer on your monitor up and running. Wayland still has a
protocol, but it’s based around UNIX sockets and local resources. The
biggest drastic change is that there is no
/usr/bin/wayland binary running like there is a
/usr/bin/Xorg.
Instead, Wayland follows the modern desktop’s advice and moves all of
this into the window manager process instead. These window managers,
more accurately called “compositors” in Wayland terms, are actually in
charge of pulling events from the kernel with a system like evdev,
setting up a frame buffer using KMS and DRM, and displaying windows on
the screen with whatever drawing stack they want, including OpenGL.
While this may sound like a lot of code, since these subsystems have
moved elsewhere, code to do all of these things would probably be on the
order of 2000-3000 SLOC. Consider that the portion of mutter just to
implement a sane window focus and stacking policy and synchronize it
with the X server is around 4000-5000 SLOC, and maybe you’ll understand
my fascination a bit more.
While Wayland does have a library that implementations of both
clients and compositors probably should use, it’s simply a reference
implementation of the specified Wayland protocol. Somebody could write a
Wayland compositor entirely in Python or Ruby and implement the
protocol in pure Python, without the help of a libwayland.
Wayland clients talk to the compositor and request a buffer. The
compositor will hand them back a buffer that they can draw into, using
OpenGL, cairo, or whatever. The compositor is at the discretion to do
whatever it wants with that buffer – display it normally because it’s
awesome, set it on fire because the application is being annoying, or
spin it on a cube because we need more YouTube videos of Linux cube
spinning.
The compositor is also in control of input and event handling. If you
tried out the thing with setting the scale of the windows in GNOME
Shell above, you may have been confused at first, and then figured out
that your mouse corresponded to the untransformed window. This is
because we weren’t *actually* affecting the X11 window itself, just
changing how it gets displayed. The X server keeps track of where
windows are, and it’s up to the compositing window manager to display
them where the X server thinks it is, otherwise, confusion happens.
Since a Wayland compositor is in charge of reading from evdev and
giving windows events, it probably has a much better idea of where a
window is, and can do the transformations internally, meaning that not
only can we spin windows on a cube temporarily, we will be able to
interact with windows on a cube.
Summary
I still hear a lot today that Xorg is very monolithic in its
implementation. While this is very true, it’s less true than it was a
long while ago. This isn’t due to the incompetence on the part of Xorg
developers, a large amount of this is due to baggage that we just have
to support, like the hardware-accelerated XRender protocol, or going
back even further, non-anti-aliased drawing commands like
XPolyFill.
While it’s very apparent that X is going to go away in favor of Wayland
some time soon, I want to make it clear that a lot of this is happening
with the acknowledgement and help of the Xorg and desktop developers.
They’re not stubborn, and they’re not incompetent. Hell, for dealing
with and implementing a 30-year-old protocol plus history, they’re doing
an excellent job, especially with the new architecture.
I also want to give a big shout-out to everybody who worked on the
stuff I mentioned in this article, and also want to give my personal
thanks to Owen Taylor, Ray Strode and Adam Jackson for being extremely
patient and answering all my dumb questions, and to Dave Airlie and Adam
Jackson for helping me technically review this article.
While I went into some level of detail in each of these pieces,
there’s a lot more here where you could study in a lot more detail if it
interests you. To pick just a few examples, you could study the the
geometry algorithms and theories that cairo exploits to convert
arbitrary shapes to trapezoids. Or maybe the fast software rendering
algorithm for trapezoids by Carl Worth and Keith Packard and investigate
why it’s fast. Consider looking at the design of DRI2, and how it
differs from DRI1. Or maybe you’re interested in the hardware itself,
and looking at graphics card architecture, and looking at the data
sheets to see how you would program one. And if you want to help out in
any of these areas, I assume all the projects listed above would be more
than happy to have contributions.
I’m planning on writing more of these in the future. A large amount
of the stacks used in the Linux and GNOME community today don’t have a
good overview document detailing them from a very high level.
Version History
- Published
- Converted CSS-based diagrams to images to make it easier for Planet GNOME readers
- Fixed Looking Glass example line
- Fixed TFP link
- Modified the tone of the policy of window managers section to better convey the point
- Corrected information about GTK+ and cairo integration (GTK+ 2.8, not “near the end of the cycle”)
- Fixed acronym expansion for “fglrx”
(sumber: http://blog.mecheye.net/2012/06/the-linux-graphics-stack/)
The globetrotter. This person wants to 
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When you're researching a VPN, you'll see terms like SSL/TLS (sometimes
referred to as OpenVPN support,) PPTP, IPSec, L2TP, and other 
