Tag Archives: X11

DRM Render- and Modeset-Nodes

Another year, another Google Summer of Code. This time I got the chance to work on something that I had on my TODO list for quite a long time: DRM Render- and Modeset-Nodes

As part of the X.org Foundation mentoring organization, I will try to pick up the work from Ilija HadzicDave AirlieKristian HoegsbergMartin Peres and others. The idea is to extend the DRM user-space API of the linux kernel to split modeset and rendering interfaces apart. The main usage is to allow different access-modes for graphics-compositors (which require the modeset API) and client-side rendering or GPGPU-users (which both require the rendering API). We currently use the DRM-Master interface to restrict the modeset API to privileged applications. However, this requires SYS_CAP_ADMIN privileges, which is roughly equivalent to root-privileges. With two different interfaces for modeset and rendering APIs, we can apply a different set of filesystem-access-modes to each of them and thus get fine-grained access-control.

Apart from fine-grained access control, we also get some other nice features almost for free:

  • We will be able to run GPGPU clients without any running compositor or event without any display controller
  • We can split modeset objects across multiple nodes to allow multi-seat setups with a single display controller
  • Efficient compositor-stacking by granting page-flip access or full modeset access temporarily to sub-compositors

There are actually a lot of other ideas how to extend this. So I decided to concentrate on the modeset-node / render-node split first. Once that is done (and fully working), I will pick different ideas that depend on this and try to implement them. Considering the lot of work others have put in this already, I think if I get the split merged into mainline, the project will already be a great success. Everything on top of it will be some bonus that will probably take more time to get merged. But lets see, maybe it turns out to be easier than I think and we end up with some of the use-cases merged upstream, too.

Thanks to the X.org Foundation, Google and my GSoC-mentor Dave Airlie for giving me the chance to work on the DRM API! I hope it will be a productive summer.


If someone is interested in more details, some excerpts from my original GSoC proposal:

Project Description:
Since several years xserver is no longer the only user-space project that makes
use of the kernel DRM API. The introduction of KMS allowed many new projects to
emerge, including plymouth, weston and kmscon. On the other side, OpenCL support
allows applications to make use of DRM without requiring any KMS APIs. Even though
both use-cases work with the current APIs, there are a lot of restrictions that
need to be worked around.

The most problematic concept is DRM-Master. KMS applications are required to be
DRM-Master to perform modesetting, but DRM-Master is tightly coupled to
CAP_SYS_ADMIN/root. On the other side, render clients are required to be assigned
to a DRM-Master so they can get authenticated. This prevents off-screen/offline
rendering without a running compositor.

One possible solution is to split render- and modeset-nodes apart. The DRM control
node can be used as the management node (which is, as far as I understand, what
it was designed for, anyway). A separate static render-node is created for each
DRM device which is restricted to ioctls specific to rendering operations. Instead
of requiring drmAuth() for authorization, we can now use filesystem access-modes.
This allows slightly more dynamic access-control, but the biggest advantage is
that we can do off-screen/offline rendering without a running

On the other side, a modeset-node is a concept to have KMS separated from DRM.
The use-case is to split modeset objects (eg., crtcs, encoders, connectors) across
different modesetting applications. This allows one compositor to use one
CRTC+connector combination, while another compositor (maybe on another seat) can
use another CRTC+external-connector. This doesn't have to be a static setup. On
the contrary, one use-case I am very interested in is a dynamic modeset-object
assignment to temporary clients. This way, a fullscreen application can be granted
page-flip rights from the compositor to avoid context-switches to the compositor
for doing trivial page-flips only.

* Working render-node clients: Preferably an offline OpenCL example and
  a wayland EGL client
* Merged kernel render-node implementation with at least i915 support
* Dynamic kernel modeset-nodes
* "Zero-context-switches" wayland/weston fullscreen client (optional)

Known Problems:
There were several attempts to push render-nodes into the kernel, but all failed
due to missing motivation to finish the user-space clients. Writing up new fancy
APIs is one part, but pushing API changes to such big projects requires the whole
environment to work well with the changes. That's why I want to concentrate on
the user-space side of render-nodes. And I want to finish the render-nodes project
before continuing with modeset-nodes. The idea has been around long enough that
it's time that we get it done.

However, one problem is that I never worked with the low-level X11 stack. The
wayland environment is great for experiments and quite active. I am very familiar
with it and know how to get examples easily running. The xserver, however, is a
huge black box to me. I know the concepts and understand the input and graphics
drivers design. But I never read xserver core code. That's something I'd like to
change during this project. I will probably be limited to DRI and graphics
drivers, but that's a good start.

Another idea that came up quite often is something like gem-fs. It's far beyond
the scope of this project, but it's something I'd like to keep in mind when
designing the API. It's hard to account for something that's only an idea, but
the concepts seem related so I will try to understand the reasons behind gem-fs
and avoid orthogonal implementations.

A few smaller implementation-specific problems are already known, including the
mmap-security problem, static "possible_encoders"/"possible_crtcs" bitsets and
missing MMUs on GPUs. However, there already have been ideas how to solve them
so I don't consider them blockers for render-nodes.


During Google Summer of Code (GSoC) 2011 I developed a Linux kernel driver for the Nintendo Wii Remote. With linux-3.1 release the driver was released with the upstream kernel sources and in a few weeks with linux-3.3 extension-support will be available. I tested the driver for a while now and despite several Bluetooth HIDP bugs I didn’t find any bugs in the hid-wiimote driver. The Bluetooth core is currently undergoing heavy changes and it might take a few weeks until the HIDP driver is stable but it works quite reliable for me.

Although the driver is available in most mainstream distributions the user-space part lagged behind for half a year. So I decided to write an X11 driver that works with the kernel Wii Remote driver. The first step was creating the xwiimote tools which provide a user-space library that allows very easy access to connected Wii Remotes and some debugging tools for connection tests. The library is still under development but the Core, Accelerometer and IR interfaces of Wii Remotes are supported. Based on this library I started the X11 input driver and released the version 0.2 yesterday. It currently only supports button input but the most challenging part was getting the X.Org module right and working around some epoll+O_SETSIG bugs.

Anyway, if you own a Nintendo Wii Remote there’s few steps you need to do to connect your Wii Remote:

  1. Install the xf86-input-xwiimote driver (this requires installing the xwiimote-tools). If you use ArchLinux they are available in the AUR. Make sure the hid-wiimote kernel driver is loaded.
  2. Install the BlueZ Bluetooth stack (this is the official Linux Bluetooth stack). See your distribution for more information.
  3. Start your Bluetooth-Applet of choice (like gnome-bluetooth, blueman or bluez-simple-agent)
  4. Search for nearby devices (device inquiry) and connect to your Wii Remote (it is called Nintendo RVL-CNT-1). If you use bluez-4.96 or newer than everything should work out-of-the box. However, if you are asked for PIN-input then you either use an older version or your Wii Remote is not detected. Simply connect to the device without Pairing/Bonding and everything should work fine. Pairing with Wii Remotes is only supported since bluez-4.96 as the Wii Remote does not follow the standards and needs special BlueZ-plugins.
  5. If your Wii Remote is connected dmesg should show some information about the Wii Remote. You can also use the xwiishow tool from the xwiimote-tools project (See man xwiishow).
  6. Your X-Server should automatically pick up the Wii Remote and load the xwiimote driver. The D-Pad should work as Left/Right/Up/Down keys and the other keys should also have useful mappings. Seeman xorg-xwiimote for configuration options.

That’s all you need to do to enable your Wii Remote as input device. I must admit that the most interesting parts (getting the IR cam and accelerometer as mouse-emulation, sound support, extension support) are still not supported by the X.Org driver. However, the kernel driver does support all this (except sound support) so it shouldn’t be very difficult to add support for these to xf86-input-xwiimote. At least the Linux user-space now has support for Nintendo Wii Remotes based on the hid-wiimote kernel driver and the most requested feature (button/key input) is now available and can be mapped to arbitrary buttons/keys.

If the software is not working on your distribution, please don’t hesitate to fill bug reports at http://github.com/dvdhrm/xwiimote or contact me directly per email.