Tag Archives: modesetting

Splitting DRM and KMS device nodes

While most devices of the 3 major x86 desktop GPU-providers have GPU and display-controllers merged on a single card, recent development (especially on ARM) shows that rendering (via GPU) and mode-setting (via display-controller) are not necessarily bound to the same device. To better support such devices, several changes are being worked on for DRM.

In it’s current form, the DRM subsystem provides one general-purpose device-node for each registered DRM device: /dev/dri/card<num>. An additional control-node is also created, but it remains unused as of this writing. While in general a kernel driver is allowed to register multiple DRM devices for a single physical device, no driver made use of this, yet. That means, whatever hardware you use, both mode-setting and rendering is done via the same device node. This entails some rather serious consequences:

  1. Access-management to mode-setting and rendering is done via the same file-system node
  2. Mode-setting resources of a single card cannot be split among multiple graphics-servers
  3. Sharing display-controllers between cards is rather complicated

In the following sections, I want to look closer at each of these points and describe what has been done and what is still planned to overcome these restrictions. This is a highly technical description of the changes and serves as outline for the Linux-Plumbers session on this topic. I expect the reader to be familiar with DRM internals.

1) Render-nodes

While render-nodes have been discussed since 2009 on dri-devel, several mmap-related security-issues have prevented it from being merged. Those have all been fixed and 3-days ago, the basic render-node infrastructure has been merged. While it’s still marked as experimental and hidden behind the drm.rnodes module parameter, I’m confident we will enable it by default in one of the next kernel releases.

What are render-nodes?

From a user-space perspective, render-nodes are “like a big FPU” (krh) that can be used by applications to speed up computations and rendering. They are accessible via /dev/dri/renderD<num> and provide the basic DRM rendering interface. Compared to the old card<num> nodes, they lack some features:

  • No mode-setting (KMS) ioctls allowed
  • No insecure gem-flink allowed (use dma-buf instead!)
  • No DRM-auth required/supported
  • No legacy pre-KMS DRM-API supported

So whenever an application wants hardware-accelerated rendering, GPGPU access or offscreen-rendering, it no longer needs to ask a graphics-server (via DRI or wl_drm) but can instead open any available render node and start using it. Access-control to render-nodes is done via standard file-system modes. It’s no longer shared with mode-setting resources and thus can be provided for less-privileged applications.

It is important to note that render-nodes do not provide any new APIs. Instead, they just split a subset of the already available DRM-API off to a new device-node. The legacy node is not changed but kept for backwards-compatibility (and, obviously, for mode-setting).

It’s also important to know that render-nodes are not bound to a specific card. While internally it’s created by the same driver as the legacy node, user-space should never assume any connection between a render-node and a legacy/mode-setting node. Instead, if user-space requires hardware-acceleration, it should open any node and use it. For communication back to the graphics-server, dma-buf shall be used. Really! Questions like “how do I find the render-node for a given card?” don’t make any sense. Yes, driver-specific user-space can figure out whether and which render-node was created by which driver, but driver-unspecific user-space should never do that! Depending on your use-cases, either open any render-node you want (maybe allow an environment-variable to select it) or let the graphics-server do that for you and pass the FD via your graphics-API (X11, wayland, …).

So with render-nodes, kernel drivers can now provide an interface only for off-screen rendering and GPGPU work. Devices without any display-controller can avoid any mode-setting nodes and just provide a render-node. User-space, on the other hand, can finally use GPUs without requiring any privileged graphics-server running. They’re independent of the kernel-internal DRM-Master concept!

2) Mode-setting nodes

While splitting off render-nodes from the legacy node simplifies the situation for most applications, we didn’t simplify it for mode-setting applications. Currently, if a graphics-server wants to program a display-controller, it needs to be DRM-Master for the given card. It can acquire it via drmSetMaster() and drop it via drmDropMaster(). But only one application can be DRM-Master at a time. Moreover, only applications with CAP_SYS_ADMIN privileges can acquire DRM-Master. This prevents some quite fancy features:

  • Running an XServer without root-privileges
  • Using two different XServers to control two independent monitors/connectors of the same card

The initial idea (and Ilija Hadzic’s follow-up) to support this were mode-setting nodes. A privileged ioctl on the control-node would allow applications to split mode-setting resources across different device-nodes. You could have /dev/dri/modesetD1 and /dev/dri/modesetD2 to split your KMS CRTC and Connector resources. An XServer could use one of these nodes to program the now reduced set of resources. We would have one DRM-Master per node and we’d be fine. We could remove the CAP_SYS_ADMIN restriction and instead rely on file-system access-modes to control access to KMS resources.

Another discussed idea to avoid creating a bunch of file-system nodes, is to allocate these resources on-the-fly. All mode-setting-resources would now be bound to a DRM-Master object. An application can only access the resources available on the DRM-Master that it is assigned to. Initially, all resources are bound to the default DRM-Master as usual, which everyone gets assigned to when opening a legacy node. A new ioctl DRM_CLONE_MASTER is used to create a new DRM-Master with the same resources as the previous DRM-Master of an application. Via a DRM_DROP_MASTER_RESOURCE an application can drop KMS resources from their DRM-Master object. Due to their design, neither requires a CAP_SYS_ADMIN restriction as they only clone or drop privileges, they never acquire new privs! So they can be used by any application with access to the control node to create two new DRM-Master resources and pass them to two independent XServers. These use the passed FD to access the card, instead of opening the legacy or mode-setting nodes.

From the kernel side, the only thing that changes is that we can have multiple active DRM-Master objects. In fact, per DRM-Master one open-file might be allowed KMS access. However, this doesn’t require any driver-modifications (which were mostly “master-agnostic”, anyway) and only a few core DRM changes (except for vmwgfx-ttm-lock..).

3) DRM infrastructure

The previous two chapters focused on user-space APIs, but we also want the kernel-internal infrastructure to account for split hardware. However, fact is we already have anything we need. If some hardware exists without display-controller, you simply omit the DRIVER_MODESET flag and only set DRIVER_RENDER. DRM core will only create a render-node for this device then. If your hardware only provides a display-controller, but no real rendering hardware, you simply set DRIVER_MODESET but omit DRIVER_RENDER (which is what SimpleDRM is doing).

Yes, you currently get a bunch of unused DRM code compiled-in if you don’t use some features. However, this is not because DRM requires it, but only because no-one sent any patches for it, yet! DRM-core is driven by DRM-driver developers!

There is a reason why mid-layers are frowned upon in DRM land. There is no group of core DRM developers, but rather a bunch of driver-authors who write fancy driver-extensions. And once multiple drivers use them, they factor it out and move it to DRM core. So don’t complain about missing DRM features, but rather extend your drivers. If it’s a nice feature, you can count on it being incorporated into DRM-core at some point. It might be you doing most of the work, though!

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.

GSoC-Proposal

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
compositor/DRM-Master.

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.

Deliverables
* 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.

Advanced DRM Mode-Setting API

I recently wrote a short How-To that introduces the linux DRM Mode-Setting API. It didn’t use any advanced techniques but I got several responses that it is a great introduction if you want to get started with linux DRM Mode-Setting. So I decided to go further and extend the examples to use double-buffering and vsync’ed page-flips.

The first extension that I wrote can be found here:

https://github.com/dvdhrm/docs/blob/master/drm-howto/modeset-double-buffered.c

It extends the old example to use two buffers so we no longer render into the front-buffer. It reduces a lot of tearing that you get when using the single-buffered example. However, it is still not perfect as you might swap the front and back buffer during a scanout-period and the display-controller will use the new buffer in the middle of the screen. So I extended this further to do the page-flip during a vertical-blank period using drmModePageFlip(). You can find this example here:

https://github.com/dvdhrm/docs/blob/master/drm-howto/modeset-vsync.c

This example also shows how to wait for page-flip events and integrate it into any select(), poll() or epoll based event-loop. Everything regarding page-flip-timing beyond that point depends on the use-cases and can get very hard to get right. I recommend reading Owen Taylor’s posts #1 and #2 on frame-timing for compositors.

There are still many more things like “DRM hardware-accelerated rendering”, “DRM planes/overlays/sprites”, “DRM flink/dmabuf buffer-passing” that I want to write How-Tos for. But time is short around Christmas so that’ll have to wait until next year.

Feedback is always welcome and you can find my email address in all the examples. Happy reading!

Linux DRM Mode-Setting API

The Direct Rendering Manager (DRM) is a subsystem of the linux kernel that manages access to graphics cards (GPUs). It is the main video API used by X.org‘s xserver and the xf86-video-* video drivers. However, it can also be used by independent programs to program video output without using the xserver or wayland. In the past most other projects used the much older fbdev API, however, with more and more drivers being added to the DRM subsystem, there is really no reason to avoid DRM on modern computers, anymore. Unfortunately, there hasn’t been any documentation of the DRM API, yet.

DRM-Modesetting HowTo

I have written a short introduction into the DRM mode-setting API, which can be found on github. It is a full C-file with detailed comments on what is needed to perform simple mode-setting with the DRM-API. I embedded the documentation directly into the source file as this makes reading a lot more convenient:

https://github.com/dvdhrm/docs/blob/master/drm-howto/modeset.c

This document does not describe the whole DRM API. There are parts like the OpenGL-rendering-pipeline, which are driver-dependent and which should almost never be accessed outside of the mesa-3D implementation. Instead, this document describes the API which is needed to write simple applications performing software-rendering similar to fbdev but with the DRM API.

Furthermore, this document is not free of errors. So please contact me if something is wrong, if essential parts are missing or if you intend to extend this documentation.

More tutorials will follow, including “DRM double/triple-buffering”, “DRM vsync’ed pageflips”, “DRM hardware-accelerated rendering”, “DRM planes/overlays/sprites” and more.

I hope you enjoy this short introduction.

Edit: For a new series of How-To’s, see http://dvdhrm.wordpress.com/2012/12/21/advanced-drm-mode-setting-api/