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Ozone Overview

Ozone is a platform abstraction layer beneath the Aura window system that is used for low level input and graphics. Once complete, the abstraction will support underlying systems ranging from embedded SoC targets to new X11-alternative window systems on Linux such as Wayland or Mir to bring up Aura Chromium by providing an implementation of the platform interface.

Guiding Principles

Our goal is to enable chromium to be used in a wide variety of projects by making porting to new platforms easy. To support this goal, ozone follows the following principles:

  1. Interfaces, not ifdefs. Differences between platforms are handled by calling a platform-supplied object through an interface instead of using conditional compilation. Platform internals remain encapsulated, and the public interface acts as a firewall between the platform-neutral upper layers (aura, blink, content, etc) and the platform-specific lower layers. The platform layer is relatively centralized to minimize the number of places ports need to add code.
  2. Flexible interfaces. The platform interfaces should encapsulate just what chrome needs from the platform, with minimal constraints on the platform's implementation as well as minimal constraints on usage from upper layers. An overly prescriptive interface is less useful for porting because fewer ports will be able to use it unmodified. Another way of stating is that the platform layer should provide mechanism, not policy.
  3. Runtime binding of platforms. Avoiding conditional compilation in the upper layers allows us to build multiple platforms into one binary and bind them at runtime. We allow this and provide a command-line flag to select a platform (--ozone-platform) if multiple are enabled. Each platform has a unique build define (e.g. ozone_platform_foo) that can be turned on or off independently.
  4. Easy out-of-tree platforms. Most ports begin as forks. Some of them later merge their code upstream, others will have an extended life out of tree. This is OK, and we should make this process easy to encourage ports, and to encourage frequent gardening of chromium changes into the downstream project. If gardening an out-of-tree port is hard, then those projects will simply ship outdated and potentially insecure chromium-derived code to users. One way we support these projects is by providing a way to inject additional platforms into the build by only patching one ozone_extra.gni file.

Ozone Platform Interface

Ozone moves platform-specific code behind the following interfaces:

  • PlatformWindow represents a window in the windowing system underlying chrome. Interaction with the windowing system (resize, maximize, close, etc) as well as dispatch of input events happens via this interface. Under aura, a PlatformWindow corresponds to a WindowTreeHost. Under mojo, it corresponds to a NativeViewport. On bare hardware, the underlying windowing system is very simple and a platform window corresponds to a physical display.
  • SurfaceFactoryOzone is used to create surfaces for the Chrome compositor to paint on using EGL/GLES2 or Skia.
  • GpuPlatformSupportHost provides the platform code access to IPC between the browser & GPU processes. Some platforms need this to provide additional services in the GPU process such as display configuration.
  • CursorFactoryOzone is used to load & set platform cursors.
  • OverlayManagerOzone is used to manage overlays.
  • InputController allows to control input devices such as keyboard, mouse or touchpad.
  • SystemInputInjector converts input into events and injects them to the Ozone platform.
  • NativeDisplayDelegate is used to support display configuration & hotplug.
  • PlatformScreen is used to fetch screen configuration.
  • ClipboardDelegate provides an interface to exchange data with other applications on the host system using a system clipboard mechanism.

Ozone in Chromium

Our implementation of Ozone required changes concentrated in these areas:

  • Cleaning up extensive assumptions about use of X11 throughout the tree, protecting this code behind the USE_X11 ifdef, and adding a new USE_OZONE path that works in a relatively platform-neutral way by delegating to the interfaces described above.
  • a WindowTreeHostOzone to send events into Aura and participate in display management on the host system, and
  • an Ozone-specific flavor of GLSurfaceEGL which delegates allocation of accelerated surfaces and refresh syncing to the provided implementation of SurfaceFactoryOzone.

Porting with Ozone

Users of the Ozone abstraction need to do the following, at minimum:

  • Write a subclass of PlatformWindow. This class (I'll call it PlatformWindowImpl) is responsible for window system integration. It can use MessagePumpLibevent to poll for events from file descriptors and then invoke PlatformWindowDelegate::DispatchEvent to dispatch each event.
  • Write a subclass of SurfaceFactoryOzone that handles allocating accelerated surfaces. I'll call this SurfaceFactoryOzoneImpl.
  • Write a subclass of CursorFactoryOzone to manage cursors, or use the BitmapCursorFactoryOzone implementation if only bitmap cursors need to be supported.
  • Write a subclass of OverlayManagerOzone or just use StubOverlayManager if your platform does not support overlays.
  • Write a subclass of NativeDisplayDelegate if necessary or just use FakeDisplayDelegate, and write a subclass of PlatformScreen, which is used by aura::ScreenOzone then.
  • Write a subclass of GpuPlatformSupportHost or just use StubGpuPlatformSupportHost.
  • Write a subclass of InputController or just use StubInputController.
  • Write a subclass of SystemInputInjector if necessary.
  • Write a subclass of OzonePlatform that owns instances of the above subclasses and provide a static constructor function for these objects. This constructor will be called when your platform is selected and the returned objects will be used to provide implementations of all the ozone platform interfaces. If your platform does not need some of the interfaces then you can just return a Stub* instance or a nullptr.

Adding an Ozone Platform to the build (instructions for out-of-tree ports)

The recommended way to add your platform to the build is as follows. This walks through creating a new ozone platform called foo.

  1. Fork chromium/src.git.
  2. Add your implementation in ui/ozone/platform/ alongside internal platforms.
  3. Patch ui/ozone/ozone_extra.gni to add your foo platform.

Building with Ozone

Chrome OS - (waterfall)

To build chrome, do this from the src directory:

gn args out/OzoneChromeOS --args="use_ozone=true target_os=\"chromeos\""
ninja -C out/OzoneChromeOS chrome

Then to run for example the X11 platform:

./out/OzoneChromeOS/chrome --ozone-platform=x11

Embedded

Warning: Only some targets such as content_shell or unit tests are currently working for embedded builds.

To build content_shell, do this from the src directory:

gn args out/OzoneEmbedded --args="use_ozone=true toolkit_views=false"
ninja -C out/OzoneEmbedded content_shell

Then to run for example the headless platform:

./out/OzoneEmbedded/content_shell --ozone-platform=headless \
                                  --ozone-dump-file=/tmp/

Linux Desktop - (waterfall)

Warning: Experimental support for Linux Desktop is available since m57 and still under development. The work is purely done in the upstream, but you can still find some Ozone/X11 patches in the the old ozone-wayland-dev branch.

To build chrome, do this from the src directory:

gn args out/OzoneLinuxDesktop --args="use_ozone=true use_system_minigbm=true use_system_libdrm=true"
ninja -C out/OzoneLinuxDesktop chrome

Then to run for example the X11 platform:

./out/OzoneLinuxDesktop/chrome --ozone-platform=x11

Or run for example the Wayland platform:

./out/OzoneLinuxDesktop/chrome --ozone-platform=wayland

GN Configuration notes

You can turn properly implemented ozone platforms on and off by setting the corresponding flags in your GN configuration. For example ozone_platform_headless=false ozone_platform_gbm=false will turn off the headless and DRM/GBM platforms. This will result in a smaller binary and faster builds. To turn ALL platforms off by default, set ozone_auto_platforms=false.

You can also specify a default platform to run by setting the ozone_platform build parameter. For example ozone_platform="x11" will make X11 the default platform when --ozone-platform is not passed to the program. If ozone_auto_platforms is true then ozone_platform is set to headless by default.

Running with Ozone

Specify the platform you want to use at runtime using the --ozone-platform flag. For example, to run content_shell with the GBM platform:

content_shell --ozone-platform=gbm

Caveats:

  • content_shell always runs at 800x600 resolution.
  • For the GBM platform, you may need to terminate your X server (or any other display server) prior to testing.
  • During development, you may need to configure sandboxing or to disable it.

Ozone Platforms

Headless

This platform draws graphical output to a PNG image (no GPU support; software rendering only) and will not output to the screen. You can set the path of the directory where to output the images by specifying --ozone-dump-file=/path/to/output-directory on the command line:

content_shell --ozone-platform=headless \
              --ozone-dump-file=/tmp/

DRM/GBM

This is Linux direct rending with acceleration via mesa GBM & linux DRM/KMS (EGL/GLES2 accelerated rendering & modesetting in GPU process) and is in production use on Chrome OS.

Note that all Chrome OS builds of Chrome will compile and attempt to use this. See Building Chromium for Chromium OS for build instructions.

Cast

This platform is used for Chromecast.

X11

This platform provides support for the X window system.

Wayland

This platform provides support for the Wayland display protocol. It was initially developed by Intel as a fork of chromium and then partially upstreamed. It is still actively being developed by Igalia both on the ozone-wayland-dev branch and the Chromium mainline repository, feel free to discuss with us on freenode.net, #ozone-wayland channel or on ozone-dev.

Below are some quick build & run instructions. It is assumed that you are launching chrome from a Wayland environment such as weston. Execute the following commands (make sure a system version of gbm and drm is used, which are required by Ozone/Wayland by design, when running on Linux platforms.):

gn args out/OzoneWayland --args="use_ozone=true use_system_minigbm=true use_system_libdrm=true"
ninja -C out/OzoneWayland chrome
./out/OzoneWayland/chrome --ozone-platform=wayland

Caca

This platform draws graphical output to text using libcaca (no GPU support; software rendering only). In case you ever wanted to test embedded content shell on tty. It has been removed from the tree and is no longer maintained but you can build it as an out-of-tree port.

Alternatively, you can try the latest revision known to work. First, install libcaca shared library and development files. Next, move to the git revision 0e64be9cf335ee3bea7c989702c5a9a0934af037 (you will probably need to synchronize the build dependencies with gclient sync --with_branch_heads). Finally, build and run the caca platform with the following commands:

gn args out/OzoneCaca \
        --args="use_ozone=true ozone_platform_caca=true use_sysroot=false ozone_auto_platforms=false toolkit_views=false"
ninja -C out/OzoneCaca content_shell
./out/OzoneCaca/content_shell

Note: traditional TTYs are not the ideal browsing experience.
Picture of a workstation using Ozone/caca to display the Google home page in a text terminal

Communication

There is a public mailing list: ozone-dev@chromium.org