Chapter 33. Offloading Graphics Display with RandR 1.4
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## Chapter 33. Offloading Graphics Display with RandR 1.4

Version 1.4 of the X Resize, Rotate, and Reflect Extension (RandR 1.4 for short) adds a way for drivers to work together so that one graphics device can display images rendered by another. This can be used on Optimus-based laptops to display a desktop rendered by an NVIDIA GPU on a screen connected to another graphics device, such as an Intel integrated graphics device or a USB-to-VGA adapter.

### System Requirements

- X.Org X server version 1.13 or higher.

- A Linux kernel with CONFIG_DRM enabled and the appropriate driver interfaces present:

  - drm_gem_prime_export

  - drm_gem_prime_import

  - gem_prime_pin

  - gem_prime_get_sg_table

  - gem_prime_import_sg_table

  - gem_prime_vmap

  - gem_prime_vunmap

- Version 1.4.0 of the xrandr command-line utility.

### Using the NVIDIA Driver as a RandR 1.4 Output Source Provider

To use the NVIDIA driver as an RandR 1.4 output source provider, the X server needs to be configured to use the NVIDIA driver for its primary screen and to use the “modesetting” driver for the other graphics device. This can be achieved by placing the following in `/etc/X11/xorg.conf`:

    Section "ServerLayout"
        Identifier "layout"
        Screen 0 "nvidia"
        Inactive "intel"
    EndSection

    Section "Device"
        Identifier "nvidia"
        Driver "nvidia"
        BusID "<BusID for NVIDIA device here>"
    EndSection

    Section "Screen"
        Identifier "nvidia"
        Device "nvidia"
        # Uncomment this line if your computer has no display devices connected to
        # the NVIDIA GPU.  Leave it commented if you have display devices
        # connected to the NVIDIA GPU that you would like to use.
        #Option "UseDisplayDevice" "none"
    EndSection

    Section "Device"
        Identifier "intel"
        Driver "modesetting"
    EndSection

    Section "Screen"
        Identifier "intel"
        Device "intel"
    EndSection

See “What is the format of a PCI Bus ID?” for information on determining the appropriate BusID string for your graphics card.

The X server does not automatically enable displays attached to the non-NVIDIA graphics device in this configuration. To do that, use the `xrandr` command line tool:

    $ xrandr --setprovideroutputsource modesetting NVIDIA-0
    $ xrandr --auto

This pair of commands can be added to your X session startup scripts, for example by putting them in `$HOME/.xinitrc` before running `startx`.

Use the

    $ xrandr --listproviders

command to query the capabilities of the graphics devices. If the system requirements are met and the X server is configured correctly, there should be a provider named `NVIDIA-0` with the `Source Output` capability and one named `modesetting` with the `Sink Output` capability. If either provider is missing or doesn't have the expected capability, check your system configuration.

### Caveats

- There is no synchronization between the images rendered by the NVIDIA GPU and the output device. This means that the output device can start reading the next frame of video while it is still being updated, producing a graphical artifact known as “tearing”. Tearing is currently expected due to limitations in the design of the X.Org X server.

- The NVIDIA driver currently only supports the `Source Output` capability. It does not support render offload and cannot be used as an output sink.

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