<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-gpu-accel">
  <title>GPU acceleration</title>
  <para>
    NixOS provides various APIs that benefit from GPU hardware
    acceleration, such as VA-API and VDPAU for video playback; OpenGL
    and Vulkan for 3D graphics; and OpenCL for general-purpose
    computing. This chapter describes how to set up GPU hardware
    acceleration (as far as this is not done automatically) and how to
    verify that hardware acceleration is indeed used.
  </para>
  <para>
    Most of the aforementioned APIs are agnostic with regards to which
    display server is used. Consequently, these instructions should
    apply both to the X Window System and Wayland compositors.
  </para>
  <section xml:id="sec-gpu-accel-opencl">
    <title>OpenCL</title>
    <para>
      <link xlink:href="https://en.wikipedia.org/wiki/OpenCL">OpenCL</link>
      is a general compute API. It is used by various applications such
      as Blender and Darktable to accelerate certain operations.
    </para>
    <para>
      OpenCL applications load drivers through the <emphasis>Installable
      Client Driver</emphasis> (ICD) mechanism. In this mechanism, an
      ICD file specifies the path to the OpenCL driver for a particular
      GPU family. In NixOS, there are two ways to make ICD files visible
      to the ICD loader. The first is through the
      <literal>OCL_ICD_VENDORS</literal> environment variable. This
      variable can contain a directory which is scanned by the ICL
      loader for ICD files. For example:
    </para>
    <programlisting>
$ export \
  OCL_ICD_VENDORS=`nix-build '&lt;nixpkgs&gt;' --no-out-link -A rocm-opencl-icd`/etc/OpenCL/vendors/
</programlisting>
    <para>
      The second mechanism is to add the OpenCL driver package to
      <xref linkend="opt-hardware.opengl.extraPackages" />. This links
      the ICD file under <literal>/run/opengl-driver</literal>, where it
      will be visible to the ICD loader.
    </para>
    <para>
      The proper installation of OpenCL drivers can be verified through
      the <literal>clinfo</literal> command of the clinfo package. This
      command will report the number of hardware devices that is found
      and give detailed information for each device:
    </para>
    <programlisting>
$ clinfo | head -n3
Number of platforms  1
Platform Name        AMD Accelerated Parallel Processing
Platform Vendor      Advanced Micro Devices, Inc.
</programlisting>
    <section xml:id="sec-gpu-accel-opencl-amd">
      <title>AMD</title>
      <para>
        Modern AMD
        <link xlink:href="https://en.wikipedia.org/wiki/Graphics_Core_Next">Graphics
        Core Next</link> (GCN) GPUs are supported through the
        rocm-opencl-icd package. Adding this package to
        <xref linkend="opt-hardware.opengl.extraPackages" /> enables
        OpenCL support:
      </para>
      <programlisting language="bash">
hardware.opengl.extraPackages = [
  rocm-opencl-icd
];
</programlisting>
    </section>
    <section xml:id="sec-gpu-accel-opencl-intel">
      <title>Intel</title>
      <para>
        <link xlink:href="https://en.wikipedia.org/wiki/List_of_Intel_graphics_processing_units#Gen8">Intel
        Gen8 and later GPUs</link> are supported by the Intel NEO OpenCL
        runtime that is provided by the intel-compute-runtime package.
        For Gen7 GPUs, the deprecated Beignet runtime can be used, which
        is provided by the beignet package. The proprietary Intel OpenCL
        runtime, in the intel-ocl package, is an alternative for Gen7
        GPUs.
      </para>
      <para>
        The intel-compute-runtime, beignet, or intel-ocl package can be
        added to <xref linkend="opt-hardware.opengl.extraPackages" /> to
        enable OpenCL support. For example, for Gen8 and later GPUs, the
        following configuration can be used:
      </para>
      <programlisting language="bash">
hardware.opengl.extraPackages = [
  intel-compute-runtime
];
</programlisting>
    </section>
  </section>
  <section xml:id="sec-gpu-accel-vulkan">
    <title>Vulkan</title>
    <para>
      <link xlink:href="https://en.wikipedia.org/wiki/Vulkan_(API)">Vulkan</link>
      is a graphics and compute API for GPUs. It is used directly by
      games or indirectly though compatibility layers like
      <link xlink:href="https://github.com/doitsujin/dxvk/wiki">DXVK</link>.
    </para>
    <para>
      By default, if <xref linkend="opt-hardware.opengl.driSupport" />
      is enabled, mesa is installed and provides Vulkan for supported
      hardware.
    </para>
    <para>
      Similar to OpenCL, Vulkan drivers are loaded through the
      <emphasis>Installable Client Driver</emphasis> (ICD) mechanism.
      ICD files for Vulkan are JSON files that specify the path to the
      driver library and the supported Vulkan version. All successfully
      loaded drivers are exposed to the application as different GPUs.
      In NixOS, there are two ways to make ICD files visible to Vulkan
      applications: an environment variable and a module option.
    </para>
    <para>
      The first option is through the
      <literal>VK_ICD_FILENAMES</literal> environment variable. This
      variable can contain multiple JSON files, separated by
      <literal>:</literal>. For example:
    </para>
    <programlisting>
$ export \
  VK_ICD_FILENAMES=`nix-build '&lt;nixpkgs&gt;' --no-out-link -A amdvlk`/share/vulkan/icd.d/amd_icd64.json
</programlisting>
    <para>
      The second mechanism is to add the Vulkan driver package to
      <xref linkend="opt-hardware.opengl.extraPackages" />. This links
      the ICD file under <literal>/run/opengl-driver</literal>, where it
      will be visible to the ICD loader.
    </para>
    <para>
      The proper installation of Vulkan drivers can be verified through
      the <literal>vulkaninfo</literal> command of the vulkan-tools
      package. This command will report the hardware devices and drivers
      found, in this example output amdvlk and radv:
    </para>
    <programlisting>
$ vulkaninfo | grep GPU
                GPU id  : 0 (Unknown AMD GPU)
                GPU id  : 1 (AMD RADV NAVI10 (LLVM 9.0.1))
     ...
GPU0:
        deviceType     = PHYSICAL_DEVICE_TYPE_DISCRETE_GPU
        deviceName     = Unknown AMD GPU
GPU1:
        deviceType     = PHYSICAL_DEVICE_TYPE_DISCRETE_GPU
</programlisting>
    <para>
      A simple graphical application that uses Vulkan is
      <literal>vkcube</literal> from the vulkan-tools package.
    </para>
    <section xml:id="sec-gpu-accel-vulkan-amd">
      <title>AMD</title>
      <para>
        Modern AMD
        <link xlink:href="https://en.wikipedia.org/wiki/Graphics_Core_Next">Graphics
        Core Next</link> (GCN) GPUs are supported through either radv,
        which is part of mesa, or the amdvlk package. Adding the amdvlk
        package to <xref linkend="opt-hardware.opengl.extraPackages" />
        makes amdvlk the default driver and hides radv and lavapipe from
        the device list. A specific driver can be forced as follows:
      </para>
      <programlisting language="bash">
hardware.opengl.extraPackages = [
  pkgs.amdvlk
];

# To enable Vulkan support for 32-bit applications, also add:
hardware.opengl.extraPackages32 = [
  pkgs.driversi686Linux.amdvlk
];

# Force radv
environment.variables.AMD_VULKAN_ICD = &quot;RADV&quot;;
# Or
environment.variables.VK_ICD_FILENAMES =
  &quot;/run/opengl-driver/share/vulkan/icd.d/radeon_icd.x86_64.json&quot;;
</programlisting>
    </section>
  </section>
  <section xml:id="sec-gpu-accel-common-issues">
    <title>Common issues</title>
    <section xml:id="sec-gpu-accel-common-issues-permissions">
      <title>User permissions</title>
      <para>
        Except where noted explicitly, it should not be necessary to
        adjust user permissions to use these acceleration APIs. In the
        default configuration, GPU devices have world-read/write
        permissions (<literal>/dev/dri/renderD*</literal>) or are tagged
        as <literal>uaccess</literal>
        (<literal>/dev/dri/card*</literal>). The access control lists of
        devices with the <literal>uaccess</literal> tag will be updated
        automatically when a user logs in through
        <literal>systemd-logind</literal>. For example, if the user
        <emphasis>alice</emphasis> is logged in, the access control list
        should look as follows:
      </para>
      <programlisting>
$ getfacl /dev/dri/card0
# file: dev/dri/card0
# owner: root
# group: video
user::rw-
user:alice:rw-
group::rw-
mask::rw-
other::---
</programlisting>
      <para>
        If you disabled (this functionality of)
        <literal>systemd-logind</literal>, you may need to add the user
        to the <literal>video</literal> group and log in again.
      </para>
    </section>
    <section xml:id="sec-gpu-accel-common-issues-mixing-nixpkgs">
      <title>Mixing different versions of nixpkgs</title>
      <para>
        The <emphasis>Installable Client Driver</emphasis> (ICD)
        mechanism used by OpenCL and Vulkan loads runtimes into its
        address space using <literal>dlopen</literal>. Mixing an ICD
        loader mechanism and runtimes from different version of nixpkgs
        may not work. For example, if the ICD loader uses an older
        version of glibc than the runtime, the runtime may not be
        loadable due to missing symbols. Unfortunately, the loader will
        generally be quiet about such issues.
      </para>
      <para>
        If you suspect that you are running into library version
        mismatches between an ICL loader and a runtime, you could run an
        application with the <literal>LD_DEBUG</literal> variable set to
        get more diagnostic information. For example, OpenCL can be
        tested with <literal>LD_DEBUG=files clinfo</literal>, which
        should report missing symbols.
      </para>
    </section>
  </section>
</chapter>
