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<h2 class="title"><a name="randr14" id=
"randr14"></a>Chapter&nbsp;34.&nbsp;Offloading Graphics Display
with RandR 1.4</h2>
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<p>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.</p>
<p><a name="requirements" id="requirements"></a></p>
<h3>System Requirements</h3>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>For displaying NVIDIA GPU desktop contents on a screen connected
to another graphics device, X.Org X server version 1.13 or
higher.</p>
</li>
<li>
<p>For displaying another graphics device's desktop contents on a
screen connected to an NVIDIA GPU, X.Org X server version 1.20.7 or
higher. X server version 1.20.6 is also supported using <a href=
"xconfigoptions.html#AllowPRIMEDisplayOffloadSink"><code class=
"computeroutput">Option "AllowPRIMEDisplayOffloadSink"
"boolean"</code></a>.</p>
</li>
<li>
<p>A Linux kernel, version 3.13 or higher, with CONFIG_DRM
enabled.</p>
</li>
<li>
<p>Version 1.4.0 of the xrandr command-line utility.</p>
</li>
</ul>
</div>
<p><a name="outputsource" id="outputsource"></a></p>
<h3>Using the NVIDIA Driver as an RandR 1.4 Output Source or Output
Sink Provider</h3>
<p>To use the NVIDIA driver as an RandR 1.4 output source provider,
also known as &ldquo;<span class="quote">PRIME</span>&rdquo;, the X
server needs to be configured to use the NVIDIA driver for its
primary screen and to use the &ldquo;<span class=
"quote">modesetting</span>&rdquo; driver for the other graphics
device. This can be achieved by placing the following in
<code class="computeroutput">/etc/X11/xorg.conf</code>:</p>
<pre class="screen">
Section "ServerLayout"
    Identifier "layout"
    Screen 0 "nvidia"
    Inactive "intel"
EndSection

Section "Device"
    Identifier "nvidia"
    Driver "nvidia"
    BusID "&lt;BusID for NVIDIA device here&gt;"
EndSection

Section "Screen"
    Identifier "nvidia"
    Device "nvidia"
    Option "AllowEmptyInitialConfiguration"
EndSection

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

Section "Screen"
    Identifier "intel"
    Device "intel"
EndSection
</pre>
<p></p>
<p>To use the NVIDIA driver as an RandR 1.4 output sink provider,
also known as &ldquo;<span class="quote">Reverse
PRIME</span>&rdquo;, the X server needs to be configured to use the
&ldquo;<span class="quote">modesetting</span>&rdquo; driver for its
primary screen and to use the NVIDIA driver for the other graphics
device. This can be achieved by placing the following in
<code class="computeroutput">/etc/X11/xorg.conf</code>:</p>
<pre class="screen">
Section "ServerLayout"
    Identifier "layout"
    Screen 0 "intel"
    Inactive "nvidia"
    Option "AllowNVIDIAGPUScreens"
EndSection

Section "Device"
    Identifier "intel"
    Driver "modesetting"
    BusID "&lt;BusID for Intel device here&gt;"
EndSection

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

Section "Device"
    Identifier "nvidia"
    Driver "nvidia"
EndSection

Section "Screen"
    Identifier "nvidia"
    Device "nvidia"
EndSection
</pre>
<p></p>
<p>When using the NVIDIA driver as a &ldquo;<span class=
"quote">Reverse PRIME</span>&rdquo; RandR 1.4 output sink provider
combined with an application being run via <a href=
"primerenderoffload.html" title=
"Chapter&nbsp;35.&nbsp;PRIME Render Offload">Chapter&nbsp;35,
<i>PRIME Render Offload</i></a>, an optimization known as
&ldquo;<span class="quote">Reverse PRIME Bypass</span>&rdquo; may
be used, bypassing the bandwidth overhead of both PRIME Render
Offload and PRIME Display Offload. In order for Reverse PRIME
Bypass to be used, a PRIME Render Offload application must be
unredirected, fullscreen, and visible only on a single
NVIDIA-driven PRIME Display Offload output. Usage of Reverse PRIME
Bypass is printed to the X log when verbose logging is enabled in
the X server.</p>
<p>If an NVIDIA Reverse PRIME output is the sole display in the
system then special conditions apply. That configuration is
supported if the NVIDIA driver version is 495.46 or later and the
X.Org X server version is newer than 21.1.3. Failing these
conditions the NVIDIA Reverse PRIME output will not be synchronized
to the native refresh rate of the NVIDIA graphics card in which
case X.Org will revert the display to a default rate of 1 frame per
second.</p>
<p>Note that at the time of writing the latest X.Org X server is
21.1.3 so there is no official X.Org release yet where this
configuration is supported. For maintainers of Linux distributions
and others who are willing to compile the X.Org X server locally,
please cherry-pick this Git commit to support the configuration:
<a href=
"https://gitlab.freedesktop.org/xorg/xserver/-/commit/69774044716039fa70655b3bc6dd6a4ff4535cfd"
target=
"_top">https://gitlab.freedesktop.org/xorg/xserver/-/commit/69774044716039fa70655b3bc6dd6a4ff4535cfd</a>.
The commit already lives in the branch where the next X.Org X
server release after 21.1.3 will come from.</p>
<p>See <a href="faq.html#busid">&ldquo;What is the format of a PCI
Bus ID?&rdquo;</a> for information on determining the appropriate
BusID string for your graphics card.</p>
<p>The nvidia-xconfig(1) utility can be used to update the X
configuration file for using the NVIDIA driver as an output source
provider.</p>
<pre class="screen">
$ nvidia-xconfig --prime
</pre>
<p>See the nvidia-xconfig(1) man page for details.</p>
<p>The X server does not automatically enable displays attached
using the output sink in this configuration. To do that, use the
<code class="computeroutput">xrandr</code> command line tool.</p>
<p>For NVIDIA as an output source:</p>
<pre class="screen">
$ xrandr --setprovideroutputsource modesetting NVIDIA-0
$ xrandr --auto
</pre>
<p></p>
<p>For NVIDIA as an output sink:</p>
<pre class="screen">
$ xrandr --setprovideroutputsource NVIDIA-G0 modesetting
$ xrandr --auto
</pre>
<p></p>
<p>This pair of commands can be added to your X session startup
scripts, for example by putting them in <code class=
"computeroutput">$HOME/.xinitrc</code> before running <code class=
"computeroutput">startx</code>.</p>
<p>Use the</p>
<pre class="screen">
$ xrandr --listproviders
</pre>
<p>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 <code class=
"computeroutput">NVIDIA-0</code> or <code class=
"computeroutput">NVIDIA-G0</code> with the <code class=
"computeroutput">Source Output</code> or <code class=
"computeroutput">Sink Output</code> capability, respectively, and
one named <code class="computeroutput">modesetting</code> with the
<code class="computeroutput">Sink Output</code> and/or <code class=
"computeroutput">Source Output</code> capabilities. If either
provider is missing or doesn't have the expected capability, check
your system configuration.</p>
<p><a name="randr14sync" id="randr14sync"></a></p>
<h3>Synchronized RandR 1.4 Outputs</h3>
<p>When running against X.Org X server with video driver ABI 23 or
higher, synchronization is supported with compatible drivers. At
the time of writing, synchronization is compatible with the
&ldquo;<span class="quote">modesetting</span>&rdquo; driver with
Intel devices on Linux version 4.5 or newer. If all requirements
are met, synchronization will be used automatically.</p>
<p>X.Org X server version 1.19 or newer is required to support
synchronization. Without synchronization, displays are prone to
&ldquo;<span class="quote">tearing</span>&rdquo;. See <a href=
"randr14.html#caveats" title="Caveats">Caveats</a> for details.</p>
<p>If synchronization is being used but is not desired, it can be
disabled with:</p>
<pre class="screen">
$ xrandr --output &lt;output&gt; --set "PRIME Synchronization" 0
</pre>
<p>and re-enabled with:</p>
<pre class="screen">
$ xrandr --output &lt;output&gt; --set "PRIME Synchronization" 1
</pre>
<p></p>
<p>See <a href="openglenvvariables.html#vblanksyncing" title=
"Vblank syncing">Vblank syncing</a> for information on how OpenGL
applications can synchronize with sink-provided outputs.</p>
<p><a name="caveats" id="caveats"></a></p>
<h3>Caveats</h3>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>Support for PRIME Synchronization relies on DRM KMS support. See
<a href="kms.html" title=
"Chapter&nbsp;36.&nbsp;Direct Rendering Manager Kernel Modesetting (DRM KMS)">
Chapter&nbsp;36, <i>Direct Rendering Manager Kernel Modesetting
(DRM KMS)</i></a> for more information.</p>
</li>
<li>
<p>Some Intel i915 DRM driver versions, such as that included with
Linux 4.5, have a bug where drmModeMoveCursor() and
drmModePageFlip() interfere with each other, resulting in only one
occurring per frame. If choppy performance is observed in
configurations using PRIME Synchronization and i915, it is
suggested to add <code class="computeroutput">Option
"SWCursor"</code> to Intel's device section in xorg.conf. The bug
appears to be fixed as of Linux 4.6.</p>
</li>
<li>
<p>When running against X.Org X server version 1.18.x or lower,
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
&ldquo;<span class="quote">tearing</span>&rdquo;. Tearing is
expected due to limitations in the design of the X.Org X server
prior to video driver ABI 23.</p>
</li>
<li>
<p>NVIDIA's implementation of PRIME requires support for DRM render
nodes, a feature first merged in Linux 3.12. However, the feature
was not enabled by default until Linux 3.17. To enable it on
earlier supported kernels, specify the <code class=
"computeroutput">drm.rnodes=1</code> kernel boot parameter.</p>
</li>
<li>
<p>PRIME Synchronization is compatible with xf86-video-amdgpu as an
output sink. xf86-video-amdgpu implements a separate interface for
PRIME Synchronization that the RandR layer of the X server does not
recognize. As a result, X will print "randr: falling back to
unsynchronized pixmap sharing", despite the fact that PRIME is
synchronized. Additionally, the "PRIME Synchronization" output
property will not function to disable PRIME Synchronization when
set to 0.</p>
</li>
<li>
<p>The NVIDIA driver only exposes the <code class=
"computeroutput">Output Sink</code> capability by default on X
server version 1.20.7 or later, but can be used without issue on X
server version 1.20.6 with <code class="computeroutput">Option
"AllowPRIMEDisplayOffloadSink"</code>. See <a href=
"xconfigoptions.html#AllowPRIMEDisplayOffloadSink"><code class=
"computeroutput">Option "AllowPRIMEDisplayOffloadSink"
"boolean"</code></a> for more information.</p>
</li>
<li>
<p>The NVIDIA driver requires DRM KMS support to operate as an
output sink when the output source driver is either NVIDIA or
AMDGPU. See <a href="kms.html" title=
"Chapter&nbsp;36.&nbsp;Direct Rendering Manager Kernel Modesetting (DRM KMS)">
Chapter&nbsp;36, <i>Direct Rendering Manager Kernel Modesetting
(DRM KMS)</i></a> for more information.</p>
</li>
</ul>
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