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<h2 class="title"><a name="randr14" id=
"randr14"></a>Chapter 33. 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.
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.</p>
<p><a name="requirements" id="requirements"></a></p>
<h3>System Requirements</h3>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>X.Org X server version 1.13 or higher.</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 a RandR 1.4 Output Source
Provider</h3>
<p>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 “<span class=
"quote">modesetting</span>” 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 "<BusID for NVIDIA device here>"
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>See <a href="faq.html#busid">“What is the format of a PCI
Bus ID?”</a> for information on determining the appropriate
BusID string for your graphics card.</p>
<p>The X server does not automatically enable displays attached to
the non-NVIDIA graphics device in this configuration. To do that,
use the <code class="computeroutput">xrandr</code> command line
tool:</p>
<pre class="screen">
$ xrandr --setprovideroutputsource modesetting NVIDIA-0
$ xrandr --auto
</pre>
<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> with the <code class=
"computeroutput">Source Output</code> capability and one named
<code class="computeroutput">modesetting</code> with the
<code class="computeroutput">Sink Output</code> capability. 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
“<span class="quote">modesetting</span>” 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
“<span class="quote">tearing</span>”. 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 <output> --set "PRIME Synchronization" 0
</pre>
<p>and re-enabled with:</p>
<pre class="screen">
$ xrandr --output <output> --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 34. Direct Rendering Manager Kernel Modesetting (DRM KMS)">
Chapter 34, <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
“<span class="quote">tearing</span>”. 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>The NVIDIA driver currently only supports the <code class=
"computeroutput">Source Output</code> capability. It does not
support render offload and cannot be used as an output sink.</p>
</li>
<li>
<p>Some versions of the “<span class=
"quote">modesetting</span>” driver try to load a sub-module
called “<span class="quote">glamor</span>”, which
conflicts with the NVIDIA GLX implementation. Please ensure that
the <code class="filename">libglamoregl.so</code> X module is not
installed.</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>
</ul>
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