Dual-GPU support: Launch on the discrete GPU automatically

*reality TV show deep voice guy*

In 2016, we added a way to launch apps on the discrete GPU.

*swoosh effects*

In 2019, we added a way for that to work with the NVidia drivers.

*explosions*

In 2020, we're adding a way for applications to launch automatically on the discrete GPU.

*fast cuts of loads of applications being launched and quiet*

Introducing the (badly-named-but-if-you-can-come-up-with-a-better-name-youre-ready-for-computers) “PrefersNonDefaultGPU” desktop entry key.

From the specifications website:

If true, the application prefers to be run on a more powerful discrete GPU if available, which we describe as “a GPU other than the default one” in this spec to avoid the need to define what a discrete GPU is and in which cases it might be considered more powerful than the default GPU. This key is only a hint and support might not be present depending on the implementation. 

And support for that key is coming to GNOME Shell soon.

TL;DR

Add “PrefersNonDefaultGPU=true” to your application's .desktop file if it can benefit from being run on a more powerful GPU.

We've also added a switcherooctl command to recent versions of switcheroo-control so you can launch your apps on the right GPU from your scripts and tweaks.

Dual-GPU support follow-up: NVIDIA driver support

If you remember, back in 2016, I did the work to get a “Launch on Discrete GPU” menu item added to application in gnome-shell.

This cycle I worked on adding support for the NVIDIA proprietary driver, so that the menu item shows up, and the right environment variables are used to launch applications on that device.

Tested with another unsupported device...

Behind the scenes

There were a number of problems with the old detection code in switcheroo-control:
- it required the graphics card to use vga_switcheroo in the kernel, which the NVIDIA driver didn't do
- it could support more than 2 GPUs
- and it didn't really actually know which GPU was going to be the “main” one

And, on top of all that, gnome-shell expected the Mesa OpenGL stack to be used, so it only knew the right environment variables to do that, and only for one secondary GPU.

So we've extended switcheroo-control and its API to do all this.

(As a side note, commenters asked me about the KDE support, and how it would integrate, and it turns out that KDE's code just checks for the presence of a file in /sys, which is only present when vga_switcheroo is used. So I would encourage KDE to adopt the switcheroo-control D-Bus API for this)

Closing

All this will be available in Fedora 32, using GNOME 3.36 and switcheroo-control 2.0. We might backport this to Fedora 31 after it's been tested, and if there is enough interest.

Dual-GPU integration in GNOME

Thanks to the work of Hans de Goede and many others, dual-GPU (aka NVidia Optimus or AMD Hybrid Graphics) support works better than ever in Fedora 25.

On my side, I picked up some work I originally did for Fedora 24, but ended up being blocked by hardware support. This brings better integration into GNOME.

The Details Settings panel now shows which video cards you have in your (most likely) laptop.

dual-GPU Graphics

The second feature is what Blender and 3D video games users have been waiting for: a contextual menu item to launch the application on the more powerful GPU in your machine.

Mooo Powaa!

This demonstration uses a slightly modified GtkGLArea example, which shows which of the GPUs is used to render the application in the title bar.

on the integrated GPU

on the discrete GPU

Behind the curtain

Behind those 2 features, we have a simple D-Bus service, which runs automatically on boot, and stays running to offer a single property (HasDualGpu) that system components can use to detect what UI to present. This requires the "switcheroo" driver to work on the machine in question.

Because of the way applications are launched on the discrete GPU, we cannot currently support D-Bus activated applications, but GPU-heavy D-Bus-integrated applications are few and far between right now.

Future plans

There's plenty more to do in this area, to polish the integration. We might want applications to tell us whether they'd prefer being run on the integrated or discrete GPU, as live switching between renderers is still something that's out of the question on Linux.

Wayland dual-GPU support, as well as support for the proprietary NVidia drivers are also things that will be worked on, probably by my colleagues though, as the graphics stack really isn't my field.

And if the hardware becomes more widely available, we'll most certainly want to support hardware with hotpluggable graphics support (whether gaming laptop "power-ups" or workstation docks).

Availability

All the patches necessary to make this work are now available in GNOME git (targeted at GNOME 3.24), and backports are integrated in Fedora 25, due to be released shortly.