Hybrid GPU systems: DX12 capability probing uses first Vulkan device rather than discrete GPU

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Summary

On hybrid GPU systems (Intel iGPU + NVIDIA dGPU), Proton/VKD3D-Proton appears to be relying on Vulkan device enumeration order when probing DX12 capabilities. Even when Steam and games are explicitly launched on the discrete GPU (confirmed via system tools), Vulkan seems to be enumerating the iGPU first. Proton then probes that device and reports missing DX12 support, despite the discrete GPU fully supporting it.

As a result, games that require DirectX12 on hybrid GPU systems are unable to launch.

This behavior reproduces across multiple distributions and does not appear to be caused by missing drivers or ICDs, though the timing does roughly line up with recent NVIDIA driver issues that began to manifest post-580.

Question

Is Proton expected to implement additional heuristics or checks for hybrid GPU systems (e.g. preferring discrete GPUs when available), or will this be considered the responsibility of the OS / Vulkan layer?

Downstream issue for context

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It seems that the GPU selection issue was raised with Vulkan around 2019 - 2020, and while a fix was implemented for Windows to prefer the dGPU, a fix was not made for Linux due to technical limitations at the time.

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Update:

After extensive testing on a hybrid GPU system (Intel iGPU + NVIDIA dGPU), I have some new findings that help clarify why DX12 games are now failing under Proton on certain hybrid systems, even when the discrete GPU is available.

First, Vulkan correctly enumerates all devices. vulkaninfo | grep deviceName returns:

deviceName = Intel(R) UHD Graphics (CML GT2)
deviceName = NVIDIA GeForce RTX 3070 Laptop GPU
deviceName = llvmpipe (LLVM 19.1.1, 256 bits)

So all three Vulkan devices (Intel iGPU, NVIDIA dGPU, llvmpipe) are visible to the Vulkan loader. This matches what we would expect to see on a hybrid system and confirms there is no driver installation or ICD problem; Vulkan is functioning and sees every GPU.

Second, Native Vulkan apps prefer the NVIDIA GPU. vkcube returns:

Selected GPU 1: NVIDIA GeForce RTX 3070 Laptop GPU, type: DiscreteGpu

This demonstrates that native Vulkan applications like vkcube are capable of selecting the dGPU by default, even when multiple GPUs are present. The NVIDIA Vulkan Optimus layer is not used for basic selection in simple Vulkan apps like this.

Third, the NVIDIA Vulkan Optimus layer is present, but now acts as advisory only. VK_LAYER_NV_optimus=NVIDIA_only vkcube vulkaninfo | grep -A5 Optimus shows:

VK_LAYER_NV_optimus (NVIDIA Optimus layer) Vulkan version 1.4.312, layer version 1:
Devices: count = 3
GPU id = 0 (Intel(R) UHD Graphics (CML GT2))

The layer is loaded and recognizes all 3 GPUS, but its current behavior is now advisory. In other words, it tags the NVIDIA device, but leaves all 3 GPUs visible. This explains why setting VK_LAYER_NV_optimus=NVIDIA_only does not change device enumeration.

Summarizing the Data

Historically, the NVIDIA Vulkan Optimus layer (VK_LAYER_NV_optimus) hid non-NVIDIA devices when NVIDIA_only was set. This effectively forced all Vulkan apps to see only the discrete GPU. Proton and other DX12/Vulkan wrappers would then probe a single device (the NVIDIA GPU) and everything worked correctly.

However, under the new NVIDIA driver versions and observed in the above tests, the Optimus layer no longer hides non-NVIDIA devices. All devices (Intel iGPU, NVIDIA dGPU, and lvmpipe) are enumerated regardless of the environment variable, and the layer is now advisory; it marks the NVIDIA dGPU but does not filter devices. This is consistent with anecdotal reports from other users that DX12 Proton games started failing recently on hybrid systems; the timeline correlates with the newer NVIDIA driver versions.

When NVIDIA’s Optimus layer used to hide the iGPU, the first enumerated device was always the dGPU, so Proton never ran into this failure. Now that the iGPU is visible first, Proton probes the iGPU, fails DX12 capability checks, and aborts the game process instead of retrying the dGPU.

So the root cause seems to be upstream: NVIDIA changed the Optimus layer behavior, which exposed a latent issue in Proton’s device probing logic.

Combining these observations, we see that:

• The Optimus layer is now advisory-only and is not correctly enforcing NVIDIA_only
• Vulkan exposes all GPUs (with the iGPU being first and the NVIDIA dGPU second)
• Proton / VKD3D-Proton probes devices in order of enumeration
• The first device (Intel iGPU) fails DX12 capability checks
• Proton aborts initialization and the game never launches
• Even if Steam or the system is running on the NVIDIA GPU, the DX12 game fails because Proton never retries the discrete GPU after the first device fails
• Native Vulkan apps that don't rely on the Optimus layer are unaffected, unlike Proton DX12 games

Possible Fixes

• NVIDIA could provide a Vulkan layer or extension that enforces discrete-only enumeration, or restore device filtering behavior in the Optimus Vulkan layer so that VK_LAYER_NV_optimus=NVIDIA_only hides non-NVIDIA devices like before.

• NVIDIA should also document advisory-only behavior clearly for developers so they understand that NVIDIA_only no longer guarantees device hiding as it did in prior versions.

• Proton should allow user override via Steam launch options to explicitly select the dGPU, or to prefer dGPUs ahead of iGPUs when multiple Vulkan devices are present.

• Proton should also retry device selection if the first enumerated GPU fails DX12 capability checks, and update the DX12 initialization logic to be aware of hybrid systems and multiple GPU scenarios.

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UPDATE WITH POTENTIAL FIX IDENTIFIED:

For comparison purposes: I attempted to launch World of Warcraft and World of Warcraft Classic using Crossover. Note the original tests on Steam Proton were not with World of Warcraft; this is simply to illustrate that a DX12 application can function correctly on the same system while DX12 games inside Steam Proton did not.

When I first tried in Crossover 25.1.0, both WoW programs failed with an error. I then:

1. Launched Crossover Preview 20251106 with the NVIDIA GPU while on Pop!_OS hybrid graphics mode.
2. Launched the Battle.net app as a child process under Crossover Preview
3. Launched World of Warcraft and World of Warcraft Classic as child processes under Battle.net

Both games launched correctly, and the game is running on the NVIDIA GPU under DirectX 12 (see screenshot).

Inspection of the working process under Crossover Preview shows that it explicitly sets:

__VK_LAYER_NV_optimus=NVIDIA_only
__NV_PRIME_RENDER_OFFLOAD=1

and maps winevulkan.dll, vulkan-1.dll, and /dev/nvidia0, indicating DX12 → Vulkan → NVIDIA is active.

This suggests that Crossover Preview applies a policy that forces Vulkan device selection to the discrete GPU, avoiding DX12 initialization on the integrated GPU. (Crossover 25.1.0 appears not to do this) Vulkan enumeration still lists Intel as GPU0, so success depends on overriding enumeration order, not on drivers or hardware.

This aligns with the Steam DX12 failure reported in pop-os/cosmic-epoch#2696 and suggests that Proton’s DX12 path should either prefer PHYSICAL_DEVICE_TYPE_DISCRETE_GPU during Vulkan selection during initial DXGI adapter enumeration, or defer DX12 capability rejection until all Vulkan physical devices are evaluated.

The key point is that DX12 failure in Steam is not due to missing support, but due to early Vulkan device selection on the wrong adapter. Crossover Preview demonstrates a working policy that Proton can mirror.