D828 - The host GPU is chosen by ORBISTOUN_GPU, discrete first by default

Status: assumed Date: 2026-10-13

The Vulkan session opens the device ORBISTOUN_GPU names: discrete, integrated, a device's place in the host's enumeration (as vulkaninfo --summary lists it), or part of a device's name, compared without case. Unset, it opens the first discrete GPU, or the first device where there is none. A choice the host has no device for refuses to open one, saying so, rather than running on another. The device opened is logged with its place, name and type.

Why: the session opened whichever device the host listed first. On a desktop with an integrated GPU beside a discrete one that is an accident of driver order, and a measurement taken on it says nothing certain about which GPU it measured. Choosing on purpose also makes the integrated GPU a low-end host on demand: a desktop's integrated RDNA2 GPU is weaker than a handheld's, and a run on it shows what a GPU-bound host costs without leaving the desk. A handheld's only GPU is integrated, so the default falls back to the first device rather than refusing.

Rejected:

• The first device, as before: the default changes nothing on hosts that list the discrete GPU first, and on the rest it was the integrated GPU by accident.
• Falling back to another device when the named one is absent: a run would measure a GPU nobody asked for, and say it measured the one they did.