ADR-0008 — PCI Enumeration for GPU Selection

Date: Sprint 4
Status: Accepted
Deciders: PlayOS core team

Context

The compositor needs to select the correct DRM device. The simplest approach is to hardcode /dev/dri/card0. The correct approach is enumeration.

Decision

Always enumerate DRM devices and select by PCI vendor identity and active display connector. Never hardcode /dev/dri/card0 or any other device path.

Rationale

• card0 is not guaranteed: On systems with multiple DRM devices, or depending on driver load order, the integrated GPU may not be card0
• Intel expansion (Sprint 13): Supporting Intel graphics requires that the compositor works without knowing the GPU vendor at compile time — enumeration is the only correct approach
• Future-proofing: If PlayOS ever supports dual-GPU setups (dGPU + iGPU), enumeration is required
• Correctness on real hardware: Tested on the ROG Ally, where card0 is currently correct, but this should not be relied upon

Selection Algorithm

For each DRM device in drmGetDevices2():
    Resolve PCI vendor ID
    Check if a connector on this device is connected to an active display
    If connected:
        Select this device
        Break
If no connected device found:
    Select first AMD (0x1002) device
    Else select first Intel (0x8086) device
    Else select first valid DRM device
    Else fatal error

Log the selected device path, PCI ID, connector name, and preferred mode.

Consequences

• Compositor startup takes slightly longer (one DRM enumeration call)
• Code is more robust on all hardware configurations
• The selection algorithm must be tested on: single AMD GPU, single Intel GPU, and eventually multi-GPU setups
• PCI_VENDOR_AMD = 0x1002 and PCI_VENDOR_INTEL = 0x8086 are defined in compositor code