`auto` mode previously only detected NVLink — PCIe P2P (a patched consumer-GPU driver:
tinygrad/geohot/aikitoria) required setting NVLINK_MODE=pcie_p2p by hand. Now, after the
NVLink check comes up empty, `auto` probes `nvidia-smi topo -p2p r`; if P2P is reported OK
between ALL pairs, it auto-promotes to PCIe P2P (custom all-reduce ON, NCCL_P2P_LEVEL=PHB).
So any user on a P2P-capable layout (shared root complex / switch) + a patched driver gets
it automatically.
- New `_pcie_p2p_available()` helper: parses the p2p matrix, skips header/legend rows (a data
row carries the self-"X"), requires every off-diagonal cell == OK (full P2P; partial → off).
- `pcie_p2p` stays as a FORCE override (assert the patch is loaded, bypass the probe).
- No-op for stock GeForce drivers (software-disable P2P → "CNS") and cards on separate root
complexes — both correctly stay off. Verified on this 2× 3090 rig: `auto` still resolves
"P2P DISABLED" (CNS).
- New test-detect-nvlink-autop2p.sh (6 mocked cases: enable on OK, stay off on CNS, NVLink
wins, multi-GPU all-OK vs mixed/none). Existing alloc-conf test unchanged. Gate 58/58.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EfF565T9eSLaqGzidyJ1Pm
