conftest: only resolve the device when a collected test restricts to one

conftest.py

@@ -175,10 +175,19 @@ def pytest_configure(config):
 
 
 def pytest_collection_modifyitems(config, items):
+    marked_items = [
+        (item, item.get_closest_marker("supported_devices")) for item in items
+    ]
+    marked_items = [(item, marker) for item, marker in marked_items if marker]
+    if not marked_items:
+        # Nothing collected needs the NPU. Resolving one here would open the
+        # single-tenant device at collection time, contending with whatever
+        # else holds it and erroring out when none is attached.
+        return
+
     device = aie_utils.DefaultNPURuntime.device().resolve().name
-    for item in items:
-        marker = item.get_closest_marker("supported_devices")
-        if marker and device not in marker.args:
+    for item, marker in marked_items:
+        if device not in marker.args:
             item.add_marker(
                 pytest.mark.skip(
                     reason=f"Not supported on {device} (supported: {', '.join(marker.args)})"

iron/tests/infrastructure/conftest_lazy_device.py

@@ -0,0 +1,104 @@
+#!/usr/bin/env python3
+# SPDX-FileCopyrightText: Copyright (C) 2026 Advanced Micro Devices, Inc. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+"""The root conftest.py's pytest_collection_modifyitems must not resolve a
+device unless some collected test restricts itself to specific devices via
+@pytest.mark.supported_devices. Resolving one unconditionally opens the
+single-tenant NPU on every plain `pytest` in this tree, whatever was selected.
+
+pytest loads the root conftest.py for these tests too, so the hook under test
+is imported by path instead and called directly, against fake items and a
+stubbed aie_utils.DefaultNPURuntime that raises if .device() is reached.
+"""
+
+import importlib.util
+import sys
+from pathlib import Path
+from types import SimpleNamespace
+
+import pytest
+
+_ROOT_CONFTEST = Path(__file__).resolve().parents[3] / "conftest.py"
+
+
+def _load_root_conftest():
+    spec = importlib.util.spec_from_file_location(
+        "_root_conftest_under_test", _ROOT_CONFTEST
+    )
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+class _FakeMarker:
+    def __init__(self, *args):
+        self.args = args
+
+
+class _FakeItem:
+    def __init__(self, marker=None):
+        self._marker = marker
+        self.markers_added = []
+
+    def get_closest_marker(self, name):
+        assert name == "supported_devices"
+        return self._marker
+
+    def add_marker(self, marker):
+        self.markers_added.append(marker)
+
+
+class _DeviceCalledError(Exception):
+    pass
+
+
+def _stub_runtime_that_forbids_device_calls(root_conftest, monkeypatch):
+    def _raise():
+        raise _DeviceCalledError(
+            "DefaultNPURuntime.device() was called with no marked test collected"
+        )
+
+    monkeypatch.setattr(
+        root_conftest.aie_utils,
+        "DefaultNPURuntime",
+        SimpleNamespace(device=_raise),
+    )
+
+
+def test_no_device_probe_when_nothing_is_device_restricted(monkeypatch):
+    root_conftest = _load_root_conftest()
+    _stub_runtime_that_forbids_device_calls(root_conftest, monkeypatch)
+
+    items = [_FakeItem(), _FakeItem(), _FakeItem()]
+    root_conftest.pytest_collection_modifyitems(config=None, items=items)
+    assert all(item.markers_added == [] for item in items)
+
+
+def test_device_probed_and_unsupported_items_skipped_when_a_test_is_restricted(
+    monkeypatch,
+):
+    root_conftest = _load_root_conftest()
+
+    class _FakeDevice:
+        def resolve(self):
+            return SimpleNamespace(name="npu2")
+
+    monkeypatch.setattr(
+        root_conftest.aie_utils,
+        "DefaultNPURuntime",
+        SimpleNamespace(device=lambda: _FakeDevice()),
+    )
+
+    unrestricted = _FakeItem()
+    matches_device = _FakeItem(_FakeMarker("npu1", "npu2"))
+    excludes_device = _FakeItem(_FakeMarker("npu1"))
+
+    root_conftest.pytest_collection_modifyitems(
+        config=None, items=[unrestricted, matches_device, excludes_device]
+    )
+
+    assert unrestricted.markers_added == []
+    assert matches_device.markers_added == []
+    assert len(excludes_device.markers_added) == 1
+    assert excludes_device.markers_added[0].name == "skip"