Add a selftest for the ethtool loopback UAPI exercising module
loopback via the loopback GET/SET netlink commands.

Works on any device that reports module loopback entries. Tests cover
enable near-end and far-end, disable, direction switching (mutual
exclusivity), idempotent enable, and rejection while interface is up.
Devices without module loopback support are skipped.

Signed-off-by: Björn Töpel <bjorn@kernel.org>
---
 .../selftests/drivers/net/hw/loopback_drv.py  | 227 ++++++++++++++++++
 1 file changed, 227 insertions(+)
 create mode 100755 tools/testing/selftests/drivers/net/hw/loopback_drv.py

diff --git a/tools/testing/selftests/drivers/net/hw/loopback_drv.py b/tools/testing/selftests/drivers/net/hw/loopback_drv.py
new file mode 100755
index 000000000000..ab105664e07e
--- /dev/null
+++ b/tools/testing/selftests/drivers/net/hw/loopback_drv.py
@@ -0,0 +1,227 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+"""Tests for ethtool loopback GET/SET with CMIS modules.
+
+Works on any device that reports module loopback entries. On devices
+without CMIS loopback support, tests are skipped.
+"""
+
+import errno
+
+from lib.py import ksft_run, ksft_exit, ksft_eq
+from lib.py import KsftSkipEx, KsftFailEx, ksft_disruptive
+from lib.py import EthtoolFamily, NlError
+from lib.py import NetDrvEnv, ip, defer
+
+# Direction flags as YNL returns them (sets of flag name strings)
+DIR_NONE = set()
+DIR_NEAR_END = {'near-end'}
+DIR_FAR_END = {'far-end'}
+
+
+def _get_loopback(cfg):
+    """GET loopback and return the list of entries."""
+    result = cfg.ethnl.loopback_get({
+        'header': {'dev-index': cfg.ifindex}
+    })
+    return result.get('entry', [])
+
+
+def _set_loopback(cfg, component, name, direction):
+    """SET loopback for a single entry."""
+    cfg.ethnl.loopback_set({
+        'header': {'dev-index': cfg.ifindex},
+        'entry': [{
+            'component': component,
+            'name': name,
+            'direction': direction,
+        }]
+    })
+
+
+def _require_module_entries(cfg):
+    """Return module loopback entries, skip if none available."""
+    try:
+        entries = _get_loopback(cfg)
+    except NlError as e:
+        if e.error == errno.EOPNOTSUPP:
+            raise KsftSkipEx("Device does not support loopback")
+        raise
+    mod_entries = [e for e in entries if e['component'] == 'module']
+    if not mod_entries:
+        raise KsftSkipEx("No module loopback entries")
+    return mod_entries
+
+
+@ksft_disruptive
+def test_set_near_end(cfg):
+    """SET a module entry to near-end and verify via GET."""
+    mod_entries = _require_module_entries(cfg)
+
+    near = [e for e in mod_entries
+            if 'near-end' in e['supported']]
+    if not near:
+        raise KsftSkipEx("No near-end capable module entry")
+
+    ip(f"link set dev {cfg.ifname} down")
+    defer(ip, f"link set dev {cfg.ifname} up")
+
+    target = near[0]
+    _set_loopback(cfg, 'module', target['name'], 'near-end')
+    defer(_set_loopback, cfg, 'module', target['name'], 0)
+
+    entries = _get_loopback(cfg)
+    updated = [e for e in entries
+               if e['name'] == target['name']
+               and 'near-end' in e['supported']]
+    ksft_eq(len(updated), 1)
+    ksft_eq(updated[0]['direction'], DIR_NEAR_END)
+
+
+@ksft_disruptive
+def test_set_far_end(cfg):
+    """SET a module entry to far-end and verify via GET."""
+    mod_entries = _require_module_entries(cfg)
+
+    far = [e for e in mod_entries
+           if 'far-end' in e['supported']]
+    if not far:
+        raise KsftSkipEx("No far-end capable module entry")
+
+    ip(f"link set dev {cfg.ifname} down")
+    defer(ip, f"link set dev {cfg.ifname} up")
+
+    target = far[0]
+    _set_loopback(cfg, 'module', target['name'], 'far-end')
+    defer(_set_loopback, cfg, 'module', target['name'], 0)
+
+    entries = _get_loopback(cfg)
+    updated = [e for e in entries
+               if e['name'] == target['name']
+               and 'far-end' in e['supported']]
+    ksft_eq(len(updated), 1)
+    ksft_eq(updated[0]['direction'], DIR_FAR_END)
+
+
+@ksft_disruptive
+def test_set_disable(cfg):
+    """Enable then disable loopback and verify."""
+    mod_entries = _require_module_entries(cfg)
+
+    near = [e for e in mod_entries
+            if 'near-end' in e['supported']]
+    if not near:
+        raise KsftSkipEx("No near-end capable module entry")
+
+    ip(f"link set dev {cfg.ifname} down")
+    defer(ip, f"link set dev {cfg.ifname} up")
+
+    target = near[0]
+    _set_loopback(cfg, 'module', target['name'], 'near-end')
+    defer(_set_loopback, cfg, 'module', target['name'], 0)
+
+    # Disable
+    _set_loopback(cfg, 'module', target['name'], 0)
+
+    entries = _get_loopback(cfg)
+    updated = [e for e in entries if e['name'] == target['name']]
+    ksft_eq(updated[0]['direction'], DIR_NONE,
+            "Direction should be off after disable")
+
+
+@ksft_disruptive
+def test_set_direction_switch(cfg):
+    """Enable near-end, then switch to far-end. The kernel must disable
+    near-end before enabling far-end (mutual exclusivity).
+    """
+    mod_entries = _require_module_entries(cfg)
+
+    both = [e for e in mod_entries
+            if 'near-end' in e['supported'] and 'far-end' in e['supported']]
+    if not both:
+        raise KsftSkipEx("No entry with both near-end and far-end support")
+
+    ip(f"link set dev {cfg.ifname} down")
+    defer(ip, f"link set dev {cfg.ifname} up")
+
+    target = both[0]
+    _set_loopback(cfg, 'module', target['name'], 'near-end')
+    defer(_set_loopback, cfg, 'module', target['name'], 0)
+
+    entries = _get_loopback(cfg)
+    updated = [e for e in entries if e['name'] == target['name']]
+    ksft_eq(updated[0]['direction'], DIR_NEAR_END)
+
+    # Switch to far-end
+    _set_loopback(cfg, 'module', target['name'], 'far-end')
+
+    entries = _get_loopback(cfg)
+    updated = [e for e in entries if e['name'] == target['name']]
+    ksft_eq(updated[0]['direction'], DIR_FAR_END,
+            "Should have switched to far-end")
+
+
+@ksft_disruptive
+def test_set_idempotent(cfg):
+    """Enable the same direction twice. Second call should not fail."""
+    mod_entries = _require_module_entries(cfg)
+
+    near = [e for e in mod_entries
+            if 'near-end' in e['supported']]
+    if not near:
+        raise KsftSkipEx("No near-end capable module entry")
+
+    ip(f"link set dev {cfg.ifname} down")
+    defer(ip, f"link set dev {cfg.ifname} up")
+
+    target = near[0]
+    _set_loopback(cfg, 'module', target['name'], 'near-end')
+    defer(_set_loopback, cfg, 'module', target['name'], 0)
+
+    # Second enable of the same direction should succeed
+    _set_loopback(cfg, 'module', target['name'], 'near-end')
+
+    entries = _get_loopback(cfg)
+    updated = [e for e in entries
+               if e['name'] == target['name']
+               and 'near-end' in e['supported']]
+    ksft_eq(updated[0]['direction'], DIR_NEAR_END,
+            "Direction should still be near-end")
+
+
+@ksft_disruptive
+def test_set_while_up(cfg):
+    """SET while interface is UP should fail."""
+    mod_entries = _require_module_entries(cfg)
+
+    target = mod_entries[0]
+    direction = 'near-end'
+    if direction not in target['supported']:
+        direction = 'far-end'
+
+    try:
+        _set_loopback(cfg, 'module', target['name'], direction)
+        raise KsftFailEx("Should have rejected SET while interface is up")
+    except NlError as e:
+        ksft_eq(e.error, errno.EBUSY,
+                "Expected EBUSY when interface is up")
+
+
+def main() -> None:
+    with NetDrvEnv(__file__, nsim_test=False) as cfg:
+        cfg.ethnl = EthtoolFamily()
+
+        ksft_run([
+            test_set_near_end,
+            test_set_far_end,
+            test_set_disable,
+            test_set_direction_switch,
+            test_set_idempotent,
+            test_set_while_up,
+        ], args=(cfg, ))
+    ksft_exit()
+
+
+if __name__ == "__main__":
+    main()
-- 
2.53.0