@@ -0,0 +1,249 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+"""Netdevsim-specific tests for ethtool CMIS module loopback.
+
+Seeds the CMIS EEPROM via debugfs and verifies register-level
+behavior that can only be checked with controlled EEPROM contents.
+"""
+
+import errno
+import os
+
+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
+
+# CMIS register constants matching net/ethtool/cmis_loopback.c
+SFF8024_ID_QSFP_DD = 0x18
+
+# Page 01h, Byte 142: bit 5 = Page 13h supported
+CMIS_DIAG_PAGE13_BIT = 1 << 5
+
+# Page 13h, Byte 128: loopback capability bits
+CMIS_LB_CAP_MEDIA_OUTPUT = 1 << 0
+CMIS_LB_CAP_MEDIA_INPUT = 1 << 1
+CMIS_LB_CAP_HOST_OUTPUT = 1 << 2
+CMIS_LB_CAP_HOST_INPUT = 1 << 3
+CMIS_LB_CAP_ALL = (CMIS_LB_CAP_MEDIA_OUTPUT | CMIS_LB_CAP_MEDIA_INPUT |
+                    CMIS_LB_CAP_HOST_OUTPUT | CMIS_LB_CAP_HOST_INPUT)
+
+# 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 _nsim_dfs_path(cfg):
+    """Return the per-port debugfs path for the netdevsim device."""
+    return cfg._ns.nsims[0].dfs_dir
+
+
+def _nsim_write_page_byte(cfg, page, offset, value):
+    """Write a single byte to a netdevsim EEPROM page via debugfs."""
+    if offset < 128:
+        page_file = os.path.join(_nsim_dfs_path(cfg),
+                                 "ethtool/module/pages/0")
+        file_offset = offset
+    else:
+        page_file = os.path.join(_nsim_dfs_path(cfg),
+                                 f"ethtool/module/pages/{page}")
+        file_offset = offset - 128
+
+    with open(page_file, "r+b") as f:
+        f.seek(file_offset)
+        f.write(bytes([value]))
+
+
+def _nsim_read_page_byte(cfg, page, offset):
+    """Read a single byte from a netdevsim EEPROM page via debugfs."""
+    if offset < 128:
+        page_file = os.path.join(_nsim_dfs_path(cfg),
+                                 "ethtool/module/pages/0")
+        file_offset = offset
+    else:
+        page_file = os.path.join(_nsim_dfs_path(cfg),
+                                 f"ethtool/module/pages/{page}")
+        file_offset = offset - 128
+
+    with open(page_file, "rb") as f:
+        f.seek(file_offset)
+        return f.read(1)[0]
+
+
+def _nsim_seed_cmis(cfg, caps=CMIS_LB_CAP_ALL):
+    """Seed the netdevsim EEPROM with CMIS module identity and
+    loopback capabilities.
+    """
+    _nsim_write_page_byte(cfg, 0x00, 0, SFF8024_ID_QSFP_DD)
+    _nsim_write_page_byte(cfg, 0x01, 0x8E, CMIS_DIAG_PAGE13_BIT)
+    _nsim_write_page_byte(cfg, 0x13, 0x80, caps)
+
+
+def _nsim_clear_cmis(cfg):
+    """Clear CMIS identity bytes left by previous tests."""
+    _nsim_write_page_byte(cfg, 0x00, 0, 0)
+    _nsim_write_page_byte(cfg, 0x01, 0x8E, 0)
+    _nsim_write_page_byte(cfg, 0x13, 0x80, 0)
+
+
+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 test_get_no_module(cfg):
+    """GET on a device with no CMIS module returns no entries."""
+    _nsim_clear_cmis(cfg)
+
+    try:
+        entries = _get_loopback(cfg)
+        ksft_eq(len(entries), 0, "Expected no entries without CMIS module")
+    except NlError as e:
+        ksft_eq(e.error, errno.EOPNOTSUPP,
+                "Expected EOPNOTSUPP without CMIS module")
+
+
+def test_get_all_caps(cfg):
+    """GET with all four CMIS loopback capabilities seeded."""
+    _nsim_seed_cmis(cfg, CMIS_LB_CAP_ALL)
+
+    entries = _get_loopback(cfg)
+    mod_entries = [e for e in entries if e['component'] == 'module']
+
+    # Expect 2 entries (one per name), each with both directions
+    ksft_eq(len(mod_entries), 2, "Expected 2 module loopback entries")
+
+    host = [e for e in mod_entries if e['name'] == 'cmis-host']
+    media = [e for e in mod_entries if e['name'] == 'cmis-media']
+    ksft_eq(len(host), 1, "Expected 1 cmis-host entry")
+    ksft_eq(len(media), 1, "Expected 1 cmis-media entry")
+
+    ksft_eq(host[0]['supported'], DIR_NEAR_END | DIR_FAR_END)
+    ksft_eq(media[0]['supported'], DIR_NEAR_END | DIR_FAR_END)
+
+    for e in mod_entries:
+        ksft_eq(e['direction'], DIR_NONE,
+                f"Expected direction=off for {e['name']}")
+
+
+def test_get_partial_caps(cfg):
+    """GET with only host-input capability advertised."""
+    _nsim_seed_cmis(cfg, CMIS_LB_CAP_HOST_INPUT)
+
+    entries = _get_loopback(cfg)
+    mod_entries = [e for e in entries if e['component'] == 'module']
+    ksft_eq(len(mod_entries), 1, "Expected 1 module loopback entry")
+    ksft_eq(mod_entries[0]['name'], 'cmis-host')
+    ksft_eq(mod_entries[0]['supported'], DIR_NEAR_END)
+
+
+@ksft_disruptive
+def test_set_verify_eeprom(cfg):
+    """SET near-end and verify the EEPROM control byte directly."""
+    _nsim_seed_cmis(cfg, CMIS_LB_CAP_ALL)
+
+    ip(f"link set dev {cfg.ifname} down")
+    defer(ip, f"link set dev {cfg.ifname} up")
+
+    _set_loopback(cfg, 'module', 'cmis-host', 'near-end')
+    defer(_set_loopback, cfg, 'module', 'cmis-host', 0)
+
+    # Host Side Input = Page 13h, Byte 183
+    val = _nsim_read_page_byte(cfg, 0x13, 183)
+    ksft_eq(val, 0xFF, "Host Side Input control byte should be 0xFF")
+
+    # Disable and verify
+    _set_loopback(cfg, 'module', 'cmis-host', 0)
+    val = _nsim_read_page_byte(cfg, 0x13, 183)
+    ksft_eq(val, 0x00, "Host Side Input should be 0x00 after disable")
+
+
+@ksft_disruptive
+def test_set_direction_switch_eeprom(cfg):
+    """Switch directions and verify both EEPROM bytes."""
+    _nsim_seed_cmis(cfg, CMIS_LB_CAP_ALL)
+
+    ip(f"link set dev {cfg.ifname} down")
+    defer(ip, f"link set dev {cfg.ifname} up")
+
+    _set_loopback(cfg, 'module', 'cmis-host', 'near-end')
+    defer(_set_loopback, cfg, 'module', 'cmis-host', 0)
+
+    # Switch to far-end
+    _set_loopback(cfg, 'module', 'cmis-host', 'far-end')
+
+    # Near-end (Host Input, Byte 183) should be disabled
+    val = _nsim_read_page_byte(cfg, 0x13, 183)
+    ksft_eq(val, 0x00, "Near-end should be disabled after switch")
+    # Far-end (Host Output, Byte 182) should be enabled
+    val = _nsim_read_page_byte(cfg, 0x13, 182)
+    ksft_eq(val, 0xFF, "Far-end should be enabled after switch")
+
+
+@ksft_disruptive
+def test_set_unsupported_direction(cfg):
+    """SET with unsupported direction should fail."""
+    _nsim_seed_cmis(cfg, CMIS_LB_CAP_HOST_INPUT)  # only near-end
+
+    ip(f"link set dev {cfg.ifname} down")
+    defer(ip, f"link set dev {cfg.ifname} up")
+
+    try:
+        _set_loopback(cfg, 'module', 'cmis-host', 'far-end')
+        raise KsftFailEx("Should have rejected unsupported direction")
+    except NlError as e:
+        ksft_eq(e.error, errno.EOPNOTSUPP,
+                "Expected EOPNOTSUPP for unsupported direction")
+
+
+@ksft_disruptive
+def test_set_no_cmis(cfg):
+    """SET on a device without CMIS loopback support should fail."""
+    _nsim_clear_cmis(cfg)
+
+    ip(f"link set dev {cfg.ifname} down")
+    defer(ip, f"link set dev {cfg.ifname} up")
+
+    try:
+        _set_loopback(cfg, 'module', 'cmis-host', 'near-end')
+        raise KsftFailEx("Should have rejected SET without CMIS support")
+    except NlError as e:
+        ksft_eq(e.error, errno.EOPNOTSUPP,
+                "Expected EOPNOTSUPP without CMIS support")
+
+
+def main() -> None:
+    with NetDrvEnv(__file__) as cfg:
+        cfg.ethnl = EthtoolFamily()
+
+        ksft_run([
+            test_get_no_module,
+            test_get_all_caps,
+            test_get_partial_caps,
+            test_set_verify_eeprom,
+            test_set_direction_switch_eeprom,
+            test_set_unsupported_direction,
+            test_set_no_cmis,
+        ], args=(cfg, ))
+    ksft_exit()
+
+
+if __name__ == "__main__":
+    main()