[Mellanox] Implement get_midplane_down_reason for DPU module

platform/mellanox/mlnx-platform-api/sonic_platform/module.py

@@ -483,6 +483,21 @@ def get_reboot_cause(self):
                 return rd
         return ChassisBase.REBOOT_CAUSE_NON_HARDWARE, ''
 
+    def get_midplane_down_reason(self):
+        """
+        Retrieves the reason for the midplane down
+
+        Returns:
+            A tuple (string, string) where the first element is one of the
+            ChassisBase.REBOOT_CAUSE_* strings and the second element is a
+            description of the midplane down reason.
+        """
+        for f, rd in self.reboot_cause_map.items():
+            if utils.read_int_from_file(f) == 1:
+                logger.log_notice(f"Midplane down reason for {self._name} is {rd[0]}")
+                return rd
+        return ChassisBase.REBOOT_CAUSE_NON_HARDWARE, ''
+
     def get_midplane_ip(self):
         """
         Retrieves the midplane IP-address of the module in a modular chassis

platform/mellanox/mlnx-platform-api/tests/test_module.py

@@ -344,6 +344,13 @@ def mock_read_int_from_file(file_path, default=0, raise_exception=False, log_fun
                 for index, file_name in enumerate(file_name_list):
                     test_file_path = file_name
                     assert m.get_reboot_cause() == reboot_cause_list[index]
+
+            # get_midplane_down_reason() shares reboot_cause_map, so it reports the same
+            # (cause, description) pairs as get_reboot_cause() for every reset-cause file.
+            with patch("sonic_platform.utils.read_int_from_file", wraps=mock_read_int_from_file):
+                for index, file_name in enumerate(file_name_list):
+                    test_file_path = file_name
+                    assert m.get_midplane_down_reason() == reboot_cause_list[index]
 
             # Test subprocess exception case
             mock_check_output.side_effect = subprocess.CalledProcessError(1, 'mlxreg')