rfnoc: python: Expose ref_clk_calibration feature

This enables the ref_clk_calibration_iface discoverable feature in
Python. It can be used like this:

```python
mb_ctrl = graph.get_mb_controller(0)
ref_cal = mb_ctrl.get_ref_clk_calibration()
ref_cal.set_ref_clk_tuning_word(0x24a)
word = ref_cal.get_ref_clk_tuning_word()
ref_cal.store_ref_clk_tuning_word(word)
```

This feature is also exposed as a devtest.

host/lib/rfnoc/rfnoc_python.hpp

@@ -11,6 +11,7 @@
 #include <uhd/features/discoverable_feature.hpp>
 #include <uhd/features/gpio_power_iface.hpp>
 #include <uhd/features/gps_iface.hpp>
+#include <uhd/features/ref_clk_calibration_iface.hpp>
 #include <uhd/rfnoc/block_id.hpp>
 #include <uhd/rfnoc/filter_node.hpp>
 #include <uhd/rfnoc/graph_edge.hpp>
@@ -280,6 +281,14 @@ void export_rfnoc(py::module& m)
         .def("get_sensors", &uhd::features::gps_iface::get_sensors)
         .def("send_cmd", &uhd::features::gps_iface::send_cmd);
 
+    py::class_<uhd::features::ref_clk_calibration_iface>(m, "ref_clk_calibration")
+        .def("set_ref_clk_tuning_word",
+            &uhd::features::ref_clk_calibration_iface::set_ref_clk_tuning_word)
+        .def("get_ref_clk_tuning_word",
+            &uhd::features::ref_clk_calibration_iface::get_ref_clk_tuning_word)
+        .def("store_ref_clk_tuning_word",
+            &uhd::features::ref_clk_calibration_iface::store_ref_clk_tuning_word);
+
     py::class_<detail::filter_node>(m, "filter_node")
         .def("get_rx_filter_names", &detail::filter_node::get_rx_filter_names)
         .def("get_rx_filter", &detail::filter_node::get_rx_filter)
@@ -331,6 +340,12 @@ void export_rfnoc(py::module& m)
             [](mb_controller& self) {
                 return &self.get_feature<uhd::features::gps_iface>();
             },
+            py::return_value_policy::reference_internal)
+        .def(
+            "get_ref_clk_calibration",
+            [](mb_controller& self) {
+                return &self.get_feature<uhd::features::ref_clk_calibration_iface>();
+            },
             py::return_value_policy::reference_internal);
 
     py::class_<timekeeper, PyTimekeeper, timekeeper::sptr>(m, "timekeeper")

host/tests/devtest/devtest_x410.py

@@ -131,3 +131,4 @@ from list_sensors_test import ListSensorsTest
 from python_api_test import UhdPythonApiTest
 from rx_multi_spc_timed_commands_test import RxMultiSpcTimedCommandsTest
 from tx_multi_spc_timed_commands_test import TxMultiSpcTimedCommandsTest
+from ref_clk_calibration_test import RefClkCalibrationTest

host/tests/devtest/devtest_x420.py

@@ -90,3 +90,4 @@ from list_sensors_test import ListSensorsTest
 from python_api_test import UhdPythonApiTest
 from rx_multi_spc_timed_commands_test import RxMultiSpcTimedCommandsTest
 from tx_multi_spc_timed_commands_test import TxMultiSpcTimedCommandsTest
+from ref_clk_calibration_test import RefClkCalibrationTest

host/tests/devtest/devtest_x440.py

@@ -67,5 +67,6 @@ from list_sensors_test import ListSensorsTest
 from python_api_test import UhdPythonApiTest
 from rx_multi_spc_timed_commands_test import RxMultiSpcTimedCommandsTest
 from tx_multi_spc_timed_commands_test import TxMultiSpcTimedCommandsTest
+from ref_clk_calibration_test import RefClkCalibrationTest
 
 from trigger_test import TxTriggerTest

host/tests/devtest/ref_clk_calibration_test.py

@@ -0,0 +1,31 @@
+#!/usr/bin/env python3
+#
+# Copyright 2026 Ettus Research, a National Instruments Brand
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+"""Test the ref_clk_calibration feature."""
+
+from uhd_test_base import UHDPythonTestCase
+
+
+class RefClkCalibrationTest(UHDPythonTestCase):
+    """Test ref_clk_calibration feature.
+
+    Reads the current tuning word and writes it back. Does not call
+    store_ref_clk_tuning_word to avoid modifying EEPROM during the test.
+    """
+
+    test_name = "RefClkCalibrationTest"
+
+    def run_test(self, test_name, test_args):
+        """Run test and report results."""
+        import uhd
+
+        usrp = uhd.usrp.MultiUSRP(self.args_str)
+        ref_cal = usrp.get_mb_controller().get_ref_clk_calibration()
+        word = ref_cal.get_ref_clk_tuning_word()
+        self.log.info("Current ref_clk tuning word: 0x%X", word)
+        ref_cal.set_ref_clk_tuning_word(word)
+
+        return {"passed": True}