lib: x400: Add fail-flag APIs to RFDC control

Two new APIs are added to x400_rfdc_control:
- get_nco_good(): Combines the reset and FAIL flags into a single "good"
  value
- get_nco_no_fail(): Provides information on the current status of the
  NCO reset FAIL flag

host/lib/include/uhdlib/usrp/common/x400_rfdc_control.hpp

@@ -77,6 +77,17 @@ public:
     //! Return true if the NCO is out of reset
     bool get_nco_reset_done();
 
+    //! Return true if the failure flag is NOT set and the NCO is out of reset
+    bool get_nco_good();
+
+    /*! Return true if the NCO fail flag is NOT set
+     *
+     * Note: The logic inversion is to stay consistent with the previous
+     * get_nco_*() methods; in all three cases we return 'true' if we have a
+     * positive status.
+     */
+    bool get_nco_no_fail();
+
     /*! \brief Set an NCO to a specific frequency.
      *
      * \param nco Which NCO to re-tune

host/lib/usrp/dboard/zbx/zbx_dboard_init.cpp

@@ -629,7 +629,8 @@ void zbx_dboard_impl::_init_lo_prop_tree(uhd::property_tree::sptr subtree,
     // The NCO gets a sub-node called 'reset'. It is read/write: Write will
     // perform a reset, and read will return the reset status. The latter is
     // also returned in the 'locked' sensor for the NCO, but the 'nco_locked'
-    // sensor node is read-only, and returns a sensor_value_t (not a bool).
+    // sensor node is read-only, and returns a sensor_value_t (not a bool), and
+    // it also includes the sync-fail flag (which this node does not).
     // This node is primarily used for debugging, but can also serve as a manual
     // reset line for the NCOs.
     const auto nco = (trx == TX_DIRECTION)

host/lib/usrp/x400/x400_rfdc_control.cpp

@@ -75,6 +75,21 @@ bool rfdc_control::get_nco_reset_done()
     return bool(_iface.peek32(regmap::NCO_RESET) & (1 << regmap::NCO_RESET_DONE_MSB));
 }
 
+bool rfdc_control::get_nco_good()
+{
+    constexpr uint32_t flag_mask = (uint32_t(1) << regmap::NCO_RESET_DONE_MSB)
+                                   | (uint32_t(1) << regmap::NCO_SYNC_FAILED_MSB);
+    const uint32_t nco_flags = _iface.peek32(regmap::NCO_RESET) & flag_mask;
+    return nco_flags == (uint32_t(1) << regmap::NCO_RESET_DONE_MSB);
+}
+
+bool rfdc_control::get_nco_no_fail()
+{
+    const uint32_t fail_flag = _iface.peek32(regmap::NCO_RESET)
+                               & (uint32_t(1) << regmap::NCO_SYNC_FAILED_MSB);
+    return fail_flag == 0;
+}
+
 double rfdc_control::set_nco_freq(const rfdc_type, const double freq)
 {
     UHD_LOG_WARNING(_log_id, "set_nco_freq() called but not yet implemented!");