In f73e327, we modified PeriphManagerBase to explicitly list all
required methods as per the MPM/UHD API. This had an unintended side
effect: Because the clocking methods on x4xx are imported from
X4xxClockMgr, and not defined on x4xx itself, the method used to import
methods from X4xxClockMgr onto x4xx would refuse to re-define API calls
such as set_clock_source(), get_clock_source(), and so on.
The solution is to allow _add_public_methods() to overwrite existing
methods, which means we can overwrite abstract methods from
PeriphManagerBase in this fashion.
Without this patch, UHD sessions could fail in the following manner:
>>> import uhd
>>> U = uhd.usrp.MultiUSRP("type=x4xx")
>>> U.get_clock_source(0)
Traceback (most recent call last):
File "<input>", line 1, in <module>
U.get_clock_source(0)
RuntimeError: RuntimeError: Error during RPC call to `get_clock_source'.
Error message: get_clock_source() not available on this device!
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