mpm: Make default clock/time source values state-less

Currently, the default clock/time source is whatever the user configured
in the last session.

This fixes the scenario were you have any MPM device and do this:

    $ benchmark_rate --args $args,clock_source=external

But whoops! You forgot to attach an external 10 MHz. PLL lock fails,
nothing works. No worries, you run it again:

    $ benchmark_rate --args $args

With the previous behaviour, this would retain the setting to
'external', because there's nothing to overwrite it. You would need to
append `clock_source=internal` to get a working device again. Calling
multi_usrp::set_clock_source("internal"), or a similar API call, might
not be sufficient because the PLL lock failure might crash the program
before updating the clock source is possible.

The problem with this is twofold:

- All non-MPM devices behave differently, i.e., they have a fixed
  default ('internal') which is always applied if no other option is
  given. This is an internal inconsistency.
- Some applications (like gr-uhd's GRC bindings) simply don't set
  a clock/time source when selecting a "default", or they try and update
  the clock/time source using the API calls.

Therefore, we align the behaviour of MPM devices with the other devices,
and fall back to an internal source if nothing else is provided.

0 files changed, 0 insertions, 0 deletions