Hi Philip,
Check out line 188:
https://github.com/osmocom/gr-osmosdr/blob/master/lib/osmosdr/osmosdr_src_c.cc
It would appear that other driver writers have followed the lead of Ettus and use the same letters to indicate the same error. I've only looked at this code for an entire five minutes, but it looks like the code is initialized with a string that is fed to a dictionary, and after that, _buf_len's size is set from the dictionary, or if there's no entry in the dictionary, sets it to 15. Perhaps you could increase your buffer size? Also, if you're building from source, perhaps you can use -O2 or -O3 when you build it? I don't see any optimization flags set in the top-level CMakeLists.txt
In my limited experience with overflows, the two solutions I have used are 1) sample the input at a lower rate, or if the input sample rate has to be at whatever level it is, 2) decimate the samples as soon as possible.
Regards,
Sean