[Discuss-gnuradio] Work on USRP source and sink stream tagging for gr-uhd

[Josh Blum]

I have been working on support of tag handling for the USRP source and
sink blocks in gr-uhd. I will be adding some test and example code, but
if anyone cares to experiment, see my uhd_features branch on jblum.git
http://gnuradio.org/cgit/jblum.git/log/?h=uhd_features

Rather than describe, its easier to paste these excerpts from the docs:

 * RX Stream tagging:
 *
 * The following tag keys will be produced by the work function:
 *  - pmt::pmt_string_to_symbol("rx_time")
 *
 * The timstamp tag value is a pmt tuple of the following:
 * (uint64 seconds, and double fractional seconds).
 * A timestamp tag is produced at start() and after overflows.


 * TX Stream tagging:
 *
 * The following tag keys will be consumed by the work function:
 *  - pmt::pmt_string_to_symbol("tx_sob")
 *  - pmt::pmt_string_to_symbol("tx_eob")
 *  - pmt::pmt_string_to_symbol("tx_time")
 *
 * The sob and eob (start and end of burst) tag values are pmt booleans.
 * When present, burst tags should be set to true (pmt::PMT_T).
 *
 * The timstamp tag value is a pmt tuple of the following:
 * (uint64 seconds, and double fractional seconds).

-Josh