Axi Streaming is missing support for tstrb

[ollie-etl]

The Axi-Streaming standard supports an optional tstrb component, for signalling data validity. The semantics differ from tkeep, although may be used in conjuntion with tkeep

tkeep	tstrb	Description
0	0	This byte is junk, and may be discarded from the stream
1	0	This is a positioning byte: it contains no information, but may not be discarded from the stream
0	1	An invalid bus state
1	1	A data byte

An implementation is available in #60: There is at least one open question around checking the validity of tkeep / tstrb pairs.