Connect tagged stream blocks and UHD sink
|Assignee:||Sean Nowlan||% Done:|
The tagged stream blocks use PDU length tags; UHD uses tx_sob and tx_eob.
We need to connect these. Two possibilities come to mind:
- Adapt usrp_sink to understand length tags
- Make a burst tagger block that adds tx_?ob tags which is connected before uhd.usrp_sink
#4 Updated by Sean Nowlan over 1 year ago
I'd like to work on this if there are no objections. I have to do it anyway for some of my own applications. I have a fairly good idea of how I'd do it within gr_uhd_usrp_sink:
- whenever a length tag is seen
- set a d_pdu_left_to_consume variable (or similar)
- set UHD meta data to indicate start-of-burst
- for every call to work(), decrement d_pdu_left_to_consume by number of samples sent
- when d_pdu_left_to_consume reaches zero, send end-of-burst mini packet to UHD
I would keep backwards compatibility with the tx_sob/tx_eob format, though I would set some guard to prevent the two models from interfering, i.e., stick with whichever tag I see first. I would also need to keep the tx_time tag. Thoughts?
#6 Updated by Johnathan Corgan over 1 year ago
Sean--please implement this the way you describe and give us a patch or git branch. It will need some extensive testing, and we won't merge it until Ettus has a chance to review and comment.
However, to avoid confusion, please don't use the term PDU in any of the new code, this is an interface to a tagged stream block, and should probably use 'tsb' for its naming. A PDU interface to a USRP sink (by receiving it as an async message) might also be a desirable feature, but is a different beast altogether.
#7 Updated by Sean Nowlan over 1 year ago
Ok, understood. I'll call it "tsb" or length-tagged stream.
The async msg feature sounds interesting, but after I finish this, I believe it could be achieved through pdu_to_tagged_stream block --> tsb-capable USRP sink. Perhaps there are merits to giving the USRP sink its own async message interface; I haven't thought them through.
I'll probably just send you a patch. The timing will be in the next 1-1.5 weeks.
#8 Updated by Sean Nowlan over 1 year ago
I made these changes on top of master, current as of today (8fda7acad4f71645bded77b5a4806b21aba46078). I used the attached Python script to test the changes, but I'm sure I haven't tested every possible corner case. Perhaps I can enlist some help testing/vetting this code?
I did confirm this works for bursts down to about 50 microseconds on a Tektronix RSA6100. Getting secondary confirmation would be helpful; hopefully the good people at Ettus Research can help. :)
#9 Updated by Johnathan Corgan over 1 year ago
Sean--thanks for doing this. I've only glanced at the patch briefly, but it appears that it may be adding extra docs/functions instead of modifying them. Can you verify the patch is correctly formed?
I'm on the road, so I can't actually apply and test this for a few days, but Martin or Tom might.
#10 Updated by Sean Nowlan over 1 year ago
I added new public factory function that takes a len_tag_key argument, similar to the OFDM blocks. The original ones are still there. The sink needs to know what length tags to look for since it should not be hard-coded and it shouldn't just assume any tag it doesn't already understand is a length tag.
#13 Updated by Sean Nowlan over 1 year ago
- File uhd_usrp_sink_changes.patch added
I discovered an issue with the previous patch. During long bursts that fill up the USRP buffer, the USRP was ACKing that zero samples were transmitted. I think this is expected behavior, but I was decrementing
ninput_items instead of
num_sent, so bursts weren't lining up correctly. I've attached a patch that fixes this issue (should apply cleanly to freshly cloned master). I also changed
_len_tag_key == "" and
_len_tag_key != "" checks to
not _len_tag_key.empty(), respectively.
#15 Updated by Sean Nowlan over 1 year ago
Fixed to work on current master (e7ea85fb752c330de2f29ca5e666a0727be0aa51). I also added length tag generation to tags_demo in gr-uhd/examples/c++. I tested with many combinations of short and long bursts, sample rates, etc. I also included a test of gracefully canceling a burst if a new length tag shows up before the end of the previous burst (must be enabled in tag_source_demo.h). The smallest interval I checked was 5 microseconds on, 245 microseconds off at 10 MSps on an N200 (50 samples). Using a smaller sample rate for these durations caused late packets, but the standard tx_sob/tx_eob approach also caused late packets on my system. The largest burst length I checked was 950 ms on, 50 ms off at 5 MSps (4.75e6 samples).
#23 Updated by Sean Nowlan 8 months ago
I went ahead and reimplemented this on my own. Personal copyright assignment form was filed w/ FSF over a week ago. I'm not sure if I need to wait to hear back from them.
Here's the branch implementing this feature:
#26 Updated by Sean Nowlan 8 months ago
Martin suggested saving the length_tag as a PMT symbol instead of a string. I think this is a good suggestion. I end up checking the length of the length_tag string a handful of times in order to keep the tx_sob/tx_eob implementation mutually exclusive from the length_tag approach. Maybe if the length_tag argument string is empty, I should save the key as a pmt::PMT_NIL type. Then all my length_tag.empty() calls could be pmt::is_null(key) calls, which seems cleaner and more readable. I'm going to update this now on my github branch.
Unrelated to the above, I also thought of this: if tag propagation isn't guaranteed to be exact, there could be an off-by-one error. To give a specific example: if I'm using a block like a pfb_arb_resampler with a noninteger relative rate, sometimes it produces 1 more or less than the relative rate implies. Due to tag propagation issues, this could end up stranding samples between packet_len tagged streams in a packet flow. Any thoughts on how to handle this? My current implementation expects all samples to be accounted for in the packet_len values.
#28 Updated by Sean Nowlan 8 months ago
Personally I think pfb_resampler should handle its own tag propagation. Currently it relies on block_executor's implementation, which is to use the relative rate. Tags drift over time, and they shouldn't be. I started issue #652 and provided a fix that stops tag offsets from drifting, but it still wouldn't preserve correct tagged stream boundaries.
#30 Updated by Sean Nowlan 8 months ago
My github branch has the update to use a PMT instead of string for the member variable. https://github.com/nowls/gnuradio/tree/usrp_sink_tagged_stream_support