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[Discuss-gnuradio] USRP transfer sizes

From: Greg Troxel
Subject: [Discuss-gnuradio] USRP transfer sizes
Date: Sun, 07 May 2006 12:15:07 -0400
User-agent: Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (berkeley-unix)

  [We're working on making USRP work well on NetBSD.]


Regarding BSD ugen(4) support for the USRP, I have a few questions
about desired transfer sizes.  With ugen now, and with Linux URBs, the
size of each USB transaction is controlled from user space.

1) Is the USB transaction size fixed by the USRP firmware?  Is it
   happy with a range of sizes?

2) Does the USRP ever send a short transaction on read?  Are there
   start/stop issues?

3) Has an optimal transaction size been determined?  This seems to be
   a latency/efficiency tradeoff, but the amount of on-board buffering
   seems key.

4) Is the write size tightly linked to the read size?  Do they have to
   be the same?

The initial proposal didn't discuss how to set the read transaction
size, and a way to do that is clearly needed.  This could easily be
part of setting the read buffer size.  For write, one also needs to
set the write transaction size, and limit the amount of buffered write
data, so perhaps a symmetric ioctl would be good; this would allow
different read and write sizes, and totally decouple reading and

        Greg Troxel <address@hidden>

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