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Re: [Discuss-gnuradio] USRP transfer sizes

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] USRP transfer sizes
Date: Wed, 10 May 2006 21:04:49 -0700
User-agent: Mutt/1.5.9i

On Sun, May 07, 2006 at 12:15:07PM -0400, Greg Troxel wrote:
>   [We're working on making USRP work well on NetBSD.]
>   http://acert.ir.bbn.com/downloads/adroit/NetBSD-USB-continuous.pdf
> 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?

We currently always use 512 bytes on the Tx and Rx end points.  This
of course only works with USB 2.0.  We may want to vary this when we
start adding fixed headers to the packets, but it wouldn't be the end
of the world if the packets stayed at 512 bytes.

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

We currently don't every send a short read.

> 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.

I haven't run the experiment.

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

No.  No.

> 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
> writing.

Decoupling them seems like a good idea.

>         Greg Troxel <address@hidden>


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