Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[cinap_lenrek@felloff.net]

> To address Hiro's comments, I have no benchmarks on Plan 9, because
> the SDR code I run does not exist there.  But I do have experience
> with running SDR on Linux and FreeBSD with hardware like the HackRF
> One.  That hardware can easily saturate a USB2 interface/driver on
> both of those operating systems.  Given my experience with USB on
> Plan 9 to date, it's a safe bet that all the variants would die
> when presented with that amount of traffic.

why? the *HOST CONTROLLER* schedules the data transfers. if the
program doesnt do a read() theres nothing to schedule... (unless
its isochronous endpoint, in which case the controller dma's for
you in the background at the specified sampling rate).

> (I can knock down a Plan9 system with 56 Kb/s USB serial traffic.)

that sounds seriously scewed up. i have no issues here reading a usb
stick on my x230 with xhci at 32MB/s, not using any fancy streaming
optimization. no load at all. and this is just some garbage from the
supermarket.

> I can see about
> twisting up some code that would read the raw I/Q data from the SDR
> via USB and see how it stands up.  But the real question is what
> kind of delay, latency, and jitter will there be, getting that raw
> I/Q data from the USB interface up to the consuming application?

is this a isochronous endpoint? in that case you would not have to
worry much as the controller does all the timing for you in hardware.

> Eliminating as much of the copy in/out WRT the kernel cannot but
> help, especially when you're doing SDR decoding near the radios
> using low-powered compute hardware (think Pies and the like).

ahhhh! we'r talking about some crappy raspi here... probably with all
caches disabled... never mind.

> --lyndon

--
cinap