I'm in a situation where I need to plug 24 high bandwidths USB devices into the same machine (they're DSL modems actually).
Since those are USB 1.1 devices they can't be "high bandwidth". That term has a specific meaning, for USB 2.0 high speed periodic transfers (lets them get up to 24 Mbyte/second each).
I'm using 6 hubs, 2 of them plugged directly on the PC, and the fours others on the first hubs. Every hub has in turn 4 devices plugged in.
The trouble is that all the bandwidth is soon taken up, in ISO mode I get "usb check bandwidth would have failed" messages, and in bulk mode, thousands of "ENXIO" messages.
ENXIO sounds like a UHCI driver saying "you need to be using USB_QUEUE_BULK in urb->transfer_flags". Which would be bug in the modem driver: if it's queueing, it should set that flag, even if it's only queueing when latencies climb. (Personally, on 2.4 I avoid using bulk queuing, there've been too many hc-specific bugs.)
You _should_ be able to hook up all those modems into a single full speed controller, and the only result in "bulk mode" should be slower-than-wanted transfers (assuming the protocol to the modem isn't broken). The fact that you don't get that means a bug.
With ISO transfers, you shouldn't be able to connect that many, since there's only about 10 Mbit/sec of reserved bandwidth (for iso and interrupt transfers).
Can anyone confirm that I'll have indeed 3x12Mbps, and can usb-uhci/usb-ohci/uhci handle this setup ?
That approach is pretty common for USB 1.1 controllers, which is part of why so many motherboards come with lots of USB connectors nowadays. Yes it works, though it seems to me those ENXIO problems mean you have at least one driver bug that could still interfere.
Or instead use USB 2.0 hubs connected to a USB 2.0 root hub, where ((480 Mbit/sec net)/24 devices) == 20 Mbit/sec per device means there's plenty of bulk bandwidth (since no DSL technology I've heard of goes above around 6 Mbit/sec)
- Dave
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