Linux USB development and users ()

Olivier DOLE | 9 Feb 21:15

USB 3.0 : host-to-host communication

Hello,

I have a question about USB 3.0 and host to host communication through a
"standard A" to "standard A" cable.
As it is defined in the norm (section 5.5.2), I thought that it would be
directly supported by Linux. Nonetheless when I plug the cable between 2
USB-3.0 ports (2 servers), nothing is seen (from lsusb point of view and
also from kernel messages point of view).
So my questions:
- Is it supported by the linux kernel or is there something missing (like
OTG/RSP implementation...) ?
- If the problem is not the kernel, any advice to make it work ?

Thanks

Olivier
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Greg KH | 9 Feb 21:23

Re: USB 3.0 : host-to-host communication

On Thu, Feb 09, 2012 at 09:15:25PM +0100, Olivier DOLE wrote:
> Hello,
> 
> I have a question about USB 3.0 and host to host communication through a
> "standard A" to "standard A" cable.

Not possible at all, on a pure electrical basis.  Please read the USB
specification at www.usb.org for why.

greg k-h
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Alan Stern | 9 Feb 21:38

Re: USB 3.0 : host-to-host communication

On Thu, 9 Feb 2012, Greg KH wrote:

> On Thu, Feb 09, 2012 at 09:15:25PM +0100, Olivier DOLE wrote:
> > Hello,
> > 
> > I have a question about USB 3.0 and host to host communication through a
> > "standard A" to "standard A" cable.
> 
> Not possible at all, on a pure electrical basis.  Please read the USB
> specification at www.usb.org for why.

Actually it _is_ possible in USB-3.  That's because unlike USB-2, USB-3
cables use separate wires for TX+, TX-, RX+, and RX-.  An A-to-A cable
simply has the TX+/TX- pair on each side connected to the RX+/RX- pair
on the other side.

Nevertheless, although such connections are possible, they require 
special software to drive them.  They are intended for debugging and 
other specialized uses; the Linux kernel does not currently support 
them.

Alan Stern

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Sarah Sharp | 13 Feb 22:08

Re: USB 3.0 : host-to-host communication

On Thu, Feb 09, 2012 at 03:38:36PM -0500, Alan Stern wrote:
> On Thu, 9 Feb 2012, Greg KH wrote:
> 
> > On Thu, Feb 09, 2012 at 09:15:25PM +0100, Olivier DOLE wrote:
> > > Hello,
> > > 
> > > I have a question about USB 3.0 and host to host communication through a
> > > "standard A" to "standard A" cable.
> > 
> > Not possible at all, on a pure electrical basis.  Please read the USB
> > specification at www.usb.org for why.
> 
> Actually it _is_ possible in USB-3.  That's because unlike USB-2, USB-3
> cables use separate wires for TX+, TX-, RX+, and RX-.  An A-to-A cable
> simply has the TX+/TX- pair on each side connected to the RX+/RX- pair
> on the other side.
> 
> Nevertheless, although such connections are possible, they require 
> special software to drive them.  They are intended for debugging and 
> other specialized uses; the Linux kernel does not currently support 
> them.

Alan's right.  The xHCI spec describes a debug port to connect two hosts
together, but a debug port is optional and almost none of the xHCI hosts
currently on the market actually have them.  Also, as Alan said, there
isn't any Linux software to support it.

Sarah Sharp
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