linux-kernel - Re: USB extension (repeater) cable

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Date:	Sat, 20 Jan 2007 23:18:05 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Greg KH <greg@...ah.com>
CC:	Udo van den Heuvel <udovdh@...all.nl>, linux-kernel@...r.kernel.org
Subject: Re: USB extension (repeater) cable

Greg KH wrote:
> On Fri, Jan 19, 2007 at 04:40:34PM +0100, Udo van den Heuvel wrote:
>> Hello,
>>
>> I just tried my shiny new usb extension cable (repeater):
>>
>> Jan 19 16:01:17 epia kernel: usb 5-1: new high speed USB device using
>> ehci_hcd and address 60
>> Jan 19 16:01:17 epia kernel: usb 5-1: configuration #1 chosen from 1 choice
>> Jan 19 16:01:17 epia kernel: hub 5-1:1.0: USB hub found
>> Jan 19 16:01:17 epia kernel: hub 5-1:1.0: 4 ports detected
>> Jan 19 16:01:18 epia kernel: hub 5-1:1.0: Cannot enable port 1.  Maybe
>> the USB cable is bad?
>> Jan 19 16:01:22 epia last message repeated 3 times
>> Jan 19 16:01:23 epia kernel: hub 5-1:1.0: Cannot enable port 2.  Maybe
>> the USB cable is bad?
>> Jan 19 16:01:26 epia last message repeated 3 times
>> Jan 19 16:01:27 epia kernel: hub 5-1:1.0: Cannot enable port 3.  Maybe
>> the USB cable is bad?
>> Jan 19 16:01:31 epia last message repeated 3 times
>>
>> The second cable does the same.
>> Of course we have just one port on this hub...
>> Any ideas?
> 
> Perhaps the kernel is not lying and this cable really is bad?  :)
> 
> Your hardware can not handle this device, there really is nothing that
> the kernel can do about this.
> 
> USB extension cables are horrible things, and usually violate the USB
> spec and do not always work, as you are finding out.  Sorry about that.
> 

Actually, what it looks like is even simpler.  The extension cable 
contains a four-port hub chip (which is the most common commodity chip) 
and haven't bothered changing the descriptor to tell the computer only 
one port is actually active.  So only one port can be activated, and the 
others are stubbed out in some evil way.  In that case, it should be 
noisy but harmless.

	-hpa
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