linux-kernel - Re: [linux-usb-devel] ThinkPad T41

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Date:	Sat, 9 Jun 2007 17:20:56 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Robert de Rooy <robert.de.rooy@...il.com>
cc:	Jiri Kosina <jikos@...os.cz>, Greg KH <greg@...ah.com>,
	<Joel.Becker@...cle.com>,
	USB development list <linux-usb-devel@...ts.sourceforge.net>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [linux-usb-devel] ThinkPad T41 - Strange USB 2.0 behaviour

On Sat, 9 Jun 2007, Robert de Rooy wrote:

> Alan Stern wrote:
> > Robert, it would help somewhat if you could build a kernel with 
> > CONFIG_USB_DEBUG turned on and post the dmesg log showing what happens 
> > when you plug in one of those non-working devices.
> >   
> Sorry, yes I should have done that before...

Unfortunately you posted the system log file instead of the dmesg log, 
and your syslogd was configured not to retain debug-level messages.

> > Yes, in principle Linux can be made to switch over to full speed when 
> > high speed fails.  But there are limitations: The switchover would work 
> > only for devices plugged directly into the computer, not for devices 
> > plugged into a high-speed hub.  And some Linux systems (not regular 
> > PCs) have EHCI implementations that don't allow such a switch -- or 
> > if they do, I'm not aware of how to accomplish it.
> >   
> Interesting, I have a USB 2.0 hub, but have not tried it, mainly because 
> I need to find a power supply for it first.
> But regardless if the hub works as USB 2.0 or 1.1, it could be that 
> another hub behaves differently, just like I have one USB 2.0 memory key 
> that still works as such.

You did not understand my point.  Regardless of how the hub behaves,
there is no way to tell it that a device plugged into a particular port
should not be allowed to run at high speed.

Alan Stern

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