linux-kernel - Re: [Linux-usb-users] OHCI root_port

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Message-ID: <20071009173925.GA6062@kroah.com>
Date:	Tue, 9 Oct 2007 10:39:25 -0700
From:	Greg KH <greg@...ah.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	David Miller <davem@...emloft.net>,
	David Brownell <david-b@...bell.net>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	USB users list <linux-usb-users@...ts.sourceforge.net>
Subject: Re: [Linux-usb-users] OHCI root_port_reset() deadly loop...

On Tue, Oct 09, 2007 at 12:01:54PM -0400, Alan Stern wrote:
> On Mon, 8 Oct 2007, David Miller wrote:
> 
> > As coicidence would have it I finally found a recipe for triggering
> > the issue, and it ties into what you're talking about here.
> > 
> > It happens only if I make sure OHCI gets loaded first and then EHCI
> > right afterwards.
> > 
> > It seems that indeed it is important for EHCI to get loaded first,
> > and in-kernel this is ensured by the link ordering.
> > 
> > However, when both OHCI and EHCI are built as modules (or, similarly
> > I guess, OHCI is built-in and EHCI is modular) there appears to be
> > nothing in userspace which makes sure EHCI gets loaded first.
> > 
> > When this triggers, in OHCI's root_port_reset(), the port status
> > register reads 0x111 in that inner-loop and the value never changes.
> > It stays like this forever.
> 
> I agree with Ben; the best way to solve this is in the kernel.  Even if 
> you did manage to make sure that modules were loaded in the right order 
> initially, that wouldn't help if somebody starts unloading and loading 
> modules by hand.
> 
> The underlying cause of the problem appears to be related to the fact
> that it is impossible in principle to detect a disconnect while a
> full/low-speed reset is in progress.  However the hardware shouldn't
> rely on this, and it should fail gracefully when an unexpected
> disconnect does occur.
> 
> For example, what would the controller do if the user unplugged the USB 
> cable right in the middle of a reset?
> 
> Anyway, it certainly would be easy enough to make port resets mutually
> exclusive with EHCI initialization.  That would work on any platform,
> PCI or not.

That's a good idea, any thoughts as to how to do this?  I think I can
find some Intel and Dell people who would be glad to test this out if
you have a proposed patch.

thanks,

greg k-h
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