linux-kernel - Re: [PATCH] usb: core: don't try to reset_device() a port that got just disconnected

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Date:	Wed, 31 Jul 2013 10:49:22 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Julius Werner <jwerner@...omium.org>
cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Vincent Palatin <vpalatin@...omium.org>,
	Benson Leung <bleung@...omium.org>
Subject: Re: [PATCH] usb: core: don't try to reset_device() a port that got
 just disconnected

On Tue, 30 Jul 2013, Julius Werner wrote:

> > Wait a moment.  Why does each of these attempts lead to a 5-second
> > timeout?  Why don't they fail immediately?
> 
> Now that you mention it, that's a very good question.

I have brought this up with Sarah on more than one occasion, but we 
never found a good answer.  The effects are quite visible when somebody 
unplugs a USB-3 disk drive in the middle of a data transfer.

>  The kernel
> enqueues a control transfer to the now disconnected address because
> it's internal bookkeeping is not yet updated, but I guess that should
> usually fail very fast from an xHC-internal transaction timeout. I
> have now tried to debug the cause of this: I see the transfer being
> enqueued and the doorbell triggered, but never get a transfer event
> back from it (until the software timeout calls usb_kill_urb() which
> stops the endpoint). With the same setup on a PantherPoint system I
> get the same U1/U2 disable control messages on unplugging, but they
> fail within <5ms with a transaction error... so I guess this must be a
> LynxPoint hardware bug.

An odd sort of bug.  You'd think that not getting a response back would 
be one of the first types of error the hardware designers would check 
for.

> Regardless, calling usb_reset_device() is still wrong and will at
> least lead to pointless transfer attempts and error messages, so I
> think my patch still has merit.
> 
> > What will happen here if udev is NULL?  Maybe you should change the
> > test to (!udev || !(portstatus & ...)).
> 
> Right... I'm not sure if that can happen in practice, but I'll change
> it just in case.

Somebody said that in theory, ports can put themselves in the Disabled 
state at any time, spontaneously.  If this happened just after a device 
was attached, you would end up with udev being NULL and the connect 
status being set.

Alan Stern

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