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

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Date:	Tue, 30 Jul 2013 19:33:46 -0700
From:	Julius Werner <jwerner@...omium.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Julius Werner <jwerner@...omium.org>,
	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

> 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. 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.

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.
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