linux-kernel - Re: [PATCH v3 1/5] xhci: add a quirk for device disconnection errata for Synopsis Designware USB3 core

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Date:	Thu, 12 Feb 2015 10:18:19 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Mathias Nyman <mathias.nyman@...el.com>
cc:	Sneeker Yeh <sneeker.yeh@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Grant Likely <grant.likely@...aro.org>,
	Huang Rui <ray.huang@....com>,
	Kishon Vijay Abraham I <kishon@...com>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-usb@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	Andy Green <andy.green@...aro.org>,
	Jassi Brar <jaswinder.singh@...aro.org>,
	Sneeker Yeh <Sneeker.Yeh@...fujitsu.com>
Subject: Re: [PATCH v3 1/5] xhci: add a quirk for device disconnection errata
 for Synopsis Designware USB3 core

On Thu, 12 Feb 2015, Mathias Nyman wrote:

> On 25.01.2015 10:13, Sneeker Yeh wrote:
> > This issue is defined by a three-way race at disconnect, between
> > 1) Class driver interrupt endpoint resheduling attempts if the ISR gave an ep
> >    error event due to device detach (it would try 3 times)
> > 2) Disconnect interrupt on PORTSC_CSC, which is cleared by hub thread
> >    asynchronously
> > 3) The hardware IP was configured in silicon with
> >    - DWC_USB3_SUSPEND_ON_DISCONNECT_EN=1
> >    - Synopsys IP version is < 3.00a
> > The IP will auto-suspend itself on device detach with some phy-specific interval
> > after CSC is cleared by 2)
> > 
> > If 2) and 3) complete before 1), the interrupts it expects will not be generated
> > by the autosuspended IP, leading to a deadlock. Even later disconnection
> > procedure would detect that corresponding urb is still in-progress and issue a
> > ep stop command, auto-suspended IP still won't respond to that command.

If the Synopsys IP provides a way to do it, it would be better to turn
off the autosuspend feature entirely.  Doesn't autosuspend violate the
xHCI specification?

> So did I understand correctly that the class driver submits a new urb which
> is enqueued by xhci_urb_enqueue() before the hub thread notices the device is disconnected.
> Then hub thread clears CSC bit, controller suspends and the new urb is never given back?
> 
> Doesn't the CSC bit and PORT_CONNECT bit show the device is disconnected when we enter
> xhci_enqueue_urb(), even it the hub thread doesn't know this yet?

What if the device disconnects _after_ the new URB is enqueued?

> Would it make sense to check those bits in xhci_enqueue_urb, and just return error
> in the xhci_urb_enqueue() if device is not connected? Then there wouldn't be a need for any quirk
> at all.

That wouldn't help URBs that were already enqueued when the disconnect 
occurred.

Alan Stern

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