linux-kernel - Re: [PATCH] USB: core: Add warm reset while reset-resuming SuperSpeed HUBs

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Date:	Fri, 13 Dec 2013 10:34:22 -0800
From:	David Cohen <david.a.cohen@...ux.intel.com>
To:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Julius Werner <jwerner@...omium.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Vikas Sajjan <sajjan.linux@...il.com>,
	Vikas Sajjan <vikas.sajjan@...aro.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Vincent Palatin <vpalatin@...omium.org>,
	Lan Tianyu <tianyu.lan@...el.com>,
	Ksenia Ragiadakou <burzalodowa@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Vivek Gautam <gautam.vivek@...sung.com>,
	Douglas Anderson <dianders@...omium.org>,
	Felipe Balbi <balbi@...com>, sunil joshi <joshi@...sung.com>
Subject: Re: [PATCH] USB: core: Add warm reset while reset-resuming
 SuperSpeed HUBs

On Fri, Dec 13, 2013 at 10:00:28AM -0800, David Cohen wrote:
> Hi Sarah,
> 
> On Fri, Dec 13, 2013 at 09:48:15AM -0800, Sarah Sharp wrote:
> > On Thu, Dec 12, 2013 at 11:05:04AM -0500, Alan Stern wrote:
> > > On Wed, 11 Dec 2013, Julius Werner wrote:
> > > 
> > > > >> ...although, the spec says that it does not wait for the port resets
> > > > >> to complete.  As far as I can see re-issuing a warm reset and waiting
> > > > >> is the only way to guarantee the core times the recovery.  Presumably
> > > > >> the portstatus debounce in hub_activate() mitigates this, but that
> > > > >> 100ms is less than a full reset timeout.
> > > > 
> > > > It's definitely not just a timing issue for us. I can't reproduce all
> > > > the same cases as Vikas, but when I attach a USB analyzer to the ones
> > > > I do see the host controller doesn't even start sending a reset.
> > > > 
> > > > >>> The xHCI spec requires that when the xHCI host is reset, a USB reset is
> > > > >>> driven down the USB 3.0 ports.  If hot reset fails, the port may migrate
> > > > >>> to warm reset.  See table 32 in the xHCI spec, in the definition of
> > > > >>> HCRST.  It sounds like this host doesn't drive a USB reset down USB 3.0
> > > > >>> ports at all on host controller reset?
> > > > 
> > > > Oh, interesting, I hadn't seen that yet. So I guess the spec itself is
> > > > fine if it were followed to the letter.
> > > > 
> > > > I did some more tests about this on my Exynos machine: when I put a
> > > > device to autosuspend (U3) and manually poke the xHC reset bit, I do
> > > > see an automatic warm reset on the analyzer and the ports manage to
> > > > retrain to U0. But after a system suspend/resume which calls
> > > > xhci_reset() in the process, there is no reset on the wire. I also
> > > > noticed that it doesn't drive a reset (even after manual poking) when
> > > > there is no device connected on the other end of the analyzer.
> > > > 
> > > > So this might be our problem: maybe these host controllers (Synopsys
> > > > DesignWare) issue the spec-mandated warm reset only on ports where
> > > > they think there is a device attached. But after a system
> > > > suspend/resume (where the whole IP block on the SoC was powered down),
> > > > the host controller cannot know that there is still a device with an
> > > > active power session attached, and therefore doesn't drive the reset
> > > > on its own.
> > 
> > Ok, that makes some sense.  I could see why host controllers wouldn't
> > want to drive reset on an unconnected port.
> > 
> > > > Even though this is a host controller bug, we still have to deal with
> > > > it somehow. I guess we could move the code into xhci_plat_resume() and
> > > > hide it behind a quirk to lessen the impact. But since reset_resume is
> > > > not a common case for most host controllers, it's hard to say if this
> > > > is DesignWare specific or a more widespread implementation mistake.
> > > 
> > > I was going to suggest something along these lines too.  This seems to 
> > > be a bug in xHCI.  Therefore the fix belongs in xhci-hcd, not in the 
> > > hub driver.
> > 
> > I agree.  Is there a chance that the Synopsys DesignWare will be a PCI
> > device instead of a platform device?  If so, it would be better to put
> > the code into xhci_resume instead of xhci_plat_resume.  That also allows
> 
> DWC3 on Intel Baytrail and Merrifield is PCI device.

But it actually registers xHCI's platform device to probe it. So,
nevermind.

Br, David
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