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

[David Cohen <david.a.cohen@linux.intel.com>]

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.

Br, David