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SubjectRe: [PATCH] USB: core: Add warm reset while reset-resuming SuperSpeed HUBs
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.
> 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.

Alan Stern

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