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

[Julius Werner]

>> ...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.
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