Re: [PATCH] xhci: clear port_remote_wakeup after resume failure

[Mathias Nyman]

On 27.5.2019 14.28, Nicolas Saenz Julienne wrote:
> Hi Matthias,
> thanks for the review.
>
> On Mon, 2019-05-27 at 14:16 +0300, Mathias Nyman wrote:
> > On 24.5.2019 17.52, Nicolas Saenz Julienne wrote:
> > > This was seen on a Dell Precision 5520 using it's WD15 dock. The dock's
> > > Ethernet device interfaces with the laptop through one of it's USB3
> > > ports. While idle, the Ethernet device and HCD are suspended by runtime
> > > PM, being the only device connected on the bus. Then, both are resumed on
> > > behalf of the Ethernet device, which has remote wake-up capabilities.
> > >
> > > The Ethernet device was observed to randomly disconnect from the USB
> > > port shortly after submitting it's remote wake-up request. Probably a
> > > weird timing issue yet to be investigated. This causes runtime PM to
> > > busyloop causing some tangible CPU load. The reason is the port gets
> > > stuck in the middle of a remote wake-up operation, waiting for the
> > > device to switch to U0. This never happens, leaving "port_remote_wakeup"
> > > enabled, and automatically triggering a failure on any further suspend
> > > operation.
> > >
> > > This patch clears "port_remote_wakeup" upon detecting a device with a
> > > wrong resuming port state (see Table 4-9 in 4.15.2.3). Making sure the
> > > above mentioned situation doesn't trigger a PM busyloop.
> >
> > There was a similar case where the USB3 link had transitioned to a
> > lower power U1 or U2 state after resume, before driver read the state,
> > leaving port_remote_wakeup flag uncleared.
> >
> > This was fixed in 5.1 kernel by commit:
> >
> > 6cbcf59 xhci: Fix port resume done detection for SS ports with LPM enable
> >
> > Can you check if you have it?
> > It should be in recent stable releases as well.
>
> I was aware of that patch, unfortunately it doesn't address the same issue. In
> my case I never get a second port status event (so no PLC == 1 or any state
> change seen in PLS). The device simply disconnects from the bus.

I see, ok, then we need to clear the flag in the hub thread.

But to me it looks like this patch could cause a small race risk in the successful
device initiated resume cases.

If the hub thread, i.e. the get_port_status() function, notices the U0 state before
the interrupt handler, i.e. handle_port_status() function, then port_remote_wakeup
flag is cleared in the hub thread and the wakeup notification is never called from
handle_port_status().

Would it be enough to just check for (port_remote_wakeup flag && !PORT_CONNECT) in the hub thread?
USB3 PORT_CONNECT bit is lost in most error cases.

-Mathias