Re: debugging oops after disconnecting Nexio USB touchscreen

[Ondrej Zary]

On Thursday 03 December 2009, Alan Stern wrote:
> On Thu, 3 Dec 2009, Ondrej Zary wrote:
> > > I wish you hadn't removed all the "create qh" log messages.
> >
> > I haven't removed them - I was surprised too that they are missing. I
> > probably did something wrong (again).
>
> They probably appeared much earlier in the log.  It doesn't matter.
>
> > > Anyway, it looks like the problem is caused by your driver overwriting
> > > the data structure owned by ehci-hcd.  Here's the important part of the
> > >
> > > log:
> > > > [  151.688299] ehci_hcd 0000:00:1d.7: link qh f65cf700 (null)
> > > > [  151.688428] ehci_hcd 0000:00:1d.7: unlink qh f65cf700 (null)
> > >
> > > Here f65cf700 is the only qh on the async list (it is linked in at the
> > > head and its qh_next pointer is NULL).
> > >
> > > > [  151.688497] ehci_hcd 0000:00:1d.7: link qh f65cf080 (null)
> > >
> > > Now f65cf080 is added to the start of the list.
> > >
> > > > [  151.688534] ehci_hcd 0000:00:1d.7: end unlink qh f65cf700 (null)
> > > > [  151.688546] ehci_hcd 0000:00:1d.7: link qh f65cf700 f65cf080
> > >
> > > And f65cf700 is added to the start, preceding f65cf080.
> > >
> > > > [  151.688675] ehci_hcd 0000:00:1d.7: unlink qh f65cf700 f65cf080
> > > > [  151.688784] ehci_hcd 0000:00:1d.7: end unlink qh f65cf700 f65cf080
> > >
> > > f65cf700 is removed from the start position, leaving f65cf080 at the
> > > start.
> > >
> > > > [  151.688796] ehci_hcd 0000:00:1d.7: link qh f65cf700 f65cf080
> > >
> > > It is added again at the start, preceding f65cf080.
> > >
> > > > [  151.688923] ehci_hcd 0000:00:1d.7: unlink qh f65cf700 f65cf080
> > > > [  151.689033] ehci_hcd 0000:00:1d.7: end unlink qh f65cf700 f65cf080
> > >
> > > It is removed again from the start position.
> > >
> > > > [  151.689045] ehci_hcd 0000:00:1d.7: link qh f65cf700 f65cf080
> > >
> > > It is added again at the start.
> > >
> > > > [  151.689106] usb 1-1.1: USB disconnect, address 9
> > > > [  152.712104] prev is NULL, qh=f65cf080, ehci->async=f65cf000
> > >
> > > Evidently prev is f65cf700->qh_next.  We know that the value was set to
> > > f65cf080 just above, and you added log messages to every place where
> > > ehci-hcd changes qh_next.  Hence something your driver did must have
> > > been responsible.  Does it access urb->hcpriv anywhere?
> >
> > Thanks for explaining this.
> >
> > No, it doesn't access urb->hcpriv. The driver should not do anything
> > special. Just sends one interrupt urb, reads the replies and sends ACK (a
> > bulk urb) when touch data was received. When idle, the device sends no
> > reply most of the time, sometimes "8204abaa".
> > Here's the latest version: http://lkml.org/lkml/2009/12/3/74
>
> I don't understand.  The URBs shown in the usbmon trace are all
> bulk-IN.  But your patch adds only a bulk-OUT URB.  And the original
> usbtouchscreen driver doesn't use bulk URBs at all, only interrupt
> URBs.  So where are these URBs coming from?

The input endpoint of the device is a bulk endpoint (so the urb is filled 
using fill_int_urb but submitted as bulk). The device also has an interrupt 
endpoint (on the other of two interfaces) but it does not work (and windows 
driver does not touch it either).

> Furthermore, the patch shows that the second-to-last argument to
> usb_fill_bulk_urb() -- the completion function -- is NULL.  That is
> strictly illegal and it should have caused an oops as soon as the URB
> was used.

Thanks for catching this. usbmon showed no ACK packets so it didn't work
at all. The device did not care. It started to work after adding a
complete function.

> > > Incidentally, look at the usbmon trace:
> > > > f60eecc0 1501056647 S Bi:1:009:2 -115 128 <
> > > > f60eecc0 1501056905 C Bi:1:009:2 -32 0
> > > > f60eecc0 1501056916 S Bi:1:009:2 -115 128 <
> > > > f60eecc0 1501057172 C Bi:1:009:2 -32 0
> > > > f60eecc0 1501057183 S Bi:1:009:2 -115 128 <
> > > > f60eecc0 1501057394 C Bi:1:009:2 -32 0
> > >
> > > Why does your driver keep submitting the same request over and over
> > > again when each time it fails?
> >
> > Looks like it's resubmitting the interrupt urb. This -EPIPE case is not
> > covered in usbtouch_irq() callback. According to some other drivers,
> > -EPIPE means "halt" or "stall" which should be cleared by using
> > usb_clear_halt(). It cannot be used in interrupt context.
>
> -EPIPE does mean "halt" or "stall".  The fact that you can't use
> usb_clear_halt() in interrupt context is no excuse.  You _have_ to use
> it, otherwise the device will continue not working.  The easiest way
> would be to set up a workqueue routine to do it.

Seems that -EPIPE is returned only after the device is disconnected. Adding a 
check for -EPIPE to usbtouch_irq() and not submitting the urb again seems to 
fix the problem!



-- 
Ondrej Zary
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