Re: [PATCH] usb: core: don't try to reset_device() a port that gotjust disconnected
From: Alan Stern
Date: Wed Jul 31 2013 - 10:49:29 EST
On Tue, 30 Jul 2013, Julius Werner wrote:
> > Wait a moment. Why does each of these attempts lead to a 5-second
> > timeout? Why don't they fail immediately?
> Now that you mention it, that's a very good question.
I have brought this up with Sarah on more than one occasion, but we
never found a good answer. The effects are quite visible when somebody
unplugs a USB-3 disk drive in the middle of a data transfer.
> The kernel
> enqueues a control transfer to the now disconnected address because
> it's internal bookkeeping is not yet updated, but I guess that should
> usually fail very fast from an xHC-internal transaction timeout. I
> have now tried to debug the cause of this: I see the transfer being
> enqueued and the doorbell triggered, but never get a transfer event
> back from it (until the software timeout calls usb_kill_urb() which
> stops the endpoint). With the same setup on a PantherPoint system I
> get the same U1/U2 disable control messages on unplugging, but they
> fail within <5ms with a transaction error... so I guess this must be a
> LynxPoint hardware bug.
An odd sort of bug. You'd think that not getting a response back would
be one of the first types of error the hardware designers would check
> Regardless, calling usb_reset_device() is still wrong and will at
> least lead to pointless transfer attempts and error messages, so I
> think my patch still has merit.
> > What will happen here if udev is NULL? Maybe you should change the
> > test to (!udev || !(portstatus & ...)).
> Right... I'm not sure if that can happen in practice, but I'll change
> it just in case.
Somebody said that in theory, ports can put themselves in the Disabled
state at any time, spontaneously. If this happened just after a device
was attached, you would end up with udev being NULL and the connect
status being set.
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