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List:       linux-usb
Subject:    Re: g_serial hangs on write when the cable is disconnected
From:       Alan Stern <stern () rowland ! harvard ! edu>
Date:       2015-08-06 13:52:14
Message-ID: Pine.LNX.4.44L0.1508060951250.1844-100000 () iolanthe ! rowland ! org
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On Thu, 6 Aug 2015, Laszlo Papp wrote:

> Hi Alan,
> 
> On Wed, Aug 5, 2015 at 7:15 PM, Alan Stern <stern@rowland.harvard.edu> wrote:
> > On Wed, 5 Aug 2015, Greg KH wrote:
> >
> >> hm, wait, is this really the n_gsm line discipline?  Or is it something
> >> else?
> >>
> >> g_serial is the device side of a serial connection, there is no "cable
> >> removed" notification that it even knows about, that has to come from
> >> the gadget driver somehow, which you should listen for and then kick
> >> your userspace program.
> >
> > There is the gserial_disconnect() callback, which gets invoked when the
> > Vbus power (provided by the host) is removed.  It's a pretty good
> > indicator that the USB cable has been unplugged.
> >
> > I don't understand all the stuff that gserial_disconnect() does, but it
> > ought to be more or less equivalent to a "hangup" -- as the kerneldoc
> > says.  If it doesn't do what users expect, there's probably a bug
> > somewhere.
> >
> > Of course, it's possible that the callback does not get invoked in
> > Laszlo's case.  Then the question would be: Why not?
> 
> Hmm, that is a good question. I wonder if there had been any recent
> fixes for that lately... I suppose that I will need to skim through
> the git log. Thank you for the hints!

You should also add a printk statement to the disconnect callback so 
that you can verify whether it really is getting called.

Alan Stern

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