Re: Virtual hub, resets etc...

[Alan Stern <stern@rowland.harvard.edu>]

On Fri, 5 Jul 2019, Benjamin Herrenschmidt wrote:

> On Thu, 2019-07-04 at 12:13 -0400, Alan Stern wrote:
> > >   - bus reset: When I sense a bus reset, that's where I'm not too sure
> > > what to do. Currently I clear all the status bits of the ports
> > > except USB_PORT_STAT_SUSPEND. Thus I clear USB_PORT_STAT_ENABLE.
> > > But I'm not sure what to do with the gadget. I currently call
> > > the gadget suspend as "hinted" by the spec calling for S0 state iirc,
> > > but I don't think it's the best thing to do, it doesn't make that much
> > > sense... Should I do a gadget reset instead ? 
> > 
> > You should also clear USB_PORT_STAT_SUSPEND.  Calling the gadget's
> > suspend routine (if the gadget isn't already suspended) is the right
> > thing to do; the spec says a USB device goes into suspend if it doesn't
> > receive any packets for a period of 3 (or 5? -- something like that)  
> > ms, and that certainly would be the case here.
> > 
> > >   - If the host clears USB_PORT_STAT_ENABLE, what should I do ? I
> > > currently do a suspend as well, which isn't great... mostly it does
> > > nothing and keep potentially the gadget trying to do stuff. I could
> > > do a reset. I don't want to do a disconnect because we are still
> > > connected to the hub so that's not really the right call, but at least
> > > for composite it's the same thing...
> > 
> > As above, doing a suspend is the right thing.
> 
> It is the right HW behaviour. But with our gadget stack, it doesn't
> reset or cleanup anything. Though since the port gets disabled, I
> suppose re-enabling it will cause a reset and will sort that out.

That's right.  (Except that you got the cause and effect reversed; 
resetting the port is what causes it to be re-enabled.  :-)

> > > Now, a few things i noticed while at it:
> > > 
> > >   - At some point I had code to reject EP queue() if the device is
> > > suspended with -ESHUTDOWN. The end result was bad ... f_mass_storage
> > > goes into an infinite loop of trying to queue the same stuff in
> > > start_out_transfer() when that happens. It looks like it's not really
> > > handling errors from queue() in a particularily useful way.
> > 
> > Don't reject EP queue requests.  Accept them as you would at any time;
> > they will complete after the port is resumed.
> 
> Except the suspend on a bus reset clears the port enable. You can't
> resume from that, only reset the port no ? Or am I missing something ?

You're right.  Nevertheless, the driver should accept queue requests, 
even if they will eventually fail.  It's what the gadget drivers 
expect.

> > As for f_mass_storage, repeatedly attempting to queue an OUT transfer
> > is normal behavior.  The fact that one attempt gets an error doesn't
> > stop the driver from making more attempts; the only thing that would
> > stop it is being disabled by a config change, a suspend, a disconnect,
> > or an unbind.
> 
> Except it does that in a tight loop and locks up the machine...

Well, that wouldn't happen if your UDC accepted the requests, right?  

Besides, f_mass_storage won't repeatedly try to queue an OUT transfer 
once it knows that it is suspended.

Alan Stern

> > >   - With my current code doing suspend/resume on bus resets, when I
> > > reboot some hosts, and they re-enumerate, I tend to hit the WARN_ON
> > > drivers/usb/gadget/function/f_mass_storage.c:341
> > > 
> > > static inline int __fsg_is_set(struct fsg_common *common,
> > >                               const char *func, unsigned line)
> > > {
> > >        if (common->fsg)
> > >                return 1;
> > >        ERROR(common, "common->fsg is NULL in %s at %u\n", func, line);
> > >        WARN_ON(1);
> > >        return 0;
> > > }
> > > 
> > > This happens a little while after a successul set_configuration. Here's
> > > a trace:
> > 
> > ...
> > 
> > > I have to get my head around that code, but if one of you have a clue, I
> > > would welcome it :-)
> > > 
> > > Interestingly it recovers. The host seems to then reset the prot, then reconfigure and
> > > the second time around it all works fine.
> > 
> > I suspect this is related to the race you found.  EJ Hsu has been 
> > working on much the same thing (see the mailing list archive).
> 
> Right. I debugged the race and produced the fix I posted *after* I had
> change my code to do a reset rather than a suspend on the hub receiving
> an upstream bus reset.
> 
> I will switch back to doing suspend instead and see whether that stays
> fixed.
> 
> Cheers,
> Ben.