Re: Discussion: recovery from transient disconnects

[Greg KH <greg@xxxxxxxxx>]

On Wed, Sep 03, 2008 at 04:58:38PM -0400, Alan Stern wrote:
> The hub driver in 2.6.27 has a feature allowing it to recover from
> transient disconnections or port-enable changes.  The idea is that if
> either one occurs and after the debouncing period (100 ms or so) the
> same device is still connected to the same port, the event is handled
> as a reset rather than a disconnect.  The existing usb_device isn't
> removed and drivers aren't unbound.
> 
> In theory this should make us more resilient against errors.  In 
> practice I'm not so sure.  Port-enable changes are very rare, and 
> disconnects normally last a lot longer than 100 ms.
> 
> Furthermore, the recovery procedure can itself lead to problems.  
> While many drivers are good about detecting errors and retrying as part
> of their normal operation, most don't react so robustly when an error
> occurs in their probe routine.  As an outstanding example, consider the
> usb_generic driver, whose probe routine does little more than select a
> default configuration and install it.  If something goes wrong with the
> Set-Config call, the call is not retried; instead the device simply
> remains unconfigured.
> 
> Now under most circumstances this is all okay.  We've got a non-robust 
> reaction to a type of error that almost never occurs.  That's fine, 
> except for one thing:
> 
> 	These errors _do_ occur quite reliably when ehci-hcd is
> 	loaded after ohci-hcd or uhci-hcd!
> 
> What happens is that ehci-hcd takes over the port, disconnecting the
> device from the companion controller.  If the device can't run at high
> speed then the port is returned to the companion controller.  The
> overall result is exactly that of a transient disconnection.
> 
> A user has reported a case in which this occurred during the
> usb_generic probe routine.  As a result the device (a keyboard) never 
> got configured and consequently was nonfunctional.
> 
> Given the significant probability of bad outcomes, together with the 
> fact that real transient errors are so rare, I'm starting to think we 
> would be better off without this new recovery feature.
> 
> What do other people think?

Should we wait for .27 to be out and see how many people complain?  :)

thanks,

greg k-h
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