[Xen-devel] Re: [linux-usb-devel] Error recovery in Xen's paravirtualizing USB driver for Linux

[Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>]

On Wed, 7 Dec 2005, Harry Butterworth wrote:

> > Suspend/resume is liable to cause trouble.  For instance, what happens to 
> > the various front-ends if the back-end decides to suspend a USB device?
> 
> I don't know.  Could you explain this scenario in more detail (imagine
> that none of the 800 page USB spec sunk in when I read it :-).  What
> should happen in this case?

Suspend and reset are both troublemakers, because they act not on a device 
but on the device's parent hub.  However resets are pretty self-contained, 
so provided you recognize them and handle them properly you should be 
okay.  The back-end won't reset a device all by itself.

The problems with USB suspend are just a small subset of the problems with 
suspend in general.  What happens to a front-end if a back-end goes to 
sleep, for example?

Your best approach is to make sure that a back-end never tries to suspend
an exported device.  Then the front-end will be completely responsible for 
device power management, and no confusion will arise.

Alan Stern


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