Re: dwc3: gadget suspend/resume vs system suspend/resume

[Thinh Nguyen <Thinh.Nguyen@synopsys.com>]

On Wed, Feb 15, 2023, Alan Stern wrote:
> On Wed, Feb 15, 2023 at 07:29:52PM +0200, Roger Quadros wrote:
> > I was more interested in this case where USB is suspended and then system suspends.
> > Waking up the system on USB activity (while suspended) is taken care of by hardware.
> > But I'm not sure if gadget driver will be up in time to respond to the request
> > reasonably quickly. It would take a couple of seconds and is not hard time bound.
> > Is this time mandated by the USB Spec or is it host implementation specific?
> 
> The USB spec doesn't say very much about it.  One part of the USB 2.0 
> spec seems relevant; it says:
> 
> 	9.2.6.2 Reset/Resume Recovery Time
> 
> 	After a port is reset or resumed, the USB System Software is 
> 	expected to provide a “recovery” interval of 10 ms before the 
> 	device attached to the port is expected to respond to data 
> 	transfers. The device may ignore any data transfers during the 
> 	recovery interval.
> 
> 	After the end of the recovery interval (measured from the end 
> 	of the reset or the end of the EOP at the end of the resume 
> 	signaling), the device must accept data transfers at any time.
> 
> Accepting a data transfer doesn't necessarily mean completing it, 
> though.  The Linux USB core does send a request to a device 10 ms 
> after resuming it, but the timeout period on this request is 5 seconds.  
> This gives you some leeway.
> 

For most standard control requests, the spec indicates that the device
must respond within 500ms. But that's not the case for some real devices
so we have a 5 second timeout in Linux. For other requests, it's up to
the class drivers. For most drivers on Linux, it's typically 5 seconds
also.

IMO, the system suspend on the gadget side should take precedence. That
is, it shouldn't depend on whether the usb gadget is in suspend or not
to go through system suspend. For that to happen, the gadget must
initiate soft-disconnect. Otherwise I can see we may run into
complications from the delay from the system suspend. For example, what
if the host initiates resume right after suspend while the gadget side
is still suspending? What if there are other gadgets on the setup that
want or not want to go to suspend also? How can the system decide when
it can go into suspend then?

Thanks,
Thinh