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

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

On Wed, Feb 15, 2023 at 11:40:15AM +0200, Roger Quadros wrote:
> Hi Thinh,
> 
> Currently dwc3 gadget disconnects from the bus in the system suspend 
> path. i.e. dwc3_gadget_suspend()->dwc3_disconnect_gadget().
> 
> Is it reasonable to expect dwc3 gadget to do a USB suspend instead of 
> a USB disconnect during system suspend?

Gadgets can't initiate a USB suspend.  Only the host can.

Did you mean to ask the opposite question?  I.e., is it reasonable for a 
dwc3 gadget to go into system suspend when it gets a USB suspend?

In general, it's easiest for gadgets to disconnect from the USB bus when 
their system suspends, if they aren't already in USB suspend.  This is 
because they would be unable to respond to requests from the host while 
sleeping.

> At USB resume, system can wake up and resume and then dwc3 gadget can 
> resume.
> 
> I'd like to know your opinion please if this is a reasonable 
> expectation or something just not possible to achieve. Thanks!

There can be exceptions, but it's a dicey thing.  The UDC hardware would 
have to be able to send handshake packets back to the host while 
remaining in low power, it would have to be able to generate a wakeup 
signal to bring its system back to full power, and the system would have 
to be able to resume in time for the gadget to respond to a request from 
the host before the request times out.

The situation is different if the gadget is already in USB suspend.  
Then it only needs to be able to wake up its system when it gets a USB 
resume signal.  But the system would still have to wake up reasonably 
quickly.

Alan Stern