Re: i.MX kernel hangup caused by chipidea USB gadget driver

[Shawn Guo <shawnguo2@yeah.net>]

Hi Alan,

On Mon, Jun 09, 2025 at 09:54:45AM -0400, Alan Stern wrote:
> On Mon, Jun 09, 2025 at 07:53:22PM +0800, Xu Yang wrote:
> > Hi Shawn,
> > 
> > Thanks for your reports!
> > 
> > On Mon, Jun 09, 2025 at 01:31:06PM +0800, Shawn Guo wrote:
> > > Hi Xu, Peter,
> > > 
> > > I'm seeing a kernel hangup on imx8mm-evk board.  It happens when:
> > > 
> > >  - USB gadget is enabled as Ethernet
> > >  - There is data transfer over USB Ethernet
> > >  - Device is going in/out suspend
> 
> > During the scp process, the usb host won't put usb device to suspend state.
> > In current design, then the ether driver doesn't know the system has
> > suspended after echo mem. The root cause is that ether driver is still tring
> > to queue usb request after usb controller has suspended where usb clock is off,
> > then the system hang.
> > 
> > With the above changes, I think the ether driver will fail to eth_start_xmit() 
> > at an ealier stage, so the issue can't be triggered.
> > 
> > I think the ether driver needs call gether_suspend() accordingly, to do this,
> > the controller driver need explicitly call suspend() function when it's going
> > to be suspended. Could you check whether below patch fix the issue?
> 
> The situation is more complicated than this.
> 
> In general, a USB gadget cannot allow itself to be suspended while the 
> USB bus it is connected to remains active.  Not unless it can be set to 
> wake up when a USB packet arrives, and even that probably won't work 
> because the wakeup sequence would take too long and the USB transfer 
> would time out on the host.
> 
> The best way to fix this problem is for the gadget to disconnect itself 
> from the USB bus whenever it goes into suspend, and to reconnect when it 
> resumes.

Thank you so much for the insight!  It matches my testing pretty well.
The disconnect/reconnect gadget in udc suspend/resume is more reliable
than suspend/resume gadget.

Shawn