Re: [PATCH next] usb: usbfs: Add reset_resume for usbfs

[Alan Stern]

On Thu, Jul 11, 2024 at 10:59:56AM +0200, Oliver Neukum wrote:
> 
> 
> On 11.07.24 10:43, Hongyu Xie wrote:
> > During hibernation, usb_resume_interface will set needs_binding to 1 if
> > the usb_driver has no reset_resume implimentation. The USB interface
> > will be rebind after usb_resume_complete.
> > 
> > Normally, that's fine. But if a USB interface has a matched kernel
> > driver, and a userspace driver or application is using this USB
> > interface through usbfs during hibernation, usbfs will be
> > detatched with the USB interface after resume. And this USB interface
> > will be bind with a kernel driver instead of usbfs.
> > 
> > So add reset_resume to fix this.
> 
> The device has lost all settings, yet we continue like nothing
> changed. That strikes me as a very bad idea. If a device has undergone
> a reset user space has not requested, we need to return an error upon
> the next interaction.
> 
> I am sorry, but this implementation has some fundamental issues.

Agreed, but the solution is pretty simple.  Because the device was 
suspended, the userspace driver must have enabled suspend via the 
USBDEVFS_ALLOW_SUSPEND ioctl.  

At that point, although the driver _could_ try to do some I/O to the 
device, the safest approach is for the driver to call the 
USBDEVFS_WAIT_FOR_RESUME or USBDEVFS_FORBID_SUSPEND ioctl.  So those two 
places are where we can return a specific error code to indicate that 
the device was reset while resuming.

Alan Stern