[RFC] USB/PM: should USB interface drivers distinguish hibernation THAW from RESTORE?

[Haowen Tu <tuhaowen@uniontech.com>]

Hi,

I'm looking for feedback on a USB/PM design question that came up while
investigating a UVC hibernation issue.

In the hibernation flow, after the memory snapshot has been created, the
kernel briefly resumes devices in order to write the image to storage.
On the successful hibernation path, the system is then powered off. For
a USB camera that was actively streaming before hibernation, this means
the USB resume path runs during that intermediate THAW phase, even
though the final RESTORE path has not happened yet.

From the driver's point of view, that THAW phase is not semantically the
same as the later RESTORE path after booting from the image.

The difficulty is that USB interface drivers currently get

    int (*suspend)(struct usb_interface *intf, pm_message_t message);

but resume-side callbacks are only

    int (*resume)(struct usb_interface *intf);
    int (*reset_resume)(struct usb_interface *intf);

so by the time a USB interface driver's resume path runs, it has no
direct way to distinguish a hibernation image-write THAW from the later
RESTORE path.

The immediate trigger here is UVC, where resuming the streaming path
during that THAW phase can turn the camera LED back on and cause other
visible device activity even though the system is about to power off.
More generally, review feedback on that patch was that solving this in
individual leaf drivers would not scale well if other USB interface
drivers ever need similar behavior.

So the question is whether USB interface drivers should be able to
distinguish these two phases, and if so, what the right interface would
be.

Possible directions could include:

  1. Exposing the phase distinction to USB interface drivers
  2. Handling it inside usbcore
  3. Adding a USB-specific callback or other mechanism for this
     transition

I'm intentionally not proposing a concrete API in this RFC yet. I'd
first like to understand whether this should be considered a real USB PM
interface issue, and if so, which direction would be the least
intrusive and most maintainable.

Thanks,
Haowen Tu