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

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

On Thu, Apr 30, 2026 at 10:14:33AM +0800, Haowen Tu wrote:
> So the real issue I wanted to ask about is closer to this:
> 
>   during hibernation image writeout, should PM resume all previously
>   frozen devices, or is there any room for a more minimal resume of only
>   the devices required for writeout and their dependencies?
> 
> If writeout succeeds, the system will power off afterward, which made me
> wonder whether every previously frozen device must be resumed at that
> stage in the same way it would be for ordinary recovery. If not,
> avoiding unnecessary resume work in that phase might also reduce the
> time spent before final poweroff, although I do not have measurements
> for that. On the other hand, if writeout fails, the system needs to
> continue running, so the remaining devices would still have to be
> recovered correctly. I agree that this failure path makes the problem
> much more subtle than I described in the RFC.
> 
> I also agree with Oliver's point that this cannot be expressed as
> "storage devices only". In practice, any such approach would need to
> account for dependencies and for other classes of devices that may still
> matter during the writeout phase.
> 
> So at this point I am no longer trying to argue for a USB-specific
> interface change. Instead, I am trying to understand whether this is a
> valid PM/hibernate design question at all, namely whether the writeout
> phase should conceptually be treated as:
> 
>   1. a full THAW of the suspended system, as it is today, or
>   2. potentially a narrower resume of only the devices needed for
>      writeout, followed by broader recovery only if writeout fails.
> 
> I do not have a concrete implementation in mind yet, and I am not sure
> whether such an approach would even fit well with the current PM core
> model. I first wanted to check whether this is considered a valid
> problem to discuss.
> 
> If the answer is that the current full-THAW behavior is simply the
> intended model, that is also useful for me. In that case, I should not
> treat the UVC behavior as evidence of a missing USB-side mechanism.

As I understand it, the system works the way it currently does because 
there was no good way to tell which devices needed to be powered up for 
storing the memory image.  It's not just the storage device itself, but 
all the other things it depends on, some of which might not be its 
ancestors in the device tree.

By far, the simplest and most reliable solution was to just power 
everything up.

Alan Stern