Re: [PATCH net v1 1/1] net: usb: asix: ax88772: drop phylink use in PM to avoid MDIO runtime PM wakeups

[Oleksij Rempel <o.rempel@pengutronix.de>]

On Sat, Sep 13, 2025 at 08:45:05AM +0200, Oleksij Rempel wrote:
> On Fri, Sep 12, 2025 at 03:37:52PM +0100, Russell King (Oracle) wrote:
> > On Fri, Sep 12, 2025 at 10:29:47AM -0400, Alan Stern wrote:
> > > On Fri, Sep 12, 2025 at 09:33:05AM +0100, Russell King (Oracle) wrote:
> > > > On Thu, Sep 11, 2025 at 10:30:09PM -0400, Alan Stern wrote:
> > > > > The USB subsystem uses only one pair of callbacks for suspend and resume 
> > > > > because USB hardware has only one suspend state.  However, the callbacks 
> > > > > do get an extra pm_message_t parameter which they can use to distinguish 
> > > > > between system sleep transitions and runtime PM transitions.
> > > > 
> > > > Unfortunately, this isn't the case. While a struct usb_device_driver's
> > > > suspend()/resume() methods get the pm_message_t, a struct usb_driver's
> > > > suspend()/resume() methods do not:
> > > > 
> > > > static int usb_resume_interface(struct usb_device *udev,
> > > >                 struct usb_interface *intf, pm_message_t msg, int reset_resume)
> > > > {
> > > >         struct usb_driver       *driver;
> > > > ...
> > > >         if (reset_resume) {
> > > >                 if (driver->reset_resume) {
> > > >                         status = driver->reset_resume(intf);
> > > > ...
> > > >         } else {
> > > >                 status = driver->resume(intf);
> > > > 
> > > > vs
> > > > 
> > > > static int usb_resume_device(struct usb_device *udev, pm_message_t msg)
> > > > {
> > > >         struct usb_device_driver        *udriver;
> > > > ...
> > > >         if (status == 0 && udriver->resume)
> > > >                 status = udriver->resume(udev, msg);
> > > > 
> > > > and in drivers/net/usb/asix_devices.c:
> > > > 
> > > > static struct usb_driver asix_driver = {
> > > > ...
> > > >         .suspend =      asix_suspend,
> > > >         .resume =       asix_resume,
> > > >         .reset_resume = asix_resume,
> > > > 
> > > > where asix_resume() only takes one argument:
> > > > 
> > > > static int asix_resume(struct usb_interface *intf)
> > > > {
> > > 
> > > Your email made me go back and check the code more carefully, and it 
> > > turns out that we were both half-right.  :-)
> > > 
> > > The pm_message_t argument is passed to the usb_driver's ->suspend 
> > > callback in usb_suspend_interface(), but not to the ->resume callback in 
> > > usb_resume_interface().  Yes, it's inconsistent.
> > > 
> > > I suppose the API could be changed, at the cost of updating a lot of 
> > > drivers.  But it would be easier if this wasn't necessary, if there was 
> > > some way to work around the problem.  Unfortunately, I don't know 
> > > anything about how the network stack handles suspend and resume, or 
> > > what sort of locking it requires, so I can't offer any suggestions.
> > 
> > I, too, am unable to help further as I have no bandwidth available
> > to deal with this. Sorry.
> 
> Thanks for all the valuable input.
> 
> I’ll process the feedback and investigate possible ways to proceed. As a
> first step I’ll measure the actual power savings from USB auto-suspend
> on AX88772 to see if runtime PM is worth the added complexity.

I ran quick power measurements to check whether USB autosuspend is worth the
added complexity.

Meaning:
- "admin up/down" = ip link set dev <if> up/down.
- No link partner was attached, so the physical link was down in all tests.

Setups:
- Debian 5.10 (USB autosuspend present, no phylib).
- Debian 6.1 (phylib present, no regression).
- Power meter: Fnirsi FNB58.
- Env: QEMU 9.2.1 (USB passthrough)
       xHCI host Intel 100/C230
       device ASIX AX88772B (0b95:772b)

Legend:
- "RT: active" = runtime PM on;
- "RT: suspended" = runtime PM auto (device suspended).

Results:
- Kernel 5.10.237-1
  admin up (link down): 0.453 W (RT: active)
  admin down: 0.453 W (RT: active)
  admin down: 0.453 W (RT: suspended)

- Kernel 6.1.148-1
  admin up (link down): 0.453 W (RT: active)
  admin down: 0.248 W (RT: active)
  admin down: 0.248 W (RT: suspended)

Observations:
In this setup, USB autosuspend did not reduce power further (admin-down power
is identical with/without autosuspend).

The drop from ~0.453 W -> ~0.248 W on 6.1 appears to come from the phylib
migration (PHY powered down on admin-down), not from autosuspend.

Proposal:
Given autosuspend brings no measurable benefit here, and it hasn’t been
effectively functional for this device in earlier kernels, I suggest a minimal
-stable patch that disables USB autosuspend for ASIX driver to avoid the
PM/RTNL/MDIO issues. If someone needs autosuspend-based low-power later, they
can implement a proper device low-power sequence and re-enable it.

Would this minimal -stable patch be acceptable?

Best Regards,
Oleksij
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