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

Oleksij Rempel
2025-09-13T06:45:22Z

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.

Best Regards,
Oleksij
--
Pengutronix e.K. | |
Steuerwalder Str. 21 | http://www.pengutronix.de/ |
31137 Hildesheim, Germany | Phone: +49-5121-206917-0 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |