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

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Message-ID: <aMUS8ZIUpZJ4HNNX@pengutronix.de>
Date: Sat, 13 Sep 2025 08:45:05 +0200
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Alan Stern <stern@...land.harvard.edu>,
	Jakub Kicinski <kuba@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Hubert Wiśniewski <hubert.wisniewski.25632@...il.com>,
	stable@...r.kernel.org, kernel@...gutronix.de,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Lukas Wunner <lukas@...ner.de>, Xu Yang <xu.yang_2@....com>,
	linux-usb@...r.kernel.org
Subject: Re: [PATCH net v1 1/1] net: usb: asix: ax88772: drop phylink use in
 PM to avoid MDIO runtime PM wakeups

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
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