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: <a25b24ec-67bd-42b7-ac7b-9b8d729faba4@rowland.harvard.edu>
Date: Fri, 12 Sep 2025 10:29:47 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Jakub Kicinski <kuba@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Oleksij Rempel <o.rempel@...gutronix.de>,
	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 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.

Alan Stern