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: <aMPawXCxlFmz6MaC@shell.armlinux.org.uk>
Date: Fri, 12 Sep 2025 09:33:05 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Alan Stern <stern@...land.harvard.edu>
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 Thu, Sep 11, 2025 at 10:30:09PM -0400, Alan Stern wrote:
> On Thu, Sep 11, 2025 at 09:46:35PM +0100, Russell King (Oracle) wrote:
> > On Thu, Sep 11, 2025 at 07:55:13AM -0700, Jakub Kicinski wrote:
> > > We keep having issues with rtnl_lock taken from resume.
> > > Honestly, I'm not sure anyone has found a good solution, yet.
> > > Mostly people just don't implement runtime PM.
> > > 
> > > If we were able to pass optional context to suspend/resume
> > > we could implement conditional locking. We'd lose a lot of
> > > self-respect but it'd make fixing such bugs easier..
> > 
> > Normal drivers have the option of separate callbacks for runtime PM
> > vs system suspend/resume states. It seems USB doesn't, just munging
> > everything into one pair of suspend and resume ops without any way
> > of telling them apart. I suggest that is part of the problem here.
> > 
> > However, I'm not a USB expert, so...
> 
> 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)
{

Thanks.

-- 
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