netdev - 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: <aMM1K_bkk4clt5WD@shell.armlinux.org.uk>
Date: Thu, 11 Sep 2025 21:46:35 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Jakub Kicinski <kuba@...nel.org>
Cc: 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 07:55:13AM -0700, Jakub Kicinski wrote:
> On Thu, 11 Sep 2025 15:39:20 +0100 Russell King (Oracle) wrote:
> > I'm not surprised. I'm guessing phylib is using polled mode, and
> > removing the suspend/resume handling likely means that it's at the
> > mercy of the timings of the phylib state machine running (which is
> > what is complaining here) vs the MDIO bus being available for use.
> > 
> > Given that this happens, I'm convinced that the original patch is
> > the wrong approach. The driver needs the phylink suspend/resume
> > calls to shutdown and restart phylib polling, and the resume call
> > needs to be placed in such a location that the MDIO bus is already
> > accessible.
> 
> 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...

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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