netdev - Re: [PATCH net v1 1/1] net: usb: asix: forbid runtime PM to avoid PM/MDIO + RTNL deadlock

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Message-ID: <c94af0e9-dc67-432e-a853-e41bfa59e863@rowland.harvard.edu>
Date: Wed, 17 Sep 2025 09:54:01 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Oleksij Rempel <o.rempel@...gutronix.de>
Cc: Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Hubert Wiśniewski <hubert.wisniewski.25632@...il.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>, stable@...r.kernel.org,
	kernel@...gutronix.de, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, Lukas Wunner <lukas@...ner.de>,
	Russell King <linux@...linux.org.uk>, Xu Yang <xu.yang_2@....com>,
	linux-usb@...r.kernel.org
Subject: Re: [PATCH net v1 1/1] net: usb: asix: forbid runtime PM to avoid
 PM/MDIO + RTNL deadlock

On Wed, Sep 17, 2025 at 11:54:57AM +0200, Oleksij Rempel wrote:
> Forbid USB runtime PM (autosuspend) for AX88772* in bind.
> 
> usbnet enables runtime PM by default in probe, so disabling it via the
> usb_driver flag is ineffective. For AX88772B, autosuspend shows no
> measurable power saving in my tests (no link partner, admin up/down).
> The ~0.453 W -> ~0.248 W reduction on 6.1 comes from phylib powering
> the PHY off on admin-down, not from USB autosuspend.
> 
> With autosuspend active, resume paths may require calling phylink/phylib
> (caller must hold RTNL) and doing MDIO I/O. Taking RTNL from a USB PM
> resume can deadlock (RTNL may already be held), and MDIO can attempt a
> runtime-wake while the USB PM lock is held. Given the lack of benefit
> and poor test coverage (autosuspend is usually disabled by default in
> distros), forbid runtime PM here to avoid these hazards.
> 
> This affects only AX88772* devices (per-interface in bind). System
> sleep/resume is unchanged.

> @@ -919,6 +935,16 @@ static int ax88772_bind(struct usbnet *dev, struct usb_interface *intf)
>  	if (ret)
>  		goto initphy_err;
>  
> +	/* Disable USB runtime PM (autosuspend) for this interface.
> +	 * Rationale:
> +	 * - No measurable power saving from autosuspend for this device.
> +	 * - phylink/phylib calls require caller-held RTNL and do MDIO I/O,
> +	 *   which is unsafe from USB PM resume paths (possible RTNL already
> +	 *   held, USB PM lock held).
> +	 * System suspend/resume is unaffected.
> +	 */
> +	pm_runtime_forbid(&intf->dev);

Are you aware that the action of pm_runtime_forbid() can be reversed by 
the user (by writing "auto" to the .../power/control sysfs file)?

To prevent the user from re-enabling runtime PM, you should call 
pm_runtime_get_noresume() (and then of course pm_runtime_put() or 
equivalent while unbinding).

Alan Stern