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: <ccfd7d48-401b-4f25-ac8e-aa6aa9654956@rowland.harvard.edu>
Date: Wed, 17 Sep 2025 10:57:23 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Hubert Wiśniewski <hubert.wisniewski.25632@...il.com>
Cc: Oleksij Rempel <o.rempel@...gutronix.de>,
	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>,
	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 04:31:40PM +0200, Hubert Wiśniewski wrote:
> On Wed Sep 17, 2025 at 3:54 PM CEST, Alan Stern wrote:
> > 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)?
> 
> I have tested this. With this patch, it seems that writing "auto" to
> power/control has no effect -- power/runtime_status remains "active" and
> the device does not get suspended. But maybe there is a way to force the
> suspension anyway?

I don't know exactly what's going on in your particular case.  However, 
if you read the source code for control_store() in 
drivers/base/power/sysfs.c, you'll see that writing "auto" to the 
attribute file causes the function to call pm_runtime_allow().

If you turn on CONFIG_PM_ADVANCED_DEBUG there will be extra files in the 
.../power/ directory, showing some of the other runtime PM values.  
Perhaps they will help you to figure out what's happening.

Alan Stern